Good evening! We'll have a cool, gray, cloudy, raw, damp day on Saturday with temperatures running some 25 to 30 degrees cooler than what we had Friday. High temperature at my place in Whiteville was 83 degrees, about 25 degrees above the seasonal average. We'll have more beautiful spring-like warmth next week, but first we have to deal with Saturday.
click or tap on the images to enlarge
Saturday's weather will feature an easterly wind off the Atlantic ocean bringing lots of clouds and occasional areas of light rain and drizzle. Not looking at an all-day rain, and not looking at significant rainfall amounts, but it'll be just enough to require use of the windshield wipers every once in a while, and require the use of headlights if you're doing any traveling. The area of low pressure shown by early Saturday afternoon over northern Georgia/extreme SW North Carolina will move to just north of Virginia Beach by tomorrow evening, and then well off to the north and east by Sunday morning. The area of organized, heavier rain will move north through Virginia and not impact southeast NC or northeast SC.
IMPACTS: Rainfall -- light, spotty, intermittent, nuisance-type. No significant impacts.
Here's a look at forecast high temperatures the next few days, courtesy of NOAA (click to enlarge).
EXTENDED OUTLOOK: A subtropical ridge will be firmly in control through much of the upcoming work week. A moist flow of southerly air will crank our temperatures well above the seasonal averages. There is a chance of some showers on Washington's Birthday (Monday, aka "President's Day"), as a warm front lifts south to north over the area. This opens the doors to the dramatic warm-up for the middle of the week. The next chance for showers comes Thursday as a cold front begins to approach from the north.
GET OUT AND ENJOY IT!!
Our temperatures will take a wild ride over the next few days, but a subtropical ridge with a moist, southerly flow of air will pump our temperatures to well above seasonal norms for most of next week.
Tomorrow will be breezy and unseasonably warm... in fact, some local records may fall. I think most all of us will get up into the 80s tomorrow on southwesterly winds gusting up to 30 mph at times. Skies will feature a mix of clouds and sunshine for MOST of the day. A cold front will drop down from the north late in the afternoon through the evening, and a line of showers can be expected with this frontal passage. Winds will shift around from the north 10-15 mph with gusts into the 20s behind the front with a pretty sharp temperature drop.
SCHOOLDAY-- The morning bus stop will feature dry conditions with partly cloudy skies. There will be some patchy fog, most prevalent as you get closer to the coast, with locally dense beach fog possible. Temperatures will be a few degrees either side of 60 inland and at the coast. Click or tap on the graphics to enlarge.
As mentioned, a cold front will drop down from the north during the late afternoon / early evening hours on Friday. This will bring with it a line of showers and a wind shift. While this will be quite the cold front passage, overall instability parameters are limited so thunder doesn't appear to be an issue with this one.
For Saturday, we will experience a very sudden return to ... well ... actually relatively seasonable temps, but a high pressure "wedge" very common for springtime in NC will keep a northeast wind flow off the Atlantic. This northeast wind flow will keep lots of clouds and occasional areas of light rain and drizzle over the entire region through the day. Quite the significant change from Friday!
Our front lifts back to the north over the state Saturday night into Sunday bringing us a return to warmer temperatures for Sunday.
click/tap to enlarge
Here are your forecast graphics for southeast NC and northeast SC. Warm temperatures under variably cloudy skies will persist for at least the first half of next week... possibly even longer.
Note: Click/tap on each graphic to enlarge.
CHANGE is the theme of my forecast blog tonight, as we are on the cusp of the winter season... and maybe even the chance for some snowflakes?
Tuesday will be a very unseasonably warm day as a warm front will move through early, bringing the chance for some showers. Instability will keep a mix of clouds and sunshine on a strong southerly flow, and the chance for some isolated to widely scattered showers will persist throughout the day, enough that I'm giving us a yellow-light in the outdoor activity column on my stoplights (see below).
A strong cold front sweeps through the region late Tuesday night, clearing the coast Wednesday morning. Showers will accompany this frontal passage.
Low pressure is expected to organize on the stalled front by the end of the week and bring a cold rain to the region.
^^ Southerly flow ahead of a strong cold front will bring unseasonably warm temperatures for your Tuesday. Even with a mix of clouds and sunshine, we'll see highs in the upper 60s to lower 70s across the region, with some mid 70s possible in typically warmer locations or areas that pick up more sunshine. This will be the last day with 70-degree temperatures, possibly through the end of the calendar year.
^^ By Wednesday afternoon the frontal boundary is expected to be off the coast, and most of the precipitation should go with it. I'm doubtful that we'll see much sunshine on Wednesday, but whatever rain we get should be widely scattered. It will be MARKEDLY colder with highs only in the upper 50s.
^^ As our story plays forward into Thursday, a secondary shortwave / frontal boundary will sweep across the Carolinas and "catch up" to the first front off the coast. Low pressure will develop somewhere along this front and bring cold rain to the region much of Thursday and Friday. This model might be a bit underdone with regards to the rainfall.
There has been some social media talk recently about snow potential for the Carolinas, particularly Friday. Models have waffled back and forth with regard to a frozen precipitation potential but the chances, in my opinion, are very, very slim, and this is for a couple of reasons.
1. There is no strong Arctic high pressure to our northwest.
2. Even if there were a strong Arctic high, there is virtually zero snow pack to our north. A strong high over a snowpack would funnel particularly cold air southward. This isn't going to happen.
3. Cold air will "chase" the precipitation Friday. This almost never leads to accumulating snow. That's not to say some snowflakes won't occur, but the chances for an accumulating snowfall are very slim.
Meteorologist Tim Buckley has a neat post on his FB page that discusses what it takes to get snow in North Carolina:
Here are a couple of GFS model soundings for Friday afternoon, one for Whiteville and one for Fayetteville. Areas such as Elizabethtown, Conway, Tabor City will be similar to Whiteville. Areas such as Rockingham, Laurinburg, and Lumberton will be similar to Fayetteville.
First up... Whiteville:
This sounding shows temperature (red line) and dewpoint (green line) below freezing through the entire atmospheric column... except the last few hundred feet (very bottom of the image). Once those readings poke above freezing, that's it... snowflakes melt and we get cold rain. At the time of this sounding (1 pm Friday), it's cloudy and/or raining and 41 degrees at Whiteville.
This sounding for Fayetteville is also a no-go for snow, given the fact that there's a dry area from about 3 kilometers down to 1 kilometer above the surface. Dry air isn't exactly something that promotes precipitation, but even if that dry layer weren't there, the temperature for the final few hundred feet to the surface rises above freezing... any precipitation that falls will be liquid.
Ok.... here's my official forecast for southeast NC and northeast SC.
TONIGHT: Variably cloudy skies with areas of fog after midnight. We'll see mild temperatures with lows around 50.
TUESDAY: Mostly cloudy skies and unseasonably warm. Isolated to widely scattered showers possible. Highs in the lower 70s.
TUESDAY NIGHT: Showers becoming likely after midnight. Breezy and mild with lows in the lower to middle 50s.
WEDNESDAY: Nearly steady temperatures with cloudy skies. Showers in the morning... becoming more widely scattered late morning through the afternoon.
EXTENDED: Rain likely Thursday, Thursday night, and Friday, possibly lingering into early Friday night. It will be raw and cold with highs around 50 and lows in the mid 30s. While temperatures look close to freezing, the threat for frozen precip is so low it's not worth mentioning "officially" ... but of course, I'll be keeping an eye on that.
The cold weather is here to stay for the foreseeable future as well. The Climate Prediction Center has much of the eastern portion of the country with a high probability of temperatures being below seasonal normal values. I believe this trend will continue through the remainder of the calendar year... and with the pattern that is taking shape, I can't completely rule out that SOMEone in the Carolinas won't see some snow...
Take care and THANK YOU for viewing!
Good evening Coastal Carolinas! We have a very unremarkable weather pattern for the next several days... actually, right through next week!
Our stoplights are all green across the boards. It's really just a temperature forecast.
The ONLY interesting "thing" is a tiny ribbon of moisture that may bring a low overcast for a few hours over the coastal areas. The image below is a forecast sounding for Southport, N.C., for this evening. In fact, by the time you read this post, this might be a thing of the past.
A dry cold front will whisk across the region late Saturday evening. A southwesterly flow ahead of the front will bring mild temperatures to southeast NC... maybe even pushing 70 in some spots.
The front will move off the coast Sunday with cooler temperatures in its wake. Temperatures Sunday afternoon will struggle to reach 60, and low temperatures when you set off for work and school Monday will be in the lower 30s.
So... for tonight... I am going with variably cloudy skies for the overnight, with lows generally in the mid 30s. Some of the typically cooler spots may dip a little cooler, and of course along the coast, a bit milder... especially in areas that have more cloudiness.
Saturday looks like a real winner across southeast NC and northeast SC, with lots of sunshine and highs in the upper 60s... again, some areas may touch 70 tomorrow afternoon. Get out and enjoy it ®!
Saturday night looks variably cloudy once again with that frontal passage. The cold advection won't really begin to push in until Sunday morning, so I'm going with minimum temperatures in the mid 40s. Sunday will again feature lots of sunshine, but much cooler temperatures with highs around 60.
My extended forecast is high-and-dry. We'll see a warming trend through the week, with highs around 70 by Thursday. Notice - I did put a 10% chance of precipitation down for Wednesday as there is some indication in the modeling that there may be some atmospheric "convergence" along the coast. That's a soft call at this point and in reality there just may be a bit more cloudiness along the coast than anything else.
Click/tap on the graphics to enlarge.
LONG TERM: Weather patterns are going to be set up that cold air is going to be hard to come by for the eastern two-thirds of the country. If you're looking for snow and cold, you'll have to go well north into Canada for that. The Climate Prediction Center graphics show that temperatures should run above normal and precipitation below normal through December 8.
Explaining these graphics: These graphics show the PERCENT CHANCE that temperatures and precipitation will be above or below normal. They do NOT show "how much" above or below normal, just the percent possibility... i.e., a 60% chance of above-normal temperatures, or an 80% chance of below-normal precipitation. Click or tap on each graphic to enlarge.
Ok folks, that's it for tonight's update. I hope you have a wonderful weekend! Be safe if you're out traveling. If you have any questions or comments, hit me up on my Facebook page!
Good evening! I hope everyone was able to enjoy this chilly, but sunny, day across southeast NC.
A modestly unsettled weather pattern will affect the coastal Carolinas through the Thanksgiving holiday and into the weekend.
I kept us all green on the stoplights panel, but honestly could go with yellows for the coastal areas (mainly along and east of US-17) given the chances for rain Tuesday afternoon and again on Thanksgiving. So the panel tonight is something of a soft-call. I don't foresee any significant travel issues for Wednesday or Friday anywhere within the coastal Carolinas.
(Click/tap on each graphic to enlarge.)
While high pressure tries to maintain control of our weather, a coastal trough will develop and this will bring the potential for some rain along the coast. Meanwhile, a cold front will approach and move through the region overnight Tuesday night / early Wednesday morning. This will be a remarkably unremarkable frontal passage, but there may be some areas of light rain and fog, which may persist into Wednesday morning. This front will push offshore Wednesday.... leaving us with dry and pleasant conditions Wednesday afternoon.
A wave of low pressure is modeled to form along the frontal boundary offshore by Thanksgiving. With overrunning moisture from the south associated with this system, combined with high pressure wedging to our north (and a subsequent northeast flow)... I expect that areas along and east of I-95 will see lots of clouds, chilly temperatures, and maybe some spotty light rain. The highest rain chances will be near the coast once again. As you head inland, conditions should be dry, and there may be some more sunshine, but it's still going to be chilly thanks to that northeast flow.
All of this shown here may look semi-interesting, but as can be seen on the next graphic, rainfall amounts are expected to be quite low through 7 PM on Thanksgiving, with the highest numbers along the coast.
That all pulls away Friday and dry conditions prevail into the weekend. The next front, a much stronger one, looks to move through by the end of next weekend-(ish)... and models point to a dramatic cooldown behind this front.
Here's my official forecast for southeast NC and northeast SC. Have a great evening and thanks for viewing!
Note - Click/tap on each image to enlarge
We are in the time of year known as the "second severe weather season" for the coastal Carolinas. What does this mean? The type of severe weather changes a bit... from unstable, lightning-rich storms of the warm season to "convective showers" of the cool season.
What is this? This shows the change in wind speed as one goes from the ground surface to 4 miles up. Over Columbus County,NC, (and in areas in yellow), there is a 40-knot change in wind speed being shown. This is representative of strong increase in wind speed from the ground surface to a level 4 miles up. This is important. Why?
Stronger wind speeds with height allow thunderstorms to "tilt," which means the updraft is kept away from the downdraft. In our summertime storms, as shown by "A" on the image, the updraft and downdraft are more-or-less right next to each other, and eventually the downdraft will "choke out" the updraft, killing off the storms. When storms have a "tilt" to them ("B"), the rain falls ahead of the updraft, and therefore the storm is allowed to survive. It also allows for forward motion of the storm.
Slide #4 is a graphical representation of how wind shear affects storms. "A significant increase of wind speed with height will tilt a storm's updraft. This allows the updraft and downdraft to occur in separate regions of the storm, reducing water loading in the updraft. The downdraft will not cut off the updraft, and actually it will even enforce it. Strong upper tropospheric winds will move mass away from the top of the updraft. This reduces precipitation loading and allows the updraft to sustain itself. Directional shear in the lower atmosphere helps initiate the development of a rotating updraft. This is one component that is important to the development of a mesocyclone and the development of tornadogenesis. Strong lower tropospheric winds and directional shear together will [...] increase the tornado threat when severe storms develop" (Haby).
The situation that will present itself with this frontal boundary is known as "unidirectional shear. The speed shear will allow the storm to move. The movement insures the storm will last longer than an airmass thunderstorm. Unidirectional shear often produces storms that form into lines. Since the storm moves, outflow produces lift that enables new storms to grow on the storm's periphery. Over time, a line a storms result" (Haby).
Next up we have the European model surface-based CAPE (Convective Available Potential Energy). CAPE is just one of the ways forecasters measure instability in the atmosphere. These numbers are actually relatively low. Thunderstorm updraft strength is determined by the amount of positive "energy" in the atmosphere... large instability = large updrafts. Our summertime storms are driven by high levels of instability, but low shear (as mentioned above).
At any rate, the instability numbers as shown here are relatively low. Typically for severe thunderstorms, you'd want to see surface-based CAPE values 1000 or higher, and the higher the better.
The cold front. (Note - the graphic above is just for reference - the temperatures are fictional values.) For any kind of storms to develop, we need to get triggered! Well, the catalyst to our being triggered is a cold front, and a strong one. As Professor Haby states, "[c]old fronts tend to be the fastest movers compared to the other front types. This fast movement increases convergence along the front and results in faster storm movement, if storms do develop. The slope of a cold front is greater than that of the other frontal types. This results in convection that is more vertical (lifting associated with warm fronts has a large horizontal component). For severe weather to be associated with cold fronts, look for the following: high dewpoints ahead of the front (60 F or greater), strong upper level winds, front movement between 10 and 20 mph, and convergence along the front" (Haby). We will have all of the above.
Ok... so now that I have blinded you with science, or utterly cofounded you, lets get to the point. Slide #7 above is the surface forecast map, wind and radar "model" for 8 PM Monday evening. The frontal boundary is expected to be traversing the Carolinas by this time, with showers and thunderstorms along this front. Since we will have low instability, these won't be "thunderstorms" with lots of lightning... these will be more along the lines of "gusty showers" or "convective showers," or ... showers with some attitude. Since we will have strong winds higher in the atmosphere, downdrafts from these cells could bring the stronger winds down to the surface, causing areas of damage. There will also be the threat for isolated tornadoes. I wouldn't be surprised to see a tornado watch posted for late Monday evening through early (predawn hours) Tuesday.
A secondary cold front is shown by this model farther to the west, but that one is dry and is more of a "reinforcing shot" bringing much colder air southward.
Slide #8 above shows is the surface forecast, wind/radar model for 8 AM Tuesday. As you can see, the main cold front is already forecast to be passing off the coast by time you head to work or school on Tuesday. The secondary boundary "dies" behind it. We'll see decreasing wind/rain threat, with clearing skies.
Slide #9 is the skinny... what are the severe weather risks with this? Given a high shear/low CAPE event, we can effectively rule out large hail. You need ice in the cloud to create the prolific lightning that we typically see during summertime storms. Since we won't be looking at a lot of instability, we won't see much hail aloft, and therefore not much lightning. Since we DO have very strong winds right off the deck, downdrafts from thunderstorms could bring damaging winds to the surface and isolated tornadoes.
Finally, in terms of picking up some much-needed rainfall, the squall line(s) could bring torrential downpours, that could produce localized poor-drainage flooding.
The timing of this threat will be between 8 PM Monday and 8 AM Tuesday.
I hope this will not only alert you to the potential for some nasty weather Monday night, but explain a little bit of the "why" behind it. Thank you for reading and feel free to share!
Meteorologist Christopher Cawley
Good evening! We'll have one more day of heat and humidity before a big pattern change introduces a taste of autumn into our region. We may also be dealing with effects from a potential off-shore tropical or subtropical system early next week.
During the day Wednesday, a cold front will be bearing down on western and central North Carolina. Ahead of this, hot and humid air will continue to pump into the region. Isolated storms are expected in the early afternoon, with a better chance of showers and storms in the late afternoon into the evening. I don't anticipate any significant severe weather, but a storm or two might pulse severe.
The front slowly moves into central and east-central NC Wednesday night into Thursday morning. I expect a fairly good coverage of showers and thunderstorms during this period. By Thursday afternoon the front should be approaching the coast. As for a severe potential, similar to Wednesday, one or two storms could be severe-warned, but on the whole, I don't see an exciting severe weather event occurring.
Regarding heavy rain, the storms, of course, could produce locally heavy rain. The potential is there for some places to pick up a good couple of inches of rain, which could lead to localized ponding and flooding of poor-drainage or low-lying areas.
The front clears the coast and is a memory by Friday afternoon. We can expect perhaps some morning clouds and maybe a renegade shower or storm, but otherwise clearing conditions and much, MUCH cooler and less humid.
An absolutely beautiful weekend is on tap as high pressure of Canadian origin keeps dry conditions and temperatures below to well-below normal. Meanwhile, there is the potential that a tropcial or subtropical system develops somewhere off the southeast coast, on the remnant trough. While this shouldn't directly affect our region, coastal areas may have an increase in cloudiness early next week with some showers possible. I believe we'll all experience breezy conditions as the tropical/subtropical low interacts with the strong high pressure ridge well to our north. Temperatures should continue to run below normal.
I like to sprinkle in some extras in these blog posts, so I thought I'd show some model graphics. The first one is the GEFS 2-meter temperature anomaly chart. This is the GFS ensemble (multiple runs of the GFS model with one or more calculations "tweaked" a little bit). The blue areas show the magnitude of the below-normal temperatures at 2 PM next Monday... as determined by our models. Is this gospel? No, of course not, but since the ensembles are all pointing in the same direction, it's a pretty good bet that it comes true.
The next graphic is more-or-less another way of looking at ensemble model data, the EKDMOS chart. A forecaster can examine EKDMOS charts for hourly temps, max/min temps, wind speeds, precip chance, etc. It's just one of the tools in a forecaster's toolbox. The EKDMOS is a graphical representation of model output. The green line with triangles represents the bottom value, or the 10th percentile (90 percent of models predict values HIGHER than this number). The blue line with squares on the end indicates the 90th percentile, meaning that 90 percent of the models predict values LOWER than this number. The red, of course, is the mean... the average.
The closer the bars are to each other indicate good confidence or good agreement amongst the ensemble of models. A wide spread, of course, indicates the very opposite -- a wide range of numbers shown by the model.
I have shown here graphs for Maxton/Laurinburg, and for the Wilmington International Airport. (Other areas are similar.) The Maxton graph shows the ensemble mean maximum temperature BELOW 80 DEGREES (!!) Sunday, Monday, and Tuesday, and ensemble mean minimum temperatures around 60 degrees Sunday night/Monday morning, and Monday night/Tuesday morning.
The ILM airport graph is similar, just a little bit warmer given proximity to the very warm (lower/middle 80s) ocean water.
Are these numbers gospel? No. Again, it's just one of our tools.
Finally, here is a graphic from the NWS Climate Prediction Center, which demonstrates the confidence on below-average, average, or above-average temperatures. The darkest blue represents a 70% chance that temperatures will be below normal. It does NOT demonstrate how MUCH below normal... it merely tells us, yeah, we're really confident that it's going to be cooler than normal.
My stoplight ImpactCast highlights Thursday as a day to avoid any outdoor plans. While the heat index won't be an issue, numerous showers/storms will pretty much wipe out the chances for lawn mowing or ball games, etc. Travel conditions get a yellow light as well given the potential for isolated flooding.
If you want to go to the beach tomorrow, for the most part you should be okay. Just keep an eye to the sky and if threatening weather approaches, take shelter. "When thunder roars, go indoors."
THERE IS A MODERATE RIP CURRENT RISK for Brunswick County and New Hanover County beaches (and points north).
Here are the remainder of your forecast graphics ... and like I said, this weekend is going to be spectacular. Get out and enjoy it!
Going to try something new here. I posted these slides on the Facebook page, but I want to try setting up a blog where I go into more detail regarding the forecast for the area. We'll see how this pans out.
Click / tap on each graphic to enlarge.
Without further ado...
We had a really hot day today (Friday) across the eastern Carolinas, with high temperatures well into the 90s and heat index values upwards of 110 in some spots. This is in response to a large heat ridge located over the southern United States this afternoon. This ridge will break down a bit and allow a trough to set up over the eastern third of the country.
A trough is typically associated with stormy (or inclement) weather conditions. In our case, a cold front will be pushing toward the eastern Carolinas by Saturday afternoon. This will be the spark that brings showers and thunderstorms to portions of the region.
The main "dynamics" of this system will be with that wave of low pressure near Columbia, S.C. This low will move northeast along the frontal boundary to a position near the N.C./S.C. border by Sunday afternoon. The strongest dynamics with this system, and the greatest shower and storm threat, will exist along a weak sea-breeze boundary trying to push inland, as well as along the coast in general and particularly so toward northeast North Carolina. I believe this is where the brunt of the shower and storm activity will occur. BUT... while some areas may indeed remain dry, you should keep an umbrella handy for whatever plans you have Saturday.
If you look above at the 500 mb map, just off the map to the right of where I have written "trough," is the center of the Bermuda high pressure ridge, a semi-permanent summer feature which is responsible for heat and humidity. This will work as a "roadblock" on our front, causing it to go nowhere fast and eventually just dissipate Sunday and Monday.
(Again, click or tap on each to enlarge.)
This, of course, means not-so-good news for our eclipse viewing. The NWS office in Columbia, S.C., has a forecast page dedicated to the eclipse and their forecast probabilities. Before I post that, I want to show you what a couple of the weather models are "thinking" in regards to Monday afternoon.
For cloud cover... not looking all that optimistic as far as models go. The GFS tends to be quite overcast while the CMC (Canadian) tends to be a bit more optimistic with less in the way of cloud cover.
With the clouds come shower and thunderstorm chances. Again the GFS is quite bullish in regards to the precipitation coverage than the Canadian. My honest feel is that it will be someplace in between the two.
Again, my opinion is that we'll be somewhere in between. I think there'll be a pretty good skyscape of towering cumulus clouds, but most likely not a full overcast. There may be a scattering of showers and thunderstorms dotting the landscape as well. Pinning down who gets what is impossible at this point... some places will get a great view, and some, unfortunately, will not.
Here is the percentage of cloud cover and percent probability of precipitation from the NWS office in Columbia, S.C.
Unfortunately the only thing I am certain of is that it will be hot.
Ok, wrapping it all up and putting a bow on it, here are your forecast images for southeast N.C. and northeast S.C., including the beach forecast and tides.
Have a great evening. Any questions, comment here or contact me on Facebook at www.facebook.com/CCSkywatch.
Today we are taking a look at an atypically strong (for mid-May) mid-latitude cyclone.
In the northern hemisphere, cyclones (low pressure) have counter-clockwise winds, and anticyclones (high pressure) have clockwise winds.
Mid-latitude cyclones (MLCs) typically feature the low pressure center, a warm front, and a cold front, and the cold front usually has a faster forward speed than the warm front. When the cold front overtakes the warm front, an occlusion forms.
South of a warm front and east of a cold front we have the "warm sector" of a storm system. Winds are typically out of the south or southeast, and bring copious moisture from either the Gulf of Mexico or the mid-Atlantic ocean. This is what our area was in Thursday evening. Abundant moisture-laden, warm, unstable air was pushed into the Carolinas on the coattails of this southerly to southeasterly flow. As well, there was a lot of SHEAR, which is a change of wind speed and/or direction with height. These two combined to bring the at-times tornadic thunderstorms to the region. The winds just off the surface (within the first mile or so) were very strong and coming from a different direction when compared to the surface winds.
A warm front indicates where warm air from the south is rising up and over cooler air (relatively speaking) at the surface. This typically brings lots of clouds, along with periods of rain but generally not severe weather (although, again, there are exceptions).
A cold front is usually the focus for severe weather in the springtime across our region. (Drylines are prevalent in the Plains but that's a different story.) A cold front provides a source of "forcing," or "lift." It literally lifts up a parcel of air (think 3D), which causes the air to cool faster than it surrounding environment, and causing thunderstorms to develop. When you see a "squall line" (a narrow-ish but discernible LINE of thunderstorms) you can infer the presence of a cold front. As the front pushes eastward, the squall line pushes eastward.
So what makes this system different? Several times this spring we have had MLCs that are "vertically stacked." What? Most of the time, low pressure is only at the surface and perhaps to 5,000 or maybe 10,000 feet. This year we have had systems that have low pressure extending upwards of 18,000 to 30,000 feet. The implication is that there are many layers of low pressure from the surface to that level... they are "stacked up," like when you stack dinner plates in the sink for someone else to wash. Hence, our MLC is vertically stacked.
(Click or tap to enlarge.)
The visible satellite images above show the comma-shaped MLC quite nicely. The annotated version of the visible satellite shows the position of the isobars (lines of equal barometric pressure), the fronts, lows, and highs. A 993-millibar low pressure system is quite deep for this time of year. The term "993 millibar" is air pressure. Defining that in terms that we might hear on the nightly news weathercast, 993 millibars equals a barometric pressure of 29.32 inches of Mercury. When the pressure is low like that, we say that the MLC is very "deep." The 1013-millibar high is equivalent to a TV meteorologist saying the barometric pressure is 29.91 inches of Mercury. The "normal" barometric pressure on the Earth is 1013.25 millibars, or 29.921 inches of Mercury.
The first Earth Wind Map demonstrates the surface winds from about 4:00 PM Friday May 5. This one is not annotated, other than the major cities for reference points.
Looking at the annotated version (map #2), we see that the location of the low pressure center, and the frontal boundaries, aren't as "nice and neat" as they are on the visible satellite (or what you'd see on TV). As I noted on the slide, science and nature are perfect, where man is not, and there is no doubt I made errors in my annotations of boundaries.
Anyway, a frontal boundary is merely a shift in wind direction. In the northern hemisphere, where winds go from blowing out of the south, to suddenly blowing out of the west, one can imply a cold front lies at that junction. Winds blowing out of the south, turning to blowing from the east or southeast, indicates the presence of a warm front. Also remember I said winds around low pressure flow counter-clockwise.
If you look carefully you can see a boundary right along the spine of the Appalachian mountains. This is merely a surface trough; winds are blowing northward into the cyclone. Since the winds are parallel to the mountains, the air is rising on both sides of the mountains and funneling northward.
You'll also notice the winds over the ocean, as well as over the great lakes, are stronger, whereas the winds over the land surface are considerably slower. This is due to friction from the land surface. Over the ocean, that friction doesn't exist as strong so the winds can be much stronger. Over land, however, things get a bit more "sticky" so to speak... especially when the land features include various valleys and mountains. Look carefully north of New York City (on map #1 above) and you can make out the outline of the Hudson River Valley.
Surface weather features are "driven" by processes higher up in the atmosphere. Typically a forecaster will examine charts at 5000 feet, 10,000 feet, 18,000 feet, 30,000 feet, etc. I posted the wind map from 18,000 feet (about 3.4 miles off the ground). One of the first things we note is that friction doesn't matter here. There is a southwest wind blowing at about 92 mph over central North Carolina at 18,000 feet. We also notice our vertically stacked low center and the "upper level trough." The upper trough indicates the strength of our area of low pressure, as the lower air pressure extends the length of that trough. When you have a strong trough such as this, air movement picks up speed as it rounds the base of the trough (the southern end). In this case, the strongest winds are directly west and east of the low 'center', but we have increasing wind speeds from the base of the trough east-northeast all the way into New England.
Well what in the world does that mean? Simply put, the strengthening winds strengthen the weather features below that. As well, thunderstorms routinely reach the 18,000 foot level (sometimes much, much higher) and can "mix down" some of these very strong winds to the surface, thus making the thunderstorm classified as "severe." Furthermore, if the winds are blowing in different directions at the 18k foot level, versus the 5k foot level, versus the surface, we have that directional shear I mentioned waaaaay up above in this lengthy blog. Shear is needed for thunderstorms to become severe; shear helps keep the storm moving forward (as opposed to stationary); shear is also a factor in tornado development.
In the environment the evening of the 4th and overnight into the morning of the 5th, there wasn't much in the way of directional shear (change of direction with height), but there was a good level of speed shear (change of speed with height). However, there was just enough directional change to put a "spin" on even the relatively "shallow" thunderstorms (storms that weren't all that tall), that they generated areas of rotation seen on radar.
The last map,#4, is the jet stream level. Take a second to acclimate yourself; I realize there are a lot of colors and a lot of text on the image. I have highlighted Miami, Wilmington, and New York City in yellow font. Troughs and ridges were apparent at the 18k foot level, but are REALLY apparent at the jet stream level (34,000 feet). A ridge is a broad area of high pressure, of sinking air, and usually you have dry weather underneath a high pressure ridge. Alternatively, a trough is a broad area of low pressure, of rising air, clouds, precipitation... an unsettled weather pattern.
Our vertically stacked low is spinning its wheels over Appalachia with a very deep, broad trough extending from Canada all the way down to the Gulf of Mexico. Similar to the 18k foot chart, we see the strongest winds where they round the base of the trough and head northeastward. In this case, the southern jet stream (highlighted, over Mexico) "phases" with the northern, or "polar" jet. This is also somewhat unusual for mid-May and added additional strength to our MLC. In the middle of the Atlantic ocean you see the well-established Bermuda High pressure ridge that will eventually be responsible for our weather in June, July, August... you know... mid 90s... high humidity... etc.
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Our atmosphere is a fluid. Air is a fluid... it is water in the gaseous state. The atmosphere is a dynamic, always-changing, always-moving entity (for lack of a better word). When you see blue skies and scattered clouds, there are many different processes at work, and all are connected with each other to bring the weather you and I on the surface experience. I hope to have shown you just a sampling of this web as it relates to our classic MLC.
Thanks for reading!