Tornado Analysis for Wednesday

…Most significant threat for tornadoes in Alabama since 2020…

Warm air has been in place over Alabama for days, with beautiful sunshine and temperatures reaching 80. But it is mid-March, and these temperatures are 15 degrees above normal. Remember, 28 years ago this weekend, we had a Blizzard! Winter is not over, but with longer days and more direct sunlight over the southern U.S., the contrasts that produce severe weather are building.

With a weak upper low far to our north on Monday…we experienced isolated, but some intense, thunderstorms over Alabama. Several of these rotated, and one produced at least a wall cloud and funnel cloud just north and west of BHM, and possibly a brief tornado west of Adamsville. See radar info and pictures below. Another produced hail in the Gadsden area Monday evening.

Radar and photography of possible tornado around 2:45 pm today.

With warm, humid air in place over Alabama, we could see scattered showers and thunderstorms just about anytime today also, but most of those should not be severe.

A stronger upper-level trough with a cutoff low in the center is currently spinning near Las Vegas, NV, with a 125 mph jet at 17,000 feet to its south. The dry air being pulled in by the jet is clear on water vapor channel satellite imagery this morning. That big upper trough will cross the Rockies today and become negatively tilted over KS/OK/TX by Wednesday.

Satellite water vapor (now, top), 500 mb chart (now, middle), 500 mb chart (Wed evening, bottom)

Due to the upper-level divergence ahead of this large upper-level system, a surface low pressure area will form to the Lee of the Rockies late today, and reach Missouri by Wednesday afternoon. The air mass over the Southeast U.S. is already warm and humid in many areas (65 degrees before sunrise here in BHM) But, the deep, strong southerly flow out ahead of this surface low will bring even warmer, more humid air north from the Gulf of Mexico and Caribbean. The air in BHM on Wednesday afternoon, for example, will come from the area just west of Cuba. This will allow for dewpoints, the best measure of humidity in the atmosphere, to rise way up into the 60s over most of southern, central, and western Alabama.

NOAA Forecast Trajectories Ending in several cities in the SE US
(pivotalweather.com)

An interesting feature tknown as “cold air damming”, where very cold air in the Northeast US can’t get over the Appalachians and pours south into the Carolinas, Georgia, and eastern Alabama, will likely be present early, maybe as far west as I-65. But, the strong southerly flow will push this cold air out of the way. This may be associated with some cool, overrunning rain during the day Wednesday, delaying the threat for severe weather. However, strong thermal boundaries can serve as focal points for storm rotation later.

So how will this all play out?

Expect some rain over parts of Alabama Wednesday morning, as the main surge of warm, humid air moves in and pushes the cooler air out of the way. By Wednesday afternoon, some areas may see a bit of sunshine, warming temperatures into the 70s. Whether or not the sun comes out Wed afternoon may play a key role in the intensity of nighttime storms (the sun came out the day of the Fultondale EF-3 tornado). Either way, with the upper forcing, temperatures aloft will be cold, allowing for CAPE values of 1,000 to 2,000 J/kg. A few isolated, severe storms may develop across parts of AR/MS/and western AL Thursday afternoon in the unstable air mass with enough wind shear for storm rotation and tornadoes. This threat would mainly be west of I-65. Below is the expected atmospheric temperature and wind profile from Bluff Creek, AL, in Walker County about 25 miles west of BHM. Note the wind shear (winds increasing in speed and changing direction with height at low-levels); this causes helicity (247) that is not extreme, but enough to create storm rotation with instability in place (CAPE 2100 J/kg). This is an aggressive model…some do not have temperatures as warm as this.

(NAM model sounding for Bluff Creek, AL, 25 mi west of BHM, 7 pm Wed

The main event will likely come during the evening hours, after sunset, over north and central Alabama. The strongest forcing from the upper system will move in during the evening. Winds at 850 mb (about 5,000 feet) will increase to 60 mph, and SR helciity values may go above 300 m2/s2. With sunset and cooling, the air won’t be quite as thermodynamically unstable, but CAPE values over 1,000 J/kg are still likely. A combination of CAPE and shear that provides an indicator of tornado probability known as the Energy-Helicity Index (EHI) will still be 2-4, and only 1 is needed for tornadoes. Computer model forecasts for CAPE/SRH/EHI for 10 pm CDT Wednesday are shown below. By this time, a line of intense storms will likely be moving into Alabama ahead of the main cold front, with straight-line winds up to 75 mph, the potential for large hail, and several tornadoes. A few supercell storms may form out ahead of the line…these are often the most dangerous as they can produce strong tornadoes.

(data from pivotalweather.com)

There is still uncertainty with this system, perhaps more than normal due to 1) the closed upper low, 2) the cool Gulf waters due to our very cold February, and 3) the cold air coming in from the east. However, it looks like we will have an outbreak of severe weather, including some tornadoes, on Wednesday and Wednesday night. Even the computer models can’t agree (as shown below), with the American models more aggressive than the European model. But, they all show a risk for tornadoes and severe storms, peaking between 6 pm and 11 pm here in central Alabama.

No, this will not be April 27, 2011. The EHI that day was 11, not 3. But, it could be a dangerous weather day, and you should prepare now. Have multiple ways of getting warnings, including some that will work with no power. NOAA Weather Radio, phone apps like “Tornado” from the Red Cross, and your local TV station works best (I watch JP Dice and Wes Wyatt on Fox 6). Make sure you have batteries for flashlights, and that your phones are fully charged. Think about where you will go if a Tornado Warning is issued for your area. Remember, lowest floor, center of building, away from windows and doors, small room. Helmets, blankets, etc. to protect you from debris. And, even in a basement, get under something sturdy like a heavy pool table or in your car.

Another update tomorrow morning.

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Winter Storm Update – 3 am Mon

NWS Winter Storm Warning (pink, NOAA)

I don’t think I have ever seen so much of the country under a Winter Storm Warning in my 29 years in meteorology! This Arctic air mass just never quits. Check out 3 am temperatures. 34 here in BHM, but 30 in HSV, 30 in Meridian, MS, 21 in Natchez (with snow), and 31 in Baton Rouge (with freezing rain). Memphis has snow with 14 degrees.

Mesowest

The big upper system is digging in and a low is forming in the Gulf, bringing precipitation into the southeastern US.

NWS radar composite 300 am

Note the heavy precip near Baton Rouge and New Orleans…a lot of that is sleet and freezing rain…same near Jackson, MS and approaching Columbus, MS. Up near Memphis it is all snow. The low will continue to develop tonight, bringing more widespread precipitation to MS and AL. It should reach western AL by 5:00 am, and BHM by 9:00 am. Temperatures are key, as the air over most of MS and AL is too warm aloft for snow, but instead it will fall as rain then either refreeze before reaching the ground (sleet) or freeze on impact forming ice glaze (freezing rain). This will likely be a major ice storm from northern LA through central/eastern MS and northwest AL, with trees going down and power outages during extreme cold.

But, low pressure areas tend to follow temperature gradients, and the Arctic air has gotten so far south into Mississippi and Louisiana that it looks like the Gulf low will track more northeast than east, allowing warmer air to flow into central and eastern Alabama just above the shallow Arctic surface air, out ahead of the low. Winds are already out of the south at just 3,000 feet over central Alabama according to VAD wind profiles from the NWS Doppler Radar at Calera, AL at 3:00 am. Note the surface low position at 21Z (3 pm CST)…near Gadsden. Not at all classic for a winter storm in central Alabama. More for one in Mississippi and northwest Alabama, where the Winter Storm Warning is.

(pivotalweather.com)

Things could still change, as we are dealing with a shallow, extremely cold air mass (20 degrees and snow in south MS?). But it looks like warm air will flow into Tuscaloosa, Birmingham, and even Cullman just above the surface. With any wind mixing, plus rain drops bringing warmth to the ground, temperatures will probably stay steady or rise today, maybe as high as 40 degrees, as rain moves in. In northwest Alabama, and especially back into Mississippi and Louisiana, up to 0.5″ of ice will accumulate.

The big problem here in Birmingham could be Monday night and Tuesday, when temperatures will plummet below freezing by sunset, and into the upper teens by morning. Any water left around from the rain will freeze quickly, forming black ice on roads and causing traffic problems Tuesday, especially in the morning hours. It will stay very cold on Tuesday (maybe below freezing all day), with wind chills Tuesday morning near 10 at times. It looks like we will finally start to get out of this Arctic air, as it continues to spread out and slowly modify, by next weekend.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

colemanandknupp.com

Ice Storm Update – Sat 10 pm

(NOAA Probability of at least 0.1″ of ice accumulation through Monday night)

The brutally cold Arctic airmass that has gripped most of the Continental U.S. the past few days continues to slowly spread out now. It is a very shallow cold air mass, meaning if one goes up only 3,000 feet or so, the air is not nearly so cold. But, such air masses are very difficult to forecast. They don’t tend to move quickly, but instead spread out as a density current, like water poured on a table.

It looks like the coldest of the air will stay just north and west of our HQ here in Birmingham, but some current temperatures across SEC states include: St. Louis 5, Memphis 19, Austin 31, Nashville 25, New Orleans 35, Birmingham 35, Atlanta 43. See map below. Everything in blue is below freezing. Then there is Cooperstown, ND, where several of us guys went this Summer to the Reagan Minuteman Missile Museum…there it is currently -24 degrees!

9 pm temperatures (pivotal weather.com)

But then there is the problem with shallow cold air masses. Ice. And it looks like we will have some over parts of the Southeast Sdunay night through Monday. Winter Storm Watches or Warnings are currently in effect for areas from Texas and Oklahoma to just west of Birmingham, including Dallas, OKC, Little Rock, Memphis, Nashville, and Huntsville.

NWS Winter Storm Warnings (pink) and Watches (blue)

A pair of upper-level shortwave troughs will trigger a surface low in the Gulf of Mexico Sunday night and Monday, along the sharp temperature gradient between Gulf air and Arctic air.

(Top: 500 mb flow Monday 12Z, Bottom: Surface pressure and precip Monday 12Z)

The forcing from these upper lows, acting in tandem as a classic midlatitude storm system with the surface low in the Gulf, will produce widespread precipitation over the Southeast Sunday night and Monday. Far enough west, in places like OKC, Dallas, Little Rock, and Memphis, this system will produce a huge snowstorm, with some areas getting 6 to 12″ of snow. But, farther east, in places like Louisiana, eastern Mississippi, middle Tennessee, and western Alabama, the cold air is not deep enough for snow to form.

Vertical temperature profile in lowest 17,000 feet at KMSL Monday

Above see the computer model vertical temperature profile for Muscle Shoals, AL Monday morning. The thick blue line is the freezing line. The red line is the forecast temperature at each height. Note that there is a deep layer of temperatures just above freezing from around 3,000 feet to around 7,500 feet. This means either snow can’t form at all, or if it does form aloft it will melt on the way down. Then, it will either re-freeze just above the ground into sleet (ice pellets) if the cold air layer is deep enough, or it will freeze on contact with the ground (freezing rain), causing a glaze of ice on everything from streets to trees to power lines.

Right now, it looks like the best chance for a significant ice storm, including trees going down and power outages will be from central Louisiana through central and northeastern Mississippi, northern and western Alabama, into middle Tennessee. Some locations could get over 1/2″ of ice accumulation. This would be a bad ice storm if it plays out.

As far as Birmingham goes, right now the computer models, except for one, keep the freezing rain mainly north and west of the city. But I am a bit uneasy with the forecast given the shallow cold air and the tight gradient in temperatures. Birmingham may not be out of the woods…it will be close. There could literally be icing and power outages as close as Jasper, Hamilton, and Northport. We will have to watch this very closely. Either way, several more days of very cold air are on the way for the Southeast…at least into Wednesday…when another ice storm could happen, but more likely a bit farther north.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

No Bitter Cold Lately, Arctic Blast Next Week

Observed Temperatures at 4 pm CST Monday 1 Feb, MesoWest

We here in the Southeastern U.S. are already in a cold air mass…with 4 pm tempertures mainly in the 30s and 40s from Arkansas to the beaches of Alabama and NW Florida to the Carolinas. Here in Birmingham, the temperature has remained steady near 37 F all day, with wind chills between 15 and 25 F. So yes, it is already cold! After dropping into the 20s the next two nights over much of the Southeast, it will start to warm up a bit by Wednesday and Thursday.

It is true that we have had no serious, bitter cold outbreaks here in the Southeast since early 2018. Below is a chart of freezing degree hours at Birmingham airport for each Winter since 1980. This number was calculated by multiplying the number of degrees below freezing, for any hour with a temperature of 32 or below, by the number of hours that temperature occurred. 3 hours at 25 F would give us 24 freezing degree hours (FDH), 3 hours a 32 F, 3 FDH. Note the lack of FDH in the Winters of 2019 and 2020 (each winter includes Nov of the prior year through April of the year shown). Note the last two Winters, and 4 of the last 5 (going back to 2015-16), have been relatively mild in terms of freezing, bug killing, ground crackling cold.

Freezing degree hours by Winter, 1980 through 2020, Coleman and Knupp LLC

That may be about to change. This Winter so far has been chilly, with very few “warm” days (enough to play golf) and a lot of cold fronts bringing nightly temperatures in the 20s and 30s. But, we still have not seen a true Arctic blast yet. It looks like it may be coming next week. A large mass of extremely cold air has pooled in northern Canada and the Arctic Ocean, as shown in the 3 pm temperature analysis below. Much of the circled area is in the -30s and -40s F.

(pivotalweather.com)

And, the upper ridge that has been persistent for over a week in the Pacific will start to push eastward this week, allowing very high surface pressures (typically associated with heavy dense cold air), and a deep upper trough to help push the cold air south toward the CONUS.

300 mb winds Wed evening, UCAR/RAP

By Friday evening, the bitterly cold air mass will already be pouring into the northern Plains of the U.S., with the northerly flow being pulled by a large low pressure area over the Great Lakes, and pushed by a massive 1050 mb surface high pressure area in western Canada. By this time, according to computer models, temperatures below zero will already be as far south as Fargo and Minneapolis. but the northerly flow, the upper-level dynamics supporting it, and the simple weight of the extremely cold dense air, will help it continue to push south over the weekend.

(Sfc pressure and wind, sfc temps, Friday 6 pm CST, pivotalweather.com)

The Arctic front should, according to multiple consecutive runs of the American GFS model and the European ECMWF model, push through the Southeast Saturday night and Sunday, bringing high winds and bitterly cold air, with wind chills below zero on Sunday as far south as Birmingham, Nashville, and Atlanta. By Monday morning at 12Z (6 am CST), the cold air will be fully entrenched over the Southeast and the eastern U.S. The map below shows air temperatures, not wind chills, in the teens all the way down to Jackson, MS; Montgomery, AL; and Macon, GA, with single digits in Huntsville, Nashville, and the northern suburbs of Atlanta. Note the temperatures as low as 30 below from ND into MN, WI, IA, and IL.

An air mass this cold will come in with strong winds (as they almost always do). Wind chills will be extremely cold for an extended period. Lows could reach the single digits Monday night of next week as far south as Birmingham and Atlanta, making it the coldest night in 3 years. Below I have plotted the model forecast temperatures for next weekend and early next week from the GFS and ECMWF models at Bluff Creek, AL, about 20 miles west of Birmingham. The graph starts Saturday at 12:00.

Model forecast temps Sat noon through Tue noon (next week)

The computer models differ on just how cold it will get, but both keep temperatures below freezing for a continuous 24 hour period, and the models have gotten more aggressive with the cold air since yesterday. The computer models indicate another shot of cold air later next week, and a possibility of some snow. But, that is over a week away, so confidence is low. Either way, we have gone into a very cold pattern that will last at least the next couple of weeks. Think about insulating pipes and the usual cold precautions. And don’t let pets stay out in the extreme cold…it is often 3 or 4 degrees colder near the ground on a clear cold night than it is at 6 feet up. Bring them into the basement, garage, or inside the house!

Dr. Timothy A. Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Southern Snowfall

It has already snowed significantly in Texas! As an upper-level low pressure system in Texas interacts with a thermal gradient along the Gulf Coast, snow is occurring tonight over much of northern and central Louisiana. As of 9:00 pm CST, 1.5″ of snow are on the ground in Shreveport, and parts of northern LA may receive up to 5″ of snow overnight. As the system moves northeastward, snow will move into central Mississippi and eventually northwest Alabama.

As the system moves NE, the upper-level low is expected to weaken a bit. Since it takes significant upper forcing for heavy precipitation to fall, and the precipitation melting and evaporation is what gets surface temperatures near freezing, it appears that the less overall precipitation an area gets, the warmer it will stay. So, areas in eastern MS and western AL would get warmer temperatures and less snow due to less overall upper-level forcing and precipation than areas further west. However, closed upper lows are difficult to forecast, and a surprise or two can’t be ruled out.

The most likely areas for significant snowfall extend from northern LA (Shreveport, Monroe) into central and NE MS (Jackson, Columbus, Tupelo) then northwest AL (Florence, Hamilton, Jasper). Again, there is more than usual uncertainty given the closed upper low, and there is that small chance that snow could accumulate farther southeast (Huntsville, Birmingham, Meridan), but this is unlikely.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Hurricane Zeta and Inland Effects

Visible satellite 151 pm CDT
NEXRAD Doppler Radar 215 pm CDT

Latest reports from USAF Hurricane Hunter Aircraft, and Doppler radar imagery (right, above) indicate that Hurricane Zeta continues to intensify, even over the cooler continental shelf waters of the northern Gulf, this afternoon. Central pressure has dropped to 973 mb, and max sustained surface winds are up to 105 mph. Radar is picking up winds up to 131 mph, but that fat out the radar beam is around 11,000 feet altitude. The center is only 80 miles SW of Grand Isle, LA, and given its fast movement of 20 mph, the center will be onshore by 430 pm, and eyewall hurricane winds before that. Hurricane-force wind gusts will affect New Orleans between 5 pm and 10 pm CDT, and the Mississippi Gulf Coast between 6 pm and midnight.

Given the rapid movement of the storm and its rapid intensification over the past 24 hours, it hasn’t had time to build up as much storm surge as other storms earlier this season. However, as shown below, many areas along the Gulf Coast, including the LA delta, Gulfport, Biloxi, Pascagoula, and Dauphin Island, may see water 3 feet or more above ground level.

NOAA storm surge inundation forecast

As the storm makes landfall early this evening, hurricane-force wind gusts will affect areas from Southeast Louisiana across SE Mississippi and SW Alabama. The storm will continue to accelerate rapidly northeast, as an intense upper-level storm system, associated with the cold air and ice storm over Oklahoma the past 2 days, pushes the storm.

500 mb flow, pivotalweather.com
NHC official forecast track

Given the very dynamic storm system to the west, Hurricane Zeta will accelerate through the night, reaching forward speeds of 35 to 40 mph as it moves inland across southeast Mississippi and then Alabama. The center of the storm will be near Meridian, MS by 10 pm CDT, near Birmingham, AL by 3 am CDT, and near Chattanooga, TN by 7 am CDT. Given the extremely rapid movement, the storm will not have time to weaken below tropical storm strength until it gets to Tennessee. So, large parts of southern Alabama could see wind gusts of 50 to 65 mph, and even areas in northern Alabama, including Birmingham, Tuscaloosa, and Gadsden, could see wind gusts of 50 mph late tonight and early Thursday morning. This wind will likely cause power outages and lots of downed trees from southeast Mississippi into Alabama. The National Weather Service has issued a Tropical Storm Warning for most of Alabama for tonight, and they have issued a map showing the likely maximum winds and arrival times of the highest winds. Both shown below.

NWS Tropical Storm Warning (left, red) and max expected wind gusts (right)

Fortunately, the rapid movement means there is a low probability for widespread flooding due to rainfall, as it simply won’t rain that long. Some areas in SE Mississippi could get 3-5″ of rain, and most areas in Alabama will get 1-2″ of additional rain from the storm.

NOAA WPC 72-hour rainfall

It is quite unusual to see a Categrory 2 hurricane make landfall this late in the season, but it has been an active year for hurricanes in the western Gulf. Given that the days are shorter, the sun angle lower, and the northern latitudes are already cooling down rapidly, this allows this storm to interact with the vigorous winter-like storm system over the Plains and move so rapidly. Behind the hurricane, temperatures will drop into the 30s and 40s over Mississippi, Alabama, and Georgia by Friday night, and even colder air is on the way for early next week.

Dr. Timothy A. Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Hurricane Delta Discussion – Wed

Hurricane Delta made landfall early this morning on the Yucatan Peninsula of Mexico. Per surface and aircraft recon observations, the interaction with land has weakened the storm to 100 mph, after what had been one of the most rapid intensifications of a hurricanes ever in the Carribbean/Gulf, peaking around 140 mph yesterday. This time of year waters are very warm down there, and we’ve seen these rapid October intensifications before with Hurricanes Opal in 1995 and Michael in 2018.

It is still moving WNW around a ridge of high pressure over Florida. But, the main natural purpose of hurricanes is to move heat from the tropics to the midlatitudes and polar regions, so they tend to find a path of least resistance to move north. That opening will likely start to show up tomorrow, as a trough of low pressure over Texas, and the storm moving away from the ridge over Florida, and allows the storm to turn northward.

The computer models are in amazing agreement right now, rarely seen, showing a landfall along the Louisiana Coast, east of Lake Charles, on Friday afternoon.

Hurricanes get their intensity from heat and humidity in the center. Warm, humid air is lighter than cool, dry air, so the pressure near the surface is lower underneath the warm, humid air in a hurricane. Thunderstorms release latent heat of condensation, and some of the warm water in the ocean, released in spray by waves, makes it way into the storm. The image below shows a cross-section through a hurricane with equivalent potential temperature…a good measure of temperature and humid normalized by height. Note the extremely high values near the center, producing the low pressure underneath.

From Hawkins and Imbembo (1976)

Given the warm waters in the central Gulf, a deep humid atmosphere, and low wind shear, Delta is expected to re-intensify today and tonight into a Cat 3 or Cat 4 storm. As it approaches the coast on Friday, the shallow waters are cooler from recent cold fronts (and shorter days). And, the storm will likely encounter a bit of wind shear, tilting the warm core over to the side a little bit and spreading out the effects of low pressure at the surface out a bit. Both of these factors could weaken the storm a bit before landfall. But, it is still expected to come in as a Category 3 hurricane with winds of 120 mph.

With a large area of hurricane-force winds over the Gulf, a large area of storm surge will affect the coast from Houston all the way to Biloxi (and maybe even farther east). Storm Surge Watches have been issued along the Gulf Coast by NHC, from High Island, TX to Pascagoula, MS, along with Hurricane Watches. Given the wind direction, their could be a significant storm surge in Lake Ponchartrain near New Orleans. Wind damage is likely in Louisiana and even parts of southern Mississippi, with tropical storm force winds occurring as far inland as Shreveport, Monroe, and Jackson. The city at most risk for wind damage is probably Alexandria, LA.

Also, as the decaying storm moves inland Friday night and Saturday, tornadoes are possible over parts of Alabama, Mississippi, and NW Florida, even as far north as Tupelo, Birmingham, and Huntsville.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Hurricane Laura Update – 300 pm

Laura officially became a hurricane at 10 am this morning. It is now fully away from the mountains of Cuba that have been been keeping it from strengthening, and is now moving over the open, warm waters of the Gulf of Mexico. Water temperatures in the path of Laura are mainly between 85 and 88 F, with some apparent upwelling of cooler water over the north-central Gulf due to Marco.

Both NOAA and USAF aircraft are flying in Laura today. I peak flight-level wind of 77 knots was found this morning, and SFMR instrumentation that can estimate winds at the sea surface indicated 65 knots, the low end of hurricane intensity.

Laura is moving along quickly, toward the WNW at 16 mph. The upper-level ridge to the northeast of Laura has weakened some, and it will likely turn right, or more NW then NNW, over the next 24 hours. These hurricanes generally find the path of least resistance to the north, as redistributing heat from the tropics to the midlatitudes is one of their roles in nature.

(pivotalweather.com)

The model guidance is fairly tightly packed today, showing a landfall between Lake Charles, LA and Beaumont, TX on Thursday morning. The National Hurricane Center is following the eastern edge of this envelope, with a landfall near Lake Charles, LA around sunrise on Thursday. Despite some northerly shear affecting the storm right now, the warm water and humid air over the northwest Gulf should allow Laura to intensify quite a bit before landfall. NHC expects it to reach major hurricane status (Category 3), with sustained winds of 115 mph and gusts to 140 mph.

This storm will be moving into the low-lying bayous of southwest Louisiana. Much of south Louisiana is less than 10 feet above sea level, so storm surge will inundate hundreds of square miles of land, up nearly to I-10 in some areas. Note, in map below, that some areas of south Louisiana will be under 9 feet or more of water, and water will even move into some parts of Beaumont, TX and Lake Charles, LA. Given that the storm is moving fairly quickly, will be strong at landfall, and will not weaken as quickly in southern Louisiana as it would, say, in NW Florida due to the low elevation/storm surge waters, hurricane force wind gusts could reach well inland, including eastern parts of Houston metro and also Alexandria, LA. Tropical storm force winds could reach much farther inland, probably north of I-20. This would include Shreveport, Monroe, and parts of south Mississippi and south Arkansas. This storm will down a lot of trees and power lines over a wide area.

(NOAA/NWS)

Two other threats with this storm are flooding rainfall and tornadoes. It looks like the heaviest rainfall with the storm will stay near and just east of the center, with rainfall totals of 5 to 10″ from Lake Charles up through Shreveport and Little Rock. Some outer bands could develop and produce locally heavy rain in the very humid atmosphere moving inland from the Gulf over Mississippi and western Alabama, but the flood threat rapidly decreases as you go east of the Mississippi River. Similarly, the tornado threat will primarily be over Louisiana, but can’t be ruled out in the outer bands, primarily in Mississippi.

We are now in the prime part of hurricane season, the next 4 weeks. This will be a significant windstorm over a wide area, with the additional storm surge flooding in south Louisiana and potential flooding from rainfall in Louisiana, Arkansas, and even parts of NW Mississippi and western Tennessee.

Dr. Timothy A. Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Tropical Update – Early Sat AM

The somewhat rare situation is setting up right now that may allow two tropical cyclones (tropical storms or hurricanes) to be in the Gulf of Mexico and threatening the continental U.S. at the same time. However, there is a lot of uncertainty with the two storms. The newly-designated Tropical Storm Marco is located just off the Yucatan Peninsula (SW of Cuba), and T. S. Laura is the one in the Caribbean to the east, centered just east of Puerto Rico.

Marco is better organized this morning, with USAF C-130 hurricane hunter aircraft finding flight-level winds around 41 knots, and thunderstorms developing near the center. Most of the thunderstorms associated with Laura are to the east of its low pressure center, and that will make intensification slow for a while. It is the condensation and release of latent heat by thunderstorms that lowers the surface pressure underneath those storms, so until the storms line up with the center, strengthening is difficult.

Marco is expected to move northwest toward the Texas Coast by Tuesday. Even though the waters in the Gulf of Mexico are very warm this time of year, the upper-level trough of low pressure that has been keeping our weather unsettled here in Alabama lately will produce wind shear that will keep Marco from becoming fully vertically-stacked the way an efficient hurricane would be.

Then there is Laura. The NHC track has been shifted to the left. That is good news if it holds, as there would be land interaction through Monday (Puerto Rico, Hispaniola, Cuba), keeping the storm weaker. This thing will be moving over a lot of undisturbed, warm water (84-90 F) for the next 3-5 days, and by the time it gets into the central Gulf late Monday, wind shear will be decreasing. There is still track uncertainty in the computer models, as shown below. Most now take it toward Louisiana by next Wednesday night.

The track staying south until Monday then turning northwest indicates that the models are counting on the Bermuda High spreading west into Alabama, Georgia, and the eastern Gulf ovet the next few days, with the upper trough weakening and moving into the Ohio Valley. This would allow the storm to go around the high then move north between it and the trough next week. Tropical systems generally find the path of least resistance to go north (their large-scale role in nature is to redistribute heat upward and northward).

(Images from pivotalweather.com, annotated by me)

There is also the unknown effect of the “Fujiwhara effect” where two cyclones interact if they get close enough together. They can start to rotate around each other or even converge. That is a very complex process and impossible to forecast at this point.

It will be an interesting week along the Gulf Coast. Anyone with family, friends, or property along the Gulf Coast, pretty much anywhere in the US, but especially from about Pensacola westward, needs to pay close attention to this developing situation in the coming days.

Dr. Timothy A. Coleman

Consulting Meteorollogist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Severe weather threat Thursday?

SPC severe wx outlook for Thursday

The next upper-level storm system in this active Spring is currently moving through Oklahoma, producing thunderstorms, with a few of them severe, and even some Tornado Warnings in Texas this afternoon.

Current upper-level flow. Note wave in flow in north TX/west OK

A lead line of storms associated with this system will move through Alabama early tomorrow morning, likely reaching BHM by 7 am. However, these storms will not be surface-based nor produce any tornadoes, as the air mass will still be very stable. Currently, dewpoints are in the 40s, and they will not recover by tomorrow morning.

Current dewpoints

Even with elevated storms, as we saw on Sunday morning, we can get a lot of lightning and heavy rain, and even sometimes high winds and hail. A very unique event occurred from Clanton through Rockford, Dadeville, and Alex City Sunday morning, where the downdrafts from elevated storms produced large hail and an atmospheric wave that produced very high winds. But that is for another blog post! Either way, the storms tomorrow morning are unlikely to do that again, but expect heavy rain, possibly some hail, and local wind gusts up to 60 mph. Tornadoes should not be a threat in the morning.

The question comes during the day tomorrow. After the initial line of storms moves through, the main storm system will still be to our NW, and the air mass will start to recover (warm and humidify). By tomorrow afternoon, especially if we get any sunshine, the air will be unstable over central AL. Take a look at this NAM model sounding at Bluff Creek, AL, for tomorrow at 4 pm.

It has CAPE of 2,700 J/kg, so plenty of fuel for thunderstorm re-development. However, the wind shear by then will have decreased as surface winds veer to the SW, and 0-1 km helicity is only projected here to be around 100 m2/s2. Not really enough for any significant storm rotation. The instability will be moving in as the shear leaves. Now, if the system slows down and wind shear hangs around longer, we could see a few tornadic storms in central AL tomorrow afternoon. That is unlikely. The best chance for tornadoes tomorrow will be over SE Alabama and south Georgia, where the instability and shear will phase better.

We have been having difficulties with our computer models due to the lack of aircraft observations going into them, since most commercial airplanes are sitting idle right now and not flying. So there is higher than usual uncertainty in the forecast.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com