Cold front in July!

My forecast for temps over the next 4 days

A bona fide cold front is moving southward through the United States today, and will move all the way through most of Alabama, Mississippi, Tennessee, and Georgia tomorrow!

The term “cold front” is always used to describe the front edge of cool air masses that replace warmer ones. Of course, it is not going to get “cold” here in the Southeast U.S., but it will certainly feel much more pleasant than the sauna we have felt the past 3 weeks, with morning lows dropping near 60 degrees as far south as Birmingham and Atlanta, and daytime high temperatures only in the mid 80s.

Current temperatures (1 pm CDT)

The above map shows current temperatures from SE Kansas to NW Alabama right now. Some of the lower temperatures over parts of KY and AR are being affected by rain. But, it is 79 in Joplin, MO at 1 pm, and they have sunshine! So, this is a legitimate cool air mass. By tomorrow afternoon, winds will turn out of the north along I-20 from Jackson to Birmingham to Atlanta, and the cooler air will be moving in.

The air in places like Birmingham and Nashville will be coming from the eastern Great Lakes, as the NOAA computer model trajectories show below.

We will see some rain showers and maybe a few thunderstorms as the front moves through overnight and tomorrow morning. But, from tomorrow afternoon through Thursday, temperatures will be below normal. Some locations in the colder valleys of northeast AL, north Georgia, and east TN will easily drop well into the 50s.

Another big change that will make it feel nice will be the much lower humidity. Dewpoint is a meteorologist’s favorite measure of water vapor in the atmosphere because it is not affected by temperature, it simply indicates how much water vapor is in the air. The dewpoints over much of the Southeast U.S. the past few weeks have been between around 73, indicating a water vapor content of 17.5 grams of water vapor for every kg of air. Tuesday through Friday, dewpoints will be in the 50s, indicating water vapor contents dropping by 50%, to about 9 grams per kg. You will feel the difference!

Enjoy it, because with the strong, nearly overhead sunshine angle of July and August and warm soil, it won’t last, and we should be back into the 90s by the weekend. The humidity will come back too, with a big Gulf of Mexico nearby and water temperatures in the 80s.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Tropical Storm Barry – Sunday Update

High-resolution visible satellite picture

Barry is still considered a tropical storm right now by NHC, even though its maximum winds are out over open waters of the Gulf of Mexico. These winds are far from the center of circulation and lowest pressure near Shreveport, Louisiana, where winds are slowed by friction and thunderstorms are not as intense. Winds over most of south Louisiana as of 2 pm had dropped to 25-35 mph.

The big story with Barry now is the rain it is producing. The regional radar composite above shows a near solid wall of rain over the state of Mississippi, extending into much of eastern Louisiana and eastern Arkansas. Along the eastern edge of the area of heaviest rain, bands of thunderstorms are forming over parts of western and southern Alabama, and these may spread into central Alabama later today. There are also a few bands of thunderstorms over northern Georgia.

Some parts of south MS, extreme south AL, and southwest LA have already received 3-8″ of rainfall from Barry, and some rivers are already flooding. As Barry’s pressure continues to rise and the circulation weakens over the next 48 hours, the mass of rain will slowly decrease in size and intensity. But, even by Tuesday, with tropical moisture in place over the Southeast US, scattered afternoon storms will be increased, and some heavy rainfall amounts will occur from Memphis to Nashville and Paducah. The NOAA total rainfall forecast for the next 7 days reflects this increase centered near Memphis, with amounts over 7″ centered there, and amounts over 4″ expected in Little Rock, Paducah, and Jackson, TN. Rainfall amounts will drop off dramatically to the east over AL, middle and eastern TN, and GA, where less than 2″ is expected. Given the alignment of the rain maximum, some flooding along the Mississippi River, especially from Cape Girardeau south, is likely.

NOAA forecast 7-day rainfall starting now

One more interesting thing being picked up by upper-air weather balloons and computer models is the warm air aloft associated with Barry. Even though it has weakened to tropical storm winds, one can still see that temperatures at 500 mb (about 18,000 feet) are 10 to 15 degrees warmer above Barry than at other locations even over the Southern United States and the Gulf of Mexico. It is this warm air aloft, produced by the latent heat release in thunderstorms, that causes the air aloft to be lighter, and therefore cause the low pressure at the surface that drives tropical systems like hurricanes.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Tropical Storm Barry

USAF C-130 planes found flight level winds sufficient for the tropical depression in the Gulf to be upgraded to Tropical Storm Barry, with sustained winds near 40 mph. The center of Barry, is located south of Pascagoula, MS, and most of the strong storms are to the south of the center. This, along with wind shear and some dry air coming in from Texas, are not conducive for rapid intensification. However, Barry is over warm water in the shallow part of the northern Gulf, and the National Hurricane Center expects Barry to become a minimal hurricane by landfall sometime on Friday or Saturday in Louisiana.

The computer models are all over the place, and the center of the storm is still disorganized and, to me, unimpressive. But, my PhD is not in hurricanes like those guys, so the chance of it becoming a hurricane is still there. It looks possible that it could turn north a little earlier and come into Gulfport, Mississippi then move slowly through Mississippi as a tropical depression this weekend, but we’ll know more about the track once it develops a little more out in the Gulf.

The weather along most of the beaches from Mobile to Panama City will be OK the next few days. There will be more scattered storms than normal, but still occasional sunshine and warm temperatures. And no threat of tropical storm winds or storm surge. However, there will be rough waves and rip currents, and the water itself is closed to the public for now. Have to look at it from the beach or balcony.

The big threat to Alabama is from heavy rain over the next few days, especially Sunday through Wednesday of next week. Assuming the track stays further west near the Mississippi River, we should only see an increase in afternoon storms around here and 1-3″ of rain over the next few days. But, if it turns north earlier and comes up through eastern Mississippi, we could see a flooding threat, with 3-5″ of rain, more in west Alabama.

Another update on Barry and possible effects on the weather here in central Alabama, coming up on Saturday.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Early Heat Wave for Southeast

Current weather map across SE

For late May, today is nice with mostly sunny skies and temperatures in the upper 80s to lower 90s. But, the atmosphere is already dry (as shown by the green line, a profile of moisture in the atmosphere from the morning weather balloon at NWS Birmingham), leading to the large areas of clear to scattered clouds and no rain. About the only storms today are in middle TN and just east of Atlanta.

Weather balloon data from 7 am today at BHM. Red is temperature, green is dew point
High-resolution visible satellite (top), regional radar composite (bottom)

However, changes are coming, as a ridge of high pressure moves almost directly over the SE, cutting off any cooler, more humid air off the Gulf of Mexico. Plus, temperatures in the Gulf are still cool, in the 70s across most of the northern Gulf, and only in the lower 80s at shallow beach locations like Orange Beach, AL and Panama City Beach, FL. So, humid air will have a hard time making it in here over the next week, and our atmosphere will stay dry.

Gulf of Mexico water temperatures

This causes heat because 1) we don’t have many clouds and the sun this time of year is near maximum strength due to its high angle above the horizon, 2) the afternoon thunderstorms we normally get this time of year that cool us off won’t occur in the dry atmosphere, and 3) without afternoon storms to rain on the ground, the ground will slowly dry out, allowing more of the sun’s energy to go into heating the air than drying up soil moisture.

Surface pressure and winds on Monday (pivotalweather.com)

We still have a decent south wind today and tomorrow, bringing some of that slightly cooler air off the Gulf in here, so we should mainly have high temperatures here in Birmingham between 90 and 92. But the winds calm and turn SW by Friday and the weekend.

Computer models have a hard time with the layer of air we live in near the surface, and statistics derived from models have difficulty with abnormal events. So I take a simpler approach that has worked for years. Based on that, my best shot at high temperatures in BHM for the next several days is:

Today: 91 Thursday: 92 Friday: 94 Saturday: 95 Sunday: 97 Monday: 98

The good news is that humidity levels will remain low, with dewpoints in the 60s. So, heat index values will not be much higher, if any, than the temperature. But, in a slightly drier heat like this, people can get dehydrated because they don’t realize how much they are sweating. So, stay hydrated. Since this is coming on so quickly (last Summer it never got above 97 and that occurred in September, with most of May through August being unusually mild), check on the elderly and anyone who may not have air conditioning. Close south-facing window shades and blinds. And one final note, UAH research has shown that it can often be 4-5 degrees hotter right near the ground than the official temperature (taken at 2 meters). So, where dogs and cats are, it could easily be over 100. Bring them in during the day if you can, and make sure they have plenty of water. If you are going away for the Memorial Day weekend, make sure they have a cool place to go (basement, etc.) and enough water.

Dr. Tim Coleman

Consulting Meteorologist, Coleman and Knupp LLC

Twitter @timbhm

Sun strong, days long now!

As we move through Spring and toward Summer, the days have gotten a lot longer already. As shown by the orange curve above, today, April 24, we will have 13 hours and 6 minutes of daylight. This is after the minimum on the Winter solstice, Dec 21 2018, when we only had 9 hours and 50 minutes of daylight. The blue dashed curve indicates the change in daily daylight in minutes per day. Right now, we are still gaining almost 2 minutes per day, but that daily gain is slowing down. It will go to negative on the Summer solstice, the longest day of the year, June 21. That day, we will have 14 hours and 23 minutes of daylight, and the sunset is at 8:00 pm CDT.

For you math nerds like me out there, notice the length of day is basically a periodic function, and the change per day is its derivative, another wave function.

Sun path in sky over Birmingham on various days in 2019. Courtesy Sun Position app.

The sun is also getting higher in the sky. The above shows the sun’s path across the sky (compass direction and angle above the horizon) on 4 days in 2019. Note that today, the sun rose in the ENE at 6:07 am, reached it zenith in the sky at 70 degrees above the horizon at 12:45 pm, and then sets in the WNW at 7:23 pm. It has moved a lot since March 1, when it will still rising in the SE and setting in the SW, and its highest angle above the horizon was only 49 degrees.

The pictures below show sunset on the Warrior River at Bluff Creek looking due west from roughly the same spot on two dates, Nov 30 (left) and May 4 (right). Note that the sun is the SW sky on Nov 30, and in the NW sky on May 4.

The angles above the horizon matter, because when the sun is high in the sky, its angle is much more direct, and the incoming sun rays do not get spread out across a larger area of the ground like it does in Winter. For example, back on Jan 1, at its peak even with a clear sky the sun only produced 762 Watts of energy per square meter of ground, and the total daily solar energy was 0.0049 Watt-hours per square meter. This is why the northern hemisphere gets cold in winter. Low sun angle, short days, small amounts of incoming solar energy (none in polar regions where it stays dark for weeks or months).

Today, on the other hand, we get 1271 W/m2 at solar noon, and a total of 0.0104 Wh/m2, twice as much energy as on Jan 1. At its peak on June 21, we will get 0.0119 Wh/m2.

Some people ask why the hottest weather lags about a month or two behind the peak solar energy, and the coldest weather lags about a month or two behind the minimum. This is shown for Birmingham in the chart above (length of day and average high temperature). This is because the sun is slowly heating up the ground and the atmosphere in the northern hemisphere in the spring and early Summer, while the outgoing longwave radiation and lack of sunlight in Winter slowly cools the ground and the atmosphere in Autumn and early winter. It takes time for energy to heat things up, and loss of energy to cool things down.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Severe weather analysis – 215 pm

NOAA/SPC convective risk outlook

It has been an active year for severe weather in the Southeast, and we have another system to deal with later today and tonight.

Right now, a big area of rain and thunderstorms is occurring just east of the Mississippi River. A tornado watch is in effect for much of central and south Mississippi, and several storms have tornado warnings on them right now near and south of Jackson, MS.

This entire system is reflected by a slow-moving front and a surface low pressure area forming along it in south Louisiana. Note that pressures are falling rapidly across eastern MS into northern AL, so that is where the low pressure area in Louisiana will likely track toward over the next several hours.

How far north the low tracks is critical to how far inland the tornado threat can make it. With the low pressure area intensifying and creating imbalances in the atmosphere as it moves to our NW this evening, wind shear for storm rotation will be very high. Storm relative helicity values could be over 400 m2/s2 here in central Alabama between 7 pm and midnight.

The only limiting factor may be a lack of instability for storm updrafts. First of all, the air over central Alabama is warm right now, but somewhat dry, with dewpoints ranging from upper 50s in Anniston and Montgomery to lower 60s in Tuscaloosa and Demopolis. That makes the air more stable. Higher dewpoint air will likely move in this evening as south winds pick up, destabilizing the air somewhat, but it will be at night by then and cooling off. Also, temperatures aloft are not that cold and do not get cold rapidly with height. That will also limit instability somewhat, as shown in the expected atmospheric temperature profile for Tuscaloosa at 7 pm.

Computer model temperature profile vs. height at TCL at 00 GMT (7 pm CDT)

Given current information, it looks like the greatest threat for tornadoes will be south of I-20, especially in areas like Jackson, Meridian, Montgomery, and Troy, where it will be warmer and more humid. Storms will likely move into west Alabama by 5 pm, and the main line of storms will come through Birmingham metro between 7 pm and 11 pm. Despite the more stable air, with the wind shear in place, tornadoes will be possible even as far north as Birmingham, and I expect us to be under a tornado watch this evening. Even if we don’t get tornadoes, straight-line winds over 60 mph are likely.

Review your safety plan. Lowest floor, small interior room, no outside windows or doors, wear a helmet, etc. Make sure you have a NOAA weather radio with alert feature to get warnings from, and hopefully a phone app. I use the app called “tornado” from the American Red Cross.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Dangerous storms central AL now

Reflectivity and velocity Birmingham radar 515 pm

Even though the shear is trying to pull away, temperatures have warmed into the upper 70s in Birmingham and Tuscaloosa, with 70s in Gadsden and Anniston. This is causing very unstable air, and multiple dangerous storms right now. We have already had at least one tornado touchdown in Cullman County, and two possible others.

The most dangerous storms now are near Oneonta, Fultondale, and Bankhead Lock and Dam. On their current paths they will affect areas including Altoona and Steele; Pinson, Clay, Center Point, and Trussville; and Oak Grove, Sylvan Springs, Hueytown. The only one officially under a tornado warning as of 520 pm is the one in Blount County, but all these are supercells with tornado potential.

Dr. Tim Coleman

Twitter @timbhm