We often get questions about the three categories of risk defined by the Storm Prediction Center (SPC) in their daily convective outlooks. With some active weather expected today, this might be a good time to go over what those risks actually mean…

SLIGHT RISK: Implies well-organized severe thunderstorms are expected but in small numbers and/or low coverage. Within a slight risk area, 5-29 reports of 1 inch of larger hail, and/or 3-5 tornadoes, and/or 5-29 wind events are forecast.

MODERATE RISK: Implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather. Within a moderate risk area, at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (30 that might be associated with a squall line, bow echo or derecho) are forecast.

HIGH RISK: Almost always means a major severe weather outbreak is expected, with great coverage of severe weather and enhanced likelihood of extreme severe (i.e., violent tornadoes or extreme convective wind events over a large area). Within a high risk area, expect at least 20 tornadoes with at least 2 of them rated F3+, or an extreme derecho causing 50+ widespread wind events (50+) with numerous higher end wind (80+ mph) and structural damage reports.

The report criteria for each of those risks is valid for an area the size of Oklahoma without the panhandle, or about 50,000 square miles. As the size of the risk area increases (decreases) from 50,000 square miles, those expected severe weather numbers would increase (decrease) proportionally. Much of Alabama is in a moderate risk today, with the greatest risk posed by strong straight line winds and large hail. We will be watching the sky closely.

The Thursday afternoon web video is on the server:

http://beta.abc3340.com/weather/video.hrb

Sorry this post is a little late; active weather keeping me behind the power curve this afternoon. The strongest storms have shifted well to our south, and we should have a nice break in the storms tonight and tomorrow morning.

MODERATE RISK: As you read from J.B.'s post below, our pals at Norman at the Storm Prediction Center have upgraded our severe weather risk to moderate for tomorrow. I still think the prime threat tomorrow will come from damaging straight line winds and large hail. A few isolated tornadoes, of course, cannot be ruled out. But, the wind fields look rather unidirectional based on model soundings at this time.

Looks like the greatest chance of severe storms will come from about 2:00 p.m. until 8:00 p.m. tomorrow. Here are model extracted severe weather parameters for 6:00 p.m.:

SB CAPE: 1773
0 to 3 KM Helicity: 334
Lifted Index: -6.0
2m AGL Dewpoint: 66.1
Hail size: 1.30"
SWEAT Index: 515

The air could be very unstable... for the first time this season looks like we will have some true maritime tropical air in here, with surface dewpoints in the mid to upper 60s.

Lapse rates are very steep, and there is no doubt some high winds are very possible in the stronger storms. We will review all of this and have a closer look at the mesoscale situation early tomorrow morning.

THE WEEKEND: Much colder air rolls in here on Saturday. Looks like we stay in the 50s all day, and the NAM is showing 32 for early Sunday morning, with the GFS at 35. J.B. Elliott has lowered our forecast package low to 37. Growers and others with agricultural interests will need to watch these numbers closely.

Monday morning could be just as cold, and maybe there will be a greater risk of frost early Monday with the wind calm.

NEXT WEEK: Models are all over the map and there is little confidence in the forecast solution. Watch the video for more...

Thanks to the kids at Bessemer Academy today for being great listeners... the thunderstorm complex went right over the school during the program; the power even went out twice!

At mid afternoon, the strongest thunderstorms were very near the center of the state...in Chilton and nearby counties.

The NWS issued a Severe Thunderstorm Warning for Chilton County, including the city of Clanton until at least 3:30 pm. The thunderstorm was moving SE and may dump out large hail and produce damaging winds.

Lighting is fierce in these storms...and of course very dangerous.

Rain is also coming down in buckets. Based on doppler radar estimates, as much as 4 inches of rain has fallen with some of these storms. However, hail can alter the accuracy of those estimates and make them appear too high.

A National Weather Service retiree, Jay Shelley was driving through Chelsea, in Shelby County, around 1:15 this afternoon. He ran into rain so heavy that people were pulling off the road and the lightning was severe.

But he also encountered hail that he thought was going to break his windshield. The hail varied from 1/2 inch to 3/4 inch in diameter.

Jay, with a long-track record of knowing severe weather, reported this on his cell phone and I relayed the report to the NWS at Shelby County Airport.

In addition...The NWS also has reports of dime size hail in these places:

Downtown Northport
University of Alabama Campus, Tuscaloosa
12 miles west of Talladega
Demopolis

1/4 inch hail was also reported near Cahaba Heights and Center Point in Jefferson County.

LATE UPDATE: At 2:10 this afternoon almost all of the thunderstorms were in areas south and SE of Birmingham. The storms continue moving SE.

Showers and thunderstorms were becoming a little more numerous across parts of North and Central Alabama early this afternoon...producing some locally heavy rain and considerable lightning.

At 12:35 pm, the larger number of showers and storms were NE and East of Birmingham along I-20 to Calhoun County. Others extended northward to Cullman County but there were few if any showers north and NW of Cullman. Still other storms were across Shelby and Talladega County extending southward well into Central Alabama.

All of the showers and storms moving toward the SE.

Our stronger thunderstorms will be Friday--especially in the afternoon and night. At mid-day, the Storm Prediction Center (SPC) upgraded the risk of severe weather for most of Alabama Friday to "Moderate." The Moderate Risk area covers a large area including:

Almost all of Alabama except the Extreme South
Most of Mississippi, Tennessee and Kentucky
Extreme Southern Indiana and Ohio
About the NW half of Georgia

At 11:50 this morning some thunderstorms had already developed over NE Alabama. They were located mainly over South Etowah County and North St. Clair County. They were moving toward the SE into North and West Calhoun County. Considerable lightning indicated.

Tops on the Etowah County storms, around 33,000 feet.

Based on radar, an estimated .60 of an inch of rain has occurred from these storms north of the Gadsden area.

Another isolated thunderstorm was west of Cullman along the Cullman-Winston County line.

The Thursday morning web video is on the server:

http://beta.abc3340.com/weather/video.hrb

First off let me thank my friends at Edgewater Baptist church last night for the great hospitality. Had a great crowd and saw some old friends. The church building was destroyed by the F5 tornado on April 8, 1998... they are in a beautiful new building now and going strong.

TODAY: Moist air returns, and scattered showers and storms will form today, especially this afternoon during the peak of the daytime heating. Most of the showers will fall apart as the sun sets.

TOMORROW: SPC has all of Alabama in a slight risk of severe weather tomorrow as a strong short wave passes north of us. The surface low will be way up north, in Indiana, and our winds will be rather unidirectional. So, the main threat should be from straight line winds and hail. Here are some severe weather parameters valid for 6:00 p.m. tomorrow (from the 06Z run of the NAM):

SBCAPE 1670
LI -6.7
0 to 3 km Helicity: 352
SWEAT 512
2m Dewpoint 66
Hail size: 1.35"
Total totals: 57
850 mb wind speed: 34 kts
500 mb wind speed: 45 kts

If that 0 to 3 km helicity value is correct, there might be more veering of the wind in the lower 3 km than we expect. Instability is fairly impressive, but the wind fields are rather weak. Will be interesting to see how this all plays out tomorrow. And, of course, as we often point out here the ultimate threat will be determined by small scale boundaries that we won't be able to see until tomorrow morning.

Latest QPF guidance shows rain amounts ot 1/2 to one inch.

THE WEEKEND: Saturday will be a rather blustery, chilly day. The NAM has a high of only 59 here on Saturday. Temperatures will be determined by the cloud cover. If clouds hang tough all day, we will probably stay in the 50s. If the sun breaks out (and we are projecting that for now), we should reach the low to mid 60s. The best chance of clearing will be from Birmingham south...

The latest GFS is showing 36 here Sunday morning; we still have 39 in our forecast. The actual numbers will vary from 34 to 42; but we expect the wind to stay up during the night, which will prevent a major frost threat.

Monday morning could be just as cold, and with light wind some frost in the valleys will be more likely. For now, I have 40 degrees in the forecast for Monday morning, but I might lower that into the upper 30s later today.

NEXT WEEK: Very tricky forecast. A fast upper flow sets up over the southern U.S... and the models are all over the road. The latest GFS now keeps us dry Monday through Wednesday, and brings in rain over the latter half of the week into the following weekend. Very low confidence right now...

Will be headed out to Bessemer Academy later today to see the kids over there with a program on meteorology. Will be back in the office later today for the afternoon update!

Well, maybe not down in the lower elevations around Gatlinburg, but there are increasing indications that the high country will turn white as cold air pours across the Great Smoky Mountain National Park over the weekend.

Rain showers are forecast to mix with snow showers Saturday but moreso Saturday night and Sunday morning. This is especially true for places like Mt. Leconte (elevation 6593 feet) and Clingman's Dome, the highest point in Tennessee at an elevation of 6642 feet. But there are other lesser recognized peaks in the eastern part of the park such as Mt. Guyot (elevation 6621) and Big Cataloochee at 6122 feet.

Temperatures will fall as low as 25 in the higher elevations Saturday night with highs Sunday only near 35.

Remember, this possible snow is just for the higher elevations but that is quite a change after the recent springlike weather.