Monday, March 30, 2009

Gusty Winds and Small Hail Possible Early Tuesday:

After the threat of severe weather much of last week, strong storms could move into our area once again. A strong area of low pressure will be moving across the Northern Plains causing blizzard conditions for the Dakotas. It is this storm system that will drag a cold front down across East Texas early tomorrow morning bringing the threat of thunderstorms. At this time we are not expecting wide spread severe weather however we could see a few reports of hail up to the size of pennies and winds guts to 50 mph as this line moves through. The latest data is pointing to a small threat that a few of these storms could become severe, so we will be monitoring the line very closely as it moves through East Texas tomorrow morning. So what’s the set up for tomorrows morning’s storms?

In the image above, surface wind direction and moisture are indicated by the blue streamlines and green shading. This image is showing south southeasterly winds across East Texas and dewpoint temperatures, or the amount of moisture, in the low 60s. The next image shows the dewpoint temperatures along with the Surfaced Based Lifted Indices, or the difference in temperature of the environmental air and the lifted air from the surface. When ever the LIs are negative, instability is present, which could lead to the development of thunderstorms. The lower the LIs, the greater the instability. As the image shows, LIs are approaching -4 across the southern sections of East Texas.

In the next image below we can see that the air temperature will be in the mid 60s, outlined in the light orange color. This along with dewpoints in the low 60s will produce a low cloud base or LCL, lifted condensation levels. We look for low LCL to aid in the development of tornadoes.

Finally in the next two images we see the amount of CAPE, convective available potential energy, or buoyancy from air lifted at the surface and from air lifted at its most buoyant point. Both of these images show moderate amounts of CAPE, especially the MUCAPE, air lifted from its most buoyant point. The MUCAPE at the time of thunderstorm initiation is forecast to be over 1500 J/kg, more than sufficient for the development of hail. So why are we not anticipating a large outbreak of severe weather?

With the area of low pressure well to our north, the forcing needed to lift the air to create storms is lacking. We do see and area of added lift with a fast jet streak moving into our area but widespread severe weather is not anticipated. We will be monitoring this developing situation in case any storm gets out of hand tomorrow so stay tuned for the latest.

Sunday, March 29, 2009

A Beautiful Sunday Afternoon.

With the fantastic weather this afternoon, my family and I took off and went to the Azalea District for a nice afternoon walk. Here are some pictures, I hope you enjoy.

Thursday, March 26, 2009

Another round of severe storms possible today:

After a bumpy afternoon and evening yesterday, things could get a bit dicey this afternoon once again. The rain from last night cooled the lower levels of the atmosphere causing a boundary, or front to sag south across the Gulf coast. Out to our west we are watching a disturbance move through the upper levels of the atmosphere and this will help drag that front north as a warm front later this afternoon. There are two areas of concern for severe weather this afternoon. The first is across the southern counties of East Texas where this disturbance will move through this evening. This will cause the air to rise and form strong to severe thunderstorms, starting around 5:00PM in our extreme southwestern counties. The main threat with these storms will be strong gusty winds and isolated weak tornadoes.

The second area of concern is across the I-20 and I-30 corridors where the warm front is expected to move through this afternoon and evening. Some breaks in the clouds this afternoon should allow enough sunshine to warm temperatures into the mid to upper 70s. This along with cold mid level atmospheric temperatures will provide a large amount of energy and instability needed for thunderstorm growth. The only thing lacking is enough forcing to lift the air and get storms cranking. As the warm front moves through, and an increase in upper level winds move in, I think a few storms will develop. We will have to watch these very closely as there will be the potential for large hail, gusty winds, and tornadoes. In both areas storms should diminish around midnight. Another storm system will move towards East Texas tomorrow bringing another chance of severe weather.

Once this second storm system moves through the threat of severe weather is over for the weekend and temperatures will cool down. We will see lows in the 30s in most locations this weekend so get ready. Stay tuned to KLTV and for the latest on our severe weather threat.

Tuesday, March 24, 2009

Nacogdoches/Shelby County Storm

3D image of the storm on Nacogdoches/Shelby county line.

Inflow notch on the storm.

Monday, March 23, 2009

Severe Weather Possible This Week....

Severe weather is possible throughout the week as numerous storm systems move across the Southern Plains. The first threat of severe weather will move into East Texas tomorrow as a strong storm system to our north drags a cold front across our area. As this cold front moves in, a line of strong to severe thunderstorms will breakout, starting around midday in our northwestern counties and exiting our southeastern counties by early Wednesday morning. The main severe weather threat with this system is strong gusty winds and hail, although an isolated weak tornado can't be ruled out.

Our second chance of severe weather will enter the area late Wednesday as the cold front, now stalled across the Gulf coast, begins to move north as a warm front. Ahead of this warm front, warm air will be forced over cooler air at the surfaced causing thunderstorms to form above the planetary boundary layer, air near the surface of the Earth affected by friction. There will be a lot of what we called elevated energy available for these storms to grow, so large hail is a possibility when any of the stronger storms Wednesday night into Thursday morning. Along and south of the warm front, thunderstorms will have the best chance for rotation so we will have to watch for the possibility of tornadoes if any stronger storms can form.

By Thursday afternoon the warm front should have moved north of East Texas putting all of our area in the warm sector of our next developing storm system. With ample daytime heating, enough energy will be present for isolated to scattered thunderstorms Thursday afternoon. With the available directional wind shear, or winds turn with height, a few of these storm late Thursday could be very strong with large hail, gusty winds, and even a couple of tornadoes. The image below shows 0-1KM EHI or energy helicity index. This value takes into consideration both the amount of energy available for a storm and the amount of spin in the atmosphere. Any number greater than 2 means tornadoes are possible and a number over 4 means strong tornadoes are possible. Right now the EHI over East Texas is forecast to be as high as 3.75.(Some meteorologist will say strong tornadoes are possible with a reading greater than 2 but, the majority of strong tornadoes are associated with a higher number)

By Friday afternoon a strong cold front will move through East Texas again but this time, we should clear out and cool off for the weekend. With this front scattered to numerous strong thunderstorms will develop bringing a chance of gusty winds, hail, and even isolated tornadoes. All of East Texas should remain on guard for the threat of severe weather and stay tuned to KLTV and for the latest.

Tuesday, March 10, 2009

Slight chance of Severe Weather for parts of East Texas:

After 80 degree temperatures the last five days across East Texas, an abrupt end to the warmth is coming. A strong cold front will move into East Texas late tonight bringing a chance of showers and thunderstorms to the area, a few of which could be strong. There is a slight chance on or two storms could form later this afternoon and any storm that does develop could produce strong gusty winds and small hail. Most of the energy needed for strong to severe thunderstorms will move to our north so I am not expecting a great deal of severe weather.

I am however expecting a large area of East Texas to experience heavy rain. They way it looks now, most of East Texas should see between 2 and 3 inches of rain with a few spots receiving more than 5 inches before Saturday. This would put a huge dent in the rainfall deficit across East Texas, which is over 5 inches for most of the area.

In addition to the heavy rainfall across East Texas, much colder weather will be moving into the area. Temperatures will fall for most of the day tomorrow, reaching the upper 40s by the evening and not getting out of the 40s until late Saturday. With a northeast wind around 10 to 15 MPH, wind chill temperatures will run in the 30s for most of Thursday and Friday.

Thursday, March 5, 2009

Salem, AL EF-2 Tornado Analysis.

On Saturday February 28th of this year we saw another example of a strong tornado produced in a low instability high shear environment across East Central Alabama. Many times when we look at the threat of severe weather in the meteorological world we look for high levels of CAPE(Convective Available Potential Energy) and shear to outline an area where the greatest threat is. But there have many instances where we have seen low level of CAPE that produced strong tornadoes. The Salem, AL EF-2 tornado is just another example of why need to look at all severe weather parameters to determine the severity of impending storms.

So let’s take a look at the conditions that came together to form this severe weather event. Tornado watches were in effect throughout the early morning hours Saturday for the possibility of isolated tornadoes to form, especially along a northward moving boundary of increased moisture. In the image below you can see an area of dark green (or higher dew points) indicating a boundary between very moist are and slightly drier air to the north. This boundary was slowly moving to the north and along it, low level helicities, or atmospheric spin, would increase leading to the possibility of tornadoes.

With the added moisture to the south of this boundary, instability levels increase, as shown by the orange shading indicated LI’s of -2°C. Now this value of instability is not all that great but, when you combine it with other atmospheric conditions, severe weather can occur.

By looking at the previous two images you will notice the surface winds are out of the southwest around 15 knots. As you move up the atmosphere winds along this boundary winds were also out of the southwest and then eventually out of the west setting up what we call unidirectional shear. Usually we look to have directional shear, or winds form different direction and speed as we move up in the atmosphere. Something we must factor into the storms environment is the relative winds associated with that particular storm. In the image below, storm relive 1KM winds are indicated or, the inflow to the storm. You will notice that right along the edge of the boundary, the relative inflow backs, or turns counter-clockwise, to the southeast. This increases the storm’s relative helicity and gives the storm a better chance of producing a tornado.

As stated earlier, many times weather enthusiasts look for area of high CAPE to determine where the most severe weather will occur. CAPE basically shows the buoyancy of air. The larger the number, the faster the air will rise. We look for areas of rapid upward motion for the development of severe thunderstorms. In my opinion, many times we get caught up in just looking at areas of high CAPE and not focusing on all aspects of the storm’s environment. Prime example of this is looking at the amount of CAPE that was available for Saturday’s storms. The image below shows the surfaced based CAPE, or buoyancy available from a parcel of air lifted from the surface, was just over 500 J/kg in Lee County.

We normally look for CAPE values to be greater than 1500 J/kg for severe weather and tornadoes. But when we look at where the 500 J/kg of CAPE is located in the profile of the atmosphere and what is happening through that layer, we get a much better indication of what can happen. In the image below we can see the 0-3KM CAPE, or the amount of buoyancy from the surface to 3KM in the atmosphere, is greater than 160 J/kg. This is a fairly significant amount of CAPE for such a shallow layer of the atmosphere.

When we look at a volumetric scan of the storm as the updraft was located near Chewacla State Park, we can see the high reflectivity colors, or red, do not exceed very far beyond the 3KM mark in the atmosphere. In “text book” supercell producing thunderstorms we usually look for these areas of high reflectivity to exceed 30,000’, three times higher than the storms of this day.

So what was occurring in the 0-3KM layer of the atmosphere that lead to the development of a strong EF-2 tornado? As stated earlier, the storm relative winds were backing to the southeast in the 1st KM layer of the atmosphere. This lead to an increase in the storm relative helicity(SRH) values the storm would experience as it moved into Lee County. Notice in the image below that 0-3KM SRH were over 300 in Lee County at the time the storm was west of Auburn.

This large amount of helicity was in the same layer of the atmosphere where the buoyancy, CAPE, was the greatest. So where the updraft of the storm was the strongest, there was enough spin in the atmosphere to produce a strong mesocyclone located in the radar image below which lead to the formation of a strong EF-2 tornado in Salem a few minutes later.

So the right combinations of ingredients came together to produce a tornado who’s estimated wind speed was around 130 mph. If we were to just have looked at the CAPE and wind flow throughout the atmosphere, we might have missed the threat for a strong tornado. But by looking at all aspects needed for tornado development, we can see it does not take much CAPE to produce a significant tornado under the right conditions.

Tuesday, March 3, 2009

Wild Weather Day in Alabama....


It was a wild 24 hours across parts of the Southeast Saturday into Sunday Morning. Severe weather and tornadoes followed by snow affected much of the southeast as a strong storm system moved from the Mid South to the Southeast Atlantic Coast. Three tornadoes touched down in East Central Alabama Saturday morning and ranged from an EF-0 to an EF-2 in intensity. I took a few snap shots from the FFC Doppler velocity data to show where the rotation was as it moved through Lee County. The first shot shows an area of rotation just east of Notasulga near the Macon-Lee County line. This area of rotation would eventually touch the ground near Salem, AL becoming the strongest tornado of the day. A second area of rotation is just north of Auburn. This rotation would lead to the EF-1 tornado near The Bottle at the intersection of highway 147 and US 280. Top winds in this tornado were estimated to be near 100mph. (source National Weather Service BMX) The next image shows a strong area of rotation near exit 51 on interstate 85 or near Chewacla State Park. I have not heard of any damage reports from this area so it appears the rotation was still in the air. The final image is the area of rotation as it passed through Salem. From the damage survey performed by the National Weather Service in Birmingham, an EF-2 tornado passed through at this time. Several buildings and homes were damaged and there were a few minor injuries. If you look closely at this image and compare to the last image, the rotation does not appear to be as tight. A large area of light blue color is becoming dominate in this image showing a strong outflow which eventually lead to the demise of this tornado. The Salem tornado was on the ground for 2.65 miles and had a maximum width of 500 yards. Top winds were estimated to be near 130 mph making this a strong EF-2 tornado. (source National Weather Service BMX) If you look closely again at the last image, the large area of light blue is showing the possibility of strong straight line winds over a large area. A few minutes later this area spread across Smiths Station causing wind damage to many building and trees. Normally having a day packed with severe weather is wild enough but Mother Nature was not done with East Central Alabama. A little more than 24 hours later this area received the heaviest snow it has seen in many years. Parts of Eastern Lee County received over 5 inches of snow. Below is a map from the National Weather Service in Birmingham showing the snowfall accumulations for Alabama on Sunday. A wild weather day indeed!

Fire Danger This Week.......

Dry conditions in Texas over the past few weeks have lead to a high fire danger across our state. Nearly 80% of Texas Counties are under outdoor burning bans and we can expect to see that number increase. Low relative humidity and strong gusty winds later this week will increase the threat for grass fires across the state. As of, today much of East Texas is 5 inches or more behind on yearly rainfall and we are only 2 months into the new year.

The relative humidity at times this week will dip below 20% across East Texas and with strong gusty winds, outdoor burning is not recommended, even in counties which are not under outdoor burning bans. So is there any relief in sight? Unfortunately it appears this dry pattern will continue for a while. We will see a strong ridge of high pressure in the upper levels of the atmosphere across the southern plains producing sinking air across our region for much of the week.

By the weekend there are some signs this ridge will begin to break down and allow a trough of low pressure to move into the southern plains giving us a chance of scattered showers and thunderstorms. That’s the good news. The bad news is we will not see a strong surge of Gulf moisture ahead of this trough which would allow widespread showers and storms giving most of the area some beneficial rains.

As we head towards the middle part of next week there is the potential of a large storm system forming which could produce heavy rain and severe weather for parts of East Texas followed by some much cooler weather by the end of next week. We could do without the severe weather but the possibility of heavy rain would be great news for East Texas. So until we receive the heavy rain we need, please refrain from outdoor burning.

Nice End to Monday!

What a beautiful way to end a Monday. In case you missed it, the high clouds yesterday across East Texas caused a beautiful golden sunset across our area. We will see a few high clouds again today so be on the lookout for another beautiful sunset this evening. However, it appears some drier air is moving into the upper levels of the atmosphere so we probably will not see such a dramatic sight.