Wednesday, February 23, 2011

Strong to Severe Storms Tomorrow

A strong storm system will be moving into East Texas during the day tomorrow bringing a chance of severe thunderstorms to parts of the area. Right now it looks as though an area of low pressure will develop across northwest Texas and move into eastern Oklahoma during the afternoon. As this low moves of to the northeast a cold front will move through East Texas during the late afternoon and evening hours. This will bring the front through during the peak heating of the day giving the area more energy to fuel these storms. Most of East Texas is under a slight risk for severe storms tomorrow.
With the low pressure expected to be in eastern Oklahoma when the front moves through the area, our winds will be out of the southwest around 15 to 20 miles per hour during the day. This southwest wind will help to limit the threat of tornadoes tomorrow keeping the amount of spin available in the atmosphere much less than if the surface winds were out of the southeast.
Winds above the surface will also be out of the southwest but much faster, around 80 mph. Any thunderstorm that develops could tap into these winds bringing them down to the surface. So the threat of strong gusty winds appears to be the main threat at this time. The mid and upper levels of the atmosphere area still fairly cold meaning there will also be a threat of large hail from any storm that can develop.
This is still a developing situation and a lot could change between now and tomorrow afternoon. If the area of low pressure forecast to develop moves farther south, then our surface winds would become more favorable for tornado development. All of East Texas north of a Palestine to Nacogdoches to Center line is under a slight risk of severe storms tomorrow. Please stay tuned for the latest on this developing storm system

Monday, February 7, 2011

Another winter storm on the way

Last week’s winter weather was replaced by a very nice weekend as many areas saw temperatures soar into the 60s Sunday afternoon. A cold front moved through the area last night so today will be much colder than this past weekend but not a cold as we saw last week. Anther arctic front is heading towards East Texas late Tuesday into Wednesday morning followed by a disturbance in the upper atmosphere. It is the combination of these two events that will bring another round of wintry weather to East Texas.

With the event being 48 hours out a lot could change between now and then but based on the latest information, this is what will likely happen. Tuesday night a strong front will move through the area dropping temperatures to near or below freezing in out northern counties. Here it looks as though the precipitation will switch quickly from rain to snow. 2 to 4 inches of snow look to fall across our northern counties before this system rapidly moves out of the area.

For our central counties along and immediately south of I-20, precipitation looks to start off as rain. As the temperature falls to near freezing during the late morning hours, rain will start to freeze on elevated surfaces. As continue temperatures fall into the upper 20s by the early afternoon precipitation should transition to sleet, causing slick spots to develop on area roads. Late in the afternoon precipitation could change to snow for a couple hours allowing for 1 to 2 inches of snow to fall.

In our southern counties including the Lufkin and Nacogdoches areas, rain looks to be the primary precipitation form until late in the afternoon. During the late afternoon hours temperatures will drop below freezing allowing for rain to mix with sleet and freezing rain. Before the precipitation there could be a few flurries but little or no snow accumulation is expected at this time.

Again this forecast will likely change between now and Wednesday so please continue to stay tuned for the latest developments. A winter storm watch has been issued for our northwestern counties and more watches, advisories, or warnings could be issued tomorrow.

Wednesday, February 2, 2011

Lake Effect Snow in East Texas?

Very cold temperatures and northwest winds greeted East Texas this morning. Lows dropped down into the lower teens in many areas with wind chill values below zero. Another unusual weather phenomenon that occurred this morning was lake effect snow. Lake effect snow is very common on the leeward side of large lakes, mainly along the Great Lakes, as very cold air moves over the relatively warmer waters. However, this is very rare here in Texas. Usually the size of our man made reservoirs is not large enough to produce a lake effect. Most lakes that produce good lake effect snow have the winds traveling at least 50 miles over the water. Of course here in East Texas we have some very large reservoirs but none give the usual 50 mile long fetch of cold wind blowing across the warmer water to produce heavy snow. However, in rare cases you can get flurries or even light snow over smaller lakes and even rivers if the conditions are right.

So what happened? At 4 AM most of East Texas was under clear skies except for the extreme southern counties. Our wind was out of the northwest around 15 mph moving over a deep snow pack to the north dropping our temperatures into the middle teens in most areas. Many of our East Texas lakes are oriented from northwest to southeast, parallel to the wind flow. This maximized the available moisture fetch available. Now the greatest fetch of wind over water this morning was about 20 miles. So heavy snow is not expected however, if there is a great enough difference from the water and air temperatures, moister will rise off the lake and form very low clouds, almost appearing like steam fog.

Well, this morning we had an average of 30°F difference from the lake temperature to the air temperature. This is more than enough of a difference needed to cause moisture to rise for lake effect snow. Usually you look for a difference of 13°C or about 23°F. The surface wind speed usually needed to produce a lake effect snow event is between 10 and 40 mph. Our wind speed was on the lower end which actually helps the air mass acquire the moisture needed to produce snow down wind.

This is a radar shot from the Shreveport NEXRAD that shows two snow bands that developed this morning. Both of these bands developed off of the warmer waters of nearby lakes. The other bands that developed this morning were too far away to be detected by radar but could be seen from space. Notice the cloud streaks that formed downwind from all of the area lakes this morning.

So next time you see a big weather story in Buffalo, NY showing you how many feet of snow fell remember, we here in East Texas got a little taste of what our neighbors to the north have to deal with many times a year.