11:45 a.m. CST Thursday, February 3, 2011
This is my Thursday morning update on the winter storm expected to impact Southeast Texas later today, tonight and Friday.
...A Winter Storm Warning is in effect for all of Southeast Texas including the Houston/Galveston area from Noon CST today through Noon CST Friday. A Winter Storm Warning means that a a significant winter storm or hazardous winter weather is occurring, imminent, or likely, and is a threat to life and property...
...A wintry mix of snow, sleet and freezing rain will produce significant snow and/or ice accumulations across Southeast Texas beginning later today with the peak of the storm later tonight and in the early morning hours Friday with the storm winding down about midday Friday or early Friday afternoon...
As I mentioned in my special update yesterday evening, this winter storm event is still evolving and seriously challenging forecasters (including me) with exactly what is going to happen, the various precipitation types that can be expected and total snow and/or ice accumulations for specific locations in Southeast Texas. As you know, I revised my forecast last night to include a significant threat of freezing rain and ice accumulation along the coast while shifting the higher snow accumulations a bit further inland. This was based on model output last night that suggested a warmer layer of air aloft (a temperature inversion) will override the very cold surface air along and near the coast resulting in a more favorable scenario for freezing rain to mix with the sleet and snow there.
Before I get to this morning's forecast discussion, I will address the current weather.
At 10:00 a.m., most of Southeast Texas is covered by a scattered to broken layer of low level stratocumulus below an overcast layer of mid level nimbostratus. Current temperatures are: N/A at Houston-Bush, 32 at Houston-Hobby, 32 at Houston-Ellington, 33 at Galveston, 32 at Jamaica Beach, 31 at Angleton-Lake Jackson, 28 at Sugar Land, 30 at Fresno-Arcola, 27 at Tomball, 29 at Cleveland, 27 at Conroe, 25 at Huntsville, 27 at Crockett, 27 at Lufkin, 26 at La Grange, 23 at Giddings, 27 at Brenham, 23 at Caldwell, 24 at Hearne, 26 at Bryan-College Station, 28 at Victoria, 30 at Bay City, 30 at Wharton, 32 at Beaumont-Port Arthur and 32 at Orange. Dew point temperatures are currently in the teens and lower 20's.
The local National Weather Service Doppler Radar in League City is showing widespread returns now but most of this precipitation is not yet reaching the ground due to the very cold, dry air at the surface. This evaporating precipitation is know as "virga". However, as each hour passes, the atmosphere is becoming increasingly more saturated and this precipitation aloft will eventually start reaching the ground. Within the past hour, a few reports of ice pellets (sleet) have been received in Polk County near Ace...and in Harris County in north Houston off Tidwell Rd, west Houston near Beltway 8 and the Texas Medical Center area.
Now to the still challenging forecast.
Although some of you may see some very light snow flurries or a brief shower of ice pellets (sleet) early this afternoon, I believe the main onset of wintry precipitation in earnest will be delayed until after 6:00 p.m. This is better news for the late afternoon rush hour but tomorrow morning's rush hour will be an entirely different story.
The biggest question remains precipitation type and accumulations of snow or ice. After reviewing this morning's upper air data, I am still conflicted about the primary precipitation type along the coast but I am convinced that the vertical thermal profile of the atmosphere (aka "the column") from about 30 miles and more inland will support a mainly snow and sleet event with very little freezing rain. Once the precipitation begins falling, evaporative cooling will cause the surface air temperature to fall toward the dew point. While this happens, the dew point temperature will rise toward the air temperature. The complete saturation point (air and dew point temperature the same resulting in 100% humidity) will probably occur with the air temperature in the upper 20's.
Along the immediate coast to about 30 miles inland, I think there will be more of a wintry mix with various combinations of snow, sleet and freezing rain. Now, here's where things could get interesting. If a heavier band of snow moves through the coastal area, it could produce enough evaporative cooling to change the vertical thermal profile just cold enough to shift the primary threat to mainly snow and/or sleet instead of freezing rain along the coast. Think of putting ice cubes in a glass of ice water. One ice cube will eventually cool the water but it takes longer than if you fill the glass with ice cubes or crushed ice.
On the other hand, if precipitation remains fairly light this evening, the vertical column could become just warm enough aloft to support a better chance of freezing rain instead of sleet/snow. This is the primary forecasting dilemma at the moment for the coastal areas. I can't emphasize enough how close of a call this is going to be.
Therefore, I am going to slightly modify last night's update to decrease the duration of freezing rain and slightly increase the duration of snow and sleet again along the coast. The result of this would be somewhat less of an ice accumulation potential and a slightly better snow accumulation potential than I indicated last night.
In a typical transition phase, if the warmer air aloft expands, the primary precipitation type changes from snow to sleet to freezing rain. However, if the warmer air aloft erodes, the primary precipitation type will change from freezing rain to sleet to snow. So, if you experience a transition of precipitation types, you will have a fairly good idea of what's taking place in the atmosphere above your location.
I will now revise my total snow and ice accumulations expected for this event as follows:
Galveston Island, Texas City, League City, Anahuac, High Island, Freeport, Angleton:
Precipitation types: Wintry mix of snow, sleet and freezing rain.
Snow accumulations: average of 1 inch...with a few isolated 2 to 3 inch totals.
Ice accumulations: average of 1/10 of an inch with a few 1/4 inch totals.
Greatest potential of ice accumulation:
Gulf Freeway (IH-45) - Galveston Causeway;
State Highway 146 - Dickinson Bayou Bridge, Kemah/Seabrook Bridge.
Houston, Sugar Land, Alvin, Wharton, Victoria, Cleveland, Columbus:
Precipitation types: Primarily snow and sleet with some freezing rain.
Snow accumulations: average of 1 to 3 inches...with a few isolated 4 to 5 inch totals.
Ice accumulations: generally less than 1/10 of an inch.
Greatest potential of ice accumulation:
State Highway 146 - Kemah/Seabrook Bridge and Fred Hartman Bridge;
Beltway 8 - (Jones) Toll Bridge;
Loop 610 - Ship channel bridge at the Port of Houston.
Navasota, The Woodlands, Conroe, Huntsville, Livingston, Coldspring:
Precipitation types: Primarily snow with some sleet.
Snow accumulations: average of 2 to 4 inches...with a few isolated 5 to 6 inch totals.
Ice accumulations: less than 1/10 of an inch.
The worst of this winter storm will take place during the overnight hours with the peak of the storm between 9:00 p.m. tonight and 9:00 a.m. Friday morning.
Driving conditions will quickly deteriorate this evening since surface temperatures have been well below freezing for some time now. In areas that receive mainly snow, it will be bad enough since heavy snow can quickly reduce visibility and is difficult to maintain traction in. Freezing rain and sleet, mainly along the coast, is even more dangerous and more difficult to drive in. Having driven in a few Houston ice storms back in the mid 1980's, I can assure you that most people around here do not have the necessary skills, experience or proper automotive equipment to drive on a treacherous glaze of ice.
So, if you don't have to be out on the road for a very compelling reason, please stay home. If you must drive, remember to take a survival kit that includes a cell phone, blankets, drinking water and flashlights. If you're involved in an accident, remember that police, fire and EMS will probably be responding to hundreds of others at the same time so you may be waiting awhile for assistance. Be patient and move your vehicle off the roadway if you can.
I will send out another update later this evening as the situation unfolds.