Re: January 2013: Tracking The Texas Winter Storm
Posted: Thu Jan 03, 2013 8:30 am
Morning e-mail from Jeff:
Mix of winter precipitation possible tonight for the northern parts of SE TX.
Another piece of energy currently approaching SW TX this morning will swing ENE tonight and Friday. Surface high pressure currently over NW TX has dried the area after the light rains yesterday, but moisture is lurking just to the SW and this will return to the region this afternoon and evening. Fairly decent lift is expected to affect the area north of I-10 after midnight tonight with precipitation banding developing from SC TX into SE TX.
Current soundings have a large dry layer just above the 900mb layer up to around 750mb or roughly 8000 ft thick at IAH to around 7500 ft thick at College Station. There is a significant dry layer with RH below 20% around the 850mb level. All of this dry air above the surface is important in attempting to determine what type of precipitation may fall tonight into Friday. The soundings yesterday morning were similar to today, yet a little more moist and there was just enough cooling in the mid levels from the evaporation of the precipitation falling to allow sleet to reach the ground although the temperature profile showed a extensive area of above freezing temperatures. Since the air is slightly drier this morning in the mid levels, some precipitation falling through this layer tonight will help cool the temperatures in this layer from the 35-38 degree range toward the 31-34 degree range across the northern parts of the region. The best potential for cooling will be north of a line from Columbus to The Woodlands to Livingston where lift will be strongest and enough precipitation falling to help bring the mid layer toward saturation. Using the wet bulb surface temperature the lowest surface temperature I could get was 33-34 at College Station on the forecast sounding early Friday morning…if the GFS and NAM have the correct surface temperature and dewpoint which compared to the current conditions they seem to be aligned well. A wet bulb temperature at BUSH IAH for Friday morning is closer to 36-37 degrees. (Wet bulb temperature is the process of cooling the air temperature toward the dewpoint as a result of bringing the air column toward saturation….RH of 100% this results in the temperature cooling and the dewpoint rising until the air becomes saturated).
So with all that said, the freezing layer above the surface will be shrinking overnight as precipitation falls into the dry air and cools the air to below freezing. The forecast soundings over our northern counties from College Station to Huntsville reduce the above freezing layer to less than 3000 ft thick early Friday, but maintain surface temperatures a few degrees above freezing. Think the warm layer will be short enough to allow a mix of sleet and rain in the region from College Station to Huntsville, it is possible that the lower air column could cool enough to allow all sleet in this area. Precipitation intensity is expected to be on the lighter side (less than .10 of an inch) which suggest little if any accumulation especially if surface temperatures remain in the 33-35 range. Could be a few spots that drop to 31-32 degrees under some heavier precipitation in the northern counties and this could result in some very minor accumulation on elevated surfaces. It must be stressed that only a trace of ice on bridges and overpasses can be extremely dangerous. Should the next round of models come in slightly wetter or cooler over this region, we would likely be into Winter Weather Advisory criteria…and the trend over the past 24 hours has been toward a wetter system.
North of a line from Columbus to Kingwood to Livingston, the air column will require more work as lift will be weaker in this area and the above freezing layer is much thicker. Given the fact that sleet fell yesterday, I would not be surprised to see a mixture of light rain and sleet north of the above mentioned line. Throughout this region surface temperatures will be in the mid 30’s and any sleet will melt on contact. South of this line most of what falls will be liquid with a few ice pellets.
Precipitation Types (Tonight-mid morning Friday):
College Station to Huntsville North: rain/sleet mix possibly changing to all sleet with some snow mix. Some minor accumulation possible
Columbus to Katy to Kingwood to Livingston North: rain/sleet mix. No accumulation
Columbus to Katy to Kingwood to Livingston South: mainly rain with a few sleet pellets. No accumulation
Profile temperatures should warm enough to return the area to all rain by midday Friday. Cloudy and cold conditions will continue into Saturday as the area remains under the grips of an active sub-tropical jet stream aloft. Could see a few showers continue into Saturday before finally clearing on Sunday.
Extended:
While much focus is currently on the short term…the extended will not be outdone as models are showing a potential big weather maker arriving early to mid next week. Shear and instability profiles are looking very favorable for what could be a fairly significant severe weather outbreak over TX including the threat for tornadoes. The system has many similarities to the 1-9-12 flooding and severe weather/tornado event, but even projected stronger wind fields.
Mix of winter precipitation possible tonight for the northern parts of SE TX.
Another piece of energy currently approaching SW TX this morning will swing ENE tonight and Friday. Surface high pressure currently over NW TX has dried the area after the light rains yesterday, but moisture is lurking just to the SW and this will return to the region this afternoon and evening. Fairly decent lift is expected to affect the area north of I-10 after midnight tonight with precipitation banding developing from SC TX into SE TX.
Current soundings have a large dry layer just above the 900mb layer up to around 750mb or roughly 8000 ft thick at IAH to around 7500 ft thick at College Station. There is a significant dry layer with RH below 20% around the 850mb level. All of this dry air above the surface is important in attempting to determine what type of precipitation may fall tonight into Friday. The soundings yesterday morning were similar to today, yet a little more moist and there was just enough cooling in the mid levels from the evaporation of the precipitation falling to allow sleet to reach the ground although the temperature profile showed a extensive area of above freezing temperatures. Since the air is slightly drier this morning in the mid levels, some precipitation falling through this layer tonight will help cool the temperatures in this layer from the 35-38 degree range toward the 31-34 degree range across the northern parts of the region. The best potential for cooling will be north of a line from Columbus to The Woodlands to Livingston where lift will be strongest and enough precipitation falling to help bring the mid layer toward saturation. Using the wet bulb surface temperature the lowest surface temperature I could get was 33-34 at College Station on the forecast sounding early Friday morning…if the GFS and NAM have the correct surface temperature and dewpoint which compared to the current conditions they seem to be aligned well. A wet bulb temperature at BUSH IAH for Friday morning is closer to 36-37 degrees. (Wet bulb temperature is the process of cooling the air temperature toward the dewpoint as a result of bringing the air column toward saturation….RH of 100% this results in the temperature cooling and the dewpoint rising until the air becomes saturated).
So with all that said, the freezing layer above the surface will be shrinking overnight as precipitation falls into the dry air and cools the air to below freezing. The forecast soundings over our northern counties from College Station to Huntsville reduce the above freezing layer to less than 3000 ft thick early Friday, but maintain surface temperatures a few degrees above freezing. Think the warm layer will be short enough to allow a mix of sleet and rain in the region from College Station to Huntsville, it is possible that the lower air column could cool enough to allow all sleet in this area. Precipitation intensity is expected to be on the lighter side (less than .10 of an inch) which suggest little if any accumulation especially if surface temperatures remain in the 33-35 range. Could be a few spots that drop to 31-32 degrees under some heavier precipitation in the northern counties and this could result in some very minor accumulation on elevated surfaces. It must be stressed that only a trace of ice on bridges and overpasses can be extremely dangerous. Should the next round of models come in slightly wetter or cooler over this region, we would likely be into Winter Weather Advisory criteria…and the trend over the past 24 hours has been toward a wetter system.
North of a line from Columbus to Kingwood to Livingston, the air column will require more work as lift will be weaker in this area and the above freezing layer is much thicker. Given the fact that sleet fell yesterday, I would not be surprised to see a mixture of light rain and sleet north of the above mentioned line. Throughout this region surface temperatures will be in the mid 30’s and any sleet will melt on contact. South of this line most of what falls will be liquid with a few ice pellets.
Precipitation Types (Tonight-mid morning Friday):
College Station to Huntsville North: rain/sleet mix possibly changing to all sleet with some snow mix. Some minor accumulation possible
Columbus to Katy to Kingwood to Livingston North: rain/sleet mix. No accumulation
Columbus to Katy to Kingwood to Livingston South: mainly rain with a few sleet pellets. No accumulation
Profile temperatures should warm enough to return the area to all rain by midday Friday. Cloudy and cold conditions will continue into Saturday as the area remains under the grips of an active sub-tropical jet stream aloft. Could see a few showers continue into Saturday before finally clearing on Sunday.
Extended:
While much focus is currently on the short term…the extended will not be outdone as models are showing a potential big weather maker arriving early to mid next week. Shear and instability profiles are looking very favorable for what could be a fairly significant severe weather outbreak over TX including the threat for tornadoes. The system has many similarities to the 1-9-12 flooding and severe weather/tornado event, but even projected stronger wind fields.