#471 Postby srainhoutx » Tue Dec 01, 2009 9:46 am
E-mail from Jeff this morning...
Numerous issues to contend with this morning!
Confidence starting to grow for significant winter precipitation across Texas Thursday night –late Friday (See detailed section below)
Strong storm system to affect the area today into early Wednesday.
Arctic air mass to arrive Thursday into the area.
Today-Wednesday:
Cold upper storm system over N Mexico will move eastward today while NW Gulf surface low develops and tracks along the TX coast. Widespread rains along the coast will gradually build inland today with numerous coverage ongoing by afternoon into the evening hours. Surface low deepening over the NW Gulf will bring increasing NE winds to the coastal and offshore areas today with tides increasing. Upper storm crosses TX tonight with greatest lift coming to bear in the 600pm to 300am time period. Expect widespread rains with perhaps a clap of thunder or two. Rains will end from west to east early Wednesday as the low begins to shift east of the region.
A good widespread 1” of rain will be possible with higher totals near the coast. Along the coast the increasing ENE to NE wind will result in water level rise of 3.0-4.0 feet total producing some coastal flooding at high tide this afternoon from Rockport into SW LA. Bolivar is once again to most troubled spot with a 3.5 tide forecast at Port Bolivar…which may flood HWY 87. Winds will turn N Wednesday as the surface low moves into SC LA allowing tides to fall.
Thursday
Departing storm system allows a dump of cold arctic air into TX Thursday. Blocking is gradually developing over the higher latitudes of Alaska and NW Canada and while the pattern is not in full place by Thursday…there is enough to send a cold shot deep into TX. GFS shows a large 1040mb high building southward with very cold air in the entire profile by Thursday evening. Will undercut highs by several degrees and show only the upper 40’s even with some sun on Thursday under strong cold air advection pattern.
Thursday night-Friday night:
Forecasting nightmare at hand for this period as GFS and now other models show a significant short wave moving across the region with the cold air in place producing widespread and at times heavy frozen precipitation. The potential comes from a strong short wave that drops out of the central Rockies and then deep into TX Friday with rapid moisture advection pouring into the cold air mass. Cross sections support all snow by early Friday morning as strong cold air advection in all levels is noted. The pattern reminds me more of a polar clipper system in the Midwest than a true winter storm by TX standards..I believe if my memory is correct we had something similar in February of the late 1990’s that produced trace amounts of snow at KIAH and around an inch over our NW counties. Main issue I have is not the cold air, but the amount of available moisture…the upper air pattern is a dry one…but then it does not take much moisture in such a cold air mass to produce precipitation and lots of it. Given 00Z ECMWF now trending toward slightly more moisture and 00Z NAM/GFS nearly identical cannot deny the fact that snow is possible…this is the 3rd run in a row for the GFS showing significant snow and the other drier models are starting to trend in that direction!
Profiles support all snow nearly all counties Friday morning with the exception being our SW counties where profiles and critical thickness support a rain/snow mix and then change over to all snow by mid morning. GFS and NAM show 0C 850mb line reaching the Gulf waters on Friday which would help bring the frozen precipitation to the coast and offshore. NWSFO at Austin/San Antonio and Houston/Galveston along with the section for Victoria have all reluctantly placed 30-40% snow chances in for Friday. If the GFS is correct chances will be closer to 80% with some areas of significant accumulation.
Hate to even attempt to tackle any kind of accumulations this far out and with so much uncertainty and the fact that I do not know if I believe it will snow two years in a row in Houston and so early in the season…would be a record! Given a 10-1 ratio would suggest anywhere from 1-10 inches with the 5-1 ratio showing about 1-8 inches. Do not foresee surface temperatures below freezing Friday morning, but the onset of snow could/will result in evaporative cooling and wet bulb cooling as cold air advection continues…allowing temperatures to fall toward freezing where snow will be falling. This wet bulb cooling effect would be very similar to the historical Christmas Eve 2004 snowstorm where temperatures started in the lower 40’s and cooled to near or just below freezing in the snowfall area allowing those historical accumulations.
As one can imagine with any kind of winter weather event this far south…the uncertainty is very high and forecast confidence extremely low…we could end up cold and cloudy or we could have some significant snow.
Extended:
Upper air blocking looks to lock in place in the form of a high latitude Rex block over portions of Alaska and W Canada. GFS builds a massive 1060mb arctic dome and unleashes it into the US with a bitter outbreak of very cold air across the US next week. If even a portion of this event unfolds some serious cold will be headed southward next week. Southern stream remains active and will support additional storm systems and moisture with this potentially cold air mass.
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