#202 Postby jeff » Tue Nov 28, 2006 10:11 am
Arctic air mass to arrive on Thursday
Severe storms possible ahead of front…followed by dramatic temp. fall
Current:
Bitter cold arctic air mass is on the move this morning spilling into the US from western Canada. Current temperatures are -14F at Cut Bank, MT and -10 at Great Falls. MT with NW winds of 20-25kts. 00Z (600pm Mon) soundings over MT and W Canada show a strong arctic inversion capping the cold surface air mass which is about 4000 ft thick over MT. Water vapor tells the entire story with a strong polar jet extending from the Gulf of Alaska down the US west coast to central CA and then eastward into the Great Basin carving out a strong longwave trough. Surface pressure changes show strong low pressure forming over MN and NE CO along a polar boundary. Upper air and surface features are almost in place to release the bitter cold over MT southward down the plains…this will happen in the next 12-18 hours.
Forecast:
Deepening longwave trough over the western US will eject into the plains allowing strong surface low over NE CO to deepen and move ENE toward the Great Lakes. This unleashes the arctic air mass southward. Feel the air mass is cold and dense enough that once it is on the move it is coming fast down the plains with little in the way to stop it. Arctic boundary blasts through the panhandle Wednesday and roars through N TX before sunrise Thursday and then comes through SE TX midday Thursday. Boundary will be moving at 35-45mph with impressive cold air advection behind the front. Highs at all ob sites will be early with a dramatic temp. fall post front. A temp drop of 20-25 degrees will occur within the first 30 minutes of frontal passage dropping temps. from the mid 70’s to near 50 with howling NW winds. Freezing line marches southward during the day Thursday reaching our NW counties by late afternoon. Feel temps. will continue to slide with strong cold air advection, falling into the 30’s and low 40’s north of I-10 and near 45-50 along the coast by Thursday late afternoon.
Potent upper low ejects across NW TX and OK Thursday into Thursday night resulting in a period of post frontal precipitation. Per GFS and NAM models, critical thickness values, 2m temps, and 850mb temps are dangerously close to our northern counties Thursday evening. Warm nose above the arctic dome should preclude frozen precip. in SE TX, however surface temps. will be very close to freezing as the rain comes to an end and may result in a period of freezing rain mixed with sleet from Columbus to College Station to Huntsville to Lufkin. This aspect will need close watch as the track of the upper low across TX and OK, should it shift southward, could bring significant ice and snow to portions of TX.
Winds:
Impressive arctic ridging will build into the state behind this front with NW winds sustained at 25-35mph with gust at or above 40mph inland and likely 45mph along the coast. Wind advisory will be needed along with Gale Warnings offshore as cold air strikes warm Gulf of Mexico water. Strong winds will drive wind chills into the 20’s and 30’s. Suspect gusts of 45-50mph in the downtown canyons where winds are funneled between the tall buildings.
Severe Weather:
Although instability will be lacking due to cloud cover and time of day of frontal passage, 0-6km shear profiles and strong frontal lift may result in a few severe thunderstorms. Wind profiles are supportive of rotating updrafts and supercells, although the main threat looks to be straight line winds along a fast forward moving squall line. SPC slight risk is in effect for Thursday for most of the area.
Temperatures:
Freezing line should reach the I-10 corridor early Friday morning, however strong surface winds should preclude a hard freeze over most of the area. Latest guidance does NOT show ridge axis overhead Friday night instead it is west of the area with light northerly winds continuing suggesting lower levels may remain mixed and while a freeze is likely a killing hard freeze is now questioned.
Air mass is reinforced Sunday with another polar front with GFS 0C 850mb line all the way to the coast by Monday morning suggesting freezing temps. areawide. Will have to keep a close eye on this as possible overrunning moisture may mingle with the cold air mass Sunday into Monday.
Forecast for Harris and Surrounding counties:
Today: mostly cloudy and humid with a 20% chance of light rain and showers. Highs in the upper 70’s. S winds 10-15mph.
Wednesday: cloudy and warm with a 30% chance of showers. Highs near 80 and lows in the upper 60’s. South winds 15-20mph.
Thursday: cloudy with a 70% chance of thunderstorms some may be severe. Highs near 75 early then falling into the mid 40’s by early afternoon and near 40 by late afternoon. South winds 15-20mph shifting to the NW and increasing 25-35mph and gusty with wind chills in the 20’s and 30’s.
Thursday night: cloudy and cold with a 30% chance of light rain. Temps. falling into the low 30’s north of I-10 and in the mid to upper 30’s south of I-10. NW winds 25-35mph and gusty. Light rain may change to light freezing rain or sleet north and west of a Columbus to College Station to Lufkin line by 900pm..no accumulation is expected.
Friday: clearing skies and cold with highs near 50. NW winds 15-25mph and gusty with wind chills in the 30’s and 40’s.
Friday night: clear and cold with lows of 29-31 rural areas and 30-33 urban areas. N winds 5-10mph with wind chills in the low 20’s.
Saturday: clear and cold with highs in the low 50’s. N winds 10-15mph.
Saturday night: increasing clouds late with lows 30-32 rural areas and 31-33 urban areas. Light winds.
Sunday: increasing clouds and turning colder. Temps. in the 50’s falling in the afternoon. NE winds shifting to the NW and increasing 15-20mph and gusty.
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