November Weather Discussion. Rollercoaster Pattern.

[wxman57]

Well, the 3pm-issued Houston NWS forecast for Thanksgiving certainly won't verify. Cloudy with showers and a high in the low 60s in Canadian/Arctic air? Try about 15F cooler.

WEDNESDAY AND WEDNESDAY NIGHT...CLOUDY WITH A 40 PERCENT CHANCE
OF SHOWERS. HIGHS IN THE MID 70S. LOWS IN THE LOWER 50S.

THANKSGIVING DAY...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF
SHOWERS. COOLER. HIGHS IN THE LOWER 60S.

THURSDAY NIGHT...MOSTLY CLOUDY WITH A 30 PERCENT CHANCE OF
SHOWERS IN THE EVENING...THEN PARTLY CLOUDY AFTER MIDNIGHT.
COLDER. LOWS IN THE UPPER 30S.

[srainhoutx]

Taking a look at some of the afternoon discussions coming in, it appears that some of the WFO's are buying the magnitude of the Arctic Air and their discussions reflect that...

Houston/Galveston...

RAIN CHANCES
INCREASE WEDNESDAY NIGHT AND THURSDAY AS A COLD FRONT CROSSES THE
AREA. SHOWERS WILL FOCUS ON THE FRONT WED NIGHT AND EARLY
THURSDAY. ISENTROPIC UPGLIDE BEHIND THE FRONT LOOKS STRONGER NOW
SO WILL KEEP CLOUDS AND RAIN CHANCES GOING INTO THURSDAY NIGHT.
SOUTHEAST TEXAS WILL LIE IN A RIGHT REAR QUAD THURSDAY WHICH
SHOULD PROVIDE ADDITIONAL LIFT. ECMWF AND GFS ARE IN PRETTY GOOD
AGREEMENT NOW SO CONFIDENCE IS GROWING THAT THANKSGIVING DAY WILL
BE MUCH COOLER WITH PERIODS OF VERY LIGHT RAIN. THE UPPER LEVEL
TROUGH AXIS CROSSES EAST TEXAS THURSDAY NIGHT AND SKIES SHOULD
CLEAR FRIDAY MORNING.

Dallas/Ft Worth...

THE PRIMARY FORECAST CHALLENGE CONCERNS THE STRONG MIDWEEK COLD
FRONT. UPPER LEVEL ANALYSIS THIS MORNING DEPICTS THE POLAR VORTEX
OVER THE CANADIAN ARCTIC ARCHIPELAGO WITH STRONG RIDGING ACROSS
THE GULF OF ALASKA. MODELS ARE IN GOOD AGREEMENT THAT THE POLAR
VORTEX WILL LUMBER SOUTHWARD OVER THE NEXT COUPLE DAYS AND REACH
ALBERTA SUNDAY NIGHT BEFORE HEADING EAST. IN RESPONSE TO THE
DYNAMICS ALOFT...THE SURFACE AIR MASS OVER WESTERN CANADA WILL
UNDERGO ANTICYCLOGENESIS WITH A SURFACE HIGH BUILDING TO NEAR
1040MB AND TEMPERATURES COOLING WELL BELOW 0. THIS DENSE ARCTIC
AIR MASS WILL INITIALLY BE HELD AT BAY ACROSS THE NW US...BUT AS A
PIECE OF THE UPPER TROUGH EJECTS EASTWARD BY MIDWEEK...THE FLOOD
GATES WILL OPEN AND COLD AIR WILL PLUNGE SOUTHWARD WEDNESDAY. GFS
FRONTAL TIMING IS THE FASTEST OF THE MODELS AND IS FAVORED. SINCE
MODEL AGREEMENT IS GOOD AND THE EVOLUTION OF THE UPPER LEVEL
PATTERN IS CLIMATOLOGICALLY FAVORED FOR STRONG COLD FRONTS IN OUR
AREA...WILL NOT BE SHY ABOUT PUTTING FORECAST TEMPS WELL BELOW THE
MEX MOS AND CLOSE TO RAW OUTPUT FROM THE GFS/ECMWF. THIS MEANS
TEMPS SHOULD FALL WEDNESDAY NIGHT INTO THE 30S AREA WIDE. TEMPS ON
THANKSGIVING WILL STRUGGLE TO WARM APPRECIABLY WITH COLD ADVECTION
CONTINUING AND HIGHS REMAINING IN THE 40S. 12Z GFS/ECMWF/GFES
ARE SHOWING THE MAIN TROUGH AXIS HANGING BACK AND NOT CLEARING THE
AREA UNTIL THURSDAY EVENING. THIS MAY RESULT IN CLOUD COVER WITH
ISENTROPIC LIFT NEAR AND ABOVE 700 MB THURSDAY WITH HIGHS POSSIBLY
COLDER THAN CURRENTLY FORECAST. HOWEVER...CLEARING SKIES AND
LIGHT WINDS ARE EXPECTED THURSDAY NIGHT WITH LOWS FALLING INTO THE
20S AND LOW 30S AREA WIDE. SINCE THE UPPER LEVEL PATTERN BECOMES
SOMEWHAT PROGRESSIVE...THE VERY COLD AIR WILL NOT HANG AROUND
TOO LONG AND EXPECT A GRADUAL WARMING TREND NEXT WEEKEND.



Norman, OK...

THE EXTENDED FORECAST /MONDAY AND BEYOND/ HAS NOT BEEN CHANGED BY THIS
SHIFT...EXCEPT FOR LOADING NEW GUIDANCE DEW POINT GRIDS BEYOND THE
EXPECTED MID WEEK COLD FRONTAL PASSAGE. MED RANGE MODELS SHOW VERY
SIMILAR TIMING OF THE COLD AIR INTRUSION ANTICIPATED TO ARRIVE ON
WEDNESDAY. MODEL FORECASTS VARY WIDELY REGARDING HOW QUICKLY OUR
TEMPERATURES WILL WARM UP FOLLOWING THANKSGIVING DAY.



Midland/Odessa...

MEANWHILE...THE NORTHWESTERN U.S.
UPPER TROUGH WILL MIGRATE EASTWARD INTO THE ROCKIES AND PLAINS
STATES DURING THE MIDDLE OF THE UPCOMING WEEK. THIS SYSTEM WILL
ALSO DEEPEN AND SEND A TROUGH AXIS ACROSS OUR REGION ON THURSDAY.
WITH THIS SYSTEM...WE EXPECT SOME BIG CHANGES IN OUR TEMPERATURES
LATE NEXT WEEK. AT THE PRESENT...IT APPEARS A STRONG COLD FRONT
WILL MOVE INTO THE AREA WEDNESDAY NIGHT OR EARLY THURSDAY MORNING.
ARCTIC AIR WILL MOVE IN BEHIND THE COLD FRONT...LIKELY SENDING
TEMPERATURES WELL BELOW NORMAL ON THANKSGIVING DAY AND FRIDAY. THE
CURRENT FORECAST ACCOUNTS FOR THE ABOVE SCENARIO WELL AND THE ONLY
CHANGES MADE TO THE FORECAST WERE TO DECREASE HIGH TEMPERATURES ON
THURSDAY AND FRIDAY. 850MB TEMPERATURES MAY VERY WELL REMAIN AT OR
BELOW 0C ON THURSDAY. IF THIS TREND CONTINUES...TEMPERATURES WILL
NEED TO BE LOWERED AGAIN.

[wxman57]

They may say they're buying it, but the forecasts don't reflect that they believe it's going to be cold.

[srainhoutx]

Well, we know better don't we...

[wxman57]

Well, we know better don't we...

I know better. I've watched the area NWS offices struggle with shallow cold airmasses for 30 years now. Seems that just as they get a crew in that figures out when to trust the models (or when not to), they leave and are replaced by a new crew who make the same old mistakes with Canadian/Arctic air. I'd go with 50s early in the morning (after midnight) as the cold air arrives, dropping to the 46-49F range at IAH after sunrise with clouds and occasional light rain. Temp holding in the mid-upper 40s through the day.

[srainhoutx]

One of the things I continue watch is the potential for severe weather leading up to the Arctic Intrusion. Some of the early SREF guidance data suggests this will need to be watch very carefully as we get closer to next week. Something for Ed and our severe weather folks to watch.

[Ptarmigan]

I know better. I've watched the area NWS offices struggle with shallow cold airmasses for 30 years now. Seems that just as they get a crew in that figures out when to trust the models (or when not to), they leave and are replaced by a new crew who make the same old mistakes with Canadian/Arctic air. I'd go with 50s early in the morning (after midnight) as the cold air arrives, dropping to the 46-49F range at IAH after sunrise with clouds and occasional light rain. Temp holding in the mid-upper 40s through the day.

Why do they have a hard time with shallow cold air masses? I know big freezes were shallow cold air masses, like the February 1989 Freeze.

[redneckweather]

It's not that the NWS has a hard time forecasting shallow arctic airmasses, it's that the models have a hard time forecasting them. The so called mets who write up the AFD's are just relaying what they see in the models....and they live and breathe by the models IMO.

[wxman57]

It's not that the NWS has a hard time forecasting shallow arctic airmasses, it's that the models have a hard time forecasting them. The so called mets who write up the AFD's are just relaying what they see in the models....and they live and breathe by the models IMO.

That is often the problem. I was reviewing some old model runs I'd saved from a Thanksgiving 1993 Arctic front (a good bit colder than next week's air) and the models from 2 days out were over 24 hours too late in bringing the cold air down. They had the Arctic boundary in northern Oklahoma Thanksgiving morning of 1993 when by that time, the front was already in the Gulf.

In this case, though, the models ARE forecasting the cold air to be in place. They ARE forecasting temps holding in the upper 40s here for Thanksgiving Day. So it's not an issue with following the models blindly. They're currently NOT following the models. Maybe they don't believe it can get that cold here in late November? Probably more likely they're uncomfortable forecasting extreme cold 5 days out, regardless of what is indicated by all the models and what we've learned by watching such events unfold over the decades. The forecasters are more comfortable dropping the projected highs 3-4 degrees a day until maybe by Tuesday of next week they're finally forecasting temps closer to what will actually occur.

The current forecasts are almost guaranteed to be wrong. We don't hit highs in the 60s in such an airmass AND with clouds and rain.

[srainhoutx]

A bit chilly to our N...

[srainhoutx]

Good morning. The chatter in the morning discussions from various WFO's continue the downward trend in temps...

Houston/Galveston...

THE FORECAST GETS A LITTLE MORE INTERESTING WED NIGHT THROUGH THE
THANKSGIVING WEEKEND. THE MAIN UPPER LOW WITH THE TROUGH OVER THE
ROCKIES MOVES INTO THE PLAINS. THE GFS/ECMWF/CANADIAN ARE STILL
HAVING A TOUGH TIME DETERMINING THE TIMING OF WHEN THE TROUGH
MOVES INTO THE PLAINS WITH THE ECMWF/CANADIAN TRENDING SLOWER. AS
A RESULT...THESE MODELS ARE SLOWER WITH THE COLD FRONTAL PUSH.
GFS REMAINS CONSISTENT WITH BRINGING THE FRONT THROUGH WED NIGHT
AND OFF THE COAST BY THU MORNING. THE ECMWF SEEMS TO BE ABOUT 6
HRS SLOWER THAN THE GFS WITH THE FRONTAL PASSAGE. AS SUCH...I
WILL KEEP WITH THE GOING FORECAST THINKING AND STICK CLOSER TO
THE GFS FOR THE FORECAST. POPS OF 40 PERCENT LOOK REASONABLE
RIGHT NOW ESPECIALLY GIVEN TIMING DIFFERENCES IN THE FRONT.
POSSIBLE THAT POPS COULD GO HIGHER IF THESE FORECAST TRENDS
CONTINUE.

THE THANKSGIVING HOLIDAY DOES LOOK TO BE COLDER AND WET FOR SE
TX. IT IS NOT OUT OF THE QUESTION THAT TEMPS MAY NOT REACH THE
50S WITH CLOUD COVER AND LINGERING RAIN. FOR NOW THE FORECAST
WILL KEEP TEMPERATURES A FEW DEGREES BELOW GFS GUIDANCE IN
ANTICIPATION OF A DOWNWARD TREND IN TEMPS. MODELS DO HINT AT POST
FRONTAL RAINBANDS SETTING UP THAT CORRESPOND NICELY TO
FRONTOGENESIS AND TEMP GRADIENT AT 850MB. THE FORECAST WILL KEEP
MENTION OF RAIN ON THANKSGIVING WITH DRIER CONDITIONS FOR FRIDAY.
THE CLOUD COVER MAY BE AN ISSUE IN FORECASTING FRI MORNING LOW
TEMP AS CLOUDS MAY BE SLOW TO CLEAR OUT ESPECIALLY ALONG THE
COAST. IT IS POSSIBLE THAT NEXT SATURDAY MORNING COULD BE COLDER
THAN FRIDAY BUT STILL SOME TIME TO WORK OUT THOSE DETAILS.
OVERALL...TEMPS SHOULD BE BELOW NORMAL FOR THE END OF NEXT WEEK
WHICH WILL BE IN CONTRAST TO THE ABOVE NORMAL TEMPERATURES
EXPECTED THE NEXT FEW DAYS.



Dallas/Ft Worth...

ATTENTION THEN TURNS TO THE ARRIVAL OF COLDER AIR DURING MIDWEEK.
00Z ANALYSIS REVEALS SURFACE TEMPERATURES IN THE MINUS 20 DEGREE
RANGE UNDERNEATH THE POLAR VORTEX IN NORTHERN CANADA. EXPECT THIS
COLD AIR TO SLOWLY MOVE SOUTH OVER THE NEXT FEW DAYS UNTIL A SHORT
WAVE TROUGH MOVES THROUGH THE UPPER FLOW...ALLOWING FOR THE COLD
AIR TO MOVE SOUTHWARD. MEDIUM RANGE GUIDANCE CONTINUE TO AGREE ON
THE GENERAL PATTERN...ALLOWING FOR A 1040 HIGH TO MOVE SOUTHWARD
ALONG THE FRONT RANGE OF THE MOUNTAINS. ONCE THE COLD AIR BEGINS
TO RUSH SOUTHWARD...EXPECT FOR THE COLD AIR TO REACH THE RED RIVER
TUESDAY EVENING AND CONTINUE TO MOVE ACROSS THE AREA THROUGH THE
THANKSGIVING HOLIDAY. AREAS THAT HAVE NOT SEEN FREEZING
TEMPERATURES YET THIS SEASON COULD EASILY SEE A FREEZE BY
THANKSGIVING MORNING OR FRIDAY MORNING.



Austin/San Antonio...

RAIN CHANCES WILL PEAK WEDNESDAY AFTERNOON THROUGH THURSDAY MORNING
ASSOCIATED WITH THE APPROACH AND PASSAGE ON A STRONG COLD FRONT. THE
COLD FRONT WILL SWEEP ACROSS THE REGION WEDNESDAY NIGHT SETTING THE
STAGE FOR A COLD AND BLUSTERY THANKSGIVING DAY.

THANKSGIVING DAY WILL BE COLD AND WINDY WITH POST-FRONTAL CLOUDINESS
AND PATCHY LIGHT RAIN. THE COMBINATION OF STRONG CAA...CLOUDY TO
MOSTLY CLOUDY SKIES AND LIGHT STRATIFORM PCPN WILL RESULT IN
AFTERNOON HIGHS RANGING FROM THE UPPER 40S NORTH TO LOWER 60S
SOUTHWEST.

LIGHT STRATIFORM PCPN WILL END THURSDAY AFTERNOON WITH CLEARING SKIES
THURSDAY NIGHT AS THE UPPER LEVEL TROF AXIS MOVES EAST OF OUR CWFA.

TEMPERATURES OVER THE HOLIDAY WEEKEND WILL BE BELOW NORMAL. HIGHS
WILL BE IN THE UPPER 50S TO LOWER 60S WITH LOWS IN THE 30S...20S
ACROSS THE HILL COUNTRY FRIDAY MORNING.

THIS FCST IS BASED ON A BLEND OF THE GFS/ECMWF...THE GEM IS THE
OUTLIER WITH A LATER FROPA. WILL GO ABOVE GFS MOS FOR HIGHS
WEDNESDAY GIVEN THE WEST TO SOUTHWEST WIND COMPONENT IN THE
BOUNDARY LAYER. WILL GO SLIGHTLY BELOW GFS MOS FOR HIGHS THURSDAY
BASED ON THE DOWNWARD TEMPERATURE TREND OF SUCCESSIVE MODEL RUNS.


Norman, OK...

AFTER BEING IN DECENT AGREEMENT ON THE TIMING OF THE STRONG COLD
FRONT FOR MIDWEEK... THE LATEST GFS/ECWMF RUNS ARE DIVERGING ONCE
AGAIN. THE GFS PUSHES THE FRONT THROUGH ALL OF THE CWA BY 18Z
WED... WHILE THE ECMWF... WHICH IS ABOUT 12 HOURS SLOWER... STILL
HAS IT BACK IN NW KS... A MUCH SLOWER SOLUTION FROM PRIOR ECMWF
RUNS. THE TIMING DIFFERENCES RELATE TO HOW QUICKLY EACH OF THE
MODELS MOVE THE UPPER TROUGH MOVES EAST INTO THE PLAINS. AT THIS
POINT... WILL STILL TREND WITH THE FASTER GFS UNTIL WE SEE A FEW
MORE SUBSEQUENT MODEL RUNS.



Lubbock...

A LOW AMPLITUDE SHORTWAVE TROUGH IS RESOLVED IN THE MODELS EJECTING
FROM THE WESTERN U.S. SYNOPTIC TROUGH MONDAY. THIS FEATURE WILL BRING
ANOTHER MINOR DECREASE IN HEIGHTS/THICKNESSES AND DAYTIME TEMPS...BUT
MAXIMA WILL STILL WARM SOME 5-8 DEGREES ABOVE NORMAL. A WEAK FRONTAL
BOUNDARY ALSO WILL SAG SOUTHWARD OVER THE SOUTHERN PLAINS IN THE WAKE
OF THIS FEATURE TUESDAY...BUT NO SENSIBLE WEATHER IMPACTS ARE EXPECTED.

ATTENTION THEN TURNS TOWARD THE ANTICIPATED STRONG FROPA WEDNESDAY...
WITH SUBSEQUENTLY COOLER TEMPERATURES ON THANKSGIVING. THE AIRMASS
NOW CENTERED AT 75 DEGREES N/127 DEGREES W...CURRENTLY CHARACTERIZED
BY -30 DEGREE AIR...WILL BE DISLODGED SOUTHWARD ACROSS WESTERN CANADA
AND INTO THE INNER MOUNTAIN WEST/GREAT BASIN BY TUESDAY. ALTHOUGH THE
AIRMASS WILL MODIFY SIGNIFICANTLY...IT WILL BE PUSHED OVER THE NORTHERN/
CENTRAL ROCKIES AND SOUTHWARD THROUGH THE PLAINS BY MID WEEK AS A NOTABLE
WAVE EJECTS FROM THE WESTERN CONUS UA TROUGH. A SUBSTANTIAL PORTION
OF THIS AIRMASS IS PROGGED TO FILTER OVER THE SOUTHERN PLAINS IN THE
WAKE OF A WEDNESDAY FROPA...AS THE PARENT 1040MB SURFACE HIGH REMAINS
ENTRENCHED OVER THE GREAT BASIN. THIS SHOULD SUPPORT A NOTABLE COOL
DOWN WITH TEMPERATURES 10-15 DEGREES BELOW SEASONAL VALUES BY THURSDAY...
BUT PROGGED DRY PROFILES AND NEAR FULL INSOLATION MAY HELP TO ACTIVELY
MODERATE THE AIRMASS AND PREVENT SUB-MOS DAYTIME TEMPERATURES.

[srainhoutx]

Another shot of chilly air may happen around the 30th of November if the 00Z Euro is correct...

[wxman57]

Models haven't budged in 2 days as far as temps on Thanksgiving. Houston NWS office continues to go 10 degrees higher than guidance with their forecast of a low in the low 50s and a high in the upper 50s on Thanksgiving. Models still bring the front through Wednesday evening with rapidly-falling temps Wednesday night and a continued fall during the day Thanksgiving, down to the mid to upper 40s by Thursday afternoon. It'll probably be another 2-3 days before the NWS lowers their forecast temps to highs in the upper 40s for Thursday.

[redneckweather]

What are we looking at as far as wind on Thanksgiving...20 to 25 gusting to 35?

[srainhoutx]

That looks reasonable to me redneckweather. I expect a gale warnings offshore as well.

[wxman57]

That looks reasonable to me redneckweather. I expect a gale warnings offshore as well.

Yes, strong winds offshore. I've been forecasting winds offshore for 30 years now. The strongest winds offshore occur in November/December when cold air flows out over the still relatively warm water. This results in what is called "supergeostrophic flow". Strong cold fronts can generate winds to 50-60 kts offshore, with higher gusts. As a general rule of thumb for predicting offshore winds, take a look at the high center that's dropping south to Texas. In this case, about 1030-1035mb. Now drop the first two digits of the pressure and you have 30-35. That's a good estimate of the prevailing wind speeds offshore in the wake of the front. Early in the season, when the water is warmer (like now), those winds could be a little stronger than the rule of thumb would indicate. Late winter, when the water is colder, the winds would be a little lower.

Winds onshore will generally be about 1/2 the speed of winds offshore, by the way. That's another way to estimate winds offshore. Look behind the front at the prevailing wind. If the wind is 15-20 kts behind a front, then the wind will be 30-40 kts offshore behind the front. For Thanksgiving, I'm expecting northerly winds of 15-20 kts with gusts around 30 kts (17-22 mph gusting 30-34 mph).

Looks like a quite miserable Thanksgiving Day here. Temps falling into the 40s by sunrise and staying in the mid to upper 40s during the day. Cloudy and windy with occasional light rain. So much for biking downtown for the opening of the ice rink at Discovery Green...

[srainhoutx]

The 12Z GFS still suggests a Wednesday evening Arctic frontal passage. Of note is a robust shortwave lagging to our W that will keep precip chances going into Thanksgiving Day with a very chilly airmass in place and strong gusty winds...

[wxman57]

No big changes on the 12Z GFS. Frontal passage Wednesday evening, dropping to the upper 50s by midnight and upper 40s before sunrise. Holding in the upper 40s through the day Thursday, then falling to the upper 30s Thursday night. Just a few sprinkles Thursday morning. Disturbance passes Thursday night, so some light rain then.

[srainhoutx]

Long Range 12Z GFS La La Land suggests another strong shot of cold air the first week of December, but that will be for another Topic...

[srainhoutx]

12Z GEM ( Canadian)...hour 132...