FXUS63 KDMX 251132 AAA
AFDDMX
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE DES MOINES IA
532 AM CST WED NOV 25 2009
...UDPATED AVIATION DISCUSSION...
.SHORT TERM /TODAY/...
VIGOROUS UPPER LEVEL TROUGH OVER ERN ND EARLY THIS MORNING WILL
CONTINUE TO DIVE SOUTHWARD INTO THE MEAN TROUGH ESTABLISHED BY THE
UPPER LEVEL SYSTEM THAT AFFECTED US YESTERDAY. THERE IS QUITE AN
INJECTION OF POTENTIAL VORTICITY FROM THE STRATOSPHERE...AND VERY
COLD 500MB TEMPS /-32 TO -34C/ ASSOCIATED WITH THE TROUGH AS IT
CLOSES OFF AND SINKS OVER IOWA DURING MUCH OF THE DAYTIME HOURS. WE
SHOULD START REALIZING DEEPER FORCING UNDER THE DIFFLUENCE ALOFT
STARTING BY 12Z ACROSS THE NW AND SHIFTING SOUTHEAST QUICKLY THROUGH
THE MORNING HOURS. WHILE THE DEEPER SYNOPTIC SCALE FORCING SHIFTS
EAST BY AFTN...WEAKER LIFT AND INSTABILITY UNDER THE COLD CORE
MAINTAINS PRECIP CHANCES THROUGH THE AFTN. PRECIP TYPE TODAY IS
PARTICULARLY DIFFICULT BECAUSE TEMPS AHEAD OF THE SYSTEM UPSTREAM
ARE STILL RATHER MILD. HOWEVER...A PUSH OF COOLER AIR AT THE SFC
SHOULD OFFSET ANY NORMAL DIURNAL HEATING IN THE AFTERNOON...AND MAY
ACTUALLY ALLOW TEMPS TO FALL SOME. THE PROBLEM IS THAT SOUNDINGS
MAINTAIN AT LEAST A 800 TO 1500 FT ABOVE-FREEZING LAYER AT THE
SURFACE...WITH DECENT DEPTH TO HIGH RH AND LAPSE RATES ABOVE THIS
WARM LAYER. SO WILL THE LIFT AND FALLING SNOW BE ABLE TO COOL THE
BOUNDARY LAYER ENOUGH SEE A TRANSITION TO SNOW ACROSS PARTS OF THE
AREA? NOT SURE. THE OTHER PROBLEM...AT LEAST FOR THIS MORNING...IS
THAT RUC SOUNDINGS ARE SHOWING A CONSIDERABLE DEPTH OF DRY AIR IN
THE LOWER LEVELS THAT NO OTHER MODEL IS SHOWING. JUDGING FROM
UPSTREAM OBS...AND LACK OF SIGNIFICANT COVERAGE OF ANY PRECIP...I
HAVE TO GIVE SOME CREDENCE TO WHAT THE RUC IS SHOWING. WITH ALL THAT
SAID...WILL KEEP IT JUST RAIN SHOWERS ACROSS THE EAST TODAY...WITH
RAIN/SNOW SHOWER MIXTURE MENTIONED ELSEWHERE...SHIFTING EAST DURING
THE DAY. THE ONLY WAY I SEE BRIEF ACCUMULATIONS HAPPENING IS IF
SHOWER ACTIVITY IS HEAVY ENOUGH TO BRING THE SURFACE LAYER DOWN TO
NEAR FREEZING. IF ANY PLACE HAS THIS CHANCE...IT IS LIKELY NEAR THE
MINNESOTA BORDER.
.LONG TERM /TONIGHT THROUGH SUNDAY/...
THE UPPER LOW WILL PASS TO THE EAST TONIGHT WITH ONLY LINGERING SNOW
SHOWERS MAINLY OVER THE EAST TO CONTEND WITH. LITTLE OR NO
ACCUMULATION IS EXPECTED TONIGHT. LOCATIONS EAST OF INTERSTATE 35
AND NORTH OF HIGHWAY 30 MAY SEE UP TO A HALF INCH IF WE ARE ABLE TO
CHANGE OVER TO SNOW EARLY ENOUGH. GIVEN THE FACT THAT THE BEST LIFT
AND FORCING WILL HAVE PAST THIS IS WORST CASE SCENARIO. OTHERWISE
THANKSGIVING WILL BE COLD AS UPPER RIDGE BUILDS OUT WEST WHICH PUTS
US IN NORTHERLY FLOW. THIS RIDGE WILL THEN SHIFT FOR EARLY IN THE
WEEKEND GIVING US DRY AND MILD CONDITIONS FOR LATE NOVEMBER. HIGHS
WILL ONCE AGAIN REACH INTO THE 50S. THE SYSTEM FOR LATE IN THE
WEEKEND AND THE FIRST PART OF NEXT WEEK IS STILL IN NEED OF
RESOLUTION BUT MODEL CONCENSUS IS AT LEAST NEARING. THE GEM AND 12Z
EURO WERE SIMILAR BUT NOW THE 00Z EURO BREAKS AWAY FROM EITHER
SOLUTION. THE EURO PUSHES A LONG WAVE TROUGH THROUGH THE STATE
SUNDAY NIGHT INTO EARLY MONDAY BUT THE NORTHERN PORTION OF THE
TROUGH IS OUT OF PHASE WITH THE SOUTHERN PORTION AND CENTRAL IOWA IS
IN A SPLIT AS FAR AS PRECIP GOES. BY THE TIME THE SYSTEM GETS ITS
ACT TOGETHER THE PRECIP HAS PUSHED EAST OF IOWA. THE GFS DEVELOPS A
CLOSED LOW OVER NW IA WITH A VORT MAX OVER NORTHERN IOWA AND
GENERATES SOME LIGHT PCPN SUNDAY NIGHT OVER THAT AREA. THE GEM
DEVELOPS SUCH A SPLIT FLOW AND IS SO OUT OF PHASE WITH THE NORTHERN
AND SOUTHERN WAVES THAT IT TOTALLY LEAVES IOWA DRY. IN ANY
CASE...OR RATHER IF ANY OF THESE SOLUTIONS PAN OUT...THE WORST CASE
SCENARIO IS FOR LIGHT PRECIP EITHER ACROSS THE NORTH OR SOUTHEAST.
CONSENSUS IS NOT FOR A LOW TO DEVELOP SO I PUT MORE WEIGHT ON THE
IDEA OF POTENTIALLY SOME PRECIP WORKING ITS WAY ACROSS THE SOUTHEAST
SUNDAY NIGHT AS THE UPPER TROUGH PUSHES EAST...BUT EVEN THAT MAY BE
GENEROUS. TEMPS DO COOL BACK INTO THE 40S FOR SUNDAY...STILL NICE
FOR LATE NOVEMBER.
&&
.AVIATION...
25/12Z...IFR CEILINGS ARE PULLING AWAY FROM KALO AND KOTM AS LOW
CLOUDS SWIRLING AROUND LOW PRESSURE OVER MKE MOVES EAST.
HOWEVER...NEXT SYSTEM IS RAPIDLY APPROACHING FROM THE NW AND EXPECT
TO SEE THE ATMOSPHERE QUICKLY REACH SATURATION THROUGH A GREATER
DEPTH FROM NW TO SE FROM MID MORNING INTO EARLY AFTN. COLD CORE
NATURE OF THE UPPER LEVEL LOW TO MOVE OVER IOWA TODAY WILL YIELD
SHOWERY PRECIP. EXPECT PRECIP TO BE MAINLY RAIN EARLY ON...BUT WILL
MIX WITH OR CHANGEOVER TO SNOW IN HEAVIER SHOWERS. A GENERAL
TRANSITION TO ALL SNOW WILL OCCUR LATE THIS AFTERNOON THROUGH EARLY
EVENING NW TO SE. EXPECT FOR CIGS/VSBY TO SETTLE IN THE MVFR
CATEGORY FOR MUCH OF THE DAY INTO TONIGHT...WITH IMPROVING
CONDITIONS WORKING IN FROM THE WEST AFT 06Z.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM...MOYER
LONG TERM...FAB
AVIATION...MOYER
