Issued by NWS Des Moines, IA
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000 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
