FXUS63 KDMX 052345
AFDDMX
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE DES MOINES IA
545 PM CST SAT DEC 5 2009
...UPDATED AVIATION DISCUSSION...
.SHORT TERM /TONIGHT/...
WEAK SURFACE FRONT ACROSS FAR NW IA AT THIS TIME WILL SLOWLY MOVE
INTO CENTRAL IA...AND ESSENTIALLY BISECT THE STATE NORTHEAST TO
SOUTHWEST BY MIDNIGHT. WEAK UPPER LEVEL FORCING...BUT ONLY MODEST
AMOUNTS OF MOISTURE AT THOSE LEVELS ACROSS NORTHERN IOWA EARLY
TONIGHT WILL PROBABLY JUST RESULT IN AN INCREASE IN CLOUDS. I TOYED
WITH ADDING A CHC OF FLURRIES ACROSS THE NORTH...BUT LATEST 18Z NAM
SOUNDINGS NOW SHOWING A DRIER ATMOSPHERE SIMILAR TO 12Z GFS
INDICATIONS. OTHERWISE...FRONT WILL CONTINUE TO NEAR KOTM BY 12Z.
COMBINATION OF GETTING A LITTLE WARMER THAN ANTICIPATED TODAY AND
SOUTHWESTERLY FLOW AHEAD OF THE FRONT SHOULD KEEP MIN TEMPS UP A BIT
TONIGHT...SO WENT A LITTLE ABOVE GUIDANCE ACROSS THE SOUTHEAST
HALF...BUT CLOSE TO GUIDANCE FARTHER NORTH.
.LONG TERM /SUNDAY THROUGH SATURDAY/...
FIRST SYSTEM TO ARRIVE BY LATER ON SUNDAY. PRIOR TO THE
ARRIVAL...WILL BE LOOKING AT SOME WARM AIR ADVECTION FOR THE MORNING
AND AFTERNOON HOURS. GFS/NAM HAVE BOTH COME INTO AGREEMENT WITH THE
QPF AND THE EURO BEING A BIT LIGHTER. THE GFS/NAM CONTINUE TO PLACE
ABOUT .30 TO .35 VS THE EURO ROUGHLY .25. ZONE OF WARM AIR
ADVECTION SNOW DEVELOPS OVER WEST CENTRAL IA BY LATE SUN AFTERNOON
IN H700 FRONTAL FEATURE. CURRENT SNOW WATER RATIO IS RUNNING ABOUT
14 TO 16:1 FOR THE DURATION OF THE EVENT. TIME HEIGHT CROSS SECTION
SHOWS AN EXTENDED PERIOD OF LIFT IN THE DENDRITIC LAYER WITH 90
PERCENT RH FOR FROM SUNDAY EVENING THROUGH EARLY MONDAY MORNING.
THIS PLACES A BROAD BAND OF 3 TO 5 INCHES OF SNOW FROM WEST CENTRAL
TO NORTHEAST SECTIONS OF THE FORECAST AREA. SNOW WILL FALL OVER AN
EXTENDED PERIOD OF TIME AND REMAIN BELOW WARNING CRITERIA. IF THE
CURRENT TRENDS HOLD THROUGH TNT...WX ADV WILL COVER THE EVENT. WINDS
TO REMAIN RELATIVELY TAME THROUGH THE EVENT BUT PICK UP TOWARD
MONDAY MORNING. FOR NOW WILL LEAVE OUT MENTION OF BLOWING SNOW...
THOUGH SOME MAY STILL OCCUR FOR A TIME FROM 06-15Z MONDAY MORNING.
NEXT SYSTEM STILL LOOKING MUCH MORE IMPRESSIVE. RUN TO RUN
CONSISTENCY BETWEEN MODELS HAS BEEN VERY GOOD...WITH EACH RUN
SHOWING SOMEWHAT DIFFERING SOLNS. PRESENTLY THE MODELS ARE SHOWING A
DEEPER SYSTEM THAN 12 HOURS AGO AND STILL SHOWING A RANGE OF QPF
VALUES OF .80 TO JUST UNDER 1.10 WITH THIS RANGE FROM NEAR I80 SOUTH
TO ABOUT I70. CURRENT COBB OUTPUT FOR THE GFS MODEL IS SHOWING
ROUGHLY 6 TO 10 INCHES OVER THE REGION WITH THE AXIS OF HEAVIER
SNOWFALL ACROSS THE SOUTH...ALONG AND SOUTH OF I80 TO THE BORDER. AS
WITH ALL GUIDANCE...THIS MIGHT CHANGE OVER THE NEXT SEVERAL
DAYS...BUT THERE HAS BEEN BETTER CONSISTENCY OVER THE PAST FEW RUNS.
THE SYSTEM IS EXPECTED TO CROSS COLORADO AND DROP SOUTH TO THE KS/OK
BORDER BY 00Z WED. STRONG THETAE ADVECTION WILL COUPLE WITH H700
FORCING WHICH SHOULD RESULT IN AN EXTENDED PERIOD OF MODERATE TO
HEAVY SNOWFALL FOR THE SOUTHERN HALF OF THE FORECAST AREA GIVEN THE
CURRENT MODEL CONSISTENCY. THIS WILL BEGIN LATE MONDAY NIGHT OVER
THE SOUTHWEST/WEST CENTRAL AS WARM AIR ADVECTION INCREASES ACROSS
THE REGION IN A ZONE OF 30 TO 40KT SOUTHERLY FLOW. THE EURO MODEL
IS ONCE AGAIN DEEPENING THE LOW MORE WITH TIME...983MB OVER LAKE
HURON VS THE EURO AT 970 MB. DURING THE HEIGHT OF THE EVENT THE
MODELS ARE ALSO SUGGESTING SOME WEAK NEGATIVE EPV BETWEEN H600-H500
WHICH MAY LEAD TO SOME CONVECTIVE ELEMENTS OVER THE SOUTHEAST. OTHER
FACTORS POINTING TOWARD A ROBUST WINTER WEATHER EVENT...TROWAL WILL
EXTEND BACK OVER THE AREA THROUGH EARLY WEDNESDAY AS THE SYSTEM
WRAPS UP TO THE EAST OF IOWA. AS IT STANDS TODAY...THE PERIOD OF
GREATEST SNOWFALL PRODUCTION SHOULD BE DURING THE DAY ON TUESDAY
WITH THE SNOW GRADUALLY DIMINISHING DURING THE MID TO LATE EVENING
HOURS OVER MOST OF THE REGION. WINDS TO PICK UP ON THE BACK SIDE OF
THE LOW WITH GFS SHOWING 925MB WINDS OF 35KTS BEHIND THE SYSTEM AND
EURO 45KTS FOR THE SAME PERIOD. SFC WINDS SHOULD EASILY REACH
15-25KTS WITH GUSTS TO 30-35KTS FROM LATE TUESDAY NIGHT INTO MIDDAY
WED. THUS...SNOW THAT DOES FALL WILL CERTAINLY CREATE SIGNIFICANT
BLOWING.
&&
.AVIATION...
06/00Z...COMPLEX SCENARIO SETTING UP WITH WARM ADVECTION VFR CLOUD
COVER OVER THE SOUTH AND A COLD FRONT THAT IN FAR NW IA ATTM THAT
WILL GRADUALLY PUSH ACROSS THE TAF LOCATIONS THROUGH 06/18Z. MVFR
CEILINGS WILL OCCUR AFTER FROPA. THEN A SURFACE LOW WILL WILL MOVE
NE OUT OF THE SRN PLAINS IN RESPONSE TO AN UPPER LEVEL FEATURE
LIFTING OUT. THIS WILL CAUSE SNOW TO BREAK OUT...AS EARLY AS 18Z
OVER WESTERN TAFS BUT MORE LIKELY AFT 21Z. THE SYSTEM IS FAIRLY
FAST MOVING BUT TOWARDS THE END OF THIS TAF FORECAST CYCLE AND INTO
SUNDAY EVENING...EXPECT AT LEAST LCL IFR CONDITIONS. SFC FLOW WILL
BECOME NERLY AROUND 10KTS BY 18Z AND REMAIN THAT WAY THROUGH THE
REMAINDER OF THE FORECAST PERIOD.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM/AVIATION...FAB
LONG TERM...REV
