FXUS63 KDMX 262057
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
257 PM CST SAT DEC 26 2009
.SHORT TERM.../TONIGHT/
MAIN QUESTION THIS AFTERNOON CENTERS AROUND WHAT MAY BE ONE FINAL
SHOT OF SNOWFALL. IMPRESSIVE SHORT WAVE AND PV ANOMALY IS CURRENTLY
ROTATING IN THE HEART OF THE DEEP CYCLONE ALONG THE IA/WI BORDER AND
IS EXPECTED TO COMPLETE ITS CYCLONIC PATH DOWN THE MS RIVER BEFORE
FINALLY EJECTING AND LIFTING SYSTEM OUT SUNDAY. FORCING WITH THIS
FEATURE IS RESPONSIBLE FOR CURRENT SW-NE AREA OF SNOW IN MN AND IS
MAINLY KINEMATIC AT THIS POINT ALTHOUGH THERE IS A TRANSIENT
FRONTOGENETICAL COMPONENT AS WELL. COBB OUTPUT SUGGESTS SNOW RATIOS
WELL INTO THE TEENS TONIGHT...ALTHOUGH THIS MAINLY APPEARS TO BE DUE
TO DENDRITIC GROWTH ZONE /DGZ/ DEPTH RATHER THAN UVM. MUCH OF THE
OMEGA/FORCING SEEMS TO BE IN THE H7/H5 LAYER ABOVE THE FAVORED DGZ
WHICH DIPS AS LOW AS H7/H8. 280-290K ISENT LAYER SEEMS TO CAPTURE
DGZ BETTER AND SUGGESTS AREA OF SNOW ENTERING N CENTRAL IA SHORTLY
AFTER 00Z..INTO CENTRAL AND EASTERN SECTIONS AROUND 06Z...AND
EXITING FORECAST AREA TO THE SE BY 12Z. WITH QPF UP TO
0.05...ACCUMS SHOULD VARY FROM A DUSTING UP TO TWO INCHES WITH INCH
AMOUNTS MOST COMMON.
TEMPS WILL BE SOMEWHAT PROBLEMATIC WITH 20Z MSAS OBJECTIVE SFC ANLYS
SHOWING LOWEST THETA-E VALUES RIGHT NEAR UPR MS VALLEY TROUGH
CENTER...AND HIGHER VALUES TO THE NORTH AND WEST. THUS TEMPS MAY
BOTH FALL AND RISE AT TIMES TONIGHT WITH MINS NOT MUCH BELOW CURRENT
READINGS.
.LONG TERM.../SUNDAY THROUGH SATURDAY/
WITH LARGE STACKED LOW PRESSURE SYSTEM FINALLY DEPARTING EASTWARD
TOMORROW EXPECT GRADUAL CLEARING OF SKIES INTO SUNDAY NIGHT. A FEW
RESIDUAL LOBES OF ENERGY ROTATING AROUND THE GYRE MAY PROVIDE
ADEQUATE LIFT TO SQUEEZE OUT A FEW MORE SNOW SHOWERS PARTICULARLY
DURING THE DAY ON SUNDAY...HOWEVER FORECAST PROFILES SHOW VERY
LIMITED MOISTURE ABOVE THE LOW LEVELS AND WILL GO WITH JUST FLURRIES
IN THE FORECAST. A SURGE OF SOMEWHAT DEEPER MOISTURE TRAVERSING THE
AREA TONIGHT MAY BE HANGING AROUND IN OUR FAR SOUTHEASTERN CORNER
RIGHT AT THE 12Z BEGINNING TIME OF SUNDAY...HOWEVER IT MAY ALSO HAVE
CLEARED THE CWA BY THAT TIME AND IF NOT...IT WILL DO SO VERY SOON
AFTER 12Z. WILL THUS JUST LEAVE THE SLIGHT POPS/FLURRIES WORDING
DOWN THERE AS WELL WHICH CAN BE TOUCHED UP ON THE OVERNIGHT SHIFT IF
NECESSARY.
FROM MONDAY INTO TUESDAY RIDGING WILL DOMINATE THE AREA WITH A
SURFACE HIGH MOVING THROUGH ON MONDAY NIGHT. THIS WILL LEAD TO VERY
FAVORABLE CONDITIONS FOR RADIATIONAL COOLING ESPECIALLY WITH A FRESH
SNOW PACK IN PLACE...AND HAVE GONE WITH MONDAY NIGHT MIN TEMPS WELL
BELOW GUIDANCE ACROSS THE BOARD...PARTICULARLY ACROSS OUR WESTERN
ZONES. THE ONLY FLY IN THE OINTMENT IS THAT SOME FORECAST SOUNDINGS
ARE INDICATING POSSIBLE CIRRUS COVER...HOWEVER EVEN THESE WOULD SEEM
TO SHOW THIN CIRRUS WHILE OTHERS SHOW NONE AT ALL. THIS WILL NEED TO
BE MONITORED ON SUBSEQUENT SHIFTS AS EVEN THESE VERY LOW NUMBERS MAY
HAVE TO COME DOWN FURTHER IF THE OUTLOOK REMAINS THIS FAVORABLE FOR
RADIATIVE COOLING MONDAY NIGHT.
OUR ONLY REAL SHOT AT MEASURABLE PRECIP IN THE SEVEN DAY FORECAST
WILL COME TUESDAY NIGHT AND WEDNESDAY AT THE HANDS OF A SURFACE
TROUGH MOVING THROUGH AHEAD OF A RELATIVELY WEAK MID/UPPER WAVE.
WITH THE HIGH/RIDGE MOVING AWAY JUST AHEAD OF THIS SYSTEM MOISTURE
WILL BE LIMITED...AND IT IS DIFFICULT AT THIS TIME TO DETERMINE HOW
WIDESPREAD MEASURABLE PRECIP WILL BE AS OPPOSED TO JUST WIDESPREAD
FLURRIES. HAVE INCREASED POPS TO LOW CHANCE IN SOME AREAS...AND
THESE MAY NEED TO COME UP A BIT FURTHER IN THE COMING DAYS.
HOWEVER...EVEN IF THEY DO THIS STILL APPEARS TO BE A LOW QPF
EVENT...AND ANY MEASURABLE SNOWFALL THAT DOES OCCUR WILL LIKELY
RESULT IN ONLY LIGHT ACCUMULATION.
LONG RANGE MODELS ARE INTERMITTENTLY ATTEMPTING TO BRING ANOTHER
SHORTWAVE OVERHEAD AROUND FRIDAY...HOWEVER AT THIS TIME THERE IS TOO
MUCH INCONSISTENCY TO INSERT ANY POPS FOR THESE EXTENDED PERIODS.
&&
.AVIATION...
26/18Z...LARGE LOW PRESSURE SYSTEM CONTINUES TO ROTATE AROUND IA
WITH LOBES OF CLOUDS AND CLEARING RESULTING IN LOW CONFIDENCE
FORECAST. CURRENT NOTCH OF CLEARING HAS PUSHED INTO CENTRAL IA FROM
SERN MN WITH VARIOUS CIG LEVELS EITHER SIDE OVER WRN AND SERN IA.
AS SYSTEM PULLS AWAY TONIGHT...PLAYED A MIDDLE OF THE ROAD TREND
WITH MVFR CIGS/VSBYS BECOMING MORE PREVALENT WITH PERIODS OF SNOW
OVERNIGHT. COULD DIP TO IFR BUT LIKELIHOOD NOT GREAT ENOUGH TO
INCLUDE. SNOW MAY DIMINISH OR END ALTOGETHER TOWARD DAYBREAK...BUT
WITH AT LEAST MVFR CIGS CONTINUING.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM/AVIATION...SMALL
LONG TERM...LEE
