FXUS63 KARX 152011
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
211 PM CST TUE DEC 15 2009
.SHORT TERM...THROUGH THURSDAY NIGHT
MAIN FORECAST CONCERN IS WITH CLOUD AND TEMPERATURE TRENDS.
SURFACE ANALYSIS THIS AFTERNOON SHOWS HIGH PRESSURE CENTERED OVER
SOUTHEAST NEBRASKA.
THE SURFACE RIDGE AXIS WILL SLIDE ACROSS THE AREA TONIGHT WITH THE
CENTER EXPECTED TO BE OVER CENTRAL ILLINOIS WITH THE RIDGE AXIS
NORTH INTO WISCONSIN. AS THIS HAPPENS...A SHORT WAVE TROUGH IN THE
NORTHWEST FLOW ALOFT WILL SLIDE INTO THE NORTHERN GREAT LAKES.
WHAT FORCING THERE IS WITH THIS WAVE WILL STAY NORTH OF THE
AREA...SO THE CONCERN IS WITH CLOUDS AND TEMPERATURES AS LOW LEVEL
WARM AIR ADVECTION DEVELOPS ON THE BACK SIDE OF THE RIDGE AXIS.
BUFKIT SOUNDINGS SUGGEST THAT UNDER THE CLEAR SKIES THIS EVENING
TEMPERATURES SHOULD DROP QUICKLY. THEN AS THE WARM AIR ADVECTION
KICKS IN ABOVE 900 MB...SURFACE TEMPERATURES ARE THEN EXPECTED TO
HOLD STEADY UNTIL LATE IN THE NIGHT. BY 12Z WEDNESDAY...BOTH THE
NAM AND GFS SHOW THE WARM AIR ADVECTION PRODUCING UP TO 2 UBAR/S
OF UP GLIDE ON THE 280K SURFACE WHICH WILL BE ENOUGH TO LIFT THE
AVAILABLE MOISTURE AND PRODUCE SOME MID LEVEL CLOUDS. ONCE THESE
CLOUDS FORM...THE SURFACE TEMPERATURES SHOULD THEN START TO SLOWLY
WARM...ESPECIALLY OVER THE WESTERN SECTIONS WHERE THE CLOUDS WILL
FORM FIRST.
SIMILAR SET UP FOR WEDNESDAY NIGHT AND THURSDAY WITH ANOTHER SHORT
WAVE TROUGH COMING INTO THE UPPER GREAT LAKES. SURFACE RIDGE AXIS
EXPECTED TO NOT BE THAT FAR TO THE EAST ALLOWING DECENT WARM AIR
ADVECTION TO ONCE AGAIN DEVELOP ACROSS THE AREA. THE BEST
ISENTROPIC LIFT LOOKS TO OCCUR ACROSS NORTHERN MINNESOTA...WITH
THE GFS DISSIPATING THIS AS IT MOVES INTO WISCONSIN. STILL SHOULD
BE ENOUGH TO PRODUCE SOME MORE CLOUDS BUT ANY PRECIPITATION LOOKS
HIGH DOUBTFUL. THE GFS DOES GRAZE THE NORTHERN SECTIONS WITH SOME
LOW CONDENSATION PRESSURE DEFICITS AROUND 00Z FRIDAY. SOME LOW
SNOW CHANCES WERE ALREADY IN FOR THURSDAY AND HAVE LEFT THEM FOR
NOW...BUT WITH LITTLE TO NO FORCING CONFIDENCE IN ANYTHING MORE
THAN FLURRIES IS NOT HIGH. WITH THE INCREASE IN CLOUDS WEDNESDAY
NIGHT...MAY NEED TO HAVE SOME STEADY OR RISING TEMPERATURES AGAIN
THAT NIGHT BUT THIS CAN BE ADDRESSED WITH LATER FORECASTS.
.LONG TERM...FRIDAY THROUGH TUESDAY
MEDIUM RANGE MODELS SHOW QUITE VARYING SOLUTIONS TODAY. IMPACT ON
THE OVERALL SENSIBLE WEATHER DOES NOT SEEM TO BE THAT GREAT AS AT
THIS POINT IT APPEARS THAT ANY PRECIPITATION THAT DOES FALL WILL BE
LIGHT. AS FOR THE MODELS...THE OPERATIONAL RUN OF THE 15.06Z GFS
APPEARS TO BE OUT OF STEP WITH THE 15.00Z GFS ENSEMBLE MEANS. THE
MEANS ARE IN BETTER AGREEMENT WITH THE 15.00Z ECMWF SO HAVE FAVORED
THESE SOLUTIONS. UPPER LEVEL TROUGHING IS EXPECTED TO DEVELOP OVER
THE UPPER MIDWEST WITH THE ECMWF SHOWING A PIECE OF ENERGY BREAKING
OFF FROM A LARGE CUT OFF LOW ALONG THE NORTHEAST COAST. THIS ENERGY
THEN SWINGS AROUND THE BACK SIDE OF THE LOW TO FORM THE TROUGH OVER
THE REGION. SURFACE HIGH PRESSURE INITIALLY OVER THE AREA LOOKS TO
GET PUSHED WEST ALLOWING A MORE CYCLONIC FLOW TO DEVELOP. THIS
CYCLONIC SURFACE FLOW WITH THE UPPER LEVEL TROUGHING SHOULD RESULT
IN SOME CLOUDS AND POSSIBLY SOME LIGHT SNOW OR FLURRIES. CONFIDENCE
ON THIS OVERALL OUTCOME IS LOW SO HAVE ATTEMPTED TO MAINTAIN
CONTINUITY WITH THE PREVIOUS FORECAST AS MUCH AS POSSIBLE WITH SOME
SMALL SNOW CHANCES FROM FRIDAY THROUGH SUNDAY.
&&
.AVIATION...
HIGH PRESSURE WILL DRIFT ACROSS THE AREA TONIGHT...WITH THE RIDGE
AXIS EAST OF THE AREA ON WED. WINDS WILL SWITCH FROM WESTERLY TO
SOUTHERLY TONIGHT WITH LOWER LEVEL WARM ADVECTION SPREADING ACROSS
THE AREA. MOST OF THIS WILL GO INTO WARMING. LOWER LEVELS OF THE
AIRMASS REMAIN ON THE DRY SIDE IN THE WEST/SOUTHWEST 925-700MB FLOW
AND BULK OF CLOUDS TONIGHT/WED EXPECTED TO REMAIN AT OR ABOVE 10K
FT. MAY SEE SOME 5-6SM HZ/BR AT TAF SITES IN THE 09Z-15Z TIME-FRAME
AS THE WINDS BECOME SOUTH...BUT NOT ENOUGH CONFIDENCE TO INCLUDE
THIS IN TAFS AT THIS POINT.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
SHORT/LONG TERM...04
AVIATION...RRS
