FXUS63 KARX 110907 CCA
AFDARX

AREA FORECAST DISCUSSION...CORRECTION TO EXTENDED OUTLOOK
NATIONAL WEATHER SERVICE LA CROSSE WI
250 AM CST FRI DEC 11 2009

.SHORT TERM...THROUGH SUNDAY NIGHT

PRIMARY FORECAST CONCERNS WERE WITH PRECIPITATION CHANCE AND TYPE
THIS WEEKEND...ALONG WITH TEMPERATURE TRENDS.

EARLY MORNING SURFACE ANALYSIS HAD HIGH PRESSURE CENTERED OVER THE
MID MISSISSIPPI VALLEY. FEW CLOUDS OBSERVED IN THE 11-3.9 MICRON
SATELLITE IMAGERY ACROSS THE UPPER MISSISSIPPI VALLEY...EXCEPT A
NARROW BAND NEAR THE MN/IOWA BORDER IN VICINITY OF KAUM. MUCH
FURTHER SOUTH...LOW CLOUDS NOTED OVER MUCH OF TX AND THESE
EXPECTED TO AFFECT THE FORECAST AREA DURING THE WEEKEND.

11.00Z DETERMINISTIC NCEP AND EUROPEAN MODELS WERE IN FAIRLY GOOD
AGREEMENT WITH OVERALL MASS FIELDS THROUGH 14.00Z...SUNDAY EVENING.
AS A RESULT...A CHANCE OF LIGHT PRECIPITATION CERTAINLY SEEMS
VIABLE SATURDAY NIGHT INTO SUNDAY. FORECAST CONFIDENCE WAS GOOD IN
THIS SOLUTION...WITH BIGGEST QUESTION CONCERNING PRECIPITATION TYPE.

TONIGHT...SOUTHERLY FLOW PERSISTS IN WARM AIR ADVECTION REGIME. IN
FACT...850MB TEMPERATURES RISE TO MINUS 3C-MINUS 7C BY 12.12Z...
SATURDAY MORNING. BASED ON THIS TREND...OPTED TO UTILIZE A NON-
DIURNAL TEMPERATURE TREND...MAINLY OVER LOCATIONS WEST OF THE
MISSISSIPPI RIVER WHERE RELATIVELY WARMEST AIR RESIDES.

SATURDAY NIGHT...LOW CLOUDS SHOULD MOVE IN FROM THE SOUTH AS LOW
LEVEL RELATIVE HUMIDITY INCREASES CONSIDERABLY. TIME/HEIGHT CROSS
SECTIONS INDICATED DRY LAYER RESIDES ABOVE CLOUDS WITH TEMPERATURES
WARMER THAN MINUS 10C. IN ADDITION...THERE SHOULD BE WEAK UPWARD
VERTICAL MOTIONS ASSOCIATED WITH WARM AIR ADVECTION AND LOW LEVEL
Q-G CONVERGENCE. THEREFORE...OPTED TO INTRODUCE POTENTIAL FOR
FREEZING DRIZZLE IN THIS TIME PERIOD. ALSO...UTILIZED A NON-DIURNAL
TEMPERATURE TREND...AS CLOUDS AND SOUTHERLY FLOW SHOULD PRECLUDE
TOO MUCH OF A DROP DURING THE NIGHT.

.LONG TERM...MONDAY THROUGH THURSDAY

PRIMARY CONCERN WAS WITH PRECIPITATION CHANCES MONDAY.

11.00Z DETERMINISTIC MODELS CONTINUED TO INDICATE AN UPPER LEVEL
AND SURFACE WAVE TO AFFECT THE UPPER MISSISSIPPI VALLEY ON MONDAY.
MODEL CONSENSUS HAS INCREASED AND IT CERTAINLY APPEARS A CHANCE OF
SNOW WARRANTED AS SUPPORTED BY CURRENT DATA BASE. IN FACT...WE CHOSE
TO INCREASE SNOW CHANCES OVER ABOUT THE NORTH HALF OF THE FORECAST
AREA BASED ON CONSISTENT FORCING SIGNAL ASSOCIATED WITH THIS SYSTEM.

REMAINDER OF THE EXTENDED OUTLOOK APPEARS TO BE RATHER TRANQUIL. 
TEMPERATURES SHOULD REMAIN BELOW NORMAL THROUGH MOST OF THE WEEK WITH
NORTHWESTERLY FLOW ALOFT. COLDEST TEMPERATURES LIKELY TUESDAY 
MORNING INTO WEDNESDAY AS THE CORE OF A HIGH PRESSURE SYSTEM MOVES 
ACROSS THE MIDWEST. NO CHANGES WERE MADE TO THE REMAINDER OF THE 
EXTENDED OUTLOOK.

&&

.AVIATION...TODAY AND TONIGHT

SFC HIGH OVER THE OH RIVER VALLEY WILL KEEP VFR CONDITIONS AT 
KRST/KLSE THROUGH SATURDAY MORNING. THE WINDS WILL BECOME MORE 
SOUTHERLY TONIGHT AS THE HIGH CONTINUES TO SLIP OFF TO THE EAST.

LATER ON SATURDAY...LOW LEVEL WARM AIR ADVECTION AND INCREASING 
SATURATION COULD RESULT IN A LOW MVFR/IFR STRATUS DEVELOPING AND 
MOVING INTO THE LOCAL AREA. THIS WOULD BE ASSOCIATED WITH AN UPPER 
LEVEL SHORTWAVE TROUGH SLATED TO SPIN OUT OF THE SOUTHERN PLAINS AND 
INTO THE EASTERN GREAT LAKES BETWEEN 12-18Z SUN. TIME/HEIGHT 
X-SECTIONS AND BUFKIT SOUNDINGS PLACE THE BULK OF THE SATURATION 
BELOW 700 MB...WITH TEMPERATURES AT THE TOP OF THIS SATURATION ONLY 
AROUND -4 C. WITH GOOD LOW LEVEL WARMING AND ISENTROPIC UPGLIDE 
CURRENTLY PROGGED THROUGH THIS LAYER...SOME LIGHT PCPN COULD 
DEVELOP...AND TEMPERATURE PROFILES WOULD SUPPORT LIGHT SNOW...OR 
MORE LIKELY...FREEZING DRIZZLE. THE BETTER PCPN CHANCES CURRENTLY 
LOOK TO BE AFTER 00Z SUN...CONTINUING THROUGH ABOUT 18Z SUN. THIS 
WILL HAVE TO BE MONITORED CLOSELY AS FREEZING DRIZZLE COULD HAVE 
SIGNIFICANT IMPACTS ON AIR TRAVEL AND AIRPORT OPERATIONS. 

ONE LAST AVIATION CONCERN IS WITH THE AMPLE WARMING ACROSS A 
FRESH...DEEP SNOW COVER...NOT ONLY WOULD IFR STRATUS BE 
POSSIBLE...SO WOULD MVFR/IFR VSBY REDUCTIONS TO VISIBILITY. 
AGAIN...SOMETHING THAT WILL HAVE TO BE MONITORED.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...WIND CHILL ADVISORY UNTIL 9 AM CST THIS MORNING FOR 
WIZ017-WIZ029.

MN...NONE.
IA...NONE.
&&

$$

SHORT/LONG TERM...THOMPSON
AVIATION..........RIECK