FXUS63 KARX 120935
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
335 AM CST SAT DEC 12 2009
.SHORT TERM...THROUGH MONDAY NIGHT
FORECAST CONCERNS WERE PRIMARILY FOCUSED ON PRECIPITATION CHANCE
AND TYPE TONIGHT...THEN AGAIN SUNDAY NIGHT INTO MONDAY.
EARLY MORNING SURFACE ANALYSIS HAD HIGH PRESSURE OVER THE OHIO
VALLEY...WHILE AN ARCTIC COLD FRONT EXTENDED FROM ONTARIO ACROSS THE
NORTHWEST TIP OF MN INTO NORTH DAKOTA. SOUTHERLY FLOW ESTABLISHED
BETWEEN THESE TWO SYSTEMS...WHICH WAS BRINGING LOW CLOUDS NORTHWARD
FROM MISSOURI AND KANSAS. IN FACT...EXPANSIVE AREA OF LOW CLOUDS
COVERED ARKANSAS...ALONG WITH MUCH OF OKLAHOMA AND TEXAS.
THE 12.00Z NCEP AND EUROPEAN MODEL SUITE HAD GOOD CLUSTERING OF
UPPER LEVEL AND SURFACE FEATURES THROUGH MONDAY NIGHT. LATEST 12.03Z
SREF DATA CORROBORATED THESE DATA...WHICH LENDS OVERALL HIGH
CONFIDENCE IN FORECAST DETAILS THROUGH THE WEEKEND AND INTO MONDAY.
THUS...WE ANTICIPATE A LIGHT PRECIPITATION EVENT TONIGHT...FOLLOWED
BY ANOTHER PRECIPITATION EVENT SUNDAY NIGHT INTO MONDAY.
TONIGHT...LOW LEVEL RELATIVE HUMIDITY INCREASES AND WILL BE
MANIFESTED BY LOW CLOUDS MOVING NORTHWARD. ISENTROPIC LIFT IN THE
280-290K LAYER...ALONG WITH LOW LEVEL Q-G CONVERGENCE ASSOCIATED
WITH 850MB WARM AIR ADVECTION...SUPPORTS SUFFICIENT UPWARD VERTICAL
MOTION TO PRODUCE LIGHT PRECIPITATION. UTILIZING TOP-DOWN APPROACH
IN BUFKIT SOUNDINGS WAS SUGGESTIVE OF PRECIPITATION TYPE AS MAINLY
FREEZING DRIZZLE...WHICH HAS BEEN A CONSISTENT SIGNAL.
POTENTIAL FOR WINTER WEATHER ADVISORIES TONIGHT DUE TO FREEZING
DRIZZLE WILL BE HIGHLY DEPENDENT UPON TEMPORAL AND SPATIAL EXTENT
OF PRECIPITATION. IN ADDITION...ANTECEDENT CONDITIONS OF EXISTING
SNOW COVER AND ALREADY TREATED ROAD SYSTEMS NEED TO BE CONSIDERED
TOO...BUT THIS SITUATION WILL NEED TO BE MONITORED AS THE EVENT
UNFOLDS THIS EVENING.
SUNDAY NIGHT INTO MONDAY...ANOTHER SHORT WAVE TROUGH AND ITS
ASSOCIATED SURFACE WAVE EXPECTED TO BRING A ROUND OF WINTRY
PRECIPITATION. FORCING SIGNALS ASSOCIATED WITH THIS SYSTEM
MANIFESTED BY DEEP Q-G CONVERGENCE...DECENT ISENTROPIC LIFT IN THE
280-290K LAYER...AS WELL AS LOW LEVEL WARM AIR ADVECTION. THESE
PARAMETERS CERTAINLY SUGGEST PRECIPITATION SHOULD AFFECT MUCH OF THE
FORECAST AREA. THEREFORE...INCREASED PROBABILITIES AND SUBSEQUENT
FORECASTS MAY WARRANT FURTHER INCREASE SHOULD FORCING SIGNALS
CONTINUE TO REMAIN CONSISTENT. OF NOTE...WE OPTED TO DELAY ONSET OF
HIGHEST PRECIPITATION CHANCES SUNDAY NIGHT TO MAINLY AFTER 14.06Z.
WITH REGARD TO PRECIPITATION TYPE SUNDAY NIGHT INTO MONDAY...BASED
ON BUFKIT SOUNDINGS AND PRECIPITATION TYPE ALGORITHMS...CONFIDENCE
WAS CERTAINLY HIGH FOR A SNOW EVENT. GFS COBB OUTPUT WAS MORE ROBUST
THAN NAM/WRF AND INDICATED UPWARDS OF 2 TO 3 INCHES OF SNOWFALL...
WHICH CERTAINLY SEEMS REASONABLE. DETERMINISTIC AND PROBABILISTIC
DATA SETS APPEAR TO BE SUPPORTIVE OF FREEZING DRIZZLE POTENTIAL
SOUTH OF THE FORECAST AREA...WHICH WAS REFLECTED IN LATEST DATA
BASE. REGARDLESS...WILL STILL NEED TO MONITOR POTENTIAL FOR WINTER
WEATHER ADVISORY DUE TO SNOWFALL.
TEMPERATURES FORECASTS WERE PROBLEMATIC SINCE WE ARE DEALING WITH
SNOW COVER...AS WELL AS AN ARCTIC FRONT MOVING THROUGH THE AREA.
MAINTAINED MAXIMUM TEMPERATURES BELOW MOS GUIDANCE TODAY BASED ON
DEEP SNOW COVER...ALONG WITH POTENTIAL FOR LOW CLOUDS TO MOVE IN.
WITH SOUTHERLY FLOW AHEAD OF APPROACHING ARCTIC COLD FRONT...
TEMPERATURES TONIGHT NOT EXPECTED TO CHANGE TOO MUCH AND THUS
UTILIZED A NON-DIURNAL TREND.
IN ADDITION...PASSAGE OF ARCTIC FRONT ON SUNDAY AND SHALLOWNESS OF
AIRMASS ENHANCES DIFFICULTY OF FORECASTING TEMPERATURES. THEREFORE...
CONTINUED TO UTILIZE A NON-DIURNAL TREND THROUGH MONDAY UNTIL ARCTIC
AIR MOVES MONDAY NIGHT IN FULL FORCE BEHIND DEPARTING SURFACE WAVE.
.LONG TERM...TUESDAY THROUGH FRIDAY
PRIMARY FORECAST CONCERNS WERE WITH PRECIPITATION CHANCES DURING THE
LATTER PART OF THE EXTENDED OUTLOOK.
TUESDAY INTO WEDNESDAY...MOSTLY DRY AND COLD PERIOD EXPECTED WITH
HIGH PRESSURE BUILDING IN. CONFIDENCE WAS HIGH IN THIS SOLUTION
BASED ON EXCELLENT CLUSTERING IN THE 12.00Z GFS AND ECMWF...AS WELL
AS CONSENSUS IN PREVIOUS MEDIUM RANGE DATA SETS. IN FACT...
CONFIDENCE WAS HIGH ENOUGH TO DROP MINIMUM TEMPERATURES TUESDAY
NIGHT BASED ON SURFACE HIGH SETTLING IN OVER DEEP SNOW COVER.
THURSDAY THROUGH FRIDAY...TIMING OF SHORT WAVE TROUGHS IN NORTHWEST
FLOW ALOFT STILL PROBLEMATIC IN THIS TIME FRAME. THUS...FORECAST
CONFIDENCE REMAINS LOW WITH REGARD TO THE DETAILS ASSOCIATED WITH ANY
PRECIPITATION CHANCES. THUS...LOW END CHANCE OF SNOW ON THURSDAY MAY
NEED TO BE SHIFTED IN TIME. UNTIL MODEL CONSENSUS IMPROVES...NO
CHANGES WERE MADE TO THIS PORTION OF THE EXTENDED OUTLOOK.
&&
.AVIATION...TODAY AND TONIGHT
AVIATION CONCERNS LIE IN THE LOW STRATUS...FOG...-FZDZ POTENTIAL
TONIGHT. MOISTURE WILL BE ON THE INCREASE BY THIS EVENING...PER
TIME/HEIGHT X-SECTIONS...AHEAD OF AN UPPER AIR DISTURBANCE. THE
SATURATION IS FAIRLY SHALLOW THOUGH...MOSTLY FROM 800 MB TO THE SFC.
DECENT THERMODYNAMICS ACROSS THE AREA THIS EVENING...WITH SOME
ISENTROPIC UPGLIDE ON THE 275-290 K SFCS...WITHIN THIS SATURATED
LAYER. SOME 925-850 MB WARM AIR ADVECTION...ALTHOUGH GENERALLY
BETTER TO THE EAST/SOUTHEAST OF KRST/KLSE. BUFKIT SOUNDINGS SHOW NO
ICE BEING INTRODUCED INTO THE EXPECTED LOW CLOUD LAYER...WHICH WOULD
MEAN -FZDZ RATHER THAN -SN. BEST POTENTIAL FOR THIS WOULD APPEAR TO
LIE BETWEEN 03-09Z TONIGHT. CONFIDENCE IS RELATIVELY HIGH THAT IF
PCPN DEVELOPS TONIGHT...IT WILL BE -FZDZ...BUT UNSURE WHETHER IT
WILL BE WIDESPREAD ENOUGH TO INCLUDE IN THE TAFS AT THIS TIME. IFR
CEILINGS LOOK LIKELY THOUGH TONIGHT...AND WITH WARM AIR RUNNING OVER
THE FRESH SNOW COVER...SOME BR ALSO APPEARS LIKELY. CURRENTLY...
PROBABLY ONLY 3-5SM...BUT TRENDS WILL HAVE TO BE MONITORED.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
SHORT/LONG TERM...THOMPSON
AVIATION..........RIECK
