FXUS63 KARX 052003
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
200 PM CST SAT DEC 5 2009
.SHORT TERM...TONIGHT THROUGH TUESDAY
FORECAST CONCERNS IN THE SHORT TERM IS ON THE SNOW CHANCES/AMOUNTS
FOR SUNDAY NIGHT. ADVISORY MAY BE NEEDED.
THE MODELS HAVE BEEN VERY CONSISTENT WITH KICKING A BIT OF SHORTWAVE
ENERGY OUT OF AN UPPER LEVEL TROUGH OVER THE WEST COAST...SPINNING
THIS FEATURE ACROSS THE FORECAST AREA OVERNIGHT SUN/MONDAY MORNING.
05.12Z NAM/GFS SOLUTIONS CONTINUE THIS...WITH THE SHORTWAVE TROUGH
GETTING A KICK EAST BY A 120 KT UPPER LEVEL JET...WHICH WILL ENHANCE
THE LIFT WITH THIS FEATURE AS IT PASSES ACROSS THE FORECAST AREA. QG
CONVERGENCE IS GOOD THROUGH THE LAYERS WITH THIS
SHORTWAVE...STRONGEST BETWEEN 06-12Z MON FOR THE LOCAL AREA.
ISENTROPIC UPGLIDE ALSO HIGHLIGHTED ON THE 280-295 K SFCS DURING
THIS TIME. 2-D FRONTOGENETIC RESPONSE INDICATED MOSTLY IN THE
900-800 MB LAYER...EXTENDING FROM SOUTHERN MN INTO
CENTRAL/NORTHCENTRAL WI. A SOUTHWEST TO NORTHEAST RUNNING X-SECTION
SHOWS AN INCREASE AND DEEPENING IN THE SATURATION...FROM SOUTHWEST
TO NORTHEAST SUNDAY NIGHT...WITH THE STRONGEST OMEGA RESPONSE
BETWEEN 06-12Z. THE BEST OMEGA IS OUTSIDE OF THE DENDRITIC GROWTH
REGION //WHICH LOOKS ABOUT AVERAGE//...BUT COBB OUTPUT VIA THE GFS
AND NAM INDICATE 14/15:1 RATIOS. THIS LOOKS REASONABLE. AS FOR
AMOUNTS...NAM COBB DROPS 2 INCHES IN OUR NORTH...TO 4 IN THE SOUTH.
COBB GFS IS VERY SIMILAR...JUST A TAD LESS THAN THE NAM. SEE THE
POTENTIAL FOR A WINTER WEATHER ADVISORY FOR SOUTHERN PORTIONS OF THE
REGION FOR OVERNIGHT SUNDAY THROUGH MONDAY MORNING. WILL GIVE IT
ANOTHER RUN OR TWO...BUT CONSISTENCY WITHIN THE MODELS...AND BETWEEN
THEM...ADDS CONFIDENCE THAT AN ADVISORY COULD/WILL BE NEEDED IN THE
NEAR FUTURE.
THE NEXT SYSTEM...AND MUCH MORE SIGNIFICANT STORM...LOOKS TO IMPACT
THE REGION MOSTLY TUESDAY NIGHT/WED...ALTHOUGH SOME SNOW WILL LIKELY
MOVE IN TUE AFTERNOON. MORE DETAILS ON THIS...AND WHAT COULD BE A
SIGNIFICANT SNOWFALL...BLOWING/DRIFTING EVENT...IN THE LONG TERM
DISCUSSION BELOW.
STEPPING BACK A BIT TO TONIGHT...THE MODELS HINT AT THE POTENTIAL
FOR SOME LIGHT QPF ASSOCIATED WITH AN UPPER LEVEL SHORTWAVE THAT
SLIPS ACROSS PORTIONS OF THE REGION TONIGHT. SOME WEAK UPPER LEVEL
QG CONVERGENCE WITH THIS FEATURE...BUT THE NAM12 IS SHOWING SOME
WEST-EAST RUNNING 2-D FRONTOGENETIC FORCING THROUGH 900-600 MB. WEAK
ISENTROPIC UPGLIDE ON THE 280-295 K SFCS ALSO INDICATED. FORCING NOT
GREAT...BUT ENOUGH FOR SOME LIGHT SNOW POTENTIAL IF THERE IS
SATURATION TO WORK WITH. LOOKING AT A NORTH-SOUTH RUNNING
TIME/HEIGHT X-SECTION...BOTH THE NAM AND GFS POINT TO AN INCREASE IN
THE RH FROM THE SFC TO AT LEAST 700 MB...WITH SOME OMEGA RESPONSE
THOUGH THIS MOIST LAYER. COBB OUTPUT INDICATING MAYBE UP TO 1/2 INCH
SNOW ACCUMULATION OVERNIGHT FOR NORTHERN PARTS OF THE FORECAST AREA.
THE MODELS HAVE BEEN SLOW TO PRODUCE QPF WITH THIS FEATURE...BUT DID
START TO HINT AT IT YESTERDAY. 05.12Z SPCWRF ALSO SHOWING SOME QPF
TONIGHT. WILL ADD FLURRIES TO THE FORECAST FOR TONIGHT...ALONG WITH
EXPANDING SMALL CHANCES FOR MEASURABLE SNOW. THIS COULD END UP BEING
AN OCCASIONAL LIGHT SNOW NIGHT...WITH ONLY DUSTING TO UNDER 1/2 INCH
ACCUMULATIONS. EVENING SHIFT WILL HAVE TO MONITOR TRENDS AND MAKE
UPDATES IF NECESSARY.
.LONG TERM...TUESDAY NIGHT THROUGH SATURDAY
CONCERN IN THE LONG TERM LIES SQUARELY WITH THE POTENTIAL WINTER
STORM FOR TUESDAY NIGHT THROUGH WED. BETTER AGREEMENT BETWEEN THE
GFS AND EC THIS GO AROUND...AS OPPOSED TO SOME PREVIOUS RUNS. BOTH
BRING AN OPEN UPPER LEVEL TROUGH DUE WEST ACROSS THE
ROCKIES...DRIVING IT INTO THE OH RIVER VALLEY //BECOMING NEGATIVELY
TILTED AS IT DOES// BY 12Z WED. BOTH MODELS HAVE THE ASSOCIATED SFC
LOW NEAR CVG AT THIS TIME...AND WITHIN A MB OR TWO OF EACH OTHER.
THE GEM IS SIMILAR...BUT A TAD MORE NORTH. THIS TRACK WOULD LIKELY
PLACE THE HEAVIER SNOW BANDS WITH THIS STORM JUST SOUTHEAST OF THE
FORECAST AREA...BUT SNOW WOULD STILL BE LIKELY. COBB OUTPUT VIA THE
GFS FOR THIS TIME PERIOD IS STILL AROUND 9 INCHES. PREVIOUS RUNS OF
THE GFS RANGE FROM 6 TO 12 INCHES...SO WHILE THERE IS A VARIETY IN
AMOUNTS...ALL POINT TO SIGNIFICANT SNOWS...HEADLINE EVENTS. ONE
DIFFERENCE OF NOTE IS THAT RECENT BUFKIT SOUNDINGS WITH THE GFS DO
NOT SHOW AS DEEP OF A DENDRITIC GROWTH REGION AS IT HAD PREVIOUSLY.
IN ADDITION TO WHAT IS LOOKING LIKE THE FIRST SIGNIFICANT SNOW OF
THE SEASON...STRONG AND GUSTY WINDS SHOULD ACCOMPANY THIS STORM
SYSTEM. BLOWING AND DRIFTING WOULD BE LIKELY...AND CAN/T RULE OUT
PERIODS OF NEAR WHITE-OUT CONDITIONS. STILL...ITS A BIT EARLY AND
THE STORM TRACK HAS SHOWN SOME VARIANCE OVER THE PAST COUPLE DAYS.
HOWEVER...IF THE MODELS CONTINUE TO AGREE ON THE PRESENT TRACK...OR
EVEN MOVE IT MORE NORTHWEST...A WINTER STORM WATCH WOULD PROBABLY BE
NEEDED...ISSUED AS SOON AS SUNDAY AFTERNOON.
&&
.AVIATION...TONIGHT AND SUNDAY
PRIMARY CONCERN WAS WITH TIMING ONSET OF LOWER CEILINGS AND/OR
VISIBILITIES. SURFACE FRONT SLIDES THROUGH THE AREA TONIGHT. VARIOUS
FORECAST DATA SETS CONTINUED TO SUPPORT AN INCREASE IN LOW LEVEL
RELATIVE HUMIDITY IN THE POST FRONTAL ENVIRONMENT. AS POINTED OUT IN
PREVIOUS AVIATION DISCUSSION...CERTAINTY STILL NOT TOO GOOD AS TO
WHETHER THIS MOISTURE WILL MANIFEST ITSELF AS LOW CEILINGS AND/OR
VISIBILITIES. UNTIL CONFIDENCE INCREASES...OPTED TO MAINTAIN
FORECAST OF SCATTERED CLOUDS 2-3KFT LATE TONIGHT AND SUNDAY MORNING
AS IN CURRENT TAFS.
LOOKING AHEAD...A SNOW EVENT APPEARS PROBABLE FOR SUNDAY
NIGHT...WITH POTENTIAL FOR AN EVEN MORE SIGNIFICANT SNOW AND WIND
EVENT TUESDAY INTO WEDNESDAY. AIRPORT MANAGERS...DISPATCHERS...AND
PILOTS WILL WANT TO KEEP ABREAST OF THE LATEST FORECASTS...SINCE
THERE SHOULD BE IMPACTS TO SUCH OPERATIONS AS SNOW REMOVAL...DEICING
AIRCRAFT AND INSTRUMENT APPROACHES.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
AVIATION.......... THOMPSON
SHORT/LONG TERM... RIECK
