Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 222330
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
530 PM CST SAT NOV 22 2014

.UPDATED...
530 PM CST SAT NOV 22 2014

FOG CONTINUES THICK ACROSS NORTH-CENTRAL WI...WITH CONTINUING
WARMING MELTING EXISTING SNOW PACK AND HELPING TO SATURATE THE
BOUNDARY LAYER. INCREASING WINDS WILL MOVE IN FROM THE WEST...AND
SHOULD HELP TO IMPROVE VSBYS A BIT. HAVE ISSUED A DENSE FOG ADVISORY
FOR TONIGHT FOR CLARK/TAYLOR COUNTIES IN RESPONSE TO THE THICK FOG.
MAY NEED TO CANCEL EARLY IF IMPROVEMENT IS REALIZED AND HOLDS.

.SHORT TERM...(THIS EVENING THROUGH SUNDAY)
ISSUED AT 313 PM CST SAT NOV 22 2014

CURRENTLY AS OF 21Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED A POWERFUL TROUGH OVER THE PACIFIC NORTHWEST...NOTED
BY A 155 KT JET NEAR 300MB ON THE 12Z MEDFORD OREGON RAOB. OTHER
FEATURES OF NOTE WERE AN UPPER LOW OVER THE BIG BEND REGION OF TEXAS
AND AN UPPER RIDGE OVER THE DAKOTAS AND MN. BELOW THE UPPER
RIDGE...WEST TO SOUTHWEST WINDS OF 20-30 KT FROM 925-850MB PER
PROFILER/VWP DATA HAS BEEN ADVECTING WARMER AIR INTO THE REGION.
925MB TEMPS HAVE RISEN FROM -1 TO 2C AT 12Z TO 2-6C AT 21Z.
HOWEVER...TAMDAR SOUNDINGS SHOW MUCH OF THIS WARMING HAS BEEN
STRENGTHENING A STRONG LOW LEVEL INVERSION...TRAPPING MOISTURE
ADVECTING NORTHWARD ON SOUTHERLY WINDS. DEWPOINTS PRESENTLY RANGE IN
THE 30S TO LOW 40S ACROSS THE FORECAST AREA...BUT 40S TO MID 50S
WERE LURKING ACROSS CENTRAL AND SOUTHERN IOWA. THE INCREASED
WARMING AND MOISTURE AT THE SURFACE HAS ALSO ALLOWED THE SNOWPACK TO
BEGIN TO WARM AND MELT...ADDING ADDITIONAL MOISTURE. WE HAVE SEEN
DENSE FOG EARLIER IN THE DAY...BUT DAYTIME MIXING PLUS WARMER AIR
COMING UP FROM THE SOUTH HAS HELPED TO RAISE VISIBILITIES FOR NOW.

FOR TONIGHT...IMPRESSIVE SURFACE TO 925MB WARM AND MOIST AIR
ADVECTION REMAINS PROGGED TO OCCUR ACROSS THE FORECAST AREA...AIDED
BY THE 925MB WINDS INCREASING TO 30-40 KT. 925MB TEMPS AND
DEWPOINTS BY 12Z SUNDAY CLIMB TO 8-10C...WITH MUCH OF THE JUMP
HAPPENING BETWEEN 00-06Z. THESE DEWPOINTS ARE 2-3 STANDARD
DEVIATIONS ABOVE NORMAL. HAD TO DO SOME MAJOR TEMP/DEWPOINT
ADJUSTMENTS TO HANDLE THIS WARM/MOIST AIR ADVECTION...FOLLOWING
MORE OF THE ECMWF/CANADIAN WHICH ARE NOT AFFECTED AS MUCH BY
SNOWPACK LIKE THE NAM IS. WITH ALL OF THIS WARMING AND MOISTURE
COMING ACROSS THE SNOWPACK...ANTICIPATE FAIRLY RAPID MELTING...
THROWING MORE MOISTURE INTO THE BOUNDARY LAYER. GIVEN AN
INVERSION HOLDING IN PLACE BETWEEN 850-900MB THIS EVENING...BELIEVE
WE SHOULD GET MORE FOG TO DEVELOP...POSSIBLY DENSE...SIMILAR TO WHAT
IS GOING ON RIGHT NOW IN SOUTH CENTRAL IA ON I-35 ON THE NOSE OF
THE MOISTURE SURGE. CONSIDERED GOING WITH AN ADVISORY...BUT
AFTERNOON TRENDS HAS REDUCED CONFIDENCE. ADVISORIES ARE STILL
POSSIBLE...THOUGH. OVERNIGHT...ALL THE FOG COULD DISSIPATE AS
THAT INVERSION WEAKENS...AS INDICATED IN RAP/HIRESARW VISIBILITY
FORECASTS. IN FACT...WE SHOULD HAVE THE AIRMASS OVER MISSOURI IN
PLACE WHICH FEATURES NO VISIBILITY RESTRICTION. SINCE THERE STILL
IS SNOWPACK...HAVE KEPT SOME FOG MENTION...BUT WE MAY NOT HAVE
MUCH SNOWPACK EXCEPT IN OUR NORTH BY THE TIME 12Z ROLLS AROUND.
ALONG WITH THE FOG...THE STRONG INCREASE OF LOW LEVEL MOISTURE IS
LIKELY TO LEAD TO AREAS- WIDESPREAD DRIZZLE...ESPECIALLY
OVERNIGHT.

ON SUNDAY...GOOD MODEL CONSENSUS EXISTS FOR DEEP TROUGHING TO FORM
ACROSS THE PLAINS BY 00Z...WHICH HELPS TO PULL NORTHEASTWARD THE
UPPER LOW NEAR TEXAS INTO SOUTHERN ILLINOIS. THE 22.12Z NAM IS THE
ONLY MODEL WHICH DOESNT PULL THE UPPER LOW NORTHEASTWARD...WHICH
APPEARS TO BE AN OUTLIER AND HAS BEEN DISCOUNTED. WITH THE APPROACH
OF BOTH THE UPPER TROUGH AND LOW IN THE AFTERNOON...DPVA FOR
VERTICAL MOTION INCREASES. THIS VERTICAL MOTION COMBINED WITH
PRECIPITABLE WATER CLIMBING TO 0.75-1 INCH...HIGHEST OVER THE
SOUTHEAST HALF OF THE FORECAST AREA...WILL LEAD TO AN INCREASE IN
SHOWER ACTIVITY...ESPECIALLY IN THE AFTERNOON. NOTE THAT THERE ARE
SIGNS FOR A FRONTOGENETIC BAND TO FORM STRETCHING SOUTHWEST TO
NORTHEAST ACROSS THE FORECAST AREA...WHICH MAY LEAD TO LESS
PRECIPITATION OVER THE FAR NORTHWEST SIDE OF THE FORECAST AREA.
OTHERWISE...ANY FOG FROM TONIGHT LOOKS TO DISSIPATE WEST TO EAST
THROUGH THE AFTERNOON...IN RESPONSE TO A COLD FRONT COMING INTO THE
WESTERN FORECAST AREA AND THE RAIN MOVING IN.

.LONG TERM...(SUNDAY NIGHT THROUGH SATURDAY)
ISSUED AT 313 PM CST SAT NOV 22 2014

SUNDAY NIGHT AND MONDAY...THIS PERIOD IS BECOMING MORE INTERESTING.
THERE ARE SIGNS IN THE 22.12Z GFS AND CANADIAN OF PHASING TAKING
PLACE...BETWEEN THE OVERALL UPPER TROUGH ACROSS THE PLAINS AND THE
CURRENT STRONG JET STREAK AT MEDFORD OREGON LIFTING INTO THE OHIO
VALLEY. THE GFS EVEN TAKES IT ONCE STEP FURTHER AND PHASES IN THE
TEXAS UPPER LOW. THE RESULT OF THE PHASING IS RAPID DEEPENING OF A
SURFACE LOW BETWEEN 00-12Z MONDAY MARCHING FROM MISSOURI TO NORTHERN
LAKE MICHIGAN. PRESSURES FALL FROM THE UPPER 980S TO LOW 970S OR
UPPER 960S MB. THE 22.09Z SREF NMMB MEMBERS AND SOME OF THE
CANADIAN ENSEMBLES ARE ALSO QUITE DEEP...EVEN MORESO THAN THE
GFS/CANADIAN. THESE DEEPER MODELS WOULD RESULT IN A TROWAL
OVER THE FORECAST AREA...LEADING TO A HEAVY SNOW BAND AND STRONG
WINDS. ALTHOUGH THERE HAS BEEN A TREND OF DEEPENING IN THE
AFOREMENTIONED MODELS...THE 22.12Z ECMWF REMAINS CONSISTENT IN A
MORE PROGRESSIVE/LESS PHASED SCENARIO. THE MODEL STILL HAS SNOW FOR
THE FORECAST AREA AFTER SOME EVENING RAIN...BUT THE F-GEN BAND IS
NOT NEARLY AS IMPRESSIVE. GIVEN THE CONSISTENCY IN THE ECMWF
AND THE MODEL BEING GENERALLY ONE OF THE BEST AT SYNOPTIC
VERIFICATION...FEEL IT IS DIFFICULT TO JUMP ON THE DEEP SCENARIOS
YET. THUS...THE FORECAST FOR SUNDAY NIGHT/MONDAY REMAINS LARGELY
UNCHANGED. DID BACK OFF THE BLOWING SNOW A BIT SINCE THE SNOWPACK
WILL BE GONE...ALLOWING ANY FALLING SNOW TO ACCUMULATE IN DITCHES
AND THUS LIMIT BLOWING. TEMPERATURE WISE...EXPECT A CONTINUOUS
FALLING TREND FROM SUNDAY NIGHT THROUGH MONDAY.

SNOW SHOULD GRADUALLY TAPER OFF MONDAY NIGHT AS THE LOW LIFTS
OFF TO THE NORTHEAST...BUT CONTINUED COLD AIR ADVECTION IS
ADVERTISED. 925MB TEMPS PLUNGE TO -10 TO -12C...WHICH SHOULD BRING
TEMPS DOWN INTO THE TEENS. TUESDAY FINALLY LOOKS LIKE A DRY
DAY...ASSOCIATED WITH SURFACE RIDGING COMING ACROSS AND AHEAD OF A
POTENT SHORTWAVE DROPPING INTO THE DAKOTAS. PRECIPITABLE WATER IS
QUITE DRY TOO...AROUND 0.2 OF AN INCH...WHICH SHOULD ALLOW ANY LOW
CLOUDS TO CLEAR TUESDAY MORNING. MORE HIGH CLOUDS ARE LIKELY TO
ARRIVE IN THE AFTERNOON...THOUGH. WITH 925MB TEMPS OF -8 TO -10C AT
18Z...HIGHS ONLY LOOK TO TOP OUT IN THE 20S.

TUESDAY NIGHT AND WEDNESDAY COULD BE INTERESTING AS WELL WITH THE
22.12Z ECMWF REMAINING CONSISTENT IN DRIVING A STORM SYSTEM
FROM THE DAKOTAS THROUGH THE REGION. THE 22.12Z/18Z NAM ARE SIMILAR
TOO...THOUGH THERE REALLY IS NO RESEMBLANCE OF THE SYSTEM IN THE
22.12Z GFS. THE 22.12Z CANADIAN IS LIKE A WEAKER NAM/ECMWF. SHOULD
THE ECMWF/NAM VERIFY...WE COULD BE LOOKING AT A 1-2 INCH
SNOWFALL...WHICH COULD IMPACT THANKSGIVING TRAVEL. FOR NOW HAVE TO
KEEP THE CHANCES LOWER...20-40...UNTIL BETTER MODEL AGREEMENT OCCURS.

HOWEVER WEDNESDAY PANS OUT...GOOD MODEL AGREEMENT EXISTS FOR ANOTHER
COLD SURGE TO DIVE INTO THE REGION FOR THANKSGIVING. 925MB TEMPS OF
-14 TO -16C ARE PROGGED...WHICH WOULD RESULT IN HIGHS ONLY IN THE
MID TEENS TO LOW 20S AT BEST. THIS COLD AIR LOOKS TO LINGER INTO THE
EARLY PART OF THE WEEKEND AS CANADIAN HIGH PRESSURE BUILDS IN.
GENERALLY CONDITIONS SHOULD BE DRY...THOUGH CANNOT RULE OUT A FEW
LIGHT SNOW PERIODS THAT DO NOT ACCUMULATE UP TO MUCH.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z SUNDAY EVENING)
ISSUED AT 530 PM CST SAT NOV 22 2014

LIFR/IFR CONDITIONS LOOK TO BE PREDOMINANT TONIGHT THROUGH THE
BETTER PART OF SUNDAY. INCREASING LOW LEVEL MOISTURE TONIGHT WILL
KEEP CIGS LOW...WHILE WARMING CONTINUES TO MELT SNOW PACK AND HELPS
WITH MOISTENING OF THE BOUNDARY LAYER. CIGS SHOULD STAY SUB 1KFT.
EXPECT AREAS OF DRIZZLE TO DEVELOP LATER THIS EVENING...PERSISTING
THROUGH AT LEAST SUNDAY MORNING. FOG SHOULD REMAIN...AND COULD
BECOME DENSE AT TIMES. EXPECTING A SHIFT TO NORTHWEST WINDS WITH THE
PASSAGE OF A SFC FRONT MID/LATE SUN AFTERNOON.

ONE CAVEAT TO THE LOW CLOUDS IS LATEST SATELLITE FOG/STRATUS IMAGERY
AND SFC OBS SHOW AN AREA OF CLEARING OVER NORTHERN IA. QUESTION IS
WHETHER THIS SLOT WILL MOVE NORTHWARD TO KRST/KLSE OR STAY SOUTH.
MASS OF STRATUS JUST SOUTH OF THE SLOT...AND MAKING GOOD PROGRESS
NORTH. THIS COULD OVERTAKE THE CLEARING BEFORE IT MAKES KRST/KLSE.
FOR NOW...WILL STICK WITH KEEPING THE CLEARING SLOT SOUTH...STAYING
WITH THE LOW CLOUDS. WILL ADJUST FORECAST AS NECESSARY.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...

WI...DENSE FOG ADVISORY UNTIL 6 AM CST SUNDAY FOR WIZ017-029.
MN...NONE.
IA...NONE.
&&

$$

UPDATED....RIECK
SHORT TERM...AJ
LONG TERM....AJ
AVIATION.....RIECK



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