Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 272026
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
326 PM CDT MON JUL 27 2015

.SHORT TERM...(THIS EVENING THROUGH TUESDAY)
ISSUED AT 325 PM CDT MON JUL 27 2015

MOST OF THE FORECAST WORK TODAY WAS TRYING TO FLESH OUT THE
EVOLUTION OF WEATHER ON TUESDAY. STILL FEEL THE TIMING OF
CONVECTIVE SYSTEMS AND DEGREE OF SEVERE WEATHER THREATS IS
SOMEWHAT IN QUESTION...BUT THINK THE BIGGER PICTURE IDEAS ARE
INCREASING IN CONFIDENCE.

BOTTOM LINE FOR TUESDAY IS A MORNING CONVECTIVE SYSTEM WITH WARM
FRONTAL ADVECTION...A BREAK...AND THEN CONVECTION DEVELOPING
ALONG THE COLD FRONT IN THE EVENING NEAR I-35.

TRUE MARITIME TROPICAL AIR MASS STILL PINNED DOWN IN SRN IA AND
MO...WITH SOME BIG CONVECTION EXPECTED THERE THIS AFTERNOON. THIS
IS WHERE SUPPORT WILL CONTINUE UNTIL THE OVERNIGHT. A FEW FEATURES
ON WATER VAPOR...THE SHORTWAVE TROUGH OVER CO HEADING NEWRD AND
DEEP TROUGH AND ENERGY EJECTING OUT OF SLC AREA. A MORE MODIFIED
BUT STILL MARITIME AIR MASS RESIDES OVER IA/MN NORTH OF THE
BOUNDARY AND KMPX /12Z RAOB/ IS AT 140 PERCENT NORMAL PRECIPITABLE
WATER WITH SOME HIGHER VALUES TO THE SOUTH...BUT NOT MUCH. 70F
DEWPOINT LINE NOW UP TO CANADA-U.S. BORDER.

AS THE LARGE TROUGH SHIFTS EAST HEIGHTS FALL IN THE PLAINS AND
MOISTURE TRANSPORT IN THE LOW-LEVELS BECOMES MORE ACTIVE OVERNIGHT
BUT GENERALLY WEST OF THE FORECAST AREA. THIS WILL WORK IN
CONCERT WITH THE COLORADO SHORTWAVE TO INCREASE PROBABILITIES FOR
A CONVECTIVE COMPLEX MOVING ACROSS MN/IA BY LATE TONIGHT. THIS ALL
COINCIDES WITH MOISTURE ADVECTION PROCESS WHICH IS WEAKLY LIFTING
ISENTROPICALLY. THUS...BELIEVE MUCH OF THE AREA WILL EXPERIENCE
STORMS TUESDAY MORNING AS THIS FORCING MOVES IN. WE WILL BE
UPDATING OUR TIMING GRAPHICS FOR THESE STORMS...SUNRISE IN SERN
MN NERN IA...MID-MORNING ON MISS RIVER...EXITING CENTRAL WI BY
VERY EARLY AFTERNOON. THIS IS FAIRLY WELL AGREED UPON IN THE HIRES
MODELS...AS IS A WEAKENING TREND AS IT MOVES EAST. THERE IS SOME
GOOD ELEVATED CAPE MOVING IN...1000-1500J/KG...BUT POOR WIND
SHEAR PROFILES WILL MEAN ISOLATED STRONG STORMS AND GUSTY WINDS
POSSIBLE. BUT...MAINLY HEAVY RAINFALL RATES TO AFFECT VISIBILITY
FOR MORNING COMMUTE AND LIGHTNING ARE THE MAIN THREATS.

IT APPEARS SOME DESTABILIZATION TIME WILL THEN OCCUR. THE QUESTION
IS STILL HOW MUCH AHEAD OF THE FRONT. ONE OF MY FAVORITE GUIDANCE
CHOICES IS THE 10-MEMBER HI-RES NCAR ENSEMBLE FORECAST SYSTEM.
THIS DATA SHOWS OVER AN 80 PERCENT PROBABILITY OF 2000 J/KG OF
MLCAPE AHEAD OF THE COLD FRONT ALONG I-35 AT 00Z. SO...WE ARE
FORECASTING STORM REDEVELOPMENT. TIMING IS A BIT MORE DIFFICULT AS
THE NAM IS QUITE SLOW WITH THE FRONT...NOT COMING INTO SERN MN
AND NERN IA UNTIL LATE EVENING. THE FORECAST HAS BEEN SLOWED
SLIGHTLY WITH A EARLY EVENING STORM INITIATION THERE...SHIFTING
EASTBOUND INTO WI BY LATE EVENING. THIS MAY NEED TO BE SLOWED
MORE. BELIEVE THIS CONVECTION WILL TAKE A MORE SERLY TRACK AS
WELL WHICH ALIGNS WELL WITH THE SPC RISK AREA OF SLIGHT. THIS
SEEMS ON TARGET AND BASED ON KMCW-KRST INITIATION...THIS PLACES
THE HIGHEST RISK SOUTH OF I-90.

THE WIND SHEAR IS BETTER IN THE EVENING PRE-FRONTALLY THAN THE
MORNING...WITH 0-3KM SHEAR OF ABOUT 25 KTS ORIENTED
W-E...FAVORABLE FOR A N-S LINE ECHO. MULTICELLS WITH LINEAR
FEATURES AFTER INITIATION LOOKS TO OCCUR. SO...THREATS WOULD BE
HAIL ON INITIATION...BUT THEN WIND GUSTS TO 60 MPH...BRIEF HEAVY
RAIN...AND LIGHTNING. WOULD EXPECT ISOLATED TO SCT SEVERE STORMS
TO BE POSSIBLE.

.LONG TERM...(WEDNESDAY THROUGH MONDAY)
ISSUED AT 325 PM CDT MON JUL 27 2015

WOW...WILL WEDNESDAY BE A CONTRAST TO TUESDAY WITH DRYING AND
BREEZY CONDITIONS. A VERY NICE PERIOD OF WEATHER WITH LARGE SCALE
RIDGING OVER THE WRN U.S. AND NW FLOW OVER THE GREAT LAKES.
TEMPERATURES SHOULD BE NEAR OR SLIGHTLY ABOVE NORMAL WITH
COMFORTABLE DEWPOINTS OVERALL. NO SIGNIFICANT WEATHER SEEN AT THIS
TIME.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)
ISSUED AT 1237 PM CDT MON JUL 27 2015

LOW CLOUD AND FOG IMPACTED KRST EARLY THIS MORNING...WITH SOME
PATCHY VALLEY FOG AND STRATUS NEAR KLSE. THIS RESIDUAL BOUNDARY
LAYER MOISTURE IS MANIFESTING INTO CUMULUS TODAY WHICH SHOULD
PREVAIL FOR THE REMAINDER OF THE DAYLIGHT HOURS. MEANWHILE
CONVECTION ACROSS IA SPREADING COPIOUS HIGH CLOUD OVER THE AREA.
DO NOT ANTICIPATE ANY RAIN ACTIVITY REACHING EITHER AIRFIELD FROM
THIS. NEXT QUESTION IS CONVECTIVE POTENTIAL TOWARD SUNRISE TUESDAY
AND INTO THE MORNING HOURS. COMPLICATED SCENARIO WITH SOME
CONVECTION PERHAPS FIRING IN THE WARM/MOIST ADVECTION OUT AHEAD OF
AN APPROACHING SHORT WAVE TROUGH AND FRONTAL BOUNDARY IN THE 10Z-
15Z PERIOD. THEN A BETTER CHANCE LATER IN THE DAY AS THE MAIN
FRONT COMES THROUGH. OR...IT MAY BE THAT THE INITIAL CONVECTION
DOMINATES. ADDED SOME VCSH AND TSRA TO THE LATER PERIODS OF THE
TAFS...BUT LATER SHIFTS SHOULD BE ABLE TO ADD MORE DETAIL AND/OR
BETTER TIMING AS THE SCENARIO BECOMES A BIT MORE CLEAR.


&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&

$$

SHORT TERM...BAUMGARDT
LONG TERM...BAUMGARDT
AVIATION...MW



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