000
FXUS63 KARX 170850
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
350 AM CDT FRI MAY 17 2013

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 350 AM CDT FRI MAY 17 2013

FOCUS IS ON TRENDING CURRENT SHOWERS/STORMS ACROSS IA/MN...AND
CONVECTIVE PROBABILITIES LATER TODAY ALONG WARM FRONT.

LATEST RADAR SHOWS COMPLEX OF SHOWERS/STORMS ACROSS NORTHERN
IA/SOUTHERN MN...WHICH IS FEEDING OFF 850 MB LOW-LEVEL
JET...INTERACTING WITH A WARM FRONTAL BOUNDARY THAT STRETCHES ACROSS
NORTHERN IOWA. THESE STORMS ARE EFFICIENT HEAVY RAIN
PRODUCERS...GIVEN SLOW EASTWARD PROGRESS AND PWATS 1.4 TO 1.6
INCHES...WHICH IS 150 TO 180 PERCENT ABOVE NORMAL. THESE STORMS ARE
EXPECTED TO MOVE EASTWARD ACROSS SOUTHEAST MINNESOTA AND EXTREME
NORTHEAST IOWA...AND PARTS OF WESTERN WISCONSIN THROUGH MID/LATE
MORNING. SOME OF THE STRONGER CELLS COULD PRODUCE SMALL HAIL...BUT
NOT EXPECTING ANYTHING TO REACH SEVERE LIMITS. THE BIGGER IMPACT
WOULD BE LOCALIZED HEAVY RAINFALL...AND IN TOTAL...AMOUNTS BETWEEN 1
AND 2 INCHES ARE POSSIBLE.

THE BIG QUESTION FOR TODAY IS WHERE THE WARM FRONT ENDS UP...AND IF
ANOTHER ROUND OF SHOWERS/STORMS DEVELOPS ALONG IT THIS AFTERNOON.
LATEST RAP HAS A DECENT HANDLE RIGHT NOW...SHOWING THAT THE WARM
FRONT SETTLES ALONG THE IA/MN BORDER...AND INTO SOUTHWEST WISCONSIN
THIS AFTERNOON. ONGOING CONVECTION SHOULD KEEP NORTHWARD PROGRESS OF
THE BOUNDARY LIMITED SO RAP SOLUTION SEEMS REASONABLE. THE REMNANTS
OF THIS MORNING/S STORMS WOULD LIFT TO THE EAST/NORTHEAST...BUT
LEFTOVER OUTFLOW BOUNDARIES COMBINED WITH MODEST DAYTIME HEATING AND
A PASSING UPPER LEVEL SHORTWAVE LOOKS TO BE SUFFICIENT FOR
ADDITIONAL SHOWERS AND STORMS TO DEVELOP. THESE SHOWERS AGAIN
WOULD HAVE THE POTENTIAL TO PRODUCE HEAVY RAIN.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 350 AM CDT FRI MAY 17 2013

...SEVERE WEATHER EVENT POSSIBLE SUNDAY AND AGAIN ON MONDAY...

SATURDAY FEATURES UPPER LEVEL RIDGING ACROSS THE CENTRAL PLAINS AS
THE WEST COAST TROUGH CONTINUES TO MOVE EASTWARD. AT THE
SURFACE...LOW PRESSURE DEVELOPS IN THE LEE OF THE ROCKIES AND PUSHES
THE AFOREMENTIONED WARM FRONT WELL NORTH OF THE FORECAST AREA. THE
ATMOSPHERE BECOMES CAPPED FOR THE DAY...WITH THE WARM NOSE LAYER
EVIDENT IN THE 850 TO 750 MB LAYER. PLENTY OF ELEVATED CAPE NOTED
ACROSS THE AREA...ANYWHERE FROM 2000 TO 3000 J/KG BY SATURDAY
AFTERNOON. THE CAP REMAINS IN PLACE DURING THE DAY...BUT DOES WEAKEN
SOME BY LATE AFTERNOON AND EVENING AS CIN VALUES DROP TO LESS THAN
50 J/KG...MAINLY ALONG AND NORTH OF I-90. THE LACK OF A STRONG
FORCING MECHANISM SHOULD KEEP CONVECTIVE INITIATION PROBABILITIES
LOW...AND THIS IS WELL AGREED AMONG THE VARIOUS DETERMINISTIC
MODELS. THE ONLY EXCEPTION IS IN THE 17.00Z GFS WHERE A WEAK UPPER
LEVEL SHORTWAVE TROUGHS RIDES THE RIDGE ACROSS NORTHERN WISCONSIN.
THIS COULD PROVIDE ENOUGH UPPER LEVEL SUPPORT TO BREAK THE WEAKER
CAP AND ALLOW SHOWERS/STORMS TO INITIATE. HAVE CONTINUED CHANCE POPS
MAINLY NORTH OF I-90 TO HIGHLIGHT THIS IDEA. ANY STORM CAPABLE OF
BREAKING THROUGH THE CAP AND BECOMING SURFACE BASED HAS THE
POTENTIAL TO PRODUCE LARGE HAIL AND POSSIBLY DAMAGING WINDS GIVEN
THE INSTABILITY AND 0-6 KM BULK SHEAR NEAR 30 KTS. OTHERWISE...WARM
DAY ON TAP WITH HIGHS IN THE LOW TO MID 80S.

FOCUS TURNS TO THE POTENTIAL SEVERE WEATHER EVENT FOR SUNDAY AS THE
AFOREMENTIONED UPPER LEVEL TROUGH BECOMES INCREASINGLY NEGATIVE
TILTED AND EJECTS INTO THE CENTRAL PLAINS BY 12Z SUNDAY. THE BASE OF
THE TROUGH WILL LIKELY BE ACROSS OK/KS...BUT QUICKLY LIFTS NORTHEAST
TOWARD WESTERN IOWA BY 00Z MONDAY. AT THE SURFACE...LOW PRESSURE
LOOKS TO MOVE NORTHEASTWARD FROM NEBRASKA TO NEAR SIOUX FALLS
SD...AND A WARM FRONT SHOULD BE DRAPED ACROSS SOUTHWEST MINNESOTA
EAST/SOUTHEASTWARD INTO NORTHEAST IOWA. IT/S POSSIBLE THERE COULD BE
SOME DECAYING CONVECTION SUNDAY MORNING ACROSS WESTERN IOWA...AND
THIS COULD PLAY INTO HOW UNSTABLE THE ATMOSPHERE BECOMES LATER THAT
DAY ACROSS THE REGION. IN ANY RATE...MUCAPE VALUES ARE EXPECTED TO
RANGE 3000 TO 3500 J/KG AND 0-6 KM BULK SHEAR VALUES APPROACH 35 KTS
WEST OF THE MISSISSIPPI RIVER SUNDAY AFTERNOON AND EVENING. THE
MAJORITY OF THE SHEAR IS FOUND IN THE 0-3 KM LAYER WITH 25 KTS SEEN.
EVEN MORE EXCITING ARE THE 0-1 KM SHEAR PARAMETERS...WHICH IS AT
LEAST 20 KTS. FINALLY...THE AIRMASS ACROSS SOUTHEAST MINNESOTA AND
NORTHERN IOWA SUNDAY LOOKS TO BE SOMEWHAT JUICY WITH DEWPOINTS IN
THE 60S. HOW JUICY IS STILL A QUESTION GIVEN DETERMINISTIC MODELS
POOR HANDLING OF DEWPOINTS LATELY. 17.00Z NAM/GFS PUT UPPER 60S TO
EVEN NEAR 70F BY 21Z SUNDAY...WHICH SEEMS A BIT OVERDONE. ADJUSTING
SOUNDINGS TO WHAT COULD BE A MORE REALISTIC VALUE /LOW TO MID 60S/
BASED ON THE MOIST BIAS STILL YIELDS CAPE VALUES IN THE 1500 TO 2500
J/KG RANGE.

IN SHORT...ENVISION STORMS TO FIRE UP ACROSS CENTRAL IOWA INTO
SOUTHERN MINNESOTA BY EARLY AFTERNOON SUNDAY...THEN QUICK MOVE
EAST/NORTHEAST...LASTING INTO THE EVENING HOURS. THE HIGHER SEVERE
CHANCES ARE WEST OF THE MISSISSIPPI RIVER...AS STORMS WOULD WEAKEN
SOME MOVING INTO WISCONSIN BUT STILL COULD REMAIN SEVERE. ORGANIZED
DISCRETE STRONG/SEVERE STORMS WOULD BE THE PRIMARY MODE...MAINLY
SUPERCELLS...WITH A PREFERENCE TO THE RIGHT MOVERS GIVEN CLOCKWISE
CURVATURE SEEN IN THE HODOGRAPHS. THE MAIN THREATS LOOK TO BE VERY
LARGE HAIL AND DAMAGING WINDS BUT ALSO A FAVORABLE ENVIRONMENT FOR
TORNADOES GIVEN 0-1 KM BULK SHEAR /AT LEAST 20 KTS/ AND EHI /1.0 TO
3.0/. WILL HIGHLIGHT INCREASED THREAT ACROSS THE MANY PUBLIC
PRODUCTS /HWO...GRAPHICAST...NEWS STORY/. THOSE WITH PLANS SUNDAY
AFTERNOON/EVENING WILL NEED TO KEEP A VERY CLOSE EYE TO THE FORECAST.

ANOTHER ROUND OF STRONG/SEVERE STORMS IS POSSIBLE MONDAY EVENING AS
THE WHOLE SYSTEM SLOWLY EDGES EASTWARD. ANOTHER FAVORABLE SEVERE
WEATHER ENVIRONMENT WITH 0-6 KM MUCAPE PEGGED BETWEEN 2000 TO
3000 J/KG AND BULK SHEAR 30 TO 40 KTS. DEPENDING ON HOW MUCH
EASTWARD PROGRESSION OF THE WEATHER SYSTEM OCCURS MONDAY...A
GREATER PERCENTAGE OF THE FORECAST AREA COULD BE IN RISK FOR
SEVERE WEATHER. AT THIS TIME...HIGHER THREAT AREA LOOKS TO BE
ALONG AND SOUTH OF I-90.

MODEL TIMING DIFFERENCES CREEP UP BY TUESDAY...MAINLY IN HOW QUICKLY
THE UPPER LEVEL CLOSES OFF AND MOVES EAST OVER THE AREA. WIDE RANGE
OF POSSIBILITIES ON WHEN THE LOW MOVES OVERHEAD...WITH THE 17.00Z GFS
THE QUICKEST /TUESDAY/ AND THE ECMWF SLOWEST /WEDNESDAY/. FOR
NOW...PERIODIC SHOWERS/STORMS TUESDAY AND WEDNESDAY LOOK GOOD.
COOLER TEMPERATURES ALOFT AND THREAT OF SHOWERS SHOULD KEEP HIGHS ON
THE COOLER SIDE WEDNESDAY AND THURSDAY...5 TO 7 DEGREES BELOW
NORMALS.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z FRIDAY NIGHT)
ISSUED AT 1150 PM CDT THU MAY 16 2013

COMPLEX OF STORMS THAT DEVELOPED THIS AFTERNOON OVER NORTHWEST
IOWA HAS BEEN STEADILY ADVANCING EAST TOWARD THE AREA AND
WEAKENING THIS EVENING. STILL SOME SHOWERS AND ISOLATED
THUNDERSTORMS LEFT WITH MOST OBSERVATIONS REPORTING JUST SOME
SPRINKLES FALLING OUT OF A MID LEVEL DECK. THE HRRR THIS EVENING
SEEMS TO HAVE THE BEST HANDLE ON HOW THINGS ARE PLAYING OUT AND
INDICATES THESE SHOWERS WILL MAKE IT INTO KRST BUT MAY FALL APART
BEFORE REACHING KLSE. WILL THUS SHOW A CATEGORICAL SHOWER AT KRST
WITH NO VISIBILITY RESTRICTION AND ONLY A VCSH AT KLSE. THE OTHER
CONCERN FOR TONIGHT IS WHAT THE TRAILING CONVECTION ACROSS
SOUTHWEST MINNESOTA AND NORTHWEST IOWA WILL DO. THE 17.03Z RAP ONLY
SHOWS SOME WEAK MOISTURE TRANSPORT INTO THIS ACTIVITY AND THE HRRR
WHILE HOLDING THIS TOGETHER...FOR THE MOST PART TAKES IT NORTH OF
BOTH TAF SITES. WILL NOT GET FANCY WITH THIS WITH JUST VFR
CONDITIONS EXPECTED AND WILL HOLD THE CATEGORICAL LIGHT SHOWERS AT
KRST THROUGH THE NIGHT EVEN THOUGH IT WILL NOT RAIN THAT LONG AND
KEEP AT VCSH AT KLSE INTO FRIDAY MORNING. THE NEXT CONCERN IS WITH
THE WARM FRONT THAT CUTS ACROSS IOWA. THIS SHOULD LIFT NORTH TOWARD
THE AREA FRIDAY AFTERNOON. STILL SOME CONCERN THAT CONVECTION
COULD DEVELOP ALONG THIS BUT CONFIDENCE STILL NOT HIGH ENOUGH TO
INCLUDE IN THE FORECAST AND WILL JUST SHOW SOME LOWER LEVEL
CUMULUS CLOUDS FOR THE AFTERNOON AND EVENING.


&&

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

$$

SHORT TERM...ZT
LONG TERM...ZT
AVIATION...04