Area Forecast Discussion
Issued by NWS La Crosse, WI

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609
FXUS63 KARX 151758
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1258 PM CDT FRI MAY 15 2015

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 345 AM CDT FRI MAY 15 2015

BIGGEST CHANGE TO THIS PERIOD WILL BE TO REMOVE THE SEVERE STORM
POTENTIAL OVERNIGHT CURRENTLY IN THE SERVICE SUITE.

SATELLITE AND SURFACE OBSERVATIONS SHOWING A VERY WEAK PRESSURE
GRADIENT AND LOW CLOUD AND FOG FAIRLY WIDESPREAD ACROSS THE
REGION IN THE WAKE OF THE RAIN. STILL SOME DRIZZLE BEING REPORTED
IN WI IN WHAT THE RAP SOUNDINGS INDICATE AS A 2KM DEEP MOIST
LAYER. SPORADIC REPORTS OF NEAR ZERO VISIBILITY ARE OUT THERE AS
OF 08Z /MAINLY IOWA/...BUT WEB CAMERAS ACROSS THE AREA DONT REVEAL
WIDESPREAD REDUCTIONS TO VISIBILITY. OVERALL IDEA TODAY IS
SHORTWAVE RIDGING BUILDING IN AND DEEPER MIXING...BUT HAVE TAKEN A
PESSIMISTIC APPROACH WITH MORE SUN SEEN IN THE AFTERNOON. WILL
CONTINUE TO MONITOR FOR MORE WIDESPREAD FOG ISSUES AND FOR NOW
WILL HANDLE THIS WITH SPECIAL WX STATEMENTS.

PREVIOUS FORECAST AND CURRENT MESSAGING CONTAINS A SEVERE RISK
OVERNIGHT AS THE WARM FRONT...NOW LOCATED ROUGHLY ALONG
I-80...LIFTS INTO THE AREA. A SECONDARY MARITIME TROPICAL AIRMASS
RESIDES SOUTH OF I-70 WITH 65-70F DEWPOINTS...NOT TOO FAR AWAY.
THE 15.00Z NAM IS REALLY THE OUTLIER ON THE MUCAPE FORECAST INTO
THE AREA /THROUGH ITS ENTIRE FORECAST/ AND FEEL IT MUST BE REDUCED
ABOUT 30 PERCENT BASED ON SOUNDINGS AND LATEST MODEL CONSENSUS. AS
THE SPOKE OF ENERGY FROM THE SWRN U.S. LONG WAVE TROUGH ROTATES
NORTH //NOW IN NM/AZ//...HEIGHTS WILL FALL IN THE LOW-LEVELS AND
THE MOISTURE TRANSPORT WILL ENHANCE IN THE DAKOTAS/WRN MN...BUT
PRETTY FAR WEST. BY THE THE OVERNIGHT HOURS...AN AXIS OF MOISTURE
CONVERGENCE AND ABOUT 500 J/KG OF MUCAPE WILL RUN FROM KABR-KLSE.
BETTER CONVERGENCE ALONG THE WARM FRONT IN THE LOW LEVELS WILL BE
OCCURRING FURTHER WEST. HAVE KEPT 30S PERCENT CHANCES FOR THAT
PERIOD FOR THUNDERSTORMS...BUT THE WIND SHEAR IS VERY LOW AND THE
LESS AGGRESSIVE MODELS WITH MOISTURE /15.00Z GFS/ SUGGEST SOME
CAPPING OF 50-80J IS IN PLACE FOR 1-2KM PARCELS BEING LIFTED. SO
COVERAGE OF STORMS IS ALSO IN QUESTION. OVERALL...DIDNT SEE AN
ORGANIZED THREAT TO SPEAK OF FOR MESSAGING IN OUR SERVICE. WILL BE
PULLING THAT MENTION. SPC DAY 1 OUTLOOK WOULD AGREE WITH THIS
ASSESSMENT.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 345 AM CDT FRI MAY 15 2015

SATURDAY WILL SEE WARMER CONDITIONS AS THE WARM FRONT SHIFTS FURTHER
NORTH INTO THE NORTHERN FORECAST AREA AND NRN WI. DEWPOINTS AND
CAPE WILL BE ON THE INCREASE AND PROBABLY ABOUT 1000 J/KG OF
MLCAPE WILL BE PRESENT IN THE AFTERNOON. HOWEVER...FORCING IS
MINIMAL DURING PEAK HEATING WITH FAIRLY UNIFORM S-SE FLOW IN THE
FORECAST AREA IN THE WARM SECTOR. THE WILD CARD MAY BE ANY
CONVECTIVE VORTICITY THAT IS GENERATED IN THE SRLY FLOW. HAVE LEFT
SMALL RAIN CHANCES IN THE AFTERNOON. IT COULD BE A WARM DAY
SATURDAY AND BUMPED HIGHS A BIT WITH 19-21C SUGGESTED AT 925MB
FROM THE 15.00Z GUIDANCE /4-5C WARMER THAN FRIDAY/.

THE BIG STORY CONTINUES TO BE THE SUNDAY POTENTIAL FOR SEVERE
STORMS. POTENTIAL IS A GOOD WORD AS IT COULD BE A FAVORABLE DAY
FOR TORNADOES...BUT ALSO COULD RAIN THROUGH THE MORNING AND TURN
INTO NO STORY. THERE IS A BIG DIFFERENCE BETWEEN THE
DETERMINISTIC GUIDANCE WITH THE 15.00Z AND 14.18Z NAM RUNS SUGGESTING
A GOOD SETUP FOR SUPERCELL TORNADOES AND THE OVERALL SYNOPTICS AND
TIMING. HOWEVER...IT IS AN OUTLIER AS THE 15.00Z ECMWF/GFS AND
15.03Z/21Z SREFS ALL SUGGEST A DIFFERENT OUTCOME. THAT OUTCOME
WOULD INCLUDE STRONG MOISTURE TRANSPORT CONVERGENCE COMING INTO
THE AREA EARLY ON SUNDAY FROM THE WEST /SUNRISE/ AND SHIFTING EAST
ACROSS THE AREA. THERE WOULD PROBABLY BE ENOUGH INSTABILITY AND
INCREASING DEEPER STRONGER SHEAR FOR SOME MARGINALLY SEVERE
STORMS THEN. POSSIBLY A SMALL WINDOW IN WI FOR SEVERE STORMS TO
REDEVELOP LATER...BUT THIS SEEMS TO SHIFT NORTHEAST QUICKLY. THE
SLOWER 15.00Z AND 14.18Z NAM RUNS SUGGEST A SLOWER TIMING AND LESS
MORNING CONVECTION INTO WI...FAVORING A BETTER SUNDAY AFTERNOON
SEVERE STORM OUTBREAK.

IMPORTANTLY...SUNDAY HAS EXCELLENT DEEP SHEAR THROUGH 10KM...AND
TIMING AND INSTABILITY ARE THE MAIN PROBLEMS THAT NEED RESOLUTION. SPC
DAY 3 DISCUSSION MAINTAINS THE SLIGHT RISK OVER THE AREA...WHICH
SEEMS ON TRACK FOR THE DIFFERENCES THAT REMAIN ON SUNDAYS OUTCOME.
IT IS ONLY FRIDAY MORNING...AND THESE SWRN U.S. SYSTEMS ARE VERY
DEPENDENT ON THE ENERGY OVER THE PACIFIC BEING RESOLVED CORRECTLY
BY THE MODEL INITIALIZATION. THE NAM SKILL IN THE OUTER HOURS IS
NOT AS FAVORABLE AS WITHIN A DAY. WILL TRY TO STRESS THAT
DIFFERENCES EXIST ON HOW SUNDAY COULD PLAY OUT...BUT THAT A RISK
OF ORGANIZED SEVERE WEATHER EXISTS.

MONDAY THROUGH THURSDAY...AFTER THIS DEEPENING LOW AND SHOWER
CHANCES SHIFT EAST...THE WEEK SHOULD HAVE SLIGHTLY BELOW NORMAL
TEMPERATURES AND MAINLY DRY UNDER CONFLUENT NORTHWEST FLOW AND
CANADIAN SOURCED HIGH PRESSURE. COLDEST DAYS WILL BE MONDAY AND
TUESDAY...THEN SOME RISE EXPECTED.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON)
ISSUED AT 1258 PM CDT FRI MAY 15 2015

MVFR STRATUS WILL SCATTER OUT LATE THIS AFTERNOON WITH CLOUD
BASES IN THE 1800 TO 2500 FT RANGE. INCREASING MOISTURE ACROSS THE
REGION WILL PRODUCE BR LATE TONIGHT WITH THE POTENTIAL TO SEE
SOME PATCHY FOG AT TIMES. PLAN ON VISIBILITIES FALLING TO AROUND
3SM AT KRST AND 4SM AT KLSE AFTER MIDNIGHT. LOWER VISIBILITIES ARE
POSSIBLE. AFTER THE FOG DISSIPATES SATURDAY MORNING PLAN ON
SCATTERED CUMULUS FOR THE MID TO LATE MORNING HOURS WITH THE
CHANCE OF SEEING AN ISOLATED THUNDERSTORM OR TWO.

&&

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

$$

SHORT TERM...BAUMGARDT
LONG TERM....BAUMGARDT
AVIATION...WETENKAMP



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