Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 282029
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
329 PM CDT THU AUG 28 2014

.SHORT TERM...(THIS EVENING THROUGH FRIDAY NIGHT)
ISSUED AT 329 PM CDT THU AUG 28 2014

MAIN FOCUS FOR THE NEXT 24 TO 48 HOURS IS ON THE HEAVY RAINFALL
POTENTIAL AS A FEW ROUNDS OF STORMS LIFT THROUGH THE UPPER
MISSISSIPPI RIVER VALLEY.

AS OF MID AFTERNOON...WATER VAPOR IMAGERY OVERLAID WITH RAP 500MB
RAP HEIGHTS SHOWED THE SHORTWAVE TROUGH PUSHING OFF THE COLORADO
ROCKIES WITH WEAK IMPULSES LIFTING THROUGH THE CENTRAL PLAINS
TOWARD THE UPPER MIDWEST. ONE SUCH WAVE IS BRINGING THE SHIELD OF
MAINLY RAIN SHOWERS THIS AFTERNOON...WHILE ANOTHER ONE IS NOTED
NEAR THE TX PANHANDLE INTO SOUTHERN KANSAS. THIS ONE...ALONG WITH
THE MAIN TROUGH...WILL BE OUR PLAYER FOR FRIDAY AFTERNOON AND
EVENING.

THE SURFACE HIGH PRESSURE ANCHORED OVER THE GREAT LAKES KEPT FLOW
OF DRIER AIR...LIMITING THE FORWARD PROGRESS OF RAIN SHIELD. A LOT
OF THE MOISTURE WENT INTO SATURATION...AND COMBINED WITH THE
WEAKENING LLJ MOISTURE TRANSPORT FROM THIS MORNING...THE HEAVIER
RAINFALL DID NOT MATERIALIZE. WITH THAT SAID...THE ENVIRONMENT
WILL BEGIN TO BECOME MORE FAVORABLE TONIGHT AND RECENT 1-HR
RAINFALL TOTALS HAVE BEEN GENERALLY 0.1 TO 0.2 INCH...WITH SOME
ISOLATED 0.3-0.5 AMOUNTS.

VERY LITTLE INSTABILITY FOUND ACROSS THE AREA WHICH IS KEEPING
ACTIVITY AS SHOWERS/RAIN. WENT AHEAD AND LIMITED THUNDER CHANCES
TO ISOLATED/SCATTERED THROUGH THE OVERNIGHT HOURS...ESPECIALLY
MN/WI AREAS. IT WILL BE MORE STEADY RAINFALL. AS THE CURRENT
SHIELD OF RAIN LIFTS NORTHEAST...EXPECTING THE BACK EDGES TO FILL
IN SOME KEEPING THE HIGHER POPS IN PLACE THROUGH THE OVERNIGHT.

THE TIMING OF ANY SPECIFIC UPPER LEVEL WAVES...WITH CONFLICTING
STRENGTH IN MOISTURE TRANSPORT AND SURFACE FOCUS...LENDS A BIT
LOWER CONFIDENCE IN WHERE HEAVIEST RAINFALL WILL FALL. WHAT HAS
HIGHER CONFIDENCE IS THAT THE ATMOSPHERE SHOULD BETTER PRIMED FOR
EFFICIENT RAIN PRODUCERS. NO CHANGES TO THE IDEA THAT PWATS ARE
HIGH...WARM CLOUD DEPTHS ARE ABOVE 4 KM...AND THERE IS A
CONTINUOUS FLOW OF MOISTURE STREAMING IN FROM THE SOUTHWEST IN
ADVANCE OF THE SHORTWAVE.

PERUSAL OF 28.12Z MODELS SHOW SIGNALS FOR ANOTHER IMPULSE TO
SPARK CONVECTION OVERNIGHT ACROSS WESTERN INTO SOUTHWEST IOWA.
THIS WOULD LIFT THROUGH NORTHEAST IOWA AND SOUTHEAST MINNESOTA
EARLY MORNING. AFTER THE MORNING LLJ WEAKENS...THERE SEEMS TO NOT
BE ANOTHER KICKER UNTIL LATE FRIDAY AFTERNOON AND EVENING. AS
SUCH...THE LATE MORNING AND EARLY AFTERNOON HOURS COULD END UP
DRIER...AND HAVE TRIED TO REFLECT THIS THINKING IN THE HOURLY
POPS.

THE MORE SIGNIFICANT WAVE ARRIVES FRIDAY AFTERNOON THROUGH THE
EARLY MORNING HOURS SATURDAY. FINALLY THE MAIN SHORTWAVE TROUGH
ENERGY WILL BE FELT...AND WITH A GOOD FEED OF MOISTURE TRANSPORT
AND WARM AIR ADVECTION...THERE SHOULD BE WIDESPREAD SHOWERS AND
ISOLATED/EMBEDDED THUNDERSTORMS THROUGHOUT FROM LATE AFTERNOON
ONWARD.

ALTHOUGH THE FAVORABLE ATMOSPHERIC CONDITIONS WILL BE IN PLACE
FOR HEAVY RAINFALL TOMORROW...THE LACK OF HIGHER RAINFALL TOTALS
TODAY DAMPENS THE OVERALL FLASH FLOODING THREAT. RAINFALL TOTALS
THROUGH SATURDAY HAVE BEEN TEMPERED A BIT...AND ARE MORE IN THE
1-2 INCH RANGE. A LOT OF THE AREA CAN TAKE THIS TYPE OF RAINFALL.
OF COURSE LOCALLY HIGHER AMOUNTS WILL BE POSSIBLE...BUT FEEL THE
OVERALL THREAT IS REDUCED COMPARED TO THE THINKING IN THE PAST 24
HOURS.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 329 PM CDT THU AUG 28 2014

THE MAIN FOCUS IN THE LONG TERM IS CENTERED ON THE THREAT FOR
SEVERE WEATHER SUNDAY NIGHT INTO MONDAY. WITH THE HOLIDAY WEEKEND
APPROACHING...AND WITH MANY FOLKS PLANNING OUTDOOR ACTIVITIES...IT
IS THIS PERIOD THAT WARRANTS CLOSE WATCHING.

IN THE UPPER LEVELS...THE 28.12Z GUIDANCE FOLLOWED THE PAST
SEVERAL MODEL CYCLES IN SHOWING A DIGGING WESTERN CONUS LONGWAVE
TROUGH APPROACHING THE NORTHERN PLAINS BY SUNDAY EVENING. 250MB
JET DYNAMICS PUTS THE AREA IN A FAVORABLE RIGHT ENTRANCE
REGION...AND THIS JET STRENGTHENS BETWEEN 00-06Z MONDAY.

AT THE SURFACE...A COLD FRONT LOOKS LIKELY TO MOVE ACROSS
MINNESOTA LATE SUNDAY NIGHT...REACHING THE MISSISSIPPI RIVER EARLY
MONDAY MORNING. MUCAPE RANGES FROM 1500 TO 2000 J/KG ACROSS THE
REGION LATE SUNDAY...WITH HEALTHY PROFILES AS NCAPE VALUES
APPROACH 0.15. THIS IS AIDED BY 700-500MB LAPSE RATES AROUND
7.5C/KM. THE SHEAR ENVIRONMENT IS A BIT MUDDLED BETWEEN THE
VARIOUS GUIDANCE...BUT OVERALL 0-3KM SHEAR VALUES RANGE FROM 20 TO
30 KTS. HODOGRAPHS SHOW SOME CURVATURE BUT A LOT OF THE SHEAR IS
SPEED RELATED. OVERALL...THE CURRENT TIMING WOULD SUGGEST
CONVECTION TO DEVELOP IN THE AFTERNOON ACROSS THE EASTERN DAKOTAS
/ WESTERN INTO CENTRAL MINNESOTA...POSSIBLY AS SUPERCELLS...THEN
MERGE IN LINE SEGMENTS TOWARD THE WESTERN CWA IF MAINTAINED.
THERE ARE STRONG MOISTURE TRANSPORT SIGNALS WITH HIGHER PWATS
RETURNING...SO THERE COULD BE SOME HEALTHY RAIN PRODUCERS. THE
MAIN LIMITING FACTOR WOULD BE STORM SPEED...WHICH SHOULD BE RATHER
PROGRESSIVE. REGARDLESS...MAIN THREATS LOOK TO BE HEAVY
RAIN...DAMAGING WINDS...AND LARGE HAIL. AT THIS TIME...GIVEN
TIMING...WEST OF THE MS RIVER WOULD BE MORE FAVORABLE AND THIS
LINES UP WELL WITH THE CURRENT DAY 4 OUTLOOK FROM THE SPC.
OVERALL...WILL NEED TO WATCH THIS PERIOD.

THE FRONT SLIDES INTO WESTERN WISCONSIN MONDAY MORNING...WHICH
MAY ALLOW FOR ANOTHER ROUND OF DESTABILIZATION AND STRONG STORMS
FOR OUR EASTERN AREAS MONDAY AFTERNOON. THAT IS A BIT MORE
UNCERTAIN BUT IF THE SYSTEM SLOWS DOWN...WHICH SEEMS TO BE A
TREND...THIS PERIOD MAY NEED TO BE WATCHED CLOSER.

THE TUESDAY-THURSDAY PERIOD LOOKS TO FEATURE ZONAL UPPER LEVEL
FLOW WITH THE POSSIBILITY OF A FEW RIPPLES SLIDING THROUGH. NOT
MUCH CONSISTENCY IN THE MODEL RUNS TO ADD ANY DETAILS AT THIS
POINT. LOTS OF WEATHER TO GET THROUGH UNTIL THEN. SLIGHT CHANCES
WITH SEASONABLE TEMPERATURES LOOKS GOOD AT THIS POINT.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1225 PM CDT THU AUG 28 2014

TREND OF CONDITIONS IS DOWNWARD THIS PERIOD AS A VERY MOIST AIRMASS
AND A COUPLE ROUNDS OF FORCING/LIFT SPREAD ACROSS MN/IA/WI WITH IT.
FIRST ROUND OF -RA WAS SPREADING NORTHEAST ACROSS THE AREA LATE THIS
MORNING/EARLY THIS AFTERNOON...AND MARKED THE LEADING EDGE OF LOWER
VFR OR MVFR CIGS ALONG WITH SOME MVFR VSBYS IN -RA/-SHRA.
INSTABILITY HAS BEEN LIMITED SO FAR...THUS NOT MUCH FOR TSRA/CB
EXPECTED AT THE TAF SITES THRU THIS AFTERNOON/TONIGHT. IFR CIGS WERE
DEVELOPING ALONG/WEST OF I-35 ACROSS SOUTHERN MN/NORTHERN IA...
BEHIND THE LEADING BAND OF -RA. THESE LOW CLOUDS/CIGS EXPECTED TO
SPREAD EAST/NORTHEAST ACROSS THE TAF SITES TONIGHT AND PERSIST
THROUGH AT LEAST FRI MORNING...ALONG WITH SOME PERSISTENT -SHRA. MAY
YET NEED A PERIOD OR 2 OF TSRA AT EITHER KRST OR KLSE BETWEEN 00Z
AND 18Z...BUT CONFIDENCE IN TIMING IS LOW AT THIS POINT AND DID NOT
WANT TO PUT IN A LONGER PERIOD OF VCTS/CB IN THE TAFS FOR NOW.


&&

.HYDROLOGY...THROUGH SATURDAY MORNING
ISSUED AT 329 PM CDT THU AUG 28 2014

WITH LESS THAN EXPECTED RAINFALL TODAY...OVERALL TOTALS THROUGH
SATURDAY HAVE BEEN REDUCED TO THE 1.5 TO 2.5 INCH RANGE. THE SURGE
IN EFFICIENT RAIN PRODUCING ATMOSPHERIC CONDITIONS...CHARACTERIZED
BY WARM CLOUD DEPTHS 3.5 TO 4.0 KM...PWATS 1.5 TO 2.0 INCHES...AND
LOWER 70F DEWPOINTS...WILL REMAIN IN PLACE THROUGH EARLY SATURDAY.
OVERALL...THE THREAT FOR EXCESSIVE RAINFALL HAS BEEN DIMINISHED A
BIT AS THE ROUNDS OF STORMS LOOK PROGRESSIVE ENOUGH AND MUCH OF
THE AREA CAN HANDLE THE EXPECTED AMOUNTS. AS SUCH...WILL CONTINUE
THE HYDROLOGIC OUTLOOK /ESFARX/

&&

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

$$

SHORT TERM...ZT
LONG TERM....ZT
AVIATION.....RRS
HYDROLOGY....ZT


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