Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 070002
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
702 PM CDT WED MAY 6 2015

.SHORT TERM...(TONIGHT THRU THURSDAY NIGHT)
ISSUED AT 300 PM CDT WED MAY 6 2015

MAIN FCST CONCERNS THIS PERIOD...SHRA/TSRA CHANCES AND TEMPERATURES
THRU THE PERIOD.

DATA ANALYSIS AT 18Z HAD A 1000MB LOW OVER EASTERN CO WHILE HIGH
PRESSURE WAS LOCATED OVER THE GREAT LAKES. NUMEROUS BANDS OF SHRA
AND FEW TSRA OVER THE ROCKIES AND CENTRAL/NORTHERN PLAINS...
PINWHEELING AROUND THE CO LOW AND A BROAD MID LEVEL LOW NEAR THE CO/
NEB BORDER. WARM FRONT AHEAD OF THE LOW WAS LIFTING NORTH INTO THE
REGION. GENERAL DECREASE OF CLOUDS LIFTING NORTH WITH THE
FRONT...WITH SB CAPES ACROSS MUCH OF IA EARLY THIS AFTERNOON IN THE
500-1K J/KG RANGE. BAND OF SHRA JUST AHEAD OF THE FRONT FROM
SOUTHWEST MN TO SOUTHEAST IA LIFTING NORTH WITH IT.

NO PROBLEMS NOTED WITH 06.12Z MODEL INITIALIZATIONS. SOLUTIONS OFFER
A TIGHTENING CONSENSUS OF THEIR EARLIER RUNS AS A VIGOROUS MID LEVEL
LOW OVER NORTHEAST CO/WESTERN NEB LIFTS NORTHEAST AND OPENS UP
TONIGHT. CONSENSUS IMPROVING AS IT THEN PASSES ACROSS SD AND
NORTHWEST MN INTO ONT AS AN OPEN WAVE THU/THU NIGHT. TREND TENDS TO
FAVOR FASTER OF THE EARLIER RUNS WITH THE OPEN WAVE BY THU/THU
NIGHT. SHORT-TERM FCST CONFIDENCE IS GENERALLY GOOD THIS CYCLE.

FOR THE SHORT TERM...LARGER SCALE/SYNOPTICS QUITE SIMILAR AS THE CO/
NEB TROUGH LIFTS NORTHEAST THRU THE PERIOD. HOWEVER...GIVEN
CONVECTION THE SENSIBLE WEATHER IN THE SHORT TERM WILL BE DRIVEN BY
THE SMALLER SCALE DETAILS IN THE FLOW AND LOWER LEVELS. ONE
SHORTWAVE OVER EASTERN KS EAST OF THE MID LEVEL LOW WILL LIFT NORTH
LATE THIS AFTERNOON/TONIGHT. A VORT LOBE WITH THIS FEATURE WILL
ROTATE NORTH-NORTHEAST ACROSS IA/MN. THIS WILL BE COMBINATION WITH
THE WARM FRONT LIFTING NORTH ACROSS MN/IA/WI...INCREASING 850-700MB
MOISTURE TRANSPORT/Q-G CONVERGENCE...PW VALUES APPROACHING 1.5
INCHES AND MU CAPES OF 500-1000 J/KG. A LINE OF SHRA/TSRA IS
EXPECTED TO ROTATE INTO/ACROSS THE WEST HALF OF FCST AREA LATE THIS
AFTERNOON/EVENING...WEAKENING LATER THIS EVENING AS IT PUSHES EAST
ACROSS THE MS RIVER...CAPE WANES AND IT MOVES AWAY FROM THE MID
LEVEL SUPPORT. CARRIED HIGHEST SHRA/TSRA CHANCES WEST OF THE MS
RIVER EARLY THIS EVENING. ANOTHER ROUND OF STRONGER LOW LEVEL
MOISTURE TRANSPORT SPREADS INTO/ACROSS THE FCST AREA LATE TONIGHT/
THU...AIDED BY ANOTHER SHORTWAVE AND THE MID LEVEL TROUGH AXIS
ROTATING BY. AFTER A RELATIVE LULL IN THE RAIN CHANCES NEAR/AFTER
MIDNIGHT...SHRA/TSRA CHANCES HEAD BACK UP TOWARD MORNING AND FOR
THU...THEN TREND DOWNWARD THU NIGHT WITH THE MAIN MOISTURE AXIS/
MOISTURE TRANSPORT PUSHING EAST OF THE AREA. MU CAPE PROGGED TO
INCREASE INTO THE 500 TO AS MUCH AS 2K J/KG RANGE THU...BUT WITH
POTENTIAL FOR SHRA/TSRA TO SPREAD ACROSS THE AREA IN THE MORNING NOT
SURE HOW MUCH OF THIS CAPE MAY BE REALIZED. SOME DIFFERENCE IN THE
TIMING OF THE FORCING FEATURES AND THE SFC FRONT BY LATER THU/THU
NIGHT...SO WILL LEAVE THE SHRA/TSRA CHANCES FOR THU AFTERNOON/NIGHT
MORE GENERAL. TIGHTER PRESSURE GRADIENT AHEAD OF THE LOW IN THE
PLAINS TODAY IS OVER MUCH OF MN/IA/WI ON THU...FOR SOME BRISK/GUSTY
SOUTH WINDS.

WARMER MORE MOIST AIRMASS/CLOUDS OVER THE WILL KEEP LOWS ON THE MILD
SIDE TONIGHT/THU NIGHT. HIGHS THU WILL DEPEND ON TIMING OF
CONVECTION ACROSS THE AREA. IF THU AFTERNOON ENDS UP MORE WET THAN
DRY...CONSENSUS HIGHS IN THE MID 70S TO NEAR 80 MAY BE 5-10F TOO
WARM. STAYED WITH THE CONSENSUS OF LOWS/HIGHS FOR TONIGHT THRU THU
NIGHT.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY) ISSUED AT 300 PM CDT WED MAY
6 2015

MAIN FCST CONCERNS FOR FRIDAY THRU SATURDAY NIGHT...SHRA/TSRA
CHANCES MAINLY FRI/FRI NIGHT...TEMPERATURES.

06.12Z MODELS IN GOOD AGREEMENT FOR BRISK SOUTHWEST FLOW ALOFT OVER
THE CENTRAL CONUS FRI THRU SAT...AHEAD OF ANOTHER MID LEVEL LOW OVER
THE SOUTHERN ROCKIES AND TROUGHING ACROSS CENTRAL CAN. HOWEVER...
DETAIL DIFFERENCES WITH SHORTWAVES RIPPLING THRU THIS SOUTH FLOW.
IMPROVING CONSENSUS FOR THE ROCKIES LOW TO BE SITTING OVER NORTHEAST
CO/SOUTHWEST NEB BY 12Z SUN...VERY SIMILAR TO THIS MORNING. TREND AT
12Z SUN IS TOWARD STRONGER OF THE EARLIER RUNS AND A COMPROMISE ON
POSITION OF THE MID LEVEL LOW EJECTING INTO THE PLAINS. FRI THRU SAT
NIGHT SENSIBLE WEATHER ALSO DEPENDS ON SMALLER SCALE/LOW LEVEL
DETAILS THUS FCST CONFIDENCE THIS PERIOD IS GENERALLY AVERAGE.

WOULD NORMALLY THINK FRI/FRI NIGHT WOULD BE A DRY/QUIET PERIOD WITH
DRYING BEHIND A RATHER VIGOROUS DEPARTING SYSTEM. HOWEVER...
SHORTWAVES RIPPING THRU THE MDT/STRONG SOUTHWEST FLOW ALOFT LOOK TO
STALL THE SFC FRONTAL BOUNDARY/TROUGH JUST SOUTHEAST OF THE FCST
AREA FOR FRI/FRI NIGHT. SOME CONSENSUS FOR A STRONGER SHORTWAVE TO
RIPPLE THRU THE FLOW AND ACROSS THE AREA LATER FRI/FRI EVENING. WITH
THE LOWER LEVEL BOUNDARY CLOSE AND 1 TO 1.5 INCHES OF PW NEAR/JUST
SOUTH OF THE BOUNDARY...CONTINUING SHRA/TSRA CHANCES FRI/FRI NIGHT
MAINLY ACROSS THE SOUTHEAST HALF OF THE FCST AREA REASONABLE. BEST
CHANCE FOR A DRY DAY LOOKS TO BE SAT WITH A SOUTHWARD PUSH OF THE
SFC BOUNDARY AND WEAK HIGH PRESSURE RIDGING INTO MN/WI. GIVEN MODEL
INCONSISTENCIES ON SHORTWAVES IN THE SOUTHWEST FLOW ON SAT ADDED A
SMALL -SHRA CHANCE ACROSS THE SOUTHEAST HALF OF THE FCST AREA...
WHICH BLENDS WELL WITH NEIGHBORING GRIDS. SFC THRU MID LEVEL SYSTEM
EJECTING INTO THE CENTRAL PLAINS SAT NIGHT QUICKLY BEGINS TO SPREAD
INCREASING MOISTURE/LIFT NORTHEAST ACROSS THE UPPER MIDWEST. SHRA/
TSRA CHANCES QUICKLY SPREAD BACK INTO THE FCST AREA AS WELL SAT
NIGHT...HIGHEST ACROSS THE WEST/SOUTH ENDS OF THE FCST AREA AFTER
MIDNIGHT. AS SHORTWAVE DETAILS BECOME CLEARER...LATER CYCLES WILL BE
ABLE TO ADD MORE TIMING DETAIL TO THE FRI THRU SAT NIGHT SHRA/TSRA
CHANCES. USED A BLEND OF THE GUIDANCE HIGHS/LOWS FOR FRI THRU SAT
NIGHT...THOUGH MUCH LIKE WED/THU...IF FRI/SAT AFTERNOONS ARE SUNNIER
THAN EXPECTED...HIGHS COULD EASILY BE A CATEGORY OR 2 WARMER THAN
THOSE IN THE FCST GRIDS.

FOR SUNDAY THRU WEDNESDAY...DAYS 4 TO 7...

MAIN FCST CONCERNS THIS PERIOD...SHRA/TSRA CHANCES SUN THRU MON...
SHRA CHANCES/COOLER TEMPERATURES TUE/WED.

MEDIUM RANGE MODEL RUNS OF 06.00Z/06.12Z IN GOOD AND IMPROVING
AGREEMENT SUN/SUN NIGHT ON THE MID LEVEL LOW/TROUGH TO LIFT SLOWLY
NORTH THRU THE PLAINS. TREND IS MORE MID LEVEL LOW/SLOWER WITH THIS
FEATURE THRU SUN NIGHT. IMPROVING AGREEMENT FOR THIS LOW/TROUGH TO
OPEN UP...TURN EAST AND MOVE INTO THE UPPER MIDWEST MON/MON NIGHT.
06.12Z MODELS IN GOOD AGREEMENT FOR TROUGHING TO LINGER ACROSS THE
WESTERN GREAT LAKES THRU AT LEAST WED. GIVEN THE BETTER GFS/ECMWF
AGREEMENT THRU WED...FCST CONFIDENCE IN THE DAY 4-7 PERIOD IS
AVERAGE THIS CYCLE.

SFC THRU MID LEVEL LOW/TROUGH LIFTING THRU THE PLAINS CONTINUES TO
SPREADS MORE MOISTURE/LIFT NORTHEAST INTO THE AREA THRU SUN/SUN
NIGHT. MU CAPE ON THE ORDER OF 500-1000 J/KG SPREADS NORTH INTO THE
FCST AREA BY LATER SUN AND FOR SUN NIGHT...WITH POTENTIAL FOR SOME
CAPE TO REMAIN OVER THE AREA INTO MON DEPENDING ON DETAILS OF WHERE
THE SFC LOW ENDS UP MON. GIVEN THE DEEPER/INCREASING FORCING/LIFT
AHEAD OF THE APPROACHING SFC-MID LEVEL LOW...SHRA/TSRA CHANCES
INCREASING TO 60-70 PERCENT CENTERED ON SUN NIGHT APPEAR QUITE
REASONABLE. SFC LOW WOULD PASS WITH LOSS OF CAPE BY MON NIGHT...BUT
DEEP CYCLONIC FLOW...DEFORMATION BAND MOISTURE AND MID LEVEL TROUGH
AXIS LOOKING TO REMAIN OVER THE AREA INTO TUE. DIMINISHING SHRA
CHANCES MON NIGHT/TUE AS THE MID LEVEL LOW/TROUGH MOVES SLOWLY
EAST OF THE REGION LOOK GOOD. TUE NIGHT TRENDING TO BE A DRY
PERIOD AS CAN HIGH PRESSURE WOULD DRIFT ACROSS THE REGION. SOME
SIGNAL FOR A WEAK NORTHWEST FLOW SHORTWAVE TO DROP INTO THE AREA
WED. THIS WITH SOME LINGERING LOWER LEVEL MOISTURE. SMALL -SHRA
CHANCE WED OKAY AT THIS TIME. PASSING SFC-MID LEVEL TROUGH MON
NIGHT SPREADS SOME COLDER AIR INTO THE UPPER MIDWEST. NAEFS
ENSEMBLE MEAN WITH 850MB TEMPS 1 TO 2 STANDARD DEVIATIONS BELOW
NORMAL FOR MON NIGHT INTO WED. HIGHS/LOWS FOR MON-WED TRENDED
BELOW TO MUCH BELOW NORMAL. WITH UNCERTAINTIES ON CLOUDS AND SHRA/
TSRA TIMING MUCH OF THE PERIOD...STAYED WITH THE MODEL/ENSEMBLE
CONSENSUS OF HIGH/LOWS FOR SUN-WED. WITH MORE SUNSHINE THAN CLOUDS
ANY OF DAYS 4 TO 7...HIGHS COULD BE A CATEGORY OR TWO WARMER THAN
THOSE IN THE FCST GRIDS.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)
ISSUED AT 702 PM CDT WED MAY 6 2015

PERIODS OF SHOWERS AND THUNDERSTORMS EARLY THIS EVENING AT THE
TAF SITES WILL TAPER OFF DURING THE LATE EVENING HOURS WITH VFR
CONDITIONS EXPECTED DURING THE OVERNIGHT. LLWS IS EXPECTED AT BOTH
TAF SITES STARTING AROUND 09Z AND PERSISTING  THROUGH AROUND 14Z
THURSDAY MORNING. STRONG SOUTH WINDS AND PERIODS OF SHOWERS AND
STORMS ARE EXPECTED LATE THURSDAY MORNING INTO THURSDAY AFTERNOON
WITH IFR CONDITIONS LOOKING LIKEY IN THE STORMS.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...WETENKAMP


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