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000
FXUS63 KFSD 202355
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
655 PM CDT SUN APR 20 2014

.SHORT TERM...(THIS EVENING THROUGH MONDAY)
ISSUED AT 406 PM CDT SUN APR 20 2014

ELEVATED BAND OF SHOWERS AND ISOLATED THUNDERSTORMS DEVELOPING IN
NORTHEAST NEBRASKA THIS AFTERNOON AND GRADUALLY WORKING/DEVELOPING
NORTHEAST. NOT ANTICIPATING A WHOLE LOT OF ACTIVITY FROM THIS
CONVECTION BUT SHOULD PRODUCE SOME ISOLATED TO SCATTERED SHOWERS
WITH A FEW EMBEDDED THUNDERSTORMS. ANOTHER AREA LIKELY TO PRODUCE
ISOLATED ACTIVITY WILL BE IN CENTRAL SD NEAR AN INCOMING WEAK
CONVERGENT BOUNDARY. UPPER LEVEL SUPPORT NOT TOO GOOD AND DRY LOW
LEVELS SUGGEST THAT MAYBE NOTHING WILL GET GOING BUT STILL WORTH AN
ISOLATED THREAT AT THIS POINT.

THE MAIN AREA TO FOCUS ON WILL AGAIN BE NORTHWEST IOWA WHERE THERE
IS A BETTER COMBINATION OF UPPER LEVEL FORCING...INSTABILITY AND AND
LOW TO MID LEVEL FRONT. WILL KEEP LIKELIES GOING AND ALSO THE IDEA
THAT SEVERE THUNDERSTORMS WILL NOT BE A THREAT. CAPE VALUES MAY
APPROACH 500 TO 1000 J/KG THIS EVENING BUT WITH THE MODELS OVERDOING
THE LOW LEVEL MOISTURE THOSE VALUES COULD BE A BIT PESSIMISTIC. THE
BETTER CHANCES FOR PRECIPITATION SHOULD SHIFT SOUTHEAST OF THE AREA
LATE TONIGHT AS THE UPPER LEVEL WAVE TO THE NORTH BRINGS IN DRIER
AND MORE STABLE AIR. LOWS LIKELY TO RANGE FROM THE MID 40S IN THE
NORTHERN CWA TO THE MID 50S IN PARTS OF NORTHWEST IOWA.

THIS DRIER AND MORE STABLE AIR MASS WILL ALSO COME WITH A WELL MIXED
ATMOSPHERE AND DECENT PRESSURE GRADIENT WHICH WILL LEAD TO A WINDY
MONDAY IN MOST LOCATIONS. RECENT TRENDS IN GUIDANCE HAS BEEN TO
FORECAST DEW POINT TEMPERATURES TOO HIGH...ESPECIALLY IN A WELL
MIXED ENVIRONMENT SO WENT MUCH CLOSER TO THE BIAS CORRECTED OUTPUT
WHICH HAS BEEN HANDLING OUR DRY GROUND BETTER AS THE MODELS SEEM TO
BE HAVING TROUBLE WITH OUR DRIER THAN NORMAL BOUNDARY LAYER.
SO...BECAUSE OF THE LOW DEW POINTS AND HIGH WINDS WENT AHEAD AND
ISSUED A FIRE WEATHER WATCH FOR MOST OF THE AREA AS AFTERNOON
HUMIDITY VALUES LIKELY TO FALL BELOW 25 PERCENT IN MANY LOCATIONS AS
HIGHS CLIMB INTO THE UPPER 60S TO LOWER 70S.

.LONG TERM...(MONDAY NIGHT THROUGH SUNDAY)
ISSUED AT 406 PM CDT SUN APR 20 2014

HIGH PRESSURE WILL CONTINUE TO EASE SOUTHWARD INTO THE REGION ON
MONDAY NIGHT. DIMINISHING WINDS AND MAINLY CLEAR SKIES...ALONG WITH
A DRIER AIRMASS SHOULD LEAD TO LOWS TOWARD THE LOWER END OF THE
FORECAST ENVELOPE...MAINLY MID TO UPPER 30S.

BY TUESDAY...UPPER LEVEL RIDGING WILL BUILD EASTWARD ACROSS THE AREA
ALONG WITH AN EASTWARD RETURN OF A FAIRLY STRONG MID LEVEL FRONTAL
ZONE. MOISTURE IS PRETTY ANEMIC THROUGH THE DAY AT LOWER LEVELS...
BUT ELEVATED PARCELS AROUND 700 HPA OR A BIT HIGHER DO INDICATE
SOME CONVECTIVE INSTABILITY. HAVE NOT INCLUDED ANY PRECIPITATION
WORKING SLOWLY EASTWARD ACROSS THE CWA...BUT WOULD CERTAINLY THINK
AT LEAST A COUPLE OF SPRINKLES WOULD ACCOMPANY THE THICKENING MID
CLOUDS THROUGH THE DAY TOWARD AT LEAST I29 BY LATE DAY. SHADED
DEWPOINT FORECAST TOWARD DRIER RAW PROFILES...AS HAS BEEN THE CASE
OF LATE.

STRONG LOW LEVEL JET OF 50 TO 60 KNOTS DEVELOPS ON TUESDAY NIGHT
WITH MOISTURE SURGE WORKING RAPIDLY NORTHWARD FROM THE SOUTHERN
HIGH PLAINS. FAIRLY GOOD WAVE MOVING THROUGH RIDGE DURING THE
NIGHT...MAIN CONVERGENCE WITH LOW LEVEL JET ESTABLISHES
NORTH...BUT CONTINUED MID LEVEL INSTABILITY WITH 8-9C/KM AND
INCREASING MOISTURE SUGGESTS A BETTER COVERAGE OF SHOWERS AND
ISOLATED THUNDER WITH TIME OVERNIGHT. WINDS WILL ALSO BECOME AN
ISSUE LATER IN THE NIGHT...AND COULD GET SOMEWHAT CLOSE TO WIND
ADVISORY NEAR AND WEST OF I 29...AND EXPANDING OVER MUCH OF THE
AREA DURING THE DAY ON WEDNESDAY.

LAPSE RATES REMAIN STEEP INTO THE DAYTIME...BUT GRADUALLY MODIFY A
BIT MIDDAY WITH WARMER LAYER ALOFT BUILDING IN...WHICH COULD WORK
TO SLOW UP OR EVEN END CONVECTION FROM SOUTHWEST TO NORTHEAST DURING
THE MORNING AND LINGERING INTO THE AFTERNOON. SHOULD KEEP A FAIR
AMOUNT OF CLOUDS AROUND WITH STRONG LOW LEVEL JET...ESPECIALLY
ACROSS SOUTHWEST MINNESOTA INTO NORTHWEST IOWA. IF IT BREAKS
OUT... COULD BE APPRECIABLY WARMER AND GENERATE MORE INSTABILITY
BY WEDNESDAY AFTERNOON...TO CORRELATE WITH APPROACH OF STRONGER
UPPER LIFT FORCING AND CONVERGENT BOUNDARY TOWARD THE JAMES VALLEY
BY LATER IN THE DAY. WOULD CERTAINLY MAINTAIN AT LEAST A SMALL
SEVERE WEATHER THREAT GIVEN THE SHEAR AND INSTABILITY
POTENTIAL...ESPECIALLY FROM THE LOWER JAMES AND BIG SIOUX VALLEY
EASTWARD. TROUGH WILL LIFT PAST WEDNESDAY NIGHT...WITH DYNAMICS
SUPPORTIVE OF HIGHER POPS EAST OF THE JAMES RIVER...AND ESPECIALLY
ACROSS NORTHWEST IOWA. PRECIP THREAT GRADUALLY TAPERING OFF WEST
TO EAST OVERNIGHT...LINGERING IN THE FAR EASTERN CWA INTO THURSDAY
MORNING. UPPER LOW CONTINUES TO DEEPEN AND CLOSE OFF NORTHEAST
OF THE AREA ON THURSDAY...PLACING AREA IN A MORE SUBSIDENT FAVORED
QUADRANT WITH MOISTURE DISPLACED TO THE NORTH/EAST. HAVE TRIMMED
BACK POPS SOMEWHAT...ESPECIALLY THROUGH THE MISSOURI VALLEY...AND
TAKEN TEMPS CLOSER THE 850 HPA MIXING OF DRIER SOLUTIONS.

HANDLING OF THE INTENSE UPPER TROUGH...AND NEXT ONE LATER IN THE
WEEKEND...IS WIDELY VARIED BETWEEN MODELS AND LEADS TO QUITE LOW
CONFIDENCE IN THE OVERALL PATTERN FROM FRIDAY THROUGH SUNDAY.
COULD EVEN FIND THREAT FOR A LITTLE SNOWFALL WITHOUT STRAYING TOO
FAR FROM A COUPLE SOLUTIONS...BUT WAY TOO FAR OUT TO PUT ANY FAITH
IN THE FORECAST. DOES APPEAR AS IF IT WILL BE AN ACTIVE TIME LATER
IN THE WEEKEND...AND WITH THE ABUNDANCE OF CLOUDS AND OR COOLER
AIR IN PLACE...HAVE KEPT NEAR TO A BIT ON THE COOLER SIDE OF
GUIDANCE FOR THE WEEKEND.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z MONDAY EVENING)
ISSUED AT 653 PM CDT SUN APR 20 2014

VFR WILL PREVAIL TONIGHT THROUGH MONDAY. THERE WILL BE
THUNDERSTORMS IN NORTHWEST IOWA...THE QUESTION IS HOW LONG THEY
WILL LAST. THEY COULD LAST OFF AND ON FOR A BULK OF THE NIGHT. BUT
FOR THE KSUX TAF SITE...OPTED TO ONLY INCLUDE TWO HOURS OF TSRA
EARLY THIS EVENING COINCIDING WITH THE DAYTIME HEATING...THEN GAVE
THEM A BREAK. CONTINUED TO INCLUDE JUST SHOWERS FOR NOW LATER
TONIGHT AT KSUX WHEN THE SURFACE WIND SHIFT/COLD FRONT COMES
THROUGH. BUT ADMITTEDLY THERE IS ENOUGH ELEVATED INSTABILITY THAT
THERE COULD BE SOME ISOLATED TSRA IN THAT AREA AGAIN SHORTLY AFTER
MIDNIGHT. OTHERWISE...ALSO INCLUDED A BRIEF WINDOW OF LIGHT TSRA
AT THE KHON TAF SITE LATE THIS EVENING COINCIDING WITH THE WIND
SHIFT LINE...AS THERE IS SOME ELEVATED THUNDERSTORMS IN CENTRAL SD
RIGHT NOW. IF THEY DO NOT HOLD TOGETHER AFTER SUNSET THOUGH...WILL
UPDATE THE KHON TAF AND ELIMINATE THEM. FINALLY FOR KFSD...HAVE SHOWERS
IN OVERNIGHT BUT NOT THUNDER...AGAIN COINCIDING WITH THE WIND
SHIFT. ACTIVITY SHOULD BE LESS CONVECTIVE BY THEN.

&&

.FIRE WEATHER...
ISSUED AT 406 PM CDT SUN APR 20 2014

A STRONG NORTHWEST WIND WILL DEVELOP THROUGH THE MORNING ON MONDAY
IN A WELL MIXED ATMOSPHERE...LEADING TO DANGEROUS FIRE WEATHER
CONDITIONS. AFTERNOON RELATIVE HUMIDITIES WILL LIKELY DROP TO
BETWEEN 20 AND 25 PERCENT OF THE ENTIRE AREA WITH WINDS AVERAGING
ABOUT 20 TO 30 MPH AND A FEW GUSTS AROUND 35 MPH. SOUTH CENTRAL SD
WILL SEE WINDS MORE IN LINE WITH 15 TO 25 MPH AND WITH EXPECTED
RELATIVE HUMIDITY VALUES ABOVE 20 PERCENT A VERY HIGH FIRE DANGER
WILL BE MAINTAINED. THIS MOST DANGEROUS FIRE WEATHER CONDITIONS
SHOULD BE FROM ABOUT 100 PM TO 700 PM ON MONDAY. DEW POINTS MAY END
UP JUST A BIT HIGHER BUT THE TRENDS SO FAR THIS SPRING HAVE BEEN FOR
THE GUIDANCE TO OVER FORECAST DEW POINTS BY ABOUT 3 TO 6 DEGREES SO
TOOK THIS INTO ACCOUNT.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...FIRE WEATHER WATCH FROM MONDAY AFTERNOON THROUGH MONDAY EVENING
     FOR SDZ256>258.

MN...FIRE WEATHER WATCH FROM MONDAY AFTERNOON THROUGH MONDAY EVENING
     FOR MNZ900.

IA...FIRE WEATHER WATCH FROM MONDAY AFTERNOON THROUGH MONDAY EVENING
     FOR IAZ300-301.

NE...FIRE WEATHER WATCH FROM MONDAY AFTERNOON THROUGH MONDAY EVENING
     FOR NEZ249.

&&

$$

SHORT TERM...08
LONG TERM...CHAPMAN
AVIATION...MJ
FIRE WEATHER...08