000
FXUS63 KFSD 121745
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1245 PM CDT SUN MAY 12 2013

.UPDATE...
ISSUED AT 927 AM CDT SUN MAY 12 2013

FREEZE AND FROST HIGHLIGHTS HAVE EXPIRED AND THE WARMING
CONTINUES. LITTLE FURTHER PROBLEM FOR TODAY BESIDES FINE TUNING
TEMPERATURES.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 411 AM CDT SUN MAY 12 2013

A BENIGN WEATHER DAY IN STORE AS A SURFACE RIDGE AXIS SLIDES
SOUTHWARD ACROSS THE REGION. THIS WILL BRING MAINLY CLEAR SKIES AND
LIGHT WINDS THROUGH THE DAY. STAYED ON THE LOW SIDE OF GUIDANCE WITH
REGARD TO DEW POINTS THIS AFTERNOON...WITH GOOD MIXING BRINGING VERY
DRY AIR ALOFT DOWN TO THE SURFACE. MAIN IMPACT OF THIS WOULD BE FIRE
WEATHER CONCERNS...THOUGH THE VERY LIGHT WINDS WILL COUNTER AN
ENHANCED FIRE DANGER. MODEL 850 MB THERMAL PROFILES WARM A GOOD 4
TO 6 DEGREES OVER YESTERDAY...SO HIGHS IN THE LOWER 60S EAST TO
AROUND 70 WEST LOOKS APPROPRIATE FOR THIS AFTERNOON.

NOT A LOT GOING TONIGHT EXCEPT THE SOUTHERLY FLOW BECOMES A BIT
STRONGER. NOTING THIS...MANY GUIDANCE TEMPERATURES STILL LOOKED
TOO COOL SO TRENDED TOWARD THE ECMWF...ESPECIALLY ALONG AND WEST
OF INTERSTATE 29.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 411 AM CDT SUN MAY 12 2013

ON MONDAY...UPPER LEVEL RIDGE OF HIGH PRESSURE MOVES EASTWARD FROM
THE SPINE OF THE ROCKIES TO THE WESTERN PLAINS. AHEAD OF THIS
RIDGE...A WEAK SHORT WAVE MOVES DOWN THE NORTHWEST FLOW ALOFT WITH
A SURFACE TROUGH OF LOW PRESSURE PROGRESSING TO NEAR I 29...THEN
BASICALLY STALLING AND DYING OFF. STRONG MIXING WILL BE IN STORE
ON MONDAY AND 850MB CONTINUE TO BE QUITE WARM. WITH A WESTERLY
COMPONENT TO THE SURFACE WINDS FOR AREAS ALONG AND WEST OF I 29...
CONTINUE TO FEEL THAT MANY GUIDANCE VALUES HAVE NOT CAPTURED THE
DEGREE OF WARMING WHICH WILL LIKELY OCCUR. THEREFORE KEPT THE
PREVIOUS FORECAST GOING OF VERY WARM TEMPERATURES...BUT HIGHS
SHOULD STAY A BIT CLOSER TO CONSENSUS GUIDANCE VALUES IN OUR FAR
EASTERN ZONES. AS THE STRONGER WINDS OFF THE SURFACE AND TIGHT
PRESSURE GRADIENT MOVE EASTWARD THROUGH THE DAY...IT WILL LIKELY BE
BREEZY TO WINDY IN THE AFTERNOON ACROSS NORTHWEST IA AND THE
SOUTHERN PARTS OF SOUTHWEST MN...ENHANCING FIRE DANGER DETAILED
BELOW.

WITH THE DYING SURFACE TROUGH...WIND WILL REMAIN SOUTHERLY MONDAY
NIGHT KEEPING TEMPERATURES QUITE MILD. BUMPED UP LOWS A COUPLE OF
DEGREES AS WIND SPEEDS LOOK TO INCREASE THROUGHOUT THE FORECAST
AREA AFTER MIDNIGHT. A WELL ADVERTISED COLD FRONT THEN MOVES
QUICKLY ACROSS MUCH OF OUR FORECAST AREA ON TUESDAY. FIRE WEATHER
CONCERNS ARE ABUNDANT ON TUESDAY...AGAIN DETAILED BELOW. BUT
CERTAINLY THE AIR MASS WILL BE WELL MIXED AND DRY...PROBABLY BOTH
AHEAD AND CERTAINLY BEHIND THE WIND SHIFT. AT THIS TIME...THERE
APPEARS TO BE ENOUGH COOLING BEHIND THE COLD FRONT IN OUR
NORTHERN AND WESTERN ZONES THAT TEMPERATURES IN THOSE AREAS MAY
TOP OUT IN THE EARLY AFTERNOON...THEN BEGIN TO DECREASE A BIT.
AHEAD OF THE FRONT...VERY STRONG THERMAL RIDGING EDGES UP INTO
NORTHWEST IA FROM NEBRASKA. THEREFORE BROADENED THE HIGHS ON
TUESDAY FROM NORTHWEST TO SOUTHEAST...BY RELATIVELY COOLING OFF OUR
NORTHWEST ZONES DOWN INTO THE MID 80S...AND KEPT NORTHWEST IA IN
THE MID 90S. DEPENDING ON THE DEGREE OF MID AND UPPER LEVEL CLOUD
COVER IN NORTHWEST IA BY AFTERNOON...WHICH COULD BE FAIRLY
SUBSTANTIAL...THEY COULD GET EVEN WARMER.

SOME OF THE DETERMINISTIC MODELS CONTINUE TO PAINT A BIT OF
THUNDERSTORM ACTIVITY JUST ALONG AND BEHIND THE FRONT TUESDAY
NIGHT IN NORTHWEST IA. THIS ACTIVITY WOULD BE ALONG SOME WEAK MID
LEVEL CONVERGENCE. THEREFORE KEPT SOME LOW POPS IN FOR THAT
POSSIBILITY. MORE SIGNIFICANTLY...IT WILL BE SUBSTANTIALLY COOLER
TUESDAY NIGHT COMPARED WITH THE DEGREE OF WARMING WE WILL HAVE ON
MONDAY AND TUESDAY.

OTHERWISE THE LATE WEEK WILL JUST FEATURE A ZONAL UPPER FLOW AND
ONLY SPOTTY CHANCES FOR RAIN AT BEST. IT WILL BE COOLER...WITH
DAYTIME HIGHS NEAR NORMAL TO SLIGHTLY ABOVE NORMAL. WINDS LOOK TO
BE LIGHT. AS WHAT HAS BEEN SHOWN FOR QUITE SOME TIME...BOTH THE
ECMWF AND GFS CONTINUE TO POINT AT STRONG UPPER TROUGHING DIGGING
IN THE WESTERN U.S. BY EARLY IN THE WEEKEND...THEN MOVING EASTWARD
AS THE WEEKEND PROGRESSES. THEREFORE EXPECT THE WEATHER PATTERN TO
BECOME MORE ACTIVE...PROBABLY BEGINNING FRIDAY NIGHT IF WE CAN GET
A SHORT WAVE TO MOVE UP THE SOUTHWEST FLOW ALOFT. OTHERWISE THE
VARIOUS MODELS CONTINUE TO SHOW SUBSTANTIAL RAINFALL IN OUR
FORECAST AREA SATURDAY NIGHT AS THEY HAVE FOR QUITE SOME TIME NOW.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)
ISSUED AT 1245 PM CDT SUN MAY 12 2013

VFR THROUGH 13/18Z. ISOLATED TSRA THREAT NORTHERN PART OF AREA
WITH WARM FRONT AFTER 13/06Z IS NOT ZERO BUT STILL WAY TOO LOW TO
MENTION. LOW LEVEL WIND INCREASE AHEAD OF WARM FRONT COULD BRING
NON CONVECTIVE LLWS FAIRLY CLOSE TO MENTION THRESHOLD ROUGHLY 13
/09Z-15Z PERIOD. FOR THE MOMENT SEEMS TO NOT BE ENOUGH MAGNITUDE
BUT IT MAY HAVE TO BE LOOKED AT LATER AS THE VERTICAL RESOLUTION
OF THE PROBLEM IS LIKELY FINER THAN THE MODELS CAN HANDLE. SURFACE
WIND GUSTS SHOULD STAY BELOW 25 KNOTS BUT MAY GET CLOSE AT
THE END OF THE FORECAST AT 13/18Z...AND EXCEED THAT VALUE SHORTLY
AFTER THE FORECAST PERIOD.

&&

.FIRE WEATHER...
ISSUED AT 411 AM CDT SUN MAY 12 2013

MONDAY WILL SEE ELEVATED FIRE DANGER WITH THE GRASSLAND FIRE
DANGER INDEX REACHING THE HIGH AND VERY HIGH CATEGORIES. AT THIS
TIME...THE BEST CHANCE TO REACH VERY HIGH WILL BE ACROSS SOUTHWEST
MN...NORTHWEST IA...EXTREME NORTHEAST NE AND IN OUR THREE SOUTH
CENTRAL SD ZONES. MONDAY WILL BE A DRY...WELL MIXED DAY ACROSS
THIS AREA. IN FACT...THE VARIOUS GUIDANCES HAVE QUITE SIMPLY NOT
DONE WELL AT ALL WITH SURFACE DEW POINT FORECAST AS OF LATE...
BEING WAY TOO MOIST ON WELL MIXED DAYS. THEREFORE TOOK THE FAIRLY
DRY CONSENSUS MOS DEW POINTS ON MONDAY...AND BLENDED THEM WITH THE
EVEN DRIER MIXED NAM12 DEW POINTS TO COME UP WITH THE DEW POINT
AND HUMIDITY FORECAST. A SURFACE TROUGH WILL MOVE TO ABOUT I 29
DURING THE DAY ON MONDAY BEFORE STALLING. AHEAD OF THE TROUGH...
SOUTHWEST WINDS WILL BE RATHER STRONG IN THE AFOREMENTIONED VERY
HIGH AREAS...COUPLED WITH HUMIDITY VALUES PROJECTED TO FALL TO
ABOUT 26 TO 31 PERCENT. WE WILL HAVE TO MONITOR NORTHWEST IA
CLOSELY FOR THE POSSIBILITY OF RED FLAG CONDITIONS...ESPECIALLY
AROUND THE SPENCER AND STORM LAKE AREAS WHERE THE WIND WILL BE THE
STRONGEST. HOWEVER AT THIS TIME THOUGH...IT LOOKS LIKE THE
SOUTHWEST WINDS WILL DECREASE IN THE MID AND LATE AFTERNOON HOURS
AS THE SURFACE TROUGH WEAKENS...THUS THE LACK OF FIRE WEATHER
WATCH NEEDED.

IN OUR SOUTH CENTRAL SD COUNTIES...THE WEST/NORTHWEST WINDS
SHOULD BE A LITTLE LIGHTER BEHIND THE TROUGH ON MONDAY. HOWEVER
MINIMUM HUMIDITY VALUES MAY FALL TO NEAR 20 PERCENT...THUS GIVING
THAT AREA ALSO A VERY HIGH FIRE DANGER.

THEN ON TUESDAY...A PRETTY STRONG DRY COLD FRONT WILL MOVE ACROSS
THIS AREA. THIS DAY WILL HAVE TO BE CLOSELY MONITORED FOR THE
POTENTIAL FOR RED FLAG CONDITIONS. IT LOOKS DOWNRIGHT WINDY BEHIND
THE FRONT IN MANY OF OUR SOUTH DAKOTA COUNTIES. AHEAD OF THE
FRONT...IT SHOULD BE LESS WINDY BUT HUMIDITY VALUES COULD TOTALLY
TANK WITH THE HOT AIR IN PLACE.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.
&&

$$

UPDATE...
SHORT TERM...JM/MJF
LONG TERM...MJF
AVIATION...
FIRE WEATHER...MJF