Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 250335
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
935 PM CST TUE FEB 24 2015

.UPDATE...
ISSUED AT 935 PM CST TUE FEB 24 2015

DRY LOW LEVEL AIR MAKING IT TOUGH TO LEADING EDGE OF SNOWFALL TO
MAKE MUCH EASTWARD PROGRESS THUS FAR THIS EVENING. IN FACT...03Z
DEW POINTS IN OUR FAR WEST ACTUALLY DROPPED A FEW DEGREES FROM THE
PREVIOUS HOUR...SHOWING THE STRENGTH OF THE DRY EASTERLY FETCH AT
THIS TIME. HAVE TWEAKED POP GRIDS TO SLOW DOWN TIMING OF SNOW
START TIMES 2-3 HOURS AS A RESULT.

STRONG UPSTREAM WAVE STILL OVER MONTANA THOUGH...SO YET TO REALLY
HAVE AN IMPACT IN OUR AREA. AS THIS TRACKS SOUTHEAST INTO THE
NORTHERN PLAINS OVERNIGHT...DEEP LIFT SHOULD RAMP UP AND RAPIDLY
OVERCOME THE LOW LEVEL DRY AIR. TRACK OF THE HEAVIEST SNOWFALL IN
LATEST AVAILABLE MODELS STILL APPEARS ON TRACK...MAYBE JUST A HAIR
FARTHER NORTH THAN SEEN EARLIER TODAY...THOUGH WILL BE FIGHTING
THE DRIER AIR IN THAT DIRECTION...SO DID NOT MANY ANY SIGNIFICANT
ADJUSTMENTS TO SNOWFALL AMOUNTS NOR TO EXISTING HEADLINES AT THIS
TIME. DID LOWER SNOW AMOUNTS PRIOR TO 12Z BASED ON SLOWER ONSET...
BUT REALLY THIS JUST GETS SHIFTED INTO THE 12Z-18Z PERIOD AS THE
STRONG WAVE PASSES THROUGH SO OVERALL AMOUNTS NOT CHANGING MUCH.

&&

.SHORT TERM...(THIS EVENING THROUGH WEDNESDAY)
ISSUED AT 322 PM CST TUE FEB 24 2015

FOCUS OF THE SHORT TERM IS ON SNOW POTENTIAL LATER TONIGHT INTO
WEDNESDAY. A WAVE RIDING ALONG THE STRONG THERMAL GRADIENT WILL
RESULT IN A PERIOD OF MAINLY LIGHT TO MODERATE SNOW. EXPECTATION IS
THAT SNOW WILL DEVELOP TONIGHT ALONG THE AREA OF STRONG NORTHWEST TO
SOUTHEAST ORIENTED 850 MB FRONTOGENESIS. THEN LATER TONIGHT INTO
WEDNESDAY MORNING...BETTER UPPER FORCING FROM AN APPROACHING WAVE
WILL INTERACT WITH THE FRONT...RESULTING IN AN EXPANSION OF THE
SNOW. FAVORED THE MORE CONSISTENT NORTHERN SOLUTION OF THE
GEM...ECMWF AND WRF ARW. THE LATEST NAM AND GFS HAVE ALSO FINALLY
BEGUN TO TREND NORTH. THE INITIAL BAND OF SNOW WILL IMPACT OUR
NORTHERN AREAS BY MIDNIGHT...WITH MORE WIDESPREAD ACCUMULATING SNOW
STARTING TO IMPACT THE REGION AFTER 3 AM...LIKELY IMPACTING THE
MORNING COMMUTE.

THE FAVORING OF THE NORTHERN SOLUTIONS HAS RESULTED IN A SIGNIFICANT
SHIFT OF THE AXIS OF HIGHER SNOWFALL AMOUNTS. A WARM NOSE ALOFT WILL
LIKELY RESULT IN A DECENT GRADIENT IN SNOW RATIOS. THE WARM
TEMPERATURES BETWEEN 850MB AND 700MB WILL RESULT IN RATIOS CLOSER TO
10 TO 13:1 FOR A GOOD PORTION OF THE AREA. HOWEVER AS COLDER AIR
WORKS SOUTH...SOUTHWEST MINNESOTA AND PORTIONS OF ADJACENT NORTHWEST
IOWA SHOULD SEE RATIOS AROUND THE 15:1 RANGE. MAY EVEN SEE SOME
SLEET AND FREEZING RAIN MIX IN ALONG THE MISSOURI RIVER...AS MAX
TEMPERATURES ALOFT OFF MOST OF THE GUIDANCE GETS ABOVE ZERO FOR A
PERIOD TONIGHT. NOT EXPECTING ANY MAJOR ICE ACCUMULATIONS...BUT A
LIGHT GLAZE OF ICE CAN NOT BE RULED OUT. THE NORTHWARD SHIFT OF THE
QPF...LOWER RATIOS...AND POTENTIAL MIXING...WILL HOLD SNOW AMOUNTS
IN THE 1 TO 3 INCH RANGE ALONG THE MISSOURI RIVER AND AREAS WEST OF
INTERSTATE 29. ALONG AND EAST OF THE INTERSTATE SHOULD SEE A GENERAL
2 TO 5 INCHES....WITH 5 TO 7 INCHES IN OUR FAR EASTERN COUNTIES IN
NORTHWEST IOWA. ISSUED A WINTER WEATHER ADVISORY FOR MOST OF OUR
EASTERN AREAS AND A WINTER STORM WARNING FOR DICKINSON...CLAY AND
BUENA VISTA COUNTIES. DESPITE BEING BORDERLINE...DECIDED TO GO WITH
THE WARNING IN OUR EAST...AS FORCING IS QUITE STRONG...AND HI RES
MODEL QPF VALUES ARE TRENDING UP. SO FEEL LIKE AN AREA OF 6 INCHES
IS LIKELY...AND AT THIS TIME THE BEST BET ON LOCATION IS THOSE 3
IOWA COUNTIES. AXIS OF HEAVIER AMOUNTS MAY STILL SHIFT A BIT NORTH
OR SOUTH...SO EVENING AND OVERNIGHT SHIFTS WILL HAVE TO MONITOR
RADAR AND LATEST MODELS AND UPDATE AS NECESSARY.

EAST TO NORTHERLY WINDS WILL BEGIN GUSTING TO AROUND 30 MPH BY LATER
TONIGHT INTO WEDNESDAY. WHILE LIKELY NOT STRONG ENOUGH TO CAUSE
VISIBILITY ISSUES IN BLOWING SNOW...SOME DRIFTING OF THE SNOW THAT
FALLS DOES SEEM LIKELY. TEMPERATURES WILL FALL THROUGH TONIGHT...AND
REMAIN STEADY OR FALL INTO WEDNESDAY. MOST OF THE ACCUMULATING SNOW
SHOULD BE OVER BY NOON IN OUR SOUTH DAKOTA COUNTIES...WITH JUST SOME
FLURRIES OR VERY LIGHT SNOW LINGERING INTO THE AFTERNOON. OUR IOWA
AND MINNESOTA COUNTIES SHOULD SEE THE ACCUMULATING SNOW END BY MID
AFTERNOON.

.LONG TERM...(WEDNESDAY NIGHT THROUGH TUESDAY)
ISSUED AT 322 PM CST TUE FEB 24 2015

ON WEDNESDAY NIGHT...COLD AIR ADVECTION WITH ARCTIC HIGH PRESSURE
CONTINUES TO POUR INTO THE AREA. AS THE SHORT WAVE EXITS WEDNESDAY
EVENING...DRIER AIR ESPECIALLY IN THE MID AND UPPER LEVELS WILL
RAPIDLY DIVE SOUTHWARD. THEREFORE AT THIS TIME...THE ONLY REMAINING
POPS ARE WARRANTED IN THE EARLY EVENING AROUND THE STORM LAKE IA
LOCATION. BUT THEN THROUGH THE NIGHT...MIXED LAYER WINDS ARE
AVERAGING A BRISK 15 TO 25 MPH SO THAT WILL KEEP SURFACE WINDS
STIRRED UP AS THE COLD TEMPERATURES USHER IN. AIR TEMPERATURE LOWS
WILL LIKELY BE REALIZED TOWARD MID MORNING THURSDAY AND WILL BE
LARGELY IN THE SINGLE DIGITS BELOW ZERO AS THE WIND MODERATES THE
AIR TEMPERATURE A BIT. HOWEVER WIND CHILLS WILL BE FRIGID FOR THIS
TIME OF YEAR...WITH A BULK OF OUR FORECAST AREA IN THE 20 BELOW TO
30 DEGREES BELOW ZERO RANGE BY LATE WEDNESDAY NIGHT AND THURSDAY
MORNING. WILL VERY LIKELY NEED A WIND CHILL ADVISORY IN FUTURE
FORECASTS FOR THIS TIME FRAME...BUT DID NOT ISSUE AS WE FIRST NEED
TO GET THE SNOW SYSTEM THROUGH SO AS NOT TO CONFUSE THE ISSUE.

THE OTHER ISSUE IS HOW MUCH STRATO CUMULUS CLOUDS WILL LINGER
WEDNESDAY NIGHT AND THURSDAY. THE GFS IS VERY BULLISH ON MOISTURE
AROUND 925MB...WITH THE OTHER MODELS LESS SO. BELIEVE THERE WILL BE
ENOUGH SUBSIDENCE BEHIND THE NEAR TERM SYSTEM TO ALLOW THE CUMULUS
TO DRY UP. HOWEVER WOULD NOT BE SURPRISED TO SEE SCATTERED CLOUDS
DEVELOP ON THURSDAY NOTING THE COLD AIR ALOFT...SO DID NOT GO
TOTALLY PRISTINE WITH THE SKY COVER UNTIL THURSDAY NIGHT. IN TERMS
OF HIGHS ON THURSDAY...SO FAR MOST HIGHS LOOK TO BE IN THE SINGLE
DIGITS ABOVE ZERO...WHICH WILL PLACE MANY LOCATIONS IN THE TOP 5 FOR
RECORD COLD MAXIMUM TEMPERATURES FOR FEB 26. WITH NEW SNOW COVER
OVER MANY SPOTS AND 925MB TEMPERATURES HANGING AROUND A COUPLE OF
DEGREES EITHER SIDE OF -20C...THIS SCENARIO IS PLAUSIBLE.

LOWS THURSDAY NIGHT ARE QUITE A CHALLENGE AS THE ARCTIC HIGH MOVES
EASTWARD ACROSS THIS AREA. THE SURFACE WINDS WILL SHIFT TO THE SOUTH
AROUND MIDNIGHT IN OUR SOUTH CENTRAL SD COUNTY ZONES WHICH...COUPLED
WITH WHAT SHOULD BE A LACK OF SNOW COVER...IS LIKELY TO PUT THE
BRAKES ON THEIR TEMPERATURES. THEREFORE WENT A LITTLE WARMER THEN
CONSENSUS LOWS FOR THAT LOCATION. CONVERSELY ACROSS OUR EASTERN
ZONES...SHAVED A CATEGORY OFF OF LOW TEMPERATURES ANTICIPATING LIGHT
WINDS...CLEAR SKIES AND NEW SNOW COVER. THERE IS A CHANCE THAT LOWS
COULD BE COLDER IN FUTURE FORECASTS. BUT EVEN AS IT SITS NOW...MANY
LOWS SHOULD BE IN THE TOP 5 COLDEST FOR FEB 27 ALONG AND EAST OF
INTERSTATE 29...ALTHOUGH STILL A WAYS FROM RECORDS.

FRIDAY SHOULD SEE LIGHT WINDS BUT WITH COLD TEMPERATURES
CONTINUING...STILL UNDER THE MASSIVE INFLUENCE OF ARCTIC HIGH
PRESSURE. SATURDAY WILL BE ONE OF THOSE DAYS THAT WILL BE RELATIVELY
WARMER...WITH READINGS IN THE 20S. BUT THE SOUTHERLY WINDS WILL BE
BREEZY SO IT LIKELY WILL NOT FEEL ALL THAT GREAT.

FOR THE REST OF THE EXTENDED...THE VARIOUS DETERMINISTIC SOLUTIONS
ARE ALL OVER THE MAP WITH REGARDS TO THE NEXT SNOW MAKER. BUT
CURRENTLY EACH MODEL SHOWS THAT THE NORTHERN STREAM HAS TAKEN OVER
MUCH MORE THAN PREVIOUS RUNS. THE ECMWF PAINTS LIGHT SNOW GENERALLY
ALONG AND SOUTH OF I 90 ON SATURDAY DAY PERIOD DUE TO A LEADING
SHORT WAVE. THE GFS DOES THE SAME THING EXCEPT SATURDAY NIGHT AS IT
DOES NOT HAVE A LEAD WAVE LIKE THE ECMWF HAS. TAKEN LITERALLY RIGHT
NOW...AMOUNTS COULD BE AROUND AN INCH CLOSE TO SIOUX CITY EITHER
SATURDAY OR SATURDAY NIGHT...OR BOTH...WITH LESSER AMOUNTS HEADING
TOWARD I 90. THE GEM IS DRY FOR OUR FORECAST AREA. THIS SYSTEM IS
CERTAINLY SOMETHING TO WATCH BECAUSE IF THE NORTHERN STREAM ENERGY
SLOWS DOWN AS IT DID ON JAN 31/FEB 1...THAT WILL ALLOW THE SOUTHERN
STREAM JET TO PHASE IN BETTER WITH THE NORTHERN STREAM THUS ALLOWING
MUCH DEEPER MOISTURE TO GAIN A FOOTHOLD IN OUR FORECAST AREA. NOT
GOING FOR THAT SCENARIO RIGHT NOW HOWEVER. TEMPERATURES WILL REMAIN
BELOW NORMAL THROUGH THE EXTENDED.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z WEDNESDAY EVENING)
ISSUED AT 542 PM CST TUE FEB 24 2015

SNOW DEVELOPING INTO NORTH CENTRAL SD AT THE START OF THE PERIOD.
SNOW WILL CONTINUE TO DEVELOP AND EXPAND SOUTHEAST ACROSS THE REGION
TONIGHT AND CONTINUE INTO WEDNESDAY. SNOW WILL REDUCE CONDITIONS
INTO MVFR-IFR CATEGORY. PERIODS OF LIFR VIS LIKELY IN HEAVIER SNOW
BANDS...BUT EXACT TIMING/LOCATION OF THIS STILL UNCERTAIN SO WILL
HOLD LOWEST VISIBILITY AT 1SM FOR NOW. VISIBILITY SHOULD TREND
UPWARD IN THE LATE MORNING/AFTERNOON WEDNESDAY...THOUGH LINGERING
MVFR CEILINGS ARE ANTICIPATED.

WINDS WHILE THE SNOW IS FALLING WILL BE EASTERLY IN THE 10 TO 15 KT
RANGE. HOWEVER AS THE SYSTEM SHIFTS EAST AND WINDS SWING AROUND TO
THE NORTH IN THE LATE MORNING/AFTERNOON...SPEEDS WILL INCREASE WITH
GUSTS TO AROUND 25KTS LIKELY. THIS MAY PRODUCE SOME POCKETS OF MVFR
VISIBILITY IN BLOWING SNOW IF FALLING LIGHT SNOW IS STILL LINGERING.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...WINTER WEATHER ADVISORY UNTIL 3 PM CST WEDNESDAY FOR SDZ039-040-
     055-056-062-067-071.

MN...WINTER WEATHER ADVISORY UNTIL 3 PM CST WEDNESDAY FOR MNZ071-072-
     080-081-089-090-097-098.

IA...WINTER WEATHER ADVISORY UNTIL 3 PM CST WEDNESDAY FOR IAZ001-002-
     012-013-020-021-031-032.

     WINTER STORM WARNING FROM 3 AM TO 6 PM CST WEDNESDAY FOR IAZ003-
     014-022.

NE...WINTER WEATHER ADVISORY UNTIL 3 PM CST WEDNESDAY FOR NEZ014.

&&

$$

UPDATE...JH
SHORT TERM...CHENARD
LONG TERM...MJ
AVIATION...JH



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