000
FXUS63 KFSD 072121
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
321 PM CST SAT DEC 7 2013

.SHORT TERM...(THIS EVENING THROUGH SUNDAY)
ISSUED AT 320 PM CST SAT DEC 7 2013

MAIN FOCUS IN THE SHORT TERM WILL BE ON THE EXTENDED PERIOD OF LIGHT
TO MODERATE SNOW TONIGHT INTO SUNDAY. AS THE MAIN PV ANOMALY ADVECTS
EAST THIS EVENING...A BROAD AND LARGER AREA OF ISENTROPIC LIFT WILL
ENCOMPASS THE REGION. AND WITH THE ATMOSPHERE BECOMING
SATURATED...LIGHT SNOW SHOULD RAPIDLY EXPAND AND COVER MOST OR ALL
OF THE AREA BY 09Z...WITH PORTIONS OF THE AREA SEEING SNOW BY 03Z.
AS MENTIONED...MOST OF THIS SNOW SHOULD BE LIGHT TO AT TIMES
MODERATE...WITH THE HIGHEST QPF LIKELY IN NORTHWEST IOWA...WHERE
LARGE SCALE FORCING IS GREATER. SOME SIGNS THAT A WEAK MID LEVEL
FRONTAL BOUNDARY COULD ENHANCE SNOWFALL IN A FEW BANDS DURING THE
DAY SUNDAY...BUT WITH FRONTOGENESIS WEAK AND THE ATMOSPHERE PRETTY
STABLE ABOVE THE FRONT...STILL SHOULD NOT BE TOO HEAVY. SNOW RATIOS
WILL BE A BIG FACTOR IN DETERMINING HOW MUCH SNOW WE END UP
WITH...AND WILL BE DEPENDENT ON WHICH LEVEL THE LIFTS ENDS UP
AT...AND EXACTLY WHAT OUR THERMAL PROFILE LOOKS LIKE. THE DENDRITIC
SNOW GROWTH ZONE IS GENERALLY BETWEEN -12C AND -18Z...WITH RATIOS
AROUND 20 TO 1 POSSIBLE IF WE SEE A DEEP LAYER OF LIFT COINCIDENT
WITH THESE TEMPERATURES. THAT LOOKS TO BE THE CASE IN THIS EVENT
ACROSS THE SOUTHEAST PORTION OF OUR CWA FOR MOST OF THE EVENT...SO
WENT WITH AN AVERAGE RATIO AROUND 19 TO 1 THERE. MORE UNCERTAIN AS
YOU HEAD TOWARDS SIOUX FALLS AND POINTS NORTHWEST. SOME INDICATIONS
THAT THE THERMAL PROFILE WILL ACTUALLY BE TOO COLD FOR THE BEST SNOW
GROWTH...SUGGESTING RATIOS CLOSER TO 10 TO 1. OTHER SOLUTIONS
SUGGEST OUR THERMAL PROFILE WILL BE A BIT WARMER PUTTING US BACK
CLOSER TO 20 TO 1. SO AS A COMPROMISE KEPT THINGS CLOSE TO 16 TO 1.

DID GO AHEAD AND ISSUE A WINTER WEATHER ADVISORY FOR PORTIONS OF THE
AREA. GIVEN THE LONGER DURATION OF THE SNOW...LIMITED WIND...AND THE
WEEKEND TIMEFRAME... DID NOT SEE A REASON TO GO OVERBOARD WITH OUR
ADVISORY...AND SO STAYED PRETTY STRICT TO OUR 3 INCH CRITERIA. THUS
ISSUED FOR POINTS ALONG AND SOUTHEAST OF YANKTON TO CANTON TO WINDOM
LINE. LOOKING AT 3 TO 4 INCHES THERE. COULD SEE A FEW AMOUNTS AROUND
5 INCHES IF RATIOS END UP EVEN HIGHER OR IF WE DO SEE SOME BRIEF
FRONTAL ENHANCEMENT TO THE THE SNOW. NORTHWEST OF THE
ADVISORY...LOOKING AT AROUND AN INCH WEST OF THE JAMES...AND 2 TO 3
INCHES FROM TYNDALL TO SIOUX FALLS TO MARSHALL. AGAIN IF RATIOS END
UP A BIT HIGHER OR WE GET SOME BRIEF BAND ENHANCEMENT...COULD
POTENTIALLY SEE CLOSER TO 4 INCHES IN THAT LATTER AREA...BUT
PROBABILITY IS TOO LOW TO WARRANT AN ADVISORY AT THIS POINT.
TEMPERATURES MAY DROP OFF BRIEFLY THIS EVENING BEFORE A WARMING
TREND TAKES OVER TONIGHT INTO TOMORROW. WIND CHILLS OVERNIGHT SHOULD
BE IN THE 10 TO 20 BELOW ZERO RANGE FOR PORTIONS OF THE AREA...BUT
EXPECT THEM TO REMAIN ABOVE ADVISORY CRITERIA. SHOULD SEE HIGHS GET
INTO THE LOWER TO MIDDLE TEENS OVER THE ENTIRE CWA BY TOMORROW
AFTERNOON.

.LONG TERM...(SUNDAY NIGHT THROUGH SATURDAY)
ISSUED AT 320 PM CST SAT DEC 7 2013

WILL END SNOW QUICKLY ON SUNDAY EVENING AS THE FORCING MOVES EAST.
JUST LOOKING AT AN ADDITIONAL COUPLE OF TENTHS DURING THIS TIME.
OTHERWISE...ANOTHER STRONG SURGE OF ARCTIC AIR WILL WORK INTO THE
AREA LATE SUNDAY NIGHT AND MONDAY MORNING. ADDED SOME BLOWING SNOW
TO THE EASTERN PART OF THE CWA AS THE POTENTIALLY LIGHT AND FLUFFY
NATURE OF THE SNOW ON TOP OF THE HARD FROZEN EXISTING SNOWPACK WINDS
OF 20 TO 30 MPH COULD DROP VISIBILITIES TO A HALF OF A MILE OR SO AT
TIMES. ALONG WITH THIS BLOWING SNOW WILL BE VERY COLD WIND CHILLS
WHICH SHOULD DROP VALUES TO -25 TO -35 DEGREES OVER MOST OF THE
AREA. WILL NOT ISSUE AN ADVISORY YET BUT ONE WILL BE NEEDED STARTING
FROM ABOUT 6Z MONDAY IN THE FAR NORTHWEST TO ABOUT 9Z FROM I29 WEST
AND THE REMAINDER OF THE AREA SHORTLY THEREAFTER. BY ABOUT NOON WIND
CHILLS WILL IMPROVE INTO THE TEENS BELOW ZERO IN MOST LOCATIONS.

THE WIND CHILLS AND BLOWING SNOW WILL IMPROVE BY MONDAY AFTERNOON
BUT STILL COLD. A WEAK RIDGE OF HIGH PRESSURE WILL TURN WINDS FROM
THE NORTHWEST TO THE SOUTHWEST WITH A LULL DURING THE DAY AS THIS
TRANSITION OCCURS. DID LOWER HIGHS A BIT WITH MORE CONFIDENCE IN
FRESH SNOW COVER AND THE THOUGHT THAT ALONG AND EAST OF INTERSTATE
29 THE 925MB TEMPERATURES WILL BE ABOUT -17 OR COLDER FOR THE DAY.
SO HIGHS NEAR ZERO IN SOUTHWEST MN AND NEAR 10 ABOVE SOUTH CENTRAL
SD.

AS LOW PRESSURE RACES TO THE NORTH OF THE AREA MONDAY NIGHT INTO
TUESDAY A VERY STRONG WEST TO NORTHWEST GRADIENT DEVELOPS AGAIN
WHICH WILL BRING ANOTHER THREAT FOR BLOWING SNOW...ESPECIALLY AROUND
THE BUFFALO RIDGE WHERE ENHANCED STRONG WINDS WILL BE POSSIBLE WITH
A FAVORABLE CROSS RIDGE FLOW FROM ABOUT MIDNIGHT THROUGH EARLY
TUESDAY MORNING. LOW TEMPERATURES WILL ONLY BE A COUPLE OF DEGREES
BELOW MONDAY HIGHS AS THE FLOW DEVELOPS...BUT A SOMEWHAT MIXY
ENVIRONMENT WILL SUPPORT LESS COLD TEMPERATURES ON TUESDAY SO A
DECENT CHANCE FOR HIGHS IN THE TEENS IN MOST LOCATIONS.

A STRONG NORTH TO NORTHWEST ORIENTED JET STREAK WILL DROP INTO THE
NORTHERN PLAINS AND GREAT LAKES AREA WHICH WILL HELP REINFORCE THE
COLD AIR INITIALLY. AS THE JET STREAK EXITS THE AREA BRIEF WARMING
WILL BE POSSIBLE THURSDAY AND THURSDAY NIGHT BUT THEN ANOTHER
REINFORCING SHOT OF COLD AIR WILL MOVE IN FRIDAY AFTERNOON AND NIGHT
AND LIKELY HANG IN THROUGH SUNDAY. SOME DIFFERENCE IN TIMING OF
COLDER AIR AND JUST HOW FAR NORTH THE SLIGHTLY WARMER AIR GETS ON
THURSDAY AFTERNOON WITH THE GFS FARTHEST NORTH WITH THE DEVELOPING
SURFACE LOW AND WARMER AIR. THE GEM AND ESPECIALLY THE ECMWF KEEP
THE COLDER AIR LOCKED IN A BIT MORE AND MIGHT EVEN BRING A SMALL
THREAT FOR LIGHT SNOW OR SLEET TO THE AREA AS SOME SLIGHTLY ABOVE
FREEZING AIR MOVES IN ALOFT. TOO FAR OUT TO FOCUS ON AT THIS TIME
AND TOO SMALL OF A THREAT BUT A PERIOD TO KEEP AN EYE ON.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1130 AM CST SAT DEC 7 2013

VFR CONDITIONS EXPECTED THROUGH THE DAY...WITH SNOW MOVING IN FROM
THE SOUTHWEST TONIGHT. EXPECT VISIBILITY TO QUICKLY DROP INTO THE IFR
CATEGORY AS THE SNOW MOVES IN. COULD BE PERIODS OF LIFR VISIBILITY AS
WELL...WITH THE BEST CHANCE ACROSS NORTHWEST IOWA AND ADJACENT AREAS
OF NORTHEAST NEBRASKA AND SOUTHWEST MINNESOTA...ALTHOUGH EVEN KFSD
COULD SEE VISIBILITIES BRIEFLY BELOW A MILE. EXPECT THIS SNOW TO
PERSIST THROUGH THE REMAINDER OF THE TAF PERIOD...ENDING FROM WEST TO
EAST AFTER 18Z.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM CST SUNDAY
     FOR SDZ067-069>071.

MN...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM CST SUNDAY
     FOR MNZ081-089-090.

IA...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM CST SUNDAY
     FOR IAZ001>003-012>014-020>022-031-032.

NE...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM CST SUNDAY
     FOR NEZ013-014.

&&

$$

SHORT TERM...CHENARD
LONG TERM...08
AVIATION...CHENARD