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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KOAX 071139
AFDOAX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
539 AM CST SAT DEC 7 2013

.AVIATION...
-- Changed Discussion --
12Z TAFS FOR KOFK...KOMA AND KLNK

VFR CONDITIONS AND LIGHT SURFACE WINDS ARE EXPECTED INTO THE
AFTERNOON WITH HIGH PRESURE OVER THE AREA. A MID LAYER CLOUD DECK
WILL BE PASSING OVERHEAD THROUGHOUT THE DAY. OTHERWISE...AS AN
UPPER LEVEL DISTURBANCE APPROACHES FROM THE SOUTHWEST...CLOUDS
WILL BEGIN TO INCREASE AND CONDITIONS MAY BECOME MVFR AS EARLY AS
THIS EVENING. EVENTUALLY...IFR CONDITIONS ARE EXPECTED OVERNIGHT
AS SNOW BEGINS TO DEVELOP AND SPREAD WEST TO EAST.

KERN
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&&

.PREV DISCUSSION... 
-- Changed Discussion --
/ISSUED 321 AM CST SAT DEC 7 2013/

DISCUSSION...
SNOW LATER TONIGHT AND SUNDAY IS THE PRIMARY FORECAST CONCERN.

BITTERLY COLD ARCTIC AIR HAS ONLY STRENGTHENED ITS HOLD OVER THE CWA
THIS MORNING AS ALL REPORTING STATIONS EXCEPT FOR A COUPLE IN FAR
SOUTHEAST NEBRASKA HAVE TEMPERATURES BELOW ZERO AS OF 3 AM. DOUBLE
DIGIT SUB-ZERO TEMPS WERE NOTED IN PARTS OF NORTHEAST
NEBRASKA...WHERE CORE OF SURFACE HIGH PRESSURE WAS SETTLING IN. WITH
THIS HIGH PRESSURE CENTER...WINDS WILL ONLY SLACKEN AS THE MORNING
WEARS ON. WHILE TEMPERATURES ARE WELL BELOW ZERO...ANY WIND THAT
DOES OCCUR WILL DRIVE WIND CHILLS TO OR COLDER THAN 20 BELOW. BUT
THIS SHOULD BE BRIEF THIS MORNING. WILL GO AHEAD AND MAINTAIN OUR
CURRENT WIND CHILL ADVISORY FOR FAR NORTHEAST NEBRASKA AND WEST
CENTRAL IOWA...BUT WILL NOT EXPAND TO THE SOUTHWEST WHERE WINDS WILL
BE LIGHTEST.

ALOFT...THE BROAD UPPER TROUGH STILL COVERED MOST OF THE
COUNTRY...WITH TROUGH AXIS EXTENDING ACROSS SOUTHERN CANADA INTO
THE PACIFIC NORTHWEST. THE STRONG SHORTWAVE THAT WILL BRING OUR
SNOW CHANCES LATER TONIGHT AND SUNDAY WAS DIVING SOUTH THROUGH
NORTHERN CALIFORNIA...AND SEEMED TO BE HANDLED WELL BY 00Z MODELS.

AS THAT SHORTWAVE ROTATES SOUTH THEN EAST TOWARD THE FOUR CORNERS
REGION TODAY...SURFACE PRESSURES WILL BEGIN FALLING IN THE HIGH
PLAINS BRINING EAST OR SOUTHEAST WINDS TO OUR AREA BY LATE
AFTERNOON. MID LEVEL WARM ADVECTION REGIME WILL BE SETTING UP ACROSS
THE PLAINS THIS AFTERNOON AS WELL...LEADING TO INCREASING CLOUDS
THROUGH THE AFTERNOON. WITH WINDS TURNING MORE SOUTHERLY WITH
TIME...COLD ARCTIC PLUNGE WILL BE SHUT OFF...ALLOWING TEMPERATURES
TO REBOUND INTO THE TEENS MOST AREAS. BUT THICKENING CLOUDS WILL PUT
A HALT TO SIGNIFICANT WARMING. HOWEVER THOSE CLOUDS/WINDS WILL ALSO
KEEP TEMPERATURES FROM FALLING OFF TOO FAR THIS EVENING...AND
TEMPERATURES COULD ACTUALLY RISE OVERNIGHT.

AS SHORTWAVE APPROACHES THIS EVENING...ISENTROPIC UPGLIDE WILL BEGIN
UNDER MID LEVEL FORCING DEPICTED BY CONVERGING Q-VECTORS IN 700-
300MB LAYER. MID LEVEL SATURATION WILL CONTINUE UNDER THIS
SCENARIO...GRADUALLY SATURATING LOWER LEVELS WITH TIME OVERNIGHT.
MODELS DIFFER IN THE TIMING OF THIS LOW LEVEL SATURATION...WITH THE
GFS KEEPING SURFACE/BOUNDARY LAYER DRY THE LONGEST. HOWEVER ALL
MODELS ARE CONSISTENT IN SHOWING UPPER FORCING AND EVENTUAL
SATURATION ACROSS ALL OF THE CWA OVERNIGHT OR SUNDAY MORNING.
FORECAST SOUNDINGS ALSO SHOW A DEEP DENDRITIC LAYER FROM ABOUT 600MB
TO THE SURFACE WHEN THAT SATURATION FINALLY OCCURS. THIS SUGGESTS
EFFICIENT SNOW-MAKING REGIME WILL RESULT IN FAIRLY FLUFFY SNOW THAT
WILL TEND TO PILE UP. AND AGAIN THIS MORNING...SEVERAL SNOWFALL
ACCUMULATION PROCEDURES...COBB/GARCIA/SNOW-LIQUID RATIOS...ALL ARE
POINTING WIDESPREAD 2 TO 3 INCH AMOUNTS ACROSS ALL OF THE
CWA...WITH SOME AREAS SEEING 4 OR A LITTLE MORE.

AFTER CONVERSATIONS WITH SURROUNDING OFFICES WHERE SNOW TOTALS ARE
EXPECTED TO EXCEED 3 INCHES...HAVE DECIDED TO HOLD OFF ON ISSUING
WINTER WEATHER ADVISORY UNTIL LATER TODAY. WHILE SNOW SHOULD BREAK
OUT IN OUR WEST OVERNIGHT...LOW LEVEL DRY AIR WILL HAVE TO BE
OVERCOME IN OUR EAST...SO SNOW MAY NOT BEGIN NEAR AND EAST OF THE
MISSOURI RIVER UNTIL AFTER SUNRISE SUNDAY. PLUS WINDS SHOULD NOT BE
A FACTOR FOR OUR ADVISORY. AND THE LONG DURATION LIGHT SNOW OF THIS
EVENT WILL LIKELY NOT PRODUCE 3 OR MORE INCHES OF SNOW BEFORE LATE
SUNDAY MORNING AT THE EARLIEST. WE HAVE DECIDED TO FINE TUNE
FORECAST AMOUNTS AFTER ANOTHER UPPER AIR/MODEL RUN...AND ISSUE
ADVISORY ACCORDINGLY.

SNOW SHOULD BE EXITING THE AREA FAIRLY RAPIDLY SUNDAY EVENING...WITH
MOST OF IT GONE BY MIDNIGHT. CLEARING SKIES AND HIGH PRESSURE WILL
BEGIN TO BUILD INTO THE AREA THEN. WHILE WINDS WILL NOT GO CALM...A
FRESH SNOW COVER SHOULD PUSH LOWS BELOW ZERO AGAIN MONDAY MORNING
FOR MUCH OF THE AREA.

ANOTHER SHORTWAVE IS FORECAST TO DIVE THROUGH MINNESOTA INTO THE
NORTHERN GREAT LAKES MONDAY EVENING...INDUCING SOUTHWEST LOW LEVEL
FLOW IN THE PLAINS MONDAY AFTERNOON. THIS WILL HELP TEMPERATURES
REBOUND INTO THE TEENS MONDAY AFTERNOON DESPITE SNOW COVER. THEN
NORTHWEST WINDS MONDAY NIGHT WILL KEEP LOWS IN THE SINGLE DIGITS
ABOVE ZERO.

A SLOW WARMING TREND STILL LOOKS ON TRACK FOR THE REST OF THE WEEK
AS MID LEVEL FLOW TURNS FROM NORTHWEST TO A MORE ZONAL PATTERN. YET
ANOTHER CLIPPER-TYPE SHORTWAVE IS SHOWN MOVING THROUGH THE DAKOTAS
INTO IOWA TUESDAY NIGHT AND WEDNESDAY MORNING. THIS COULD DROP A
STRIPE OF LIGHT SNOW ACROSS OUR NORTHERN CWA...BUT HAVE LEFT THAT
PERIOD DRY FOR NOW. THE MAIN RESULT WILL BE SOMEWHAT COLDER
TEMPERATURES WEDNESDAY AFTER TUESDAY HIGHS IN THE 20S. THEN WE COULD
POTENTIALLY TOUCH 30 DEGREES THURSDAY AND FRIDAY UNDER ZONAL FLOW
ALOFT AND SOUTHERLY LOW LEVEL WINDS.

DERGAN
-- End Changed Discussion --

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...WIND CHILL ADVISORY UNTIL 9 AM CST THIS MORNING FOR NEZ011-012-
     015-018-034.

IA...WIND CHILL ADVISORY UNTIL 9 AM CST THIS MORNING FOR IAZ043-055-
     056.

&&

$$

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