Area Forecast Discussion
Issued by NWS North Platte, NE

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000
FXUS63 KLBF 281109
AFDLBF

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
509 AM CST SUN DEC 28 2014

.SYNOPSIS...
ISSUED AT 330 AM CST SUN DEC 28 2014

ANALYSIS OF THE 08Z MSL DATA SHOWED A 1049MB ARCTIC AIR MASS OVER
THE NORTHERN YUKON TERRITORY AND SOUTHEAST ALASKA. A FRONT WAS
STRETCHED ACROSS THE NORTHERN U.S FROM NORTHERN MINNESOTA THROUGH
THE DAKOTAS TO NORTHERN IDAHO WITH LOW PRESSURE ACROSS NORTHWEST
MONTANA AND THE NORTHERN IDAHO PANHANDLE. THE WATER VAPOR IMAGERY
INDICATED A MATURING WAVE OVER SOUTHWEST MONTANA WITH A POTENTIAL
VORTICITY INTRUSION TO THE SOUTHWEST OF THE WAVE.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 330 AM CST SUN DEC 28 2014

THE POTENTIAL VORTICITY IN IDAHO WILL CAUSE THE UPPER TROUGH TO DIG
IN THE PACIFIC NORTHWEST. MEANWHILE...THE DENSE ARCTIC AIR FROM THE
YUKON TERRITORY COMES INTO SOUTHERN ALBERTA AND FORCES THE FRONT
SOUTH FROM THE DAKOTAS TODAY AND TONIGHT. AS THE FRONT PLUNGES
SOUTH...STRONG SYSTEM RELATIVE ISENTROPIC MOTION CREATES DECENT LIFT
OF NEARLY SATURATED AIR IN CENTRAL AND NORTHERN NEBRASKA. A CROSS
SECTION OF THE AGEOSTROPHIC VERTICAL CIRCULATION SHOWS FAIRLY STRONG
LIFT WITH SOME FRONTOGENETIC ENHANCEMENT AROUND 285K. WITH 285-295K
MIXING RATIO 1-2G/KG...AVAILABLE MOISTURE IS LIMITED. THE FORECAST
SOUNDINGS SEEM TO SUPPORT A SNOW TO LIQUID RATIO AT OR A LITTLE
ABOVE THE SEASONAL AVERAGE OF 13 TO 1. WE WILL USE 14 TO 1 AT THIS
TIME.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 330 AM CST SUN DEC 28 2014

MAIN FORECAST CHALLENGES IN THE EXTENDED PERIOD WILL BE SNOWFALL
CHANCES AND BITTERLY COLD ARCTIC AIR TO BEGIN THE WEEK. SHORTWAVE
TROUGH CURRENTLY OVER THE PACIFIC NORTHWEST WILL CONTINUE TO DIG
SOUTH AND SET THE STAGE FOR AN ARCTIC DUMP OF COLD AIR INTO THE
CENTRAL PLAINS BY MONDAY. THE LEAD SHORTWAVE/PV DIGS SOUTH AND
BECOMES SHEARED AND ELONGATES OVER THE CENTRAL ROCKIES...LEADING TO
ALMOST ZONAL FLOW TO BEGIN THE PERIOD. AS THIS OCCURS AN ARCTIC COLD
FRONT WILL SURGE SOUTH AND CLEAR MOST OF NEBRASKA BY MONDAY MORNING.
STRONG CAA BEHIND THE FRONT WILL HOLD TEMPS STEADY IN THE SINGLE
DIGITS AND LOW TEENS THROUGH THE DAY. BLUSTERY WINDS WILL DROP WIND
CHILLS INTO THE 0 TO -15 RANGE DURING THE DAYLIGHT HOURS. WIND
SPEEDS BEGIN TO RELAX MONDAY EVENING AS THE SFC PRESSURE GRADIENT
WEAKENS...HOWEVER WIND CHILLS AT OR BELOW -20 ARE LIKELY MONDAY
NIGHT INTO TUESDAY MORNING AS SFC TEMPS FALL WELL BELOW ZERO. WIND
CHILL HEADLINES WILL BE NEEDED SOON FOR THIS TIME FRAME.

BUMPED UP SNOW AMOUNTS FOR MOST OF THE CWA AS MODELS CONTINUE TO
SHOW A LONG DURATION SNOW EVENT FOR WESTERN AND NORTH CENTRAL
NEBRASKA. THE SNOWFALL COMES IN TWO WAVES...THE FIRST WILL ALREADY
BE ONGOING AT THE BEGINNING OF THE PERIOD AS A LEAD PV ANOMALY MOVES
OUT OF THE CENTRAL ROCKIES. WEAK ISENTROPIC ASCENT WILL EXPAND SOUTH
AND LEAD TO THE DEVELOPMENT OF LIGHT SNOW AND AREAS OF BLOWING SNOW
ACROSS THE CWA MONDAY MORNING AND EARLY AFTERNOON. THE NAM CONTINUES
TO BE THE MOST AGGRESSIVE WITH SNOWFALL...DUMPING WIDESPREAD 4"+
OVER WESTERN NEBRASKA BY MONDAY NIGHT. THE 00Z GFS SOLUTION IS THE
FASTEST WITH LEAD PV ANOMALY AND PUSHES MOST SNOWFALL OUT BY LATE
MONDAY AFTERNOON AND GIVES THE AREA MUCH LOWER SNOWFALL AMOUNTS. A
SECOND PV ANOMALY TRANSLATES OUT OF THE CENTRAL ROCKIES LATE MONDAY
INTO EARLY TUESDAY AND REINFORCES AN AREA OF LOW TO MID LEVEL
FRONTOGENESIS EXTENDING FROM THE PANHANDLE INTO SOUTHWEST NEBRASKA.
ADDITIONAL LIGHT SNOWFALL ACCUMULATIONS WILL BE POSSIBLE IN OUR
SOUTHWEST ZONES DURING THIS PERIOD.

ALL MODEL SOLUTIONS AGREE ON HIGHER THAN CLIMO SLR WHICH WILL
INFLATE SNOWFALL TOTALS DESPITE RELATIVELY LIGHT QPF. AN AVERAGE
SNOW TO LIQUID OF ABOUT 19:1 WAS USED BASED ON LATEST BUFKIT
SOUNDING DATA...WHICH SHOWS A DEEP DGZ NEAR THE SFC EXTENDING UP TO
ABOUT 600MB AS EARLY AS MONDAY MORNING AS CONTINUING INTO MONDAY
NIGHT. WE COULD HAVE BEEN MORE AGGRESSIVE WITH THE SLR DURING THIS
PERIOD GIVEN THE IDEAL SOUNDING PROFILE FOR A HIGHLY ANOMALOUS
EVENT...BUT GUSTY WINDS MONDAY MORNING & AFTERNOON SHOULD COMPACT
SOME OF THE SNOWFALL. TOTAL SNOWFALL GRIDS ARE NOW IN THE 2-4" RANGE
WITH THE HIGHEST AMOUNTS FALLING LATE SUNDAY NIGHT INTO MONDAY
GENERALLY ALONG AND WEST OF HWY 83. SNOWFALL TOTALS IN THIS AREA MAY
NEED TO BE BUMPED UP FURTHER SHOULD MODELS TREND TOWARD THE LATEST
NAM SOLUTIONS.

TUESDAY AND TUESDAY NIGHT WILL BE THE COLDEST PART OF THE EXTENDED
PERIOD AS H85 TEMPS BOTTOM OUT AROUND -18C TO -20C. NUDGED TEMPS
DOWN FOR TUESDAY TO FOLLOW COOLER GUIDANCE TRENDS. WOULD NOT BE
SURPRISED IF SOME LOCATIONS NEAR THE SD BORDER DO NOT CRACK ZERO
DEGREES FOR A HIGH TUESDAY. AN IDEAL SET UP FOR BITTERLY COLD LOW
TEMPERATURES SETS UP TUESDAY NIGHT. A 1050MB SFC HIGH MOVES
SOUTHEAST AND SETTLES INTO WESTERN NEBRASKA TUESDAY EVENING. LIGHT
WESTERLY DRAINAGE WINDS...CLEARING SKIES...AND FRESH SNOWPACK WILL
PROMOTE STRONG RADIATIONAL COOLING. CURRENTLY HAVE LOWS RANGING FROM
-5 TO -15...BUT THIS MAY NEED TO BE LOWERED FURTHER IF GUIDANCE
KEEPS TRENDING COOLER.

COLDER THAN NORMAL TEMPERATURES AND MAINLY DRY CONDITIONS WILL
CONTINUE THROUGH THE END OF NEXT WEEK. THE SNOWPACK WILL LIKELY
PREVENT ANY SIGNIFICANT WARMING THURSDAY-FRIDAY DESPITE H85
TEMPERATURES WARMING ABOVE FREEZING. MODELS SHOW ANOTHER REINFORCING
SHOT OF COLD AIR AND THE POTENTIAL FOR LIGHT SNOW MOVING INTO THE
CENTRAL PLAINS FOR NEXT WEEKEND.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)
ISSUED AT 508 AM CST SUN DEC 28 2014

CEILINGS AT OR BELOW 1000 FEET AGL ARE LIKELY THIS EVENING BY 00Z IN
NORTH CENTRAL AND NORTHWEST NEBRASKA AS LIGHT SNOW DEVELOPS ALONG AN
INCOMING ARCTIC FRONT. THE LOW CEILINGS ARE THEN FORECAST TO SPREAD
INTO CENTRAL AND SOUTHWEST NEBRASKA DURING THE NIGHT. ALSO...
VISIBILITIES BELOW 3SM ARE POSSIBLE AS WELL WHERE LIGHT SNOW
DEVELOPS.

&&

.HYDROLOGY...
ISSUED AT 508 AM CST SUN DEC 28 2014

AS ARCTIC COLD SETTLES INTO CENTRAL AND WESTERN NEBRASKA...THE
LIKELIHOOD OF ICE JAM DEVELOPMENT WILL INCREASE. BASED ON PREVIOUS
ICE JAMMING EVENTS...WE BELIEVE THAT SIGNIFICANT FLOODING IS
UNLIKELY. THE USUAL SPOTS ON THE NORTH PLATTE AND SOUTH PLATTE
RIVERS SUCH AS LEWELLEN AND NORTH PLATTE WILL HAVE BANK FULL STAGES
WITH A POSSIBILITY OF MINOR FLOODING. OTHER SMALLER STREAMS MAY BE
SUSCEPTIBLE AS WELL.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS...SPRINGER
SHORT TERM...SPRINGER
LONG TERM...MARTIN
AVIATION...SPRINGER
HYDROLOGY...SPRINGER





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