Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 240237
AFDSLC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
737 PM MST TUE DEC 23 2014

.SYNOPSIS...HIGH PRESSURE WILL BUILD OVER THE AREA THROUGH TONIGHT.
THE HIGH SHIFTS EAST WEDNESDAY AHEAD OF A COLD WINTER STORM FOR
CHRISTMAS DAY AND FRIDAY. UNSETTLED WEATHER WILL CONTINUE INTO THE
WEEKEND AND EARLY NEXT WEEK.

&&

.DISCUSSION...WATER VAPOR LOOP SHOWS AN EXPANSIVE EASTERN
PACIFIC RIDGE INTO THE NORTHERN ROCKIES...WITH THE NEXT SYSTEM
PROGRESSING FROM THE GULF OF ALASKA INTO BRITISH COLUMBIA. AMDAR
400-250MB WIND OBSERVATIONS INDICATE A 110-160KT ANTICYCLONIC JET
FROM BRITISH COLUMBIA DOWN THE ROCKIES. GOES/GPS/RAP/00Z RAOB
INDICATE THE PRECIPITABLE WATER VALUE RANGES BETWEEN 0.05"
NORTHEAST MOUNTAINS...TO 0.25" NORTHWEST VALLEYS. BLENDED
PRECIPITABLE WATER PRODUCT SHOWS A MOISTURE TAP INTO THE PACIFIC
NORTHWEST...WITH VALUES ALREADY OVER 150% OF NORMAL ACROSS MUCH OF
THE WEST.

SHORT TERM FORECAST ON TRACK. MADE SOME EDITS TO WINDS TO REFLECT
REMAINING DOWNSLOPE/CANYON WINDS ACROSS PRIMARILY EAST CENTRAL
AND SOUTHERN UTAH DUE TO THE NORTHEASTERLY PRESSURE GRADIENT.
OTHERWISE SATELLITE SHOWS LEADING EDGE OF SOME HIGH CLOUDS
ENTERING FAR NORTHWEST UTAH. A FEW CHANGES TO TEMPERATURE CURVE
THROUGH THE MORNING BASED ON NEW LAMP GUIDANCE. REST UNCHANGED.

&&

PREVIOUS DISCUSSION FOLLOWS...
HIGH PRESSURE IN PLACE OVER THE FORECAST AREA THIS AFTERNOON...RESULTING
IN DRY AND STABLE CONDITIONS WITH MOSTLY CLEAR SKIES. LIMITED
MIXING IS KEEPING TEMPERATURES DOWN DESPITE A WARMING
AIRMASS...BUT EVEN SO MAXES ARE STILL RUNNING RIGHT AROUND
SEASONAL NORMS.

HIGH PRESSURE WILL CONTINUE TO BUILD THROUGH EARLY TOMORROW BEFORE
THE RIDGE AXIS SLIDES EAST AHEAD OF THE NEXT APPROACHING WINTER
STORM. SOUTHWESTERLY FLOW WILL PICK UP...ALLOWING FOR IMPROVED
MIXING. AS A RESULT...MAXES WILL TEND TO RUN AT LEAST 5F ABOVE
VALUES OBSERVED TODAY...EXCEPT OVER EASTERN UTAH WHERE CONDITIONS
WILL STAY MORE STABLE.

NOT MUCH CHANGE IN THE 12Z GUIDANCE WITH REGARD TO THE CHRISTMAS
STORM SYSTEM. THE SYSTEM NOSES INTO NORTHWEST UTAH LATE
WEDNESDAY...WITH THE FRONT MAKING ITS WAY THROUGH MUCH OF THE STATE
THURSDAY MORNING. SNOW LEVELS SHOULD DROP TO VALLEY FLOORS SOON
AFTER PRECIP BEGINS...AND THE GREATEST BURST OF SNOW SHOULD BE WITH
THE FRONT ON THURSDAY MORNING. COLD POOL INSTABILITY WILL KEEP SNOW
GOING THROUGH THE AFTERNOON ON THURSDAY. THE STORM SLOWS DOWN AND
SPLITS A BIT OVER THE AREA THURSDAY NIGHT/FRIDAY...WHICH WILL KEEP
SOME SNOW GOING...AT LEAST OVER THE HIGHER TERRAIN OF NORTHERN AND
CENTRAL UTAH. DID NOT CHANGE VALLEY/MOUNTAIN VALLEY SNOW AMOUNTS
MUCH WITH THE AFTERNOON PACKAGE BUT DID LOWER NORTHERN MOUNTAIN
TOTALS A BIT. AM CONCERNED THAT LACK OF FLOW BEHIND THE FRONT WILL
LIMIT THE OROGRAPHIC COMPONENT. GOING WINTER STORM WATCHES COVER THE
THREAT WELL AT THIS TIME.

MOISTURE LINGERS FRIDAY EVENING AS THE TROUGH LAGS IN EXITING THE
STATE. A TRAILING SHORTWAVE PROMPTLY DIVES SOUTH THROUGH THE
AREA...REINFORCING THE TROUGH AND OFFERING A CHANCE OF ADDITIONAL
PRECIPITATION IN NORTHERN UTAH FRIDAY NIGHT INTO SATURDAY MORNING.
SATURDAY AFTERNOON BRINGS A BREAK IN THE ACTION BEFORE ANOTHER
DISTURBANCE SLUMPS TOWARD UTAH SATURDAY NIGHT INTO SUNDAY.

MODELS DIFFER SIGNIFICANTLY ON THE NATURE OF THIS FEATURE...WITH THE
GFS SENDING MOST OF THE ENERGY NORTH OF THE AREA AND LEAVING A
SMATTERING OF PRECIP GRAZING NORTHERN UTAH. THE ECMWF IS MUCH MORE
CONFIDENT IN DISPLAYING THIS FEATURE AS A DEEPENING TROUGH SPREADING
ITS MOISTURE AND 500 MB COLD POOL NEARLY TO THE SOUTHEASTERN REACHES
OF THE CWA. HAVE NUDGED POPS TO CLIMO FOR THIS PERIOD TO ACCOMMODATE
THIS DISCREPANCY.

700MB TEMPS ARE WIDELY DISPARATE AS WELL...WITH THE GFS SPIRALING
UPWARDS WHILE THE EC PLUMMETS. THUS LITTLE CHANGE MADE TO TEMP GRIDS
UNTIL FURTHER MODEL AGREEMENT IS REACHED. BIGGEST IMPACT FOR A TEMP
DIFFERENCE WOULD BE FOR P-TYPE AT THE LOWEST ELEVATIONS. HIGHEST
CONFIDENCE FOR THIS LONG-TERM PERIOD IS THE PERSISTENCE OF SUB-CLIMO
TEMPS STATEWIDE.

&&

.AVIATION...NO WEATHER IMPACTS TO THE SLC TERMINAL OVERNIGHT WITH
MOSTLY CLEAR SKIES AND A LIGHT SOUTHEAST DRAINAGE FLOW. SOUTHERLY
WINDS MAY BECOME GUSTY TOMORROW AFTERNOON AHEAD OF THE CHRISTMAS
STORM.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WATCH FROM LATE WEDNESDAY NIGHT THROUGH FRIDAY
     MORNING FOR UTZ001>010-014>016-517-518.

WY...WINTER STORM WATCH FROM LATE WEDNESDAY NIGHT THROUGH FRIDAY
     MORNING FOR WYZ021.

&&

$$

SHORT TERM...ROGOWSKI/TRAPHAGAN/VERZELLA
AVIATION...ROGOWSKI

FOR MORE INFORMATION FROM NOAA/S NATIONAL WEATHER SERVICE VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY (ALL LOWER CASE)

FOR FREQUENTLY ASKED QUESTIONS ABOUT THE AREA FORECAST DISCUSSION
VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY/GENERAL/AFD_FAQS.PHP (ALL LOWER CASE)


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