Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 250251
AFDSLC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
751 PM MST WED DEC 24 2014

.SYNOPSIS...A WINTER STORM MOVING OUT OF THE PACIFIC NORTHWEST
WILL IMPACT UTAH LATE TONIGHT THROUGH FRIDAY. UNSETTLED WEATHER
WILL DEVELOP AGAIN FOR LATE IN THE WEEKEND THROUGH EARLY NEXT
WEEK.

&&

.DISCUSSION...WATER VAPOR LOOP SHOWS A STORM APPROACHING FROM THE
PACIFIC NORTHWEST AND NORTHERN CALIFORNIA. AMDAR 400-250MB WIND
OBSERVATIONS INDICATE A 125-150KT CYCLONIC JET FROM OFF THE
PACIFIC NORTHWEST COAST INTO CENTRAL CALIFORNIA. GOES/GPS/RAP/00Z
RAOB INDICATE THE PRECIPITABLE WATER VALUE RANGES BETWEEN 0.15"
EASTERN MOUNTAINS...TO 0.40" NORTHWEST VALLEYS. BLENDED
PRECIPITABLE WATER PRODUCT SHOWS A MOISTURE TAP INTO CENTRAL
CALIFORNIA AND NEVADA..WITH VALUES ALREADY OVER 200% OF NORMAL
ACROSS MUCH OF THE WEST.

MSAS INDICATES 7MB/3HR PRESSURE FALLS ACROSS NORTHWEST UTAH
(EXCEPTION FOR THIS TIME OF NIGHT)...AHEAD OF A 997MB LOW ACROSS
SOUTH CENTRAL IDAHO AND ATTENDANT COLD FRONT THROUGH ELKO. PRE-
FRONTAL SOUTHERLY WINDS HAVE RESPONDED TO THE PRESSURE FALLS AND
TIGHTENING SOUTHERLY PRESSURE GRADIENT ACROSS THE WEST DESERT
WITH GUSTS INTO THE 40MPH RANGE.

RAP 850-700MB THICKNESS/SATURATED EQUIVALENT POTENTIAL
TEMPERATURE/STREAMLINES INDICATE THE COLD FRONT WILL PLOW THROUGH
THE WASATCH FRONT BETWEEN 2AM-4AM.

UPDATED TO INCREASE POPS IN THE MOUNTAINS STARTING NOW GIVEN
EXPANSIVE RADAR RETURNS AND SNOW REACHING THE GROUND AT EVANSTON.
POPS AFTER MIDNIGHT ARE CATEGORICAL AND WERE ONLY SLIGHTLY
TWEAKED. MOST ADJUSTMENT WAS MADE TO WINDS TO EMPHASIZE PRE-
FRONTAL SOUTHERLY WINDS ALREADY BEGINNING TO RAMP UP. COULD SEE
HIGHER GUSTS WORK SOUTHEAST ACROSS THE WEST DESERT ON THE LEADING
EDGE OF PRECIPITATION POST-FRONTAL...BUT WILL ALLOW NEXT SHIFT A
CHANCE TO LOOK AT UPSTREAM OBSERVATIONS BEFORE DECIDING ON A
POSSIBLE UPGRADE OF ZONE 5.

NEXT SHIFT WILL ALSO HAVE TO TAKE ANOTHER LOOK AT THE EASTERN
VALLEYS FOR POTENTIAL ADVISORIES.

&&

PREVIOUS DISCUSSION FOLLOWS...

THE UPPER TROUGH CURRENTLY ALONG THE PACIFIC NORTHWEST/NORTHERN
CALIFORNIA COAST WILL CONTINUE SOUTHEAST INTO THE WESTERN GREAT
BASIN AS AN OPEN TROUGH THIS EVENING. FROM THERE THIS TROUGH WILL
GAIN A CLOSED CIRCULATION LATE TONIGHT...WITH THIS CENTER DRIFTING
SOUTH-SOUTHEAST TO THE FOUR CORNERS REGION BY EARLY FRIDAY MORNING.

THE ASSOCIATED SURFACE COLD FRONT BASED ON MSAS DATA EXTENDS FROM
WEST-CENTRAL MONTANA THROUGH NORTHERN CALIFORNIA LATE THIS
AFTERNOON. SOLID PRESSURE FALLS ACROSS THE GREAT BASIN THIS
AFTERNOON SUGGEST THIS FRONT IS STRENGTHENING AS EXPECTED. CURRENT
TIMING WOULD PUT THE FRONT INTO EXTREME NORTHWEST UTAH AROUND EARLY
TO MID EVENING...THEN THROUGH THE URBAN CORRIDOR BETWEEN 06Z TO 10Z.

MOIST WARM ADVECTION AHEAD OF THE FRONT WILL LIKELY GENERATE SOME
LIGHT PRE-FRONTAL PRECIP THIS EVENING. PRECIP WILL RAPIDLY INCREASE
IN AREAL COVERAGE AND INTENSITY WITH THE FRONT ACROSS
NORTHERN/WESTERN UTAH DUE TO THE STRONG DYNAMIC LIFT AND GOOD
BAROCLINIC STRUCTURE TO THIS STORM. PRECIP WILL REMAIN FAIRLY ACTIVE
NEAR THE FRONT ACROSS SOUTHERN AND EASTERN UTAH LATE TONIGHT/EARLY
FRIDAY...THOUGH LIKELY NOT AS INTENSE AS THE FRONT WILL SHOW MODEST
WEAKENING AS IT PRESSES INTO SOUTHEAST UTAH.

SNOW WILL PERSIST ACROSS THE FORECAST AREA AS THE OPEN TROUGH CLOSES
OFF OVER THE AREA. SYNOPTIC-SCALE LIFT ASSOCIATED WITH THIS FEATURE
SHOULD PROMOTE WIDESPREAD SNOW...WITH WARM ADVECTION WRAP AROUND
ALONG THE NORTHERN AND WESTERN FLANKS OF THE UPPER LOW THE AREAS OF
HEAVIEST PRECIP.

SNOW WILL BEGIN TO WIND DOWN FRIDAY AS THE UPPER LOW SETTLES INTO
THE FOUR CORNERS REGION. THE COLD AND UNSTABLE AIR MASS TRAILING THE
EXITING LOW WILL CONCENTRATE ADDITIONAL SNOWS ACROSS THE CENTRAL AND
SOUTHERN MOUNTAINS...AS WELL AS ALONG THE I-15/I-70 CORRIDORS OF
CENTRAL AND SOUTHWEST UTAH. HAVE BOOSTED SNOW TOTALS SUBSTANTIALLY
IN THOSE AREAS TO ACCOUNT FOR THE FAVORABLE SNOW CONDITIONS.

CONDITIONS SHOULD STABILIZED FAIRLY QUICKLY AS AN ANTICYCLONIC NORTH-
NORTHWEST FLOW ALOFT DEVELOPS BEHIND THE EXITING LOW. OVERRUNNING
MOISTURE WILL RETURN AGAIN LATE SATURDAY AHEAD OFT HE NEXT STORM
SYSTEM SCHEDULED FOR LATE IN THE WEEKEND.

12Z GLOBAL RUNS IN BETTER AGREEMENT REGARDING THE INITIAL STAGES OF
THE NEXT TROUGH TO IMPACT THE EASTERN GREAT BASIN SUNDAY THROUGH
TUESDAY. RAPID AMPLIFICATION OF THE WEST COAST RIDGE WILL ALLOW A
STRONG SHORT WAVE TO CARVE ALMOST DUE SOUTH THROUGH EASTERN WA/OR
SATURDAY NIGHT SETTING UP DECENT NEAR H7 BAROCLINICITY ACROSS
SOUTHERN IDAHO AT THAT TIME. THE ASSOCIATED SURFACE COLD FRONT IS
EXPECTED TO TRANSITION SOUTH ACROSS THE AREA SUNDAY NIGHT BRINGING
STRONG LOW LEVEL COLD ADVECTION AND FAIRLY WIDESPREAD LIGHT SNOW TO
AT LEAST NORTHERN UTAH. THE MORE WESTERN/AMPLIFIED SOLUTIONS OF
PREVIOUS MODEL RUNS SEEMS TO HAVE WON OUT.

THE AIRMASS TAPPED UPSTREAM WILL HAVE A STRONG CONTINENTAL
FLAVOR...AND A VERY COLD AND FAIRLY DRY ARCTIC AIRMASS ASSOCIATED.
H7 TEMPS AT KSLC FORECAST TO BE -15 TO -20 C DEPENDING ON MODEL OF
CHOICE. MODEL QPF REMAINS QUITE LOW WITH AND BEHIND THE FRONT BUT
EXPECTING HIGH SNOW RATIOS WITH ANY PRECIPITATION THAT DOES FALL.

&&

.AVIATION...A SIGNIFICANT WINTER STORM WILL IMPACT THE SLC TERMINAL
EARLY CHRISTMAS MORNING INTO FRIDAY MORNING. A STRONG COLD FRONT IS
EXPECTED TO CROSS THE TERMINAL BETWEEN 09-10Z TONIGHT...BRINGING
GUSTY NORTHWEST WINDS...ACCUMULATING SNOW AND LIFR CONDITIONS FOR
THE REMAINDER OF THE MORNING. SNOWFALL INTENSITIES ARE EXPECTED TO
DECREASE THURSDAY AFTERNOON BUT PERIODIC IFR/LIFR CONDITIONS ARE
EXPECTED TO CONTINUE INTO THE OVERNIGHT HOURS. 2 TO 5 INCHES OF SNOW
ACCUMULATION IS EXPECTED ON UNTREATED SURFACES.

IN THE MEANTIME...SOUTH WINDS WILL CONTINUE TO GUST IN EXCESS OF 30
MPH UNTIL THE FRONT ARRIVES.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WARNING FROM 2 AM THURSDAY TO NOON MST FRIDAY FOR
     UTZ001>004-006>010-014>016-517-518.

     WINTER WEATHER ADVISORY FROM 2 AM THURSDAY TO NOON MST FRIDAY
     FOR UTZ005.

WY...WINTER STORM WARNING FROM 2 AM THURSDAY TO NOON MST FRIDAY FOR
     WYZ021.

&&

$$

PUBLIC...ROGOWSKI/CONGER/MERRILL
AVIATION...SEAMAN

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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