FXUS65 KSLC 222235 CCA
AFDSLC

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
308 PM MST TUE DEC 22 2009

.SYNOPSIS...THE UPPER LEVEL LOW PRESSURE SYSTEM OVER WESTERN
NEVADA THIS EVENING...WILL TRACK ACROSS NORTHERN ARIZONA TONIGHT.
THE MOIST AIRMASS ACROSS THE GREAT BASIN AHEAD OF THE UPPER LOW
WILL RESULT IN WIDESPREAD PRECIPITATION THOUGH TONIGHT.
PRECIPITATION WILL LINGER THROUGH WEDNESDAY...FOLLOWED BY HIGH
PRESSURE ALOFT RETURNING LATE IN THE WEEK.

&&

.DISCUSSION...WATER VAPOR LOOP SHOWS AN UPPER LOW DROPPING SOUTH 
FROM THE PACIFIC NORTHWEST...WHILE ANOTHER UPPER LOW MOVES ACROSS 
BRITISH COLUMBIA. MEANWHILE...AN AMPLIFIED RIDGE DOMINATES THE 
EASTERN PACIFIC OCEAN. 400-250MB ACARS WIND OBSERVATIONS PLACES THE 
UPPER TROUGH OVER SOUTHERN CALIFORNIA...WITH A 100-130KT 
CYCLONICALLY CURVED JET NOTED FROM CALIFORNIA INTO ARIZONA. 12Z 
700MB ANALYSIS SHOWS A CYCLONE OVER WASHINGTON STATE...WITH THE COLD 
FRONT EXTENDING SOUTH ACROSS NORTHERN NEVADA AND CALIFORNIA. WARM 
ADVECTION CONTINUES WITH -7C REPORTED AT KSLC AND -5C UPSTREAM AT 
KDRA. 12Z KSLC RAOB SHOWS 5C OF WARMING BETWEEN 800MB AND 
850MB...WITH 3 TO 5C OF COOLING ALOFT DURING THE PAST 24 HOURS. A 
STRONG 9C INVERSION REMAINS IN PLACE NEAR 850MB.

GOES/RUC PRECIPITABLE WATER INDICATE VALUES RANGE FROM A QUARTER 
INCH ACROSS EASTERN UTAH TO A HALF INCH ACROSS WESTERN AND SOUTHERN 
UTAH.

THE STORM WILL BE WINDING DOWN ACROSS NORTHERN UTAH THROUGH THE 
EARLY EVENING HOURS AS LOW PRESSURE SINKS INTO SOUTHWEST UTAH AND 
CONTINUES TO DEEPEN. PERSISTENT LIGHT SNOW WILL ACCUMULATE IN THE 
UINTA BASIN AND CASTLE COUNTY...WITH HIGHER SNOW RATES DUE TO THE 
TRAPPED COLD AIR. WILL ADD THESE AREAS INTO THE ADVISORY.

EXPECT CONDITIONS WILL DETERIORATE ALONG THE INTERSTATE 15 CORRIDOR 
OF WESTERN AND SOUTHWEST UTAH THIS EVENING WITH THE ARRIVAL OF THE 
COLD FRONT. WINDS WILL ALSO BEGIN TO BECOME BREEZY WHICH AID IN 
BLOWING AND DRIFTING SNOW. GUSTS MAY APPROACH WIND ADVISORY
CRITERIA ACROSS THE NORTH SOUTH CANYONS OF WASHINGTON COUNTY BY
MORNING.

WITH THE CYCLONE DEEPENING AS IT TRAVERSES NORTHERN ARIZONA 
TONIGHT...SOUTHERN UTAH WILL SEE ENHANCED WRAP AROUND MOISTURE. WILL 
ISSUE A WINTER WEATHER ADVISORY FOR ZONE 13 AND CONTINUE THE ONE 
VALID FOR ZONES 20 TO 21.

09Z SHORT RANGE ENSEMBLES INDICATE LESS OF A CHANCE OF NEGATIVE 
LIFTED INDICES AND GREATER THAN 100 J/KG OF MUCAPE ACROSS SOUTHERN 
UTAH THROUGH THIS EVENING THAN THE PREVIOUS RUN. 12Z NAM FORECAST 
SOUNDINGS ARE NOT VERY SUPPORTIVE OF THUNDER. WILL KEEP THUNDER OUT 
OF THE FORECAST WITH THE THOUGHT THAT MUCH IF NOT ALL OF THE 
LIGHTNING OCCURS SOUTH OF THE ARIZONA BORDER.

SNOW TAPERS OFF TOMORROW AS THE STORM SYSTEM PROGRESSES INTO THE 
PLAINS. A SECONDARY SYSTEM DROPPING SOUTH FROM BRITISH COLUMBIA IS 
NOW FORECAST TO STAY EAST OF THE ROCKIES...ALLOWING COLD HIGH 
PRESSURE TO BUILD INTO THE REGION DURING THE LATE WORK WEEK.

TODAYS PRECIPITATION HAS SERVED TO IMPROVE AIR QUALITY ACROSS THE 
VALLEYS OF NORTHERN UTAH. THE NEW AIRMASS ARRIVES TONIGHT WHICH 
SHOULD RESET ATMOSPHERIC CONDITIONS. WITH COLDER SURFACE 
TEMPERATURES MID WEEK AND WARMING AT RIDGETOP LEVEL BY LATE THIS 
WORK WEEK...EXPECT INVERSION TO REDEVELOP WITH AIR QUALITY 
DETERIORATING AGAIN HEADING INTO THE WEEKEND.

MODELS CONTINUE TO AGREE THAT HIGH PRESSURE WILL MOVE INTO THE 
FORECAST AREA THROUGH THE WEEKEND. THIS WILL RESULT IN THE RETURN OF 
VALLEY INVERSIONS WITH TEMPERATURES GRADUALLY WARMING OVER THE 
HIGHER ELEVATIONS. BY TUESDAY THE RIDGE IS EXPECTED TO SHIFT EAST AS 
A PACIFIC STORM SYSTEM MOVES ONSHORE. 12Z GFS KEEPS THE CENTER OF 
THIS FEATURE WELL SOUTH OF THE AREA...WHICH WOULD KEEP PRECIP 
PRIMARILY OVER SOUTHERN UTAH. HOWEVER...THE EC KEEPS IT FARTHER 
NORTH...WHICH WOULD SPREAD MOISTURE OVER THE ENTIRE STATE. WITH BOTH 
MODELS SHOWING AN UNSETTLED PATTERN...HAVE KEPT POPS AROUND CLIMO 
FOR TUESDAY. BY WEDNESDAY...THE STORM SYSTEM IS PROGGED TO EXIT THE 
AREA...WITH HIGH PRESSURE AGAIN DEVELOPING BEHIND IT.

&&

.AVIATION...PERIODS OF LIFR/IFR SNOWFALL IS EXPECTED TO PERSIST AT 
KSLC THROUGH 00Z BEFORE TURNING MORE SHOWERY HEADING INTO THE 
EVENING HOURS. THAT SAID...A 30 PERCENT CHANCE EXISTS TEMPORARY IFR 
SNOWFALL WILL OCCUR THROUGH 12Z TOMORROW MORNING. NOT LOOKING FOR 
MUCH MORE THAN AN INCH OR TWO OF ACCUMULATION TOTAL...BUT COLD 
SURFACE TEMPS WILL CONTINUE TO SUPPORT ACCUMULATIONS ON NON-TREATED 
SURFACES.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WARNING UNTIL 4 PM WEDNESDAY FOR UTZ517-UTZ518.

     WINTER STORM WARNING UNTIL 11 AM WEDNESDAY FOR UTZ005-UTZ014-
     UTZ015-UTZ016.

     WINTER WEATHER ADVISORY UNTIL 11 AM WEDNESDAY FOR UTZ003-UTZ004-
     UTZ011-UTZ012-UTZ013-UTZ020-UTZ021.

     WINTER WEATHER ADVISORY UNTIL 4 PM WEDNESDAY FOR UTZ008-UTZ009-
     UTZ010.

WY...NONE.

&&

$$

PUBLIC...ROGOWSKI/TRAPHAGAN
AVIATION...MERRILL

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