FXUS65 KSLC 221701
AFDSLC
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
1001 AM MST TUE DEC 22 2009
.SYNOPSIS...THE UPPER LEVEL LOW PRESSURE SYSTEM OVER OREGON THIS
MORNING WILL MOVE SOUTH THROUGH NEVADA TODAY. THE MOIST AIRMASS
ACROSS THE GREAT BASIN AHEAD OF THE UPPER LOW WILL RESULT IN
WIDESPREAD PRECIP 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.
FOG CONTINUES TO DIMINISH THIS MORNING WITH COMBINATION OF DIURNAL
TRENDS AND AS CLOUDS THICKEN ALOFT AND PRECIPITATION DEVELOPS. HAVE
ALLOWED THE DENSE FOG ADVISORY TO EXPIRE.
PRESSURE RISE/FALL COUPLET SUGGESTS THAT THE SURFACE COLD FRONT IS
CURRENTLY CROSSING EASTERN NEVADA AND NEARING NORTHWEST UTAH.
PRESSURE IS FALLING COINCIDENTALLY ALONG A PRE-FRONTAL TROUGH
LOCATED ACROSS WESTERN UTAH EARLY THIS MORNING. CYCLOGENESIS IS
OCCURRING ALONG THIS AXIS WITH A LOW DEEPENING IN THE VICINITY OF
DELTA. COOLING CLOUD TOPS ON IR SATELLITE AND REGIONAL RADAR SUGGEST
MODERATE TO AT TIMES HEAVY SNOWFALL IS OCCURRING ACROSS THE WEST
DESERTS. FORECAST CHALLENGE IS HOW FAR EAST THIS ENHANCED
PRECIPITATION WILL REACH...BEFORE THE FOCUS SHIFTS SOUTHWARD INTO
SOUTHERN UTAH AS THE CYCLONE DEEPENS ACROSS NORTHERN ARIZONA THIS
EVENING. THE NAM HAS BEEN INDICATING THAT THE HEAVIER PRECIPITATION
REACHES THE TOOELE VALLEY THIS AFTERNOON. WILL HAVE TO MONITOR FOR A
POSSIBLE UPGRADE BASED ON OBSERVATIONAL TRENDS. HOWEVER WITH THE
RATE OF SPEED THE CYCLONE REESTABLISHES FURTHER SOUTH...OPTING TO
NOT TO JUMP ONTO THAT SOLUTION YET.
MEANWHILE...INCREASING DIVERGENCE ALOFT WILL SUPPORT ENHANCED
PRECIPITATION ACROSS SOUTHERN UTAH THIS AFTERNOON. WHILE 03Z SHORT
RANGE ENSEMBLES INDICATE A CHANCE OF NEGATIVE LIFTED INDICES AND
SLIGHT CHANCE OF GREATER THAN 100 J/KG OF MUCAPE...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.
LOOKS AS THOUGH CONDITIONS WILL DETERIORATE ALONG THE INTERSTATE 15
CORRIDOR OF WESTERN AND SOUTHWEST UTAH THIS EVENING WITH THE ARRIVAL
OF THE COLD FRONT.
WRAP AROUND MOISTURE SHOULD ENHANCE SNOW TOTALS ACROSS THE CENTRAL
AND SOUTHERN MOUNTAINS TONIGHT. ADDED ZONES 20/21 TO AN ADVISORY
THROUGH TOMORROW MORNING. WILL LOOK AT ADDITIONAL 12Z GUIDANCE AND
INCOMING OBSERVATIONS THIS AFTERNOON AND MAY HAVE TO MAKE
ADDITIONAL REVISIONS.
PREVIOUS DISCUSSION FOLLOWS...
WRAP AROUND PRECIP WILL EXPAND ACROSS
MUCH OF UTAH OVERNIGHT AS THE UPPER LOW TURNS EAST TOWARDS THE 4
CORNERS REGION. PRECIP WILL END MOST SECTIONS BY WEDNESDAY EVENING
AS AN INCREASINGLY STABLE NORTHERLY FLOW DEVELOP ACROSS THE AREA.
STRONG HIGH PRESSURE ALOFT ALONG THE WEST COAST WILL SHIFT EAST
INTO THE BASIN BY THE WEEKEND.
PLENTY OF MOISTURE EXISTS OVER UTAH AHEAD OF THE APPROACHING UPPER
LOW. INCREASINGLY COLD CLOUD TOPS...ESPECIALLY ACROSS SRN UTAH...
SUGGESTS ENOUGH LIFT EXISTS FOR PRECIP TO BEGIN THIS MORNING.
VALLEYS LOCATIONS DOWN SOUTH WILL SEE A MIX OF RAIN AND
SNOW...WITH POCKETS OF HEAVY SNOW DEVELOPING ACROSS THE FAR SRN
MOUNTAINS. UP NORTH P-TYPES LIKELY TO REMAIN MORE SNOW THAN
RAIN THROUGH EARLY AFTERNOON...WITH SNOW OVER ALL SECTIONS BY LATE
AFTERNOON OR EVENING.
THE TRACK OF THE UPPER LOW WILL KEEP THE CENTER JUST OUTSIDE OF
UTAH AS IT TURNS EAST THROUGH NRN AZ TONIGHT. DYNAMIC LIFT NOT
OPTIMAL FOR HEAVY PRECIP...BUT WITH DEEP LAYER MOISTURE AND DECENT
WARM ADVECTION IN THE WRAP AROUND FROM THE UPPER LOW...WOULD
THINK THAT SUBSTANTIAL AND WIDESPREAD PRECIP A GOOD BET FOR UTAH
INTO EARLY WEDNESDAY. WILL KEEP THE CURRENT SUITE OF WARNINGS IN
PLACE...THOUGH THEY MAY NOT CONTINUE INTO WEDNESDAY EVENING AS
THEY CURRENTLY EXIST. UP NORTH HAVE ISSUED SELECT ADVISORIES FOR
SNOW AS AMOUNTS AND DURATION EXPECTED TO BE LESS THAN OVER THE FAR
WRN AND SRN ZONES.
LOOKING AT A GENERALLY DRY HOLIDAY WEEKEND AS THE HIGH AMPLITUDE
RIDGE ALONG THE WEST COAST SLIDES EAST INTO THE BASIN BY THE
WEEKEND. TIGHTENING INVERSIONS WILL BRING HAZE/FOG BACK INTO PLAY
WITH THE HIGHER ELEVATIONS AND THE SRN VALLEYS SHOWING A WARMING
TREND THROUGH THE WEEKEND.
&&
.AVIATION...STRATUS DECK IS BREAKING UP THIS MORNING AT KSLC...BUT
STILL EXPECT BRIEF REDUCTIONS IN CIGS TO IFR/MVFR THROUGH LATE THIS
MORNING AS STRATUS REMAINS FLOATING AROUND IN THE VICINITY. REDUCED
VISIBILITIES ARE EXPECTED TO PREVAIL THROUGH THE MORNING...BUT
SHOULD REMAIN MVFR BETWEEN 2 TO 3SM. LIGHT SNOW IS EXPECTED TO
ACTUALLY IMPROVE THE VISIBILITY SLIGHTLY THROUGH THE AFTERNOON AS IT
CHEWS UP THE INVERSION...BUT A 30 PERCENT CHANCE EXISTS TEMPORARY
IFR SNOWFALL WILL OCCUR THROUGH 03Z TONIGHT. NOT LOOKING FOR MUCH
MORE THAN AN INCH OR TWO OF ACCUMULATION...BUT COLD SURFACE TEMPS
COULD SUPPORT ACCUMULATIONS ON NON-TREATED SURFACES.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WARNING UNTIL 11 PM WEDNESDAY FOR UTZ014-UTZ517-
UTZ518.
WINTER STORM WARNING UNTIL 5 PM WEDNESDAY FOR UTZ005-UTZ015-
UTZ016.
WINTER WEATHER ADVISORY UNTIL 11 AM WEDNESDAY FOR UTZ020-UTZ021.
WINTER WEATHER ADVISORY UNTIL 5 PM WEDNESDAY FOR UTZ003-UTZ004-
UTZ008-UTZ009-UTZ010.
WY...NONE.
&&
$$
PUBLIC...ROGOWSKI/CONGER
AVIATION...MERRILL
FOR MORE INFORMATION FROM NOAA/S NATIONAL WEATHER SERVICE VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY (ALL LOWER CASE)
