Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 211148
AFDSLC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
448 AM MST SAT FEB 21 2015

.SYNOPSIS...A COOL AND UNSTABLE NORTHWEST FLOW WILL REMAIN IN
PLACE ACROSS NORTHERN UTAH TODAY. A MORE ORGANIZED STORM SYSTEM
IS EXPECTED TO BRING WIDESPREAD PRECIPITATION TO THE SOUTHERN HALF
OF UTAH SUNDAY THROUGH THE FIRST HALF OF NEXT WEEK.


&&

.SHORT TERM (THROUGH 00Z WEDNESDAY)...A DEEP LAYER NORTHWEST FLOW
RESIDES ACROSS THE GREAT BASIN/INTERMOUNTAIN WEST EARLY THIS
MORNING...ALL PART OF A VERY BROAD LONGWAVE TROUGH WHICH
ENCOMPASSES MUCH OF THE CONUS. THE UPPER JET BISECTS THE FORECAST
AREA WHICH IS ALLOWING FOR A RELATIVELY COLD...MOIST AND UNSTABLE
AIRMASS ON THE POLEWARD SIDE OF THIS JET ACROSS NORTHERN UT.

SEVERAL WEAK PERTURBATIONS CONTINUE TO ROTATE THROUGH THE FLOW
RESULTING IN BURSTS OF CONVECTION...AND THIS WILL CONTINUE THROUGH
THE DAY TODAY...PERHAPS INCREASING THIS AFTERNOON WITH THE
APPROACH OF A STRONGER SHORTWAVE FEATURE CURRENTLY CROSSING THE
PACIFIC NORTHWEST. HAVE GONE WITH ISOLATED TO SCATTERED POPS
ACROSS THE NORTHERN VALLEYS...HIGHER ALONG THE WASATCH BACK...AND
CATEGORICAL ACROSS THE NORTHERN MOUNTAINS. GIVEN THE SHOWERY
NATURE OF THE PRECIPITATION SNOWFALL AMOUNTS WILL GENERALLY REMAIN
MINOR ACROSS THE HIGHER TERRAIN AND MINIMAL ACROSS THE VALLEYS.

THE PACIFIC NORTHWEST SHORTWAVE IS FORECAST TO CONTINUE SOUTHWARD
TODAY...EVENTUALLY EVOLVING INTO A CLOSED LOW AND LINGERING ACROSS
NV LATE SUNDAY THROUGH MONDAY...BEFORE EVENTUALLY PROGRESSING
SOUTHEAST THROUGH THE DESERT SOUTHWEST TUESDAY. TWO POTENTIALLY
SIGNIFICANT IMPACTS ON THE FORECAST WILL RESULT FROM THIS LOW...AN
EASTERLY WIND EVENT ACROSS PORTIONS OF NORTHERN UT AND A
PROLONGED PERIOD OF SNOWFALL ACROSS THE SOUTH.

WITH RESPECT TO THE NORTHERN UT WIND EVENT...EASTERLY LOW LEVEL
FLOW IS FORECAST TO DEVELOP LATE TONIGHT THROUGH SUNDAY AND INTO
MONDAY ACROSS NORTHERN UT AS THE AFOREMENTIONED WAVE DIGS INTO
THE GREAT BASIN. LACK OF A CRITICAL LAYER NEAR RIDGETOP COUPLED
WITH FAIRLY WEAK COLD ADVECTION AT 700MB SHOULD LIMIT WIND GUSTS
TONIGHT INTO SUNDAY MORNING INTO THE HIGH 30 TO LOW 40 MPH
RANGE...AND LIKELY KEEP THE STRONGER GUSTS CONFINED TO CANYON
MOUTHS. A SOMEWHAT BETTER SETUP IS ADVERTISED SUNDAY NIGHT INTO
MONDAY AS COLD ADVECTION STRENGTHENS SOMEWHAT. LOCAL HI-RES
GUIDANCE SUGGESTS WINDS WILL GENERALLY PEAK IN THE HIGH END
ADVISORY RANGE WITH SOME POTENTIAL FOR A FEW LOW END WARNING
GUSTS. HELD OFF ON THE ISSUANCE OF ANY HEADLINES AT THIS TIME.

REGARDING HEAVY SNOWFALL ACROSS SOUTHERN UT...THE MODELS ALL
SUGGEST A PROLONGED PERIOD OF MODERATE TO HEAVY SNOWFALL ACROSS
MUCH OF THE SOUTHERN THIRD OF THE STATE BEGINNING AS EARLY AS
LATE TONIGHT...BUT HITTING IN EARNEST SUNDAY THOUGH MONDAY BEFORE
TAPERING OFF MONDAY NIGHT INTO TUESDAY. NOT MUCH CHANGE TO
PREVIOUS FORECAST REASONING AS THE MODELS STALL A 700MB BAROCLINIC
ZONE ACROSS THIS AREA WHICH SERVES AS A FOCUS FOR PERIODS OF HEAVY
SNOWFALL. HIGHER SNOW LEVEL ALONG THE AZ BORDER WILL LIKELY KEEP
P-TYPE AS RAIN ACROSS UTAHS DIXIE AS WELL AS NEAR LAKE POWELL.
GOING FORECAST SNOW TOTALS APPEARED REASONABLE THUS MADE ONLY
MINOR CHANGES WHILE INCREASING POPS INTO THE CATEGORICAL RANGE
SUNDAY THROUGH MONDAY ACROSS MOST AREAS IN SOUTHERN AND CENTRAL
UT...BEFORE TRENDING POPS DOWNWARD LATE MONDAY NIGHT INTO TUESDAY.

&&

.LONG TERM (AFTER 00Z WEDNESDAY)...THE TROUGH LOOKS TO FULLY EXIT
THE REGION TO THE SOUTHEAST ON TUESDAY EVENING...WITH A BRIEF PERIOD
OF SHORTWAVE RIDGING EXPECTED TUESDAY NIGHT. WEDNESDAY COULD BE THE
WARMEST DAY OF THE WEEK...WITH A WARMER AIRMASS IN PLACE FOR AT
LEAST THE BEGINNING OF THE DAY...BUT THIS IS DEPENDENT ON HOW DEEP
THE NEXT TROUGH IS AND HOW QUICKLY IT MOVES INTO THE CWA.

DIFFERENCES IN GLOBAL MODELS ARE ALREADY APPARENT AS EARLY AS 12Z
WEDNESDAY...AS THEY HAVE BEEN INCONSISTENT ON THE EXACT TRACK AND
DEPTH OF THE NEXT TROUGH MOVING IN FROM THE PACIFIC NORTHWEST.
HOWEVER...THE IDEA OF THE OVERALL PATTERN IS HIGHER
CONFIDENCE...WITH A SERIES OF TROUGHS MOVING INTO THE AREA FROM THE
NORTH-NORTHWEST.

THE SHORTWAVE ON WEDNESDAY COULD BE SOMEWHAT WEAKER COMPARED TO
TROUGHS BEFORE/AFTER IT...BUT MODELS ARE IN GOOD AGREEMENT ABOUT
BRINGING THE DEEPER LONGWAVE TROUGH OVER THE INTERMOUNTAIN WEST BY
THURSDAY...AND LETTING IT LINGER IN SOME FORM OR FASHION THROUGH THE
END OF THE WEEK. THIS KEEPS A COLDER AND POTENTIALLY UNSETTLED
AIRMASS OVER THE REGION FOR MUCH OF THE EXTENDED PERIOD....WITH
SIGNIFICANT WARMING/DRYING NOT EXPECTED UNTIL SUNDAY AT THE
EARLIEST. THAT SAID...GIVEN THE CONTINENTAL ORIGINS OF THE
AIRMASS...THIS DOES NOT LOOK LIKE THE MOISTEST PATTERN IN THE
WORLD...AND PRECIPITATION WILL LIKELY BE OFF-AND-ON BASED ON
SHORTWAVES ROTATING THROUGH THE LARGER TROUGH.

HAVE INCREASED POPS AND SKY A BIT FOR THURSDAY AND
FRIDAY...ESPECIALLY IN PERIODS WHERE MODELS HAD DECENT AGREEMENT.
OTHERWISE...HAVE GENERALLY KEPT POPS AT OR ABOVE CLIMATOLOGY FOR THE
REST OF THE EXTENDED. HAVE ALSO TRENDED TEMPERATURES COLDER THROUGH
MUCH OF THE EXTENDED...AS CONFIDENCE IS INCREASING THAT A DECENTLY
COLD AIRMASS WILL BE IN PLACE FOR THE END OF THE WEEK...WITH 700MB
TEMPERATURES GENERALLY RANGING FROM -8 TO -14 CELSIUS THURSDAY
THROUGH SATURDAY.

&&

.AVIATION...THERE IS A 40 PERCENT CHANCE OF MVFR CONDITIONS AT THE
SLC TERMINAL TODAY DUE TO PASSING SHOWERS...WITH A MIX OF RAIN AND
SNOW MOST LIKELY AT THE TERMINAL. EXTENDED PERIODS OF CEILINGS BELOW
7000FT ARE ALSO LIKELY BETWEEN 11Z AND 23Z. SURFACE WINDS ARE
EXPECTED TO SWITCH TO THE NORTHWEST BETWEEN 16Z AND 18Z...BUT WILL
LIKELY NOT BE STRONGER THAN 7 KNOTS UNTIL 20Z OR LATER.

&&

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WATCH FROM THIS EVENING THROUGH MONDAY AFTERNOON
     FOR UTZ013>016-020-517-518.

WY...NONE.
&&

$$

SEAMAN/SCHOENING

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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