Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 192330
AFDSLC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
430 PM MST FRI DEC 19 2014

.SYNOPSIS...HIGH PRESSURE ALOFT OVER UTAH WILL YIELD TO AN
INCREASINGLY MOIST WEST TO NORTHWEST FLOW BEGINNING TONIGHT. AN
ACTIVE PERIOD OF VALLEY RAIN AND MOUNTAIN SNOW WILL RESULT FROM
THIS MOISTURE OVER THE WEEKEND THROUGH EARLY NEXT WEEK.

&&

.SHORT TERM (THROUGH 00Z TUESDAY)...SHORTWAVE RIDGE OVER UTAH
EARLIER THIS AFTERNOON IS SHIFTING EAST OF THE AREA...YIELDING ITS
POSITION TO A SPLITTING SHORTWAVE TROUGH. A FEW WEAK SPOTTY SHOWERS
WILL CONTINUE ACROSS NORTHERN UTAH THE NEXT FEW HOURS BEFORE
PRECIPITATION FILLS IN MORE AS 700MB COLD ADVECTION DEVELOPS LATER
THIS EVENING. PRECIPITATION TOTALS ARE GENERALLY EXPECTED TO BE
MINOR AS DYNAMICS ASSOCIATED WITH THIS WAVE IS FAIRLY WEAK.
HOWEVER...SNOW LEVELS GENERALLY NEAR 4500-5000FT OVERNIGHT COULD
OCCASIONALLY TOUCH THE VALLEY FLOORS. UNSETTLED WEATHER WILL
CONTINUE THROUGH TOMORROW AS ANOTHER WEAK DISTURBANCE RIPPLES
THROUGH NORTHERN UTAH TOMORROW AFTERNOON.

ABUNDANT MOISTURE ASSOCIATED WITH AN ATMOSPHERIC RIVER EVENT IS
STILL EXPECTED FOR THE LATTER PART OF THE WEEKEND AND INTO MONDAY.
PRECIPITATION WILL FILL IN TOMORROW NIGHT ACROSS NORTHERN PORTIONS
OF THE AREA AS 700MB WARM ADVECTION STRENGTHENS ONCE AGAIN IN
NORTHWEST FLOW ALOFT. SNOW LEVELS LOOK TO START OFF 5000-
6000FT...QUICKLY RISING TO 7000-7500FT BY SUNDAY AFTERNOON. GREATEST
PRECIPITATION RATES LOOK TO OCCUR SUNDAY THROUGH SUNDAY NIGHT WITH
THE STRONGEST MOISTURE CONVERGENCE. HOWEVER...WITH LONG-DURATION
EVENT AND ANOMALOUSLY HIGH MOISTURE CONTENT COMPARED TO
CLIMATOLOGY...NEEDLESS TO SAY PRECIPITATION AMOUNTS WILL BE
SIGNIFICANT. MODELS CONTINUE TO INDICATE OVER 2 INCHES STORM TOTAL
QPF FOR THE NORTHERN MOUNTAINS. THE MILD AIRMASS WILL YIELD
RELATIVELY HIGH SNOW DENSITIES...BUT WITH THE AMOUNT OF
PRECIPITATION EXPECTED...SNOW TOTALS WILL STILL BE SIGNIFICANT
ACROSS THE NORTHERN UTAH MOUNTAINS. IN ADDITION TO THE SNOW...WINDS
ALONG RIDGELINES WILL BE STRONG AND RESULT IN BLOWING AND DRIFTING
OF SNOW. A WINTER STORM WATCH REMAINS IN EFFECT WITH NO CHANGES MADE
DURING THIS SHIFT. MOST NORTHERN VALLEY AREAS WILL REMAIN MILD WITH
MOSTLY RAIN FORECAST FOR THIS EVENT WHILE MUCH OF THE SOUTHERN THIRD
OF UTAH WILL REMAIN ON THE DRY SIDE.

&&

.LONG TERM (AFTER 00Z TUESDAY)...RAPID AMPLIFICATION OF THE WEST
COAST RIDGE COUPLED WITH FURTHER DEEPENING OF A STRONG UPPER LOW
OVER THE NORTHERN PLAINS WILL FINALLY BREAK THE PACIFIC MOISTURE TAP
MONDAY EVENING. MODELS...ESPECIALLY THE 12/18Z GFS...HINTING AT SAID
SHORT WAVE DRIVING A BACKDOOR COLD FRONT INTO NORTHERN UTAH ALLOWING
STRONG CAA TO PUNCH SOUTHEAST ACROSS MUCH OF THE NORTHERN HALF OF
THE STATE MONDAY NIGHT. FRONTAL BOUNDARY DEPICTED AS BEING QUITE
SHALLOW AND ONLY NOTED BELOW H5 IN TIME HEIGHTS...BUT H7 TEMPS AS
COLD AS -11 C BY 06Z TUESDAY AT KSLC. AS MENTIONED ABOVE...EXPECTING
MOISTURE TO DECREASE SIGNIFICANTLY WITH THIS FRONTAL PASSAGE BUT
COULD SEE SNOW LEVELS BRIEFLY LOWER TO NEAR VALLEY LEVELS MONDAY
AFTERNOON INTO THE EVENING.

GLOBAL MODELS MORE AND MORE IN LINE TRANSITIONING THE WEST COAST
RIDGE AXIS OVERHEAD TUESDAY INTO WEDNESDAY...WITH A STRONG SURFACE
HIGH SETTLING INTO SOUTHEASTERN IDAHO. TRIMMED POPS SIGNIFICANTLY
FOR THOSE PERIODS ACROSS THE NORTH AND NOW PORTRAY A DRY FORECAST IN
GRIDS.

FOCUS IN THE LONG TERM SHIFTS TO A POTENTIALLY POTENT TROUGH MOVING
IN FROM THE NORTHWEST CHRISTMAS MORNING. MODELS CONTINUE TO STRUGGLE
WITH TROUGH EVOLUTION AND AMPLIFICATION AS IT COMES ON
SHORE...LIKELY DUE TO SLIGHT VARIATIONS IN THE HANDLING OF THE
HIGHLY AMPLIFIED EARLY WEEK PATTERN PRECEDING IT. CONFIDENCE IS HIGH
THAT A STRONG COLD FRONT WILL CROSS MUCH OF THE AREA DURING THE DAY
BRINGING A MARKED COOL DOWN AND AT LEAST A BIT OF VALLEY RAIN
CHANGING TO SNOW. HOWEVER...IF THE TROUGH REMAINS PROGRESSIVE AND
LESS AMPLIFIED...MOST GFS RUNS OF LATE...THE FRONTAL BOUNDARY COULD
BE QUITE SHALLOW WITH SIGNIFICANTLY LESS PRECIP AND OR LOW ELEVATION
SNOWFALL THAN THE DEEPER SOLUTIONS OF SOME ECMWF RUNS AS OF LATE.
MADE MINIMAL CHANGES TO GRIDS REGARDING THIS EVENT AND AWAIT MORE
CONSISTENT RUN TO RUN AND MODEL TO MODEL AGREEMENT.

&&

.AVIATION...CIGS ARE EXPECTED TO REMAIN ABOVE THE 6000FT AGL LEVEL
AT THE KSLC TERMINAL THROUGH THE EARLY EVENING HOURS BUT WILL LIKELY
LOWER BELOW THAT AFTER 03-04Z AS SHOWERS FILL IN OVER THE AREA.
THERE IS A 20 PERCENT CHANCE OF MVFR CONDITIONS IN RAIN/SNOW MIX
OVERNIGHT. SOUTHEASTERLY TO SOUTHERLY SURFACE WINDS ARE EXPECTED TO
PREVAIL THROUGH THE NIGHT.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WATCH FROM LATE SATURDAY NIGHT THROUGH MONDAY
     AFTERNOON FOR UTZ007>010.

WY...NONE.
&&

$$

CHENG/MERRILL

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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