Weather Service Report


812 
FXUS65 KSLC 200246
AFDSLC

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

.SYNOPSIS...AN ACTIVE PERIOD OF VALLEY RAIN AND MOUNTAIN SNOW
WILL RESULT FROM THIS MOISTURE OVER THE WEEKEND THROUGH EARLY NEXT
WEEK.

&&

.DISCUSSION...WATER VAPOR LOOP SHOWS A SPLITTING TROUGH
APPROACHING...WITH THE STRONGER NORTHERN PIECE MOVING INTO
NORTHERN IDAHO. AMDAR 400-250MB WIND OBSERVATIONS INDICATE A
100-155KT NOSE OF A STRONG NORTHWESTERLY PACIFIC JET EXTENDS INTO
NORTHERN CALIFORNIA. GOES/GPS/RAP/00Z RAOB INDICATE THE
PRECIPITABLE WATER VALUE RANGES BETWEEN 0.15" MOUNTAINS...TO 0.45"
MOST VALLEYS. BLENDED PRECIPITABLE WATER PRODUCT SHOWS AN
ATMOSPHERIC RIVER APPROACHING THE PACIFIC NORTHWEST...WITH VALUES
ALREADY OVER 200% OF NORMAL ACROSS MUCH OF THE WEST.

RAP 850-700MB THICKNESS/SATURATED EQUIVALENT POTENTIAL 
TEMPERATURE/STREAMLINES INDICATE THE COOL FRONT IS ON OUR
DOORSTEP...AND WILL SLIDE THROUGH THE REMAINDER OF NORTHERN UTAH
AND SOUTHWEST WYOMING THROUGH THE NIGHT.

THE RESULTANT COOL ADVECTION SHOULD BE ENOUGH FOR LIGHT 
PRECIPITATION TO SPREAD ACROSS THE NORTHERN FORECAST AREA. NAM AND 
HRRR-3KM ARE QUITE BULLISH WITH PRECIPITATION CHANCES...WITH VERY 
LITTLE FROM THE GFS AFTER 06Z. WILL BUMP UP POPS BUT KEEP THEM TAME 
DUE TO THE GFS.

OTHERWISE TWEAKED OVERNIGHT LOWS UP A LITTLE ACROSS THE NORTH CLOSER 
TO LAMP GUIDANCE. REST UNCHANGED.

PREVIOUS DISCUSSION FOLLOWS... 
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.

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 LOWER TO NEAR 6000 FEET AGL BETWEEN 
03-05Z 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.
&&

$$

PUBLIC...ROGOWSKI/CHENG/MERRILL 
AVIATION...KRUSE

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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