968
FXUS65 KSLC 211020
AFDSLC
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
320 AM MST SAT DEC 21 2024
SYNOPSIS
HIGH PRESSURE WILL GIVE WAY TO A SERIES OF
DISTURBANCES THAT WILL BEGIN TO IMPACT THE AREA LATER SUNDAY INTO
MONDAY, CONTINUING THROUGH THE WEEK AND INTO NEXT WEEKEND.
SHORT TERM (THROUGH 12Z/5AM TUESDAY)
MID LEVEL RIDGING ACROSS
THE FORECAST AREA IS BEGINNING TO SLOWLY TRANSLATE EAST THIS
MORNING, WHICH WILL LEAVE A WARM AND DRY SOUTHERLY FLOW ACROSS THE
EASTERN GREAT BASIN TODAY. TEMPERATURES ALOFT WILL COOL SLIGHTLY,
AND WITH AN INCREASE IN CIRRUS WILL SEE A SLIGHTLY COOLING TREND
ACROSS WELL MIXED AREAS SUCH AS ELEVATIONS ABOVE 7000 FEET,
CENTRAL AND SOUTHWEST VALLEYS INCLUDING RICHFIELD AND CEDAR CITY,
ETC.... THIS WARM AIR ALOFT HAS ALSO ALLOWED VALLEY INVERSIONS TO
BECOME ESTABLISHED ACROSS TYPICALLY PRONE AREAS INCLUDING MOST
NORTHERN VALLEYS BELOW 5000 FEET. AS SUCH WILL CONTINUE TO SEE
PATCHY MORNING FOG AS WELL AS REDUCED AIR QUALITY AND HAZY
CONDITIONS. DESPITE THIS, WITH THE EXCEPTION OF THE BEAR RIVER
VALLEY TEMPERATURES WILL CONTINUE TO RUN 10-15 DEGREES ABOVE
CLIMO WITHIN THESE STAGNANT AIRMASSES.
AS THE UPPER RIDGE BREAKS DOWN, A TRANSITION TO A MORE PROGRESSIVE
PATTERN WILL FOLLOW FOR THE LATTER HALF OF THE WEEKEND INTO EARLY
NEXT WEEK. A PAIR OF WEAKER SHORTWAVE TROUGHS WITHIN THIS FLOW
WILL CROSS NORTHERN UTAH/SOUTHWEST WYOMING LATE SUNDAY THROUGH
MONDAY, WITH THE LATTER SYSTEM BEING THE SOMEWHAT BETTER ORGANIZED
OF THE TWO. THE RESULT WILL BE PERIODS OF LIGHT SNOW ACROSS THE
HIGHER TERRAIN OF NORTHERN UTAH, WITH 36 HOUR TOTALS OF 2-6 INCHES
LOOKING MOST LIKELY. WITH THE MILD NATURE OF THE SYSTEM, SNOW
LEVELS WILL REMAIN ABOVE 6000 FEET MAINTAINING RAIN AS THE
PREDOMINANT PRECIP TYPE IN THE NORTHERN VALLEYS. IF COLD AIR CAN
REMAIN TRAPPED IN AREAS SUCH AS THE CACHE AND BEAR RIVER VALLEYS,
THERE WOULD BE SOME POTENTIAL FOR PERIODS OF LIGHT FREEZING RAIN
DEPENDING ON SURFACE TEMPERATURES.
WITH THE INCREASE IN CLOUD COVER AND GRADUALLY COOLING
TEMPERATURES ALOFT, MOST NORTHERN VALLEYS SHOULD GRADUALLY MIX OUT
BETWEEN SUNDAY AFTERNOON AND LATE MONDAY, WITH THE POSSIBLE
EXCEPTION OF MORE SHELTERED LOCATIONS MENTIONED ABOVE. AS SUCH
EXPECT AN IMPROVEMENT IN AIR QUALITY WITH A REDUCED THREAT FOR
OVERNIGHT/MORNING FOG ALONG MOST OF THE WASATCH FRONT SUNDAY INTO
MONDAY.
LONG TERM (AFTER 12Z/5AM TUESDAY)
LONG-TERM PORTION OF THE
FORECAST IS PUNCTUATED BY TWO PRIMARY PERIODS OF ACTIVE WEATHER,
NAMELY IN THE LATE TUESDAY-WEDNESDAY TIMEFRAME AS WELL AS THE
THURSDAY-SATURDAY TIMEFRAME, WITH PERIODS OF BRIEF RIDGING IN
BETWEEN. THE LONGWAVE PATTERN IS FORECAST TO BE VERY PROGRESSIVE
DURING THE LONG-TERM FORECAST PERIOD, WHICH IS FAVORABLE FOR
BUILDING SNOWPACK AND PREVENTING PROLONGED INVERSION CONDITIONS.
KICKING THINGS OFF ON TUESDAY, ENSEMBLE AND DETERMINISTIC SOLUTIONS
ARE IN GOOD AGREEMENT REGARDING POSITION AND STRENGTH OF LARGE-SCALE
FEATURES, WITH CONSENSUS PLACING A HIGH PRESSURE RIDGE AXIS OVERHEAD.
THIS RIDGE WILL NOT BE IN PLACE FOR LONG, OWING TO THE PROGRESSIVE
NATURE OF THE PATTERN ADDRESSED ABOVE, AS THE NEXT SYSTEM
(REPRESENTED BY A DECAYING LONGWAVE TROUGH AND WEAKENING ATMOSPHERIC
RIVER) WILL ALREADY BEGIN TO EXERT SOME INFLUENCE ON OUR WEATHER AS
EARLY AS TUESDAY AFTERNOON. DURING THIS TIME, SOUTHWEST DEEP-LAYER
FLOW INCREASES, WITH 700MB WINDS INCREASING INTO THE 25-45KT RANGE
IN THE PRE-FRONTAL WARM ADVECTION REGIME. THIS WILL LEAD TO LOCALLY
ENHANCED WINDS OF SIMILAR MAGNITUDES ON EXPOSED TERRAIN AS WELL AS
IN THE LEE OF TERRAIN FEATURES ORIENTED PERPENDICULAR TO THE FLOW.
THERE IS MINIMAL SPREAD IN TERMS OF ONSET OF PRECIPITATION TIMING
WITH THIS SYSTEM, WITH PRECIPITATION BLOSSOMING ACROSS WESTERN UTAH
TUESDAY EVENING, BEFORE OVERSPREADING MOST OF THE REMAINDER OF UTAH
AND SW WYOMING BY THE TIME WE GET INTO WEDNESDAY MORNING. MOISTURE
QUALITY IS RELATIVELY POOR (WITH RESPECT TO CLIMO) WITH THIS SYSTEM,
AND PRECIPITATION PRODUCTION WILL BE RELIANT ON LINGERING MOISTURE,
WEAKENING DYNAMICS AND OROGRAPHICS. THE BIG QUESTION REMAINS WHITE
CHRISTMAS (DEFINED AS 1" OF SNOW ON THE GROUND AT 12Z/5AM MST)
CHANCES. FOR THE WASATCH FRONT, THE BULK OF THE PRECIPITATION WITH
THIS SYSTEM WILL FALL AS RAIN. HOWEVER, THERE REMAINS A LOW
PROBABILITY SCENARIO IN PLAY IN WHICH THE COMBINATION OF RATE-DRIVEN
PRECIPITATION AND COLD ADVECTION BEHIND THE WEDNESDAY MORNING COLD
FRONT ALLOWS FOR AN EARLIER CHANGEOVER TO SNOW PRIOR TO SUNRISE
WEDNESDAY MORNING. THERE IS ROUGHLY A 10% CHANCE FOR ALL SNOW BY
SUNRISE CHRISTMAS MORNING ACROSS THE VALLEY FLOORS OF THE WASATCH
FRONT. DO THINK THE PROBABILITY FOR ACCUMULATING SNOW ON BENCHES IS
HIGHER, GENERALLY IN THE 20-40% RANGE. WILL CONTINUE TO MONITOR
SUCCESSIVE MODEL RUNS TO SEE IF THE LOW PROBABILITY OUTCOME ODDS
INCREASE OR NOT. ON THE WHOLE, THIS SYSTEM WILL BE ONE THAT WILL ACT
TO SLIGHTLY INCREASE MOUNTAIN SNOWPACK, AS EVEN MOUNTAINS ARE
EXPECTED TO EXPERIENCE SUB-ADVISORY SNOWFALL AMOUNTS, WITH ONLY THE
BEAR RIVER RANGE, UPPER COTTONWOODS AND TUSHARS BLANKETED WITH >50%
PROBABILITIES OF 6" OF SNOW WITH THIS SYSTEM. DESPITE THE GOOD MODEL
AGREEMENT IN THE LARGE-SCALE FEATURES, THERE CONTINUES TO BE SOME
WOBBLING IN EITHER DIRECTION WITH SUBSEQUENT MODEL RUNS,
PARTICULARLY WITH RESPECT TO THE UPPER END OF THE QPF DISTRIBUTION.
FOR EXAMPLE, LATEST PQPF DOES INDICATE A ~25% OF REACHING 1" OF QPF
ACROSS THE TYPICAL NW UPSLOPE OVERACHIEVER REGIONS SUCH AS THE BEAR
RIVER RANGE, UPPER COTTONWOODS AND TUSHARS, SO WE CANNOT COMPLETELY
RULE OUT A SLIGHTLY MORE PRODUCTIVE EVENT VS THE LATEST
DETERMINISTIC FORECAST OUTPUT.
ANOTHER PERIOD OF BRIEF RIDGING FOR THE FIRST HALF OF THURSDAY WILL
QUICKLY BE REPLACED BY DEEP LAYER WARM/MOIST ADVECTION AS THE NEXT
DECAYING SYSTEM APPROACHES FROM THE WNW. THIS WILL REINTRODUCE
PRECIPITATION CHANCES FROM WEST TO EAST ACROSS THE AREA BY LATE
THURSDAY. THERE REMAIN STRENGTH DIFFERENCES WITH THIS SYSTEM, WITH
WEAKER SOLUTIONS CONSTRAINING LIGHT PRECIPITATION AMOUNTS TO
NORTHERN UTAH/SW WYOMING, WHILE STRONGER SOLUTIONS SUGGEST MORE OF
AN AREA-WIDE EVENT. EVEN THE STRONGEST SOLUTIONS ARE LIMITED IN
TERMS OF AMOUNTS AND IMPACT/HAZARD POTENTIAL WITH THIS WAVE.
THE JUICIEST OF SYSTEMS (AS EXPRESSED BY QUANTITIES OF TOTAL
ATMOSPHERIC MOISTURE/PWAT/IWV)IS FORECAST TO CRASH INTO THE NORTHERN
HALF OF THE WESTERN CONUS LATE IN THE WEEK, BEFORE LINGERING
MOISTURE BEGINS TO ADVECT ACROSS OUR AREA IN THE LATE FRIDAY INTO
SATURDAY TIMEFRAME. LIKE MOST OF THE SYSTEMS THIS SEASON, THIS ONE
WILL BE IN A DECAYING PHASE BY THE TIME IT ARRIVES ACROSS THE
INTERMOUNTAIN WEST, AND THERE ARE LINGERING DIFFERENCES AMONG THE
MODEL SUITE REGARDING BOTH HOW MUCH MOISTURE CAN MAKE IT HERE, AS
WELL AS HOW THE MOISTURE WILL BE ORIENTED. THESE FACTORS WILL PLAY A
LARGE ROLE IN HOW PRODUCTIVE THIS PERIOD WILL BE. AN EARLY LOOK AT
PROBABILITIES SUGGESTS THIS SYSTEM WILL HAVE THE BEST CHANCE TO PUSH
AMOUNTS CLOSER TO ADVISORY CRITERIA IN THE MOUNTAINS (RELATIVE TO
THE OTHER SYSTEMS IN THE FORECAST PERIOD), WITH >50% OF 8" OF SNOW
ACROSS THE BEAR RIVER RANGE, UPPER COTTONWOODS AND OGDEN AREA
MOUNTAINS. FOR NORTHERN VALLEYS, ODDS FAVOR ADDITIONAL RAIN EVENTS,
WITH SNOW CHANCES REMAINING LARGELY 10% OR LESS THROUGH NEXT
WEEKEND.
AVIATION...KSLC
LIGHT WINDS AND HAZE DUE TO INVERSION CONDITIONS
WILL PREVAIL AT KSLC THROUGH THE TAF PERIOD. THERE IS ROUGHLY A 25%
CHANCE OF SEEING LIFR CONDITIONS IN DENSE FOG THROUGH EARLY SATURDAY
MORNING, AS SHALLOW FOG LURKS NEARBY IN LOW SPOTS NEAR BODIES OF
WATER, SUCH AS THE FARMINGTON BAY. ANY FOG THAT DEVELOPS MAY BE MORE
PREVALENT OVER THE NORTHWEST PORTION OF THE AIRFIELD AT TIMES
COMPARED TO THE SOUTHEAST PORTION. WINDS WILL BE LIGHT THROUGH THE
PERIOD, MAINLY LESS THAN 6KTS, WITH NORTHERLY SFC FLOW DEVELOPING
AROUND 18Z, AND EVENING SOUTHERLY FLOW DEVELOPING AROUND 04Z.
PERIODS OF VARIABLE WINDS LIKELY THROUGH THE MORNING HOURS, AND
PERIODS OF LIGHT NORTH WINDS WILL BE MOST FAVORABLE FOR ADVECTING
NEARBY FOG ACROSS THE TERMINAL.
REST OF UTAH AND SOUTHWEST WYOMING
DRY CONDITIONS WITH LIGHT
WINDS AND SCATTERED MID AND HIGH CLOUDS WILL PREVAIL. VALLEY
INVERSIONS WITH HAZE WILL LIMIT VISIBILITY SLIGHTLY MAINLY IN THE
5SM-6SM RANGE), PRIMARILY FOR THE URBAN CORRIDOR OF NORTHERN UTAH.
ALONG WITH HAZE, PATCHY TO LOCALLY DENSE FOG COULD LOWER CONDITIONS
INTO MVFR RANGE OR LOWER FOR THE CACHE VALLEY IN PARTICULAR THROUGH
18Z.
SLC WATCHES/WARNINGS/ADVISORIES
UT...NONE.
WY...NONE.
SEAMAN/ADESMET
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