517
FXUS65 KSLC 191748
AFDSLC
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
1048 AM MST TUE NOV 19 2019
SYNOPSIS
A DEEPENING LOW PRESSURE SYSTEM WILL SETTLE INTO
CALIFORNIA TONIGHT, THEN TURN EAST AND WORK SLOWLY ACROSS SOUTHERN
AND CENTRAL UTAH MIDWEEK. WIDESPREAD RAIN AND SNOW WILL IMPACT
MAINLY SOUTHERN AND CENTRAL UTAH LATE TONIGHT THROUGH THURSDAY.
DISCUSSION
MORNING WATER VAPOR AND H5 ANALYSIS DEPICT A COMPLEX
SETUP ACROSS THE SOUTHWESTERN CONUS, AND THE BEGINNINGS OF STORM
MORPHOLOGY ACROSS THE DESERT SOUTHWEST. AN UPPER LOW OFF THE BAJA
COAST IS THE FIRST PIECE, AND THE DRIVER FOR A SUB-TROP MOISTURE
TAP THAT IS EXTENDING NORTHEAST INTO ARIZONA ATTM. UPSTREAM AND TO
THE NORTH OF THIS IS A NORTHERN BRANCH TROUGH IS ELONGATING ALONG
THE WEST COAST WHILST DEEPENING OVER NORCAL THIS MORNING. THIS
LATTER TROUGH WILL BE THE DOMINANT FEATURE MOVING FORWARD THROUGH
MID/LATE WEEK, FUELED BY THE FORMER INITIALLY AS IT FILLS AND
EJECTS ACROSS THE 4-CORNERS REGION LATER TONIGHT/WED.
ALTHOUGH THE BULK OF FORMER TROPICAL STORM RAYMOND'S TROPICAL
MOISTURE WILL ADVECT SOUTH AND EAST OF US LOCALLY (THROUGH AZ/NM)
OVER THE NEXT 24HRS, AM NOTING A TONGUE WRAPPING INTO THE NORTHERN
PERIPHERY OF THE BAJA LOW THIS MORNING, WITH PWAT INCREASING TO
NEAR 1" OVER SOUTHWESTERN AZ. AS THE NORTHERN BRANCH TROUGH
CONTINUES TO DEEPEN SOUTH ALONG THE CALI COAST THROUGH THIS EVE,
TRAJECTORIES IN ADVANCE OF THIS WILL BECOME INCREASING FAVORABLE
TO TRANSPORT THIS DEEP LAYER MOISTURE NORTH ALONG THE LOWER
COLORADO VALLEY AND INTO PRIMARILY SOUTHWESTERN UTAH TONIGHT, THIS
SETTING UP THE DEEP LAYER MOISTURE STAGE.
THE DEVELOPMENT OF QUASI-STATIONARY DEFORMATION AXIS IN ADVANCE OF
THE DEEPENING TROUGH OVER CALI COUPLED WITH INCREASING DIFFLUENCE
ALOFT REMAINS THE FOCUSED DRIVER FOR A MODERATE TO HEAVY PRECIP
EVENT ALONG ITS AXIS, WITH PLACEMENT MOST LIKELY BETWEEN KANAB AND
ST GEORGE AND POINTS NORTHEAST OVERNIGHT TONIGHT INTO WEDNESDAY
MORNING. LATEST SUITE OF HI-RES GUIDANCE HAVE RETAINED SIMILAR
TRENDS IN THIS PLACEMENT THAT BEGAN LAST EVE WITH A SLIGHT
WESTWARD NUDGE VS. 24 HOURS AGO. MODEL QPF AND INTENSITIES REMAIN
SIMILAR AS WELL WITH TOTALS THROUGH THURSDAY IN THE 0.5 TO 1.5
INCH RANGE (LOCALLY IN EXCESS OF 2") BENEATH THIS AXIS, WITH BULK
FALLING OVERNIGHT TONIGHT THROUGH WEDNESDAY MORNING. ALTHOUGH
THESE AMOUNTS ARE QUITE HIGH OVER A PROBLEMATIC AREA, LIMITED
CONVECTIVE POTENTIAL SHOULD BOOKEND RATES TO BELOW .20"/HR WITHIN
A LARGELY STRATIFORM PRECIP SHIELD. ANTICIPATING SOME PONDING OF
WATER IN POOR DRAINAGE AREAS AROUND DIXIE, MODEST RISES TO
MAINSTEM RIVERS, AND SOME FLOWS IN NORMALLY DRY WASHES, WITH THE
RAINFALL BEING MORE BENEFICIAL THAN NOT. NO SIGNIFICANT FLOODING
IS ANTICIPATED, BUT WILL MONITOR CLOSELY MOVING FORWARD AND
RETAIN THE GOING FLASH FLOOD WATCH AS A SLIGHT POTENTIAL FOR
POCKETY CONVECTION CAPABLE OF HIGHER RATES EXISTS FOR A FEW HOURS
TONIGHT.
NO CHANGES TO GOING WINTER WEATHER HEADLINES IN PLACE EITHER,
THOUGH LOWER CONFIDENCE EXISTS REGARDING PRECIP AMOUNTS NORTH OF
I-70, ESPECIALLY OVER CENTRAL/NORTHERN UTAH. PRIMARY FORECAST
CHALLENGE THIS SHIFT WILL NOT ONLY FOCUS ON RAINFALL POTENTIAL
ACROSS THE SOUTH MENTIONED ABOVE, BUT ALSO SNOW ACCUMS FROM THE
CENTRAL WASATCH PLATEAU NORTH INTO THE NORTHERN MTNS.
PREVIOUS DISCUSSION
NORTHERN UTAH WILL BEGIN TO SEE ORGANIZED
PRECIP BY WEDNESDAY NIGHT AS THE UPPER LOW WORK INTO SOUTHERN
UTAH. THE DEFORMATION AXIS ALONG THE NORTHERN FLANK OF THE LOW
WILL BECOME THE FOCUS FOR PRECIP, WITH THE PEAK PERIOD LIKELY LATE
WEDNESDAY NIGHT/EARLY THURSDAY AS LIFT INCREASES ALONG THE MID-
LEVEL BAROCLINIC ZONE SOUTH OF THE DEFORMATION.
PRECIP WILL BEGIN TO TREND TO A MORE CONVECTIVE NATURE THURSDAY
ALONG THE EASTERN FLANK OF THE UPPER LOW AND EMBEDDED WITHIN THE
MID-LEVEL COLD POOL. TRAILING THE UPPER LOW, A MOIST MID-LEVEL
WRAP AROUND WARM ADVECTION PATTERN WILL SET UP ACROSS EASTERN
NEVADA AND WESTERN UTAH. ENHANCED THERMALLY-DRIVEN LIFT SHOULD
PRODUCE ONE LAST PERIOD OF WIDESPREAD PRECIP, MOSTLY AS SNOW,
ALONG THE I-15 CORRIDOR IN CENTRAL AND SOUTHWEST UTAH THROUGH
EARLY EVENING. PRECIP SHOULD WIND DOWN FAIRLY QUICKLY LATER
THURSDAY NIGHT AS A SIGNIFICANTLY DRIER NORTH-NORTHEAST FLOW WRAPS
INTO THE BACK SIDE OF THE EXITING UPPER LOW ACROSS NORTHERN AND
WESTERN UTAH THROUGH SOUTHWEST WYOMING.
LONG TERM (AFTER 00Z SATURDAY)
GLOBAL MODELS INDICATE HIGH
PRESSURE BUILDING INTO THE PACNW FRIDAY EVENING WHILE A CLOSED LOW
MEANDERS OFF THE CENTRAL CALIFORNIA COAST. HOWEVER, DETERMINISTIC
MODELS AND ENSEMBLE MEANS DIFFER ON THE POSITIONING OF THIS LOW, AND
IN GENERAL, THE LATEST 00Z RUNS ARE A BIT FARTHER EAST THAN WHAT
MODEL RUNS FROM 24 HOURS AGO WERE SHOWING. AS SUCH, THERE IS SOME
UNCERTAINTY REGARDING THE LARGE-SCALE PATTERN IN THE EXTENDED
FORECAST PERIOD, AND MODEL SPREAD ONLY INCREASE HEADING INTO DAYS 6
AND 7.
IN GENERAL, EXPECT MOSTLY QUIET WEATHER OVER THE FORECAST AREA
SATURDAY THROUGH SUNDAY IN A NORTHWESTERLY FLOW ALOFT AND
TEMPERATURES NEAR TO SLIGHTLY BELOW NORMAL. THEREAFTER, THE EC AND
ITS ENSEMBLE MEAN BOTH HAVE THE LOW RETROGRADING WESTWARD AND
FORMING A REX BLOCK OVER THE EASTERN PACIFIC BY SUNDAY. THIS PATTERN
WOULD KEEP UTAH UNDER NORTHWESTERLY FLOW ALOFT, BUT THE BLOCK WOULD
BE FAR ENOUGH WEST TO ALLOW SOME TROUGHS IN THE NORTHWESTERLIES TO
AT LEAST GRAZE UTAH. SUCH IS THE SOLUTION PRESENTED IN THE EC/EPS
FOR MONDAY, WHERE A GRAZING TROUGH PUSHES A COLD FRONT INTO UTAH
WITH A POTENTIAL FOR SOME LIGHT SNOW ACROSS NORTHEAST UTAH AND
SOUTHWEST WYOMING.
THE GFS AND GEFS, HOWEVER, IS MORE PROGRESSIVE WITH THE LOW, AND
THIS ALLOWS THE NEXT TROUGH, ON MONDAY, TO CARVE SOUTH AS IT MOVES
INLAND. THIS STORM SPLITS STRONGLY ON THE OPERATIONAL GFS. HOWEVER,
A COLD FRONT IS STILL PROGGED TO CROSS UTAH AND PRECIPITATION
COULD BE MORE WIDESPREAD WITH SNOW POSSIBLY DOWN TO 4000-4500 FEET,
BUT AMOUNTS WOULD STILL BE LIGHT AS THE DYNAMICS WOULD BE FAIRLY
DIFFUSE. MORE INTERESTING IS THE GEFS ENSEMBLE MEAN WHICH IS NOT AS
STRONGLY SPLIT AS THE OPERATIONAL GFS AND THE TROUGH ENDS UP WITH A
MORE FAVORABLE TRACK INTO UTAH. HOWEVER, AS THIS STORM WILL NOT
IMPACT THE AREA UNTIL TOWARDS THE END OF THE FORECAST PERIOD, AND
GIVEN THE UNCERTAINTY WITH THE LARGE-SCALE PATTERN, JUST MAINTAINED
A BLENDED SOLUTION FOR POPS AND A GENERAL COOLING TREND FOR EARLY
NEXT WEEK
AVIATION
HIGH CLOUDS AT THE SLC TERMINAL THIS MORNING ARE
EXPECTED TO THICKEN AND GRADUALLY LOWER THROUGH THE DAY. OTHERWISE,
VFR CONDITIONS WILL BE MAINTAINED. INCREASING SOUTH WINDS MAY BECOME
VARIABLE IN DIRECTION UNTIL A NORTHWEST SHIFT TAKES OVER BETWEEN 18Z-
19Z. EXPECTING CEILINGS TO LOWER TO 10 KFT BY 00Z OR SO.
SLC WATCHES/WARNINGS/ADVISORIES
UT...FLASH FLOOD WATCH FROM 9 PM MST THIS EVENING THROUGH WEDNESDAY
EVENING FOR UTZ016-019-020.
WINTER WEATHER ADVISORY FROM 5 PM THIS AFTERNOON TO 4 PM MST
THURSDAY FOR UTZ009-010-517.
WINTER STORM WARNING FROM 5 PM THIS AFTERNOON TO 4 PM MST
THURSDAY FOR UTZ518.
WY...NONE.
DISCUSSION/AVIATION...MERRILL/DEWEY
PREVIOUS DISCUSSION...CONGER/CHENG
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