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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HTTP://WEATHER.GOV/SALTLAKECITY/GENERAL/AFD_FAQS.PHP  
 
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