791  
FXUS02 KWBC 270658  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 AM EST TUE NOV 27 2018  
 
VALID 12Z FRI NOV 30 2018 - 12Z TUE DEC 4 2018  
 
GENERAL WEATHER PATTERN  
-----------------------  
QUASI-ZONAL FLOW ALOFT ENCOMPASSES MUCH OF THE CONTINENTAL U.S.  
AFTER THE DEPARTURE OF THE LARGE STORM SYSTEM FROM THE NORTHEAST  
STATES LATE ON THURSDAY, WITH A LOW AMPLITUDE PERTURBATION  
CROSSING THE PLAINS. THIS QUICKLY CHANGES GOING INTO FRIDAY AS A  
POTENT PACIFIC DISTURBANCE CROSSES THE INTERMOUNTAIN WEST AND THEN  
REACHES THE CENTRAL PLAINS BY SATURDAY. AFTER A BRIEF RESPITE, A  
SECOND STORM SYSTEM WILL LIKELY AFFECT THE PACIFIC NORTHWEST AND  
ALLOW FOR THE TROUGH TO REBUILD ACROSS MUCH OF THE WEST BY SUNDAY,  
WHICH MAY EVENTUALLY AFFECT THE CENTRAL TO EASTERN U.S. BY THE END  
OF THE FORECAST PERIOD NEXT MONDAY.  
 
MODEL GUIDANCE EVALUATION  
-------------------------  
THE EVOLUTION AND TRACK OF THE PACIFIC LOW PRESSURE SYSTEM  
CROSSING THE INTERMOUNTAIN WEST ON FRIDAY AND THEN THE CENTRAL  
U.S. IS BECOMING BETTER REFINED BY THE DETERMINISTIC GUIDANCE  
COMPARED TO 24 HOURS AGO. THE CMC, ECMWF AND THE EC MEAN ARE ON  
THE NORTHWEST SIDE OF THE GUIDANCE SPREAD, AND THE GFS AND THE  
GEFS MEAN ON THE EASTERN SIDE AND MORE PROGRESSIVE. THERE IS ALSO  
SOME INDICATION OF A COLD AIR DAMMING SET-UP NEAR THE APPALACHIANS  
AHEAD OF THIS STORM SYSTEM, WHICH THE GUIDANCE IS LIKELY  
UNDER-DOING.  
 
THERE IS ALSO GROWING CONSENSUS FOR A SECOND PACIFIC STORM SYSTEM  
TO AFFECT THE NORTHWESTERN U.S. BY SATURDAY, DESPITE SOME  
PLACEMENT AND TIMING DIFFERENCES. THE CMC IS SLIGHTLY STRONGER  
THAN THE GFS AND ECMWF REGARDING THE UPPER LEVEL TROUGH BY SUNDAY  
MORNING OVER THE INTERMOUNTAIN WEST. THE ENSEMBLE MEANS PROVIDED  
A REASONABLE PLACEMENT FOR THIS FEATURE FOR THE SUNDAY AND MONDAY  
TIME FRAME.  
 
SENSIBLE WEATHER  
----------------  
ACROSS THE WESTERN U.S., THE FIRST ROUND OF HEAVY PRECIPITATION IS  
EXPECTED TO ABATE BY THE BEGINNING OF THE MEDIUM RANGE PERIOD ON  
FRIDAY AS THE UPPER LEVEL TROUGH MOVES ACROSS THE GREAT BASIN AND  
THE ROCKIES. THE SECOND STORM SYSTEM THAT WILL LIKELY AFFECT THE  
PACIFIC NORTHWEST THIS WEEKEND IS EXPECTED TO PRODUCE WIDESPREAD  
MOUNTAIN SNOW ACROSS THE NORTHWESTERN QUADRANT OF THE NATION, AND  
HEAVY RAIN FOR THE COASTAL REGION.  
 
ACTIVE WEATHER IS ALSO ANTICIPATED FROM THE SOUTHERN PLAINS TO THE  
OHIO VALLEY FRIDAY NIGHT INTO SUNDAY AS A SURFACE LOW IS FORECAST  
TO DEVELOP AND INTENSIFY AS IT TRACKS NORTHEASTWARD. THERE IS AN  
INCREASING SIGNAL IN THE MODEL GUIDANCE FOR A BAND OF MODERATE TO  
HEAVY RAIN IN THE WARM SECTOR OF THIS LOW, AND SPC IS MONITORING  
THE POTENTIAL FOR SEVERE WEATHER GIVEN THE STRONG DYNAMICS ALOFT  
THAT ARE EXPECTED IN COMBINATION WITH ENHANCED MOISTURE ADVECTION.  
SNOW IS BECOMING MORE LIKELY TO THE NORTHWEST OF THE LOW TRACK  
ACROSS THE CENTRAL PLAINS AND UPPER MIDWEST, HOWEVER PLACEMENT  
DIFFERENCES LIMIT CONFIDENCE AS TO WHERE THE BAND OF HEAVIEST SNOW  
WOULD BECOME ESTABLISHED. WINDY CONDITIONS IN THE WAKE OF THE  
DEPARTING STORM SYSTEM ALSO APPEAR LIKELY FOR PARTS OF THE PLAINS  
BY SUNDAY.  
 
TEMPERATURES ARE EXPECTED TO BE ON THE ORDER OF 5 TO 15 DEGREES  
ABOVE AVERAGE AHEAD OF THE WESTERN SYSTEM FRIDAY THROUGH SUNDAY,  
BEGINNING OVER THE SOUTHERN PLAINS AND THEN ACROSS THE MISSISSIPPI  
AND OHIO RIVER VALLEYS BEFORE REACHING THE EASTERN U.S. A RENEWED  
SURGE OF COLDER WEATHER WILL FILTER IN BEHIND IT AS TROUGHING  
SETTLES ACROSS THE CENTRAL STATES. TEMPERATURES MAY BE ON THE  
ORDER OF 10-20F BELOW AVERAGE IN THE INTERIOR WEST BY NEXT WEEK.  
 
D. HAMRICK  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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