263  
FXUS02 KWBC 171600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SAT NOV 17 2018  
 
VALID 12Z TUE NOV 20 2018 - 12Z SAT NOV 24 2018  
 
...FRIGID TEMPERATURES EXPECTED IN TIME FOR THANKSGIVING FOR MUCH  
OF THE NORTHEAST...  
 
   
..OVERVIEW
 
 
THE UPPER-LEVEL PATTERN ACROSS NORTH AMERICA WILL BE IN FLUX  
DURING THE EXTENDED PERIOD, WITH EVOLUTION TOWARD A MORE CHAOTIC  
PATTERN (ALTHOUGH NOT NECESSARILY SIGNIFICANTLY LESS AMPLIFIED).  
WEAKENING OF THE RIDGE ACROSS WESTERN NORTH AMERICA WILL OPEN THE  
CONUS UP TO FLOW OFF THE PACIFIC, WITH A RAPID SUCCESSION OF  
PACIFIC SHORTWAVES EXPECTED TO ARRIVE BY THE MIDDLE TO END OF NEXT  
WEEK.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
CONSENSUS HAS IMPROVED DURING THE DAY 3-5 (TUE-THU) TIME PERIOD  
WITH RESPECT TO THE FLOW EVOLUTION ALONG THE WEST COAST AND STRONG  
HEIGHT FALLS ARRIVING BY MID-WEEK, WITH THE ECMWF SLOWING A BIT  
AND THE GFS SPEEDING UP SLIGHTLY. A LEADING UPPER LOW MOVING  
ONSHORE IN THE SOUTHWEST EARLY TUE WILL QUICKLY MOVE EAST ACROSS  
THE SOUTHERN TIER. CONFIDENCE IS NOW BETTER THAT HEIGHT FALLS WILL  
MOVE ONSHORE ALONG THE WEST COAST LATE WED, PUSHING A SURFACE  
FRONT INTO THE GREAT BASIN BY THU MORNING. UPPER RIDGING WILL  
QUICKLY ERODE/SHIFT EASTWARD AS THIS OCCURS. FARTHER EAST, MODELS  
HAVE SHOWN A NEARLY UNIVERSAL TREND TOWARD GREATER AMPLIFICATION  
OF A SYSTEM NEAR THE CANADIAN MARITIMES WED-THU, WITH MOST  
SOLUTIONS NOW SHOWING A CLOSED UPPER LOW DEVELOPING. THE ECMWF WAS  
THE FIRST MODEL TO PICK UP ON THIS, WITH THE GFS COMING ON BOARD  
AS OF THE 06Z RUN. THIS DEGREE OF AMPLIFICATION SEEMS TO MAKE  
SENSE GIVEN BLOCKING FLOW ACROSS THE NORTH CENTRAL ATLANTIC  
EVIDENT ON D+8 ANOMALIES FROM BOTH THE ECENS AND GEFS. THE  
IMPLICATION OF THIS FOR THE CONUS WILL BE EVEN STRONGER COLD  
ADVECTION ACROSS THE MID-ATLANTIC/NORTHEAST JUST IN TIME FOR  
THANKSGIVING. THE WPC FORECAST DURING DAYS 3-5 WAS BASED ON A  
BLEND OF THE LATEST ECMWF/GFS/FV3 SOLUTIONS.  
 
DURING DAYS 6-7 (FRI-SAT), AS DESCRIBED ABOVE, THE FLOW BEGINS TO  
BECOME MORE CHAOTIC AS SMALLER SCALE SHORTWAVES ORIGINATING OVER  
THE PACIFIC PLAY A LARGER ROLE GIVEN THE BREAKDOWN OF THE WESTERN  
RIDGE. CONSENSUS IS RELATIVELY GOOD THAT THE PACIFIC TROUGH WILL  
REACH THE CENTRAL U.S. THU NIGHT FRI, ALTHOUGH SOLUTIONS CONTINUE  
TO DIFFER AS TO HOW AMPLIFIED THIS FEATURE WILL BE. AT THIS POINT  
A MIDDLE-OF-THE ROAD APPROACH WAS PREFERRED HERE. ANOTHER  
SHORTWAVE LOOKS LIKELY TO REACH THE PACIFIC NORTHWEST ON THU WITH  
ANOTHER ON FRI. THE GFS BRINGS YET ANOTHER BY SAT BUT THIS  
SOLUTIONS SEEMS FAST IN CONTEXT OF THE RANGE OF MODEL SOLUTIONS SO  
IT IS NOT PREFERRED AT THIS TIME. FARTHER EAST, THE LEADING  
SHORTWAVE WHICH SHOULD REACH THE WEST COAST ON DAY 3 WILL BE IN  
THE VICINITY OF THE SOUTHEAST/GULF COAST BY FRI. MODELS DIFFER ON  
THE AMPLITUDE OF THIS FEATURE AND WHETHER IT WILL TRIGGER  
DEVELOPMENT OF A WAVE OF LOW PRESSURE ALONG A SURFACE FRONT IN THE  
GULF OF MEXICO, WITH PERHAPS SIGNIFICANT IMPLICATIONS IN TERMS OF  
PRECIPITATION FOR PORTIONS OF SOUTH FLORIDA. THE ECMWF WAS MOST  
AGGRESSIVE WITH DEVELOPING THIS WAVE WHILE THE GFS WAS MUCH LESS  
ENTHUSIASTIC. ECENS MEMBERS SHOW A FAIR DEGREE OF SUPPORT FOR SOME  
DEGREE OF WAVE DEVELOPMENT. WEIGHT IN THE FORECAST WAS SHIFTED  
TOWARD ECENS AND GEFS MEANS DURING DAYS 6-7, WITH A BIT MORE  
WEIGHT TOWARD THE ECENS MEAN GIVEN SOMEWHAT MORE CONSISTENCY SHOWN  
OVER THE PAST FEW RUNS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THANKSGIVING SUMMARY (THU 11/22) - COLD IN THE NORTHEAST WITH HIGH  
TEMPERATURES 20 TO NEAR 30 DEG F BELOW AVERAGE, AND MANY AREAS  
STAYING WELL BELOW FREEZING. WET ALONG THE WEST COAST INTO THE  
GREAT BASIN AND ROCKIES WITH NEAR AVERAGE TEMPERATURES AND HIGHER  
ELEVATION SNOW. MILDER AND DRY FOR THE HIGH PLAINS WITH 40S/50S  
COMMON. RAIN CHANCE FOR THE GULF COAST FROM TEXAS EASTWARD THROUGH  
MUCH OF FLORIDA TO THE NORTH OF A FRONT IN THE FLORIDA STRAITS.  
 
RATHER QUIET TO START DOMINATED BY NW FLOW OUT OF CENTRAL CANADA  
EAST OF 100W. PESKY STATIONARY BOUNDARY AND INVERTED TROUGH IN THE  
WESTERN GULF WILL KEEP A HEAVY RAINFALL THREAT FOR ESPECIALLY THE  
COAST WHERE INSTABILITY IS HIGHEST. THAT WILL CONTINUE THROUGH THE  
WEEK WITH PERHAPS SOME ENHANCEMENT BY THE INCOMING SOUTHERN STREAM  
SHORTWAVE ON WEDNESDAY.  
 
THE ENTIRE WEST COAST WILL SEE AN INCREASE IN FIRST CLOUDS THEN  
RAIN/ELEVATION SNOW BY WEDNESDAY. FOCUS APPEARS TO BE AROUND  
NORTHERN CALIFORNIA AND SOUTHWESTERN OREGON AS THE UPPER TROUGH  
SPLITS N-S. MULTI-DAY RAINFALL AMOUNTS MAY BE IN THE 3-6 INCH  
RANGE OVER FAVORED AREAS AND EVEN ABOUT AND INCH TO AN INCH AND A  
HALF IN THE CENTRAL VALLEY. SNOW LEVELS WILL FALL THROUGH THE  
PERIOD WITH SEVERAL FEET OF SNOW LIKELY AT THE HIGHEST ELEVATIONS.  
 
BELOW AVERAGE TEMPERATURES IN THE GREAT LAKES/NORTHEAST WILL BE  
SLOW TO EASE BUT WILL TREND BACK TOWARD TYPICAL LATE NOVEMBER  
VALUES BY NEXT WEEKEND. RECORD COLD VALUES ARE POSSIBLE ON TUESDAY  
AND MAYBE WEDNESDAY. TEMPERATURES IN THE WEST WILL BE ABOVE  
AVERAGE BY ABOUT 5-10 DEGREES BUT WILL MOVE EAST AS TROUGHING  
MOVES INLAND. THE PLAINS AND UPPER MIDWEST WILL SEE MILDER  
TEMPERATURES WEDNESDAY-SATURDAY, GENERALLY ABOVE FREEZING FOR  
AFTERNOON HIGHS.  
 
RYAN/FRACASSO  
 
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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