827  
FXUS02 KWBC 161544  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1044 AM EST FRI NOV 16 2018  
 
VALID 12Z MON NOV 19 2018 - 12Z FRI NOV 23 2018  
   
..16 UTC UPDATE
 
 
MODEL AGREEMENT IS RELATIVELY GOOD FOR THE FIRST PORTION OF THE  
EXTENDED PERIOD AND THEN GOES QUICKLY DOWNHILL BY THE LATER  
PORTION. SIMILAR TO THE OVERNIGHT ISSUANCE, THE FORECAST DURING  
DAYS 3-5 (MON-WED) WAS BASED ON A MULTI-MODEL DETERMINISTIC BLEND  
(00Z ECMWF/FV3 AND 06Z GFS). MODELS SHOWED SIMILAR SOLUTIONS FOR A  
COUPLE SHORTWAVES CROSSING THE GREAT LAKES AT THE BASE OF THE  
HUDSON BAY MEAN TROUGH DURING THIS PERIOD, AND WITH AN ELONGATED  
AREA OF SHORTWAVE ENERGY CROSSING THE SOUTH CENTRAL/SOUTHEASTERN  
U.S. THE MAIN FORECAST DIFFICULTY ARISES BY DAYS 6-7 (THU-FRI)  
WITH RESPECT TO THE WEAKENING OF WESTERN U.S. RIDGING AND THE  
ARRIVAL OF HEIGHT FALLS FROM THE PACIFIC. THE DETERMINISTIC ECMWF  
CONTINUES TO BE AMONG THE FASTER SOLUTIONS (JOINED BY THE CMC) TO  
BRING A LEADING SHORTWAVE INTO THE WEST BY WED/THU, QUICKLY  
ERODING THE RIDGE. THE GFS AND FV3, WHILE PREVIOUSLY SHOWING A  
RATHER CONSISTENT SLOWER SOLUTION, HAVE NOW BECOME MORE VARIABLE.  
IN GENERAL, A TREND AWAY FROM THE SLOWER SOLUTIONS SEEMS TO HAVE  
BEEN OBSERVED OVER THE LAST COUPLE MODEL CYCLES, ALTHOUGH THIS IS  
BY NO MEANS UNIVERSAL. THIS IS PERHAPS BEST DEMONSTRATED BY THE  
PERSISTENCE OF THE CMC ENSEMBLE MEAN WITH A STRONG RIDGE AND  
SLOWER TROUGH ARRIVAL DESPITE THE OPERATIONAL CMC MORE RESEMBLING  
THE ECMWF. NONETHELESS, ARRIVAL OF HEIGHT FALLS AND A COLD FRONT  
ARE LIKELY BY THE MIDDLE OF NEXT WEEK ALONG THE WEST COAST. IF A  
FASTER ECMWF/CMC-LIKE SOLUTION WERE TO VERIFY, THE DEVELOPMENT OF  
A LOW PRESSURE SYSTEM ACROSS THE CENTRAL U.S. BY DAY 6-7 WOULD  
BECOME MORE LIKELY IN RESPONSE TO A FAST/AMPLIFIED LEADING  
SHORTWAVE. NEEDLESS TO SAY, FORECAST CONFIDENCE IS LOW BY DAY 6-7,  
AND WEIGHTING OF ENSEMBLE MEANS (ECENS AND GEFS) WAS INCREASED BY  
THAT TIME. MORE WEIGHT WAS PLACED TOWARD THE GEFS MEAN IN ORDER TO  
SHOW SOME DEFERENCE TO CONTINUITY GIVEN THE SIGNIFICANT OBSERVED  
FLUCTUATIONS.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0554 UTC)  
   
..OVERVIEW
 
 
WESTERN RIDGING AND EASTERN TROUGHING WILL SLOWLY TRANSLATE  
EASTWARD NEXT WEEK IN A MOSTLY DRY PATTERN FOR MUCH OF THE LOWER  
48. MUCH NEEDED RAIN MAY FINALLY WORK INTO CALIFORNIA BY LATER IN  
THE WEEK NEXT WEEK.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
FORECAST FOR MON-TUE WAS FORMED FROM A MULTI-MODEL DETERMINISTIC  
BLEND OF THE RECENT 12Z/18Z GUIDANCE. WEAK CLIPPER WILL DIP  
THROUGH THE GREAT LAKES/NORTHEAST AS SOUTHERN FRONT SINKS INTO THE  
FLORIDA STRAITS. A WEAKENING SOUTHERN UPPER LOW WILL OPEN UP OFF  
SOUTHERN CALIFORNIA AND PUSH INLAND.  
 
FOR WED-FRI, ENSEMBLES STILL DIVERGE ON HOW QUICKLY TO LOWER  
HEIGHTS INTO THE WEST. THE ECMWF AND ITS ENSEMBLES (WITH THE 12Z  
CANADIAN) REMAIN QUICKER THAN THE GEFS/CANADIAN ENSEMBLES WHILE  
THE DETERMINISTIC GFS RUNS WERE SLOWEST. THE FV3 AND UKMET WERE  
SOMEWHAT IN BETWEEN, NEAR THE 18Z GEFS MEAN. TREND OVER THE PAST  
SEVERAL CYCLES HAS BEEN SLOWER IN THE ECMWF ENSEMBLES AND JUST A  
BIT QUICKER IN THE GEFS MEMBERS, WHICH SUGGESTS LEANING AWAY FROM  
THE QUICKER MEMBERS (12Z ECMWF INCLUDED). IT HAS BEEN SEEN AT  
LEAST A COUPLE TIMES IN AS MANY MONTHS THAT WHEN THE ECMWF  
ENSEMBLES WERE QUICKER THAN THE GEFS ENSEMBLES VERIFICATION  
FAVORED THE GEFS (THOUGH PERHAPS NOT COMPLETELY). OPTED TO  
CONTINUE WITH THE PREVIOUS SHIFT PREFERENCE WITH A SLOWER  
BREAKDOWN OF THE RIDGE ALONG THE LINE OF THE 18Z GEFS MEAN AND  
SOMEWHAT THE 12Z FV3 GFS. INDEED THE 00Z GFS WAS CONSIDERABLY  
QUICKER THAN THE PREVIOUS RUNS, FURTHER EXHIBITING THE  
UNCERTAINTY. EITHER WAY, FRONT SHOULD EVENTUALLY MAKE ITS WAY TO  
THE COAST, PERHAPS AS SOON AS WEDNESDAY BUT MORE LIKELY EARLY  
THURSDAY. IN THE EAST, HEIGHTS SHOULD SLOWLY RISE BUT THERE  
REMAINS DIFFERENCES ACROSS CANADA WITH HOW MUCH NORTHERN STREAM  
TROUGHING MAY LINGER IN THE NORTHEAST. LEFTOVER SOUTHERN STREAM  
REFLECTION OF THE WEAKENING UPPER LOW THROUGH THE SOUTHWEST EARLY  
IN THE WEEK MAY ATTEMPT TO CARRY INTO THE GULF OF MEXICO LATER IN  
THE WEEK.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
RATHER QUIET PERIOD EARLY IN THE WEEK DOMINATED BY NW FLOW OUT OF  
CENTRAL CANADA. EXCEPTION MAY BE IN THE WESTERN GULF ALONG AN  
INVERTED TROUGH TO THE NORTH OF A STATIONARY BOUNDARY. POTENTIAL  
FOR HEAVIEST RAIN WOULD BE ALONG THE COAST WHERE INSTABILITY IS  
HIGHEST. THAT WILL CONTINUE THROUGH THE WEEK WITH PERHAPS SOME  
ENHANCEMENT BY THE INCOMING SOUTHERN STREAM SHORTWAVE. ENTIRE WEST  
COAST WILL SEE IN INCREASE IN THE CHANCE OF RAIN AND ELEVATION  
SNOW STARTING AS SOON AS LATE WEDNESDAY. FOCUS APPEARS TO BE  
AROUND NORTHERN CALIFORNIA BUT THIS MAY EXTEND INTO SW OREGON  
AND/OR CENTRAL CALIFORNIA GIVEN THE ENSEMBLE SPREAD. AMOUNTS MAY  
BE IN THE 1-3 INCH RANGE THU-FRI OVER FAVORED AREAS AND EVEN ABOUT  
A HALF INCH IN THE CENTRAL VALLEY.  
 
TEMPERATURES WILL BE BELOW AVERAGE IN THE GREAT LAKES/NORTHEAST  
FOR MUCH OF THE WEEK AS TROUGHING WILL BE SLOW TO LIFT OUT.  
DAYTIME HIGHS IN THE 20S AWAY FROM THE COASTAL PLAIN MAY BE NEAR  
RECORD COLD VALUES TUE/WED WITH RECORD LOWS OVERNIGHT POSSIBLE AS  
WELL ESPECIALLY WEDNESDAY MORNING IN NEW ENGLAND WITH WIDESPREAD  
TEENS AND SINGLE DIGITS POSSIBLE. TEMPERATURES IN THE WEST WILL BE  
ABOVE AVERAGE (BY ABOUT 5-15F) ALONG/INLAND FROM THE CENTRAL WEST  
COAST THROUGH AT LEAST THE FIRST PART OF NEXT WEEK. COOLER  
TEMPERATURES WILL ACCOMPANY UPPER TROUGHING AND THE FRONTAL  
APPROACH, NUDGING THE WARMER THAN AVERAGE TEMPERATURES INTO THE  
PLAINS BY THE END OF THE WEEK.  
 
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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