481  
FXUS02 KWBC 221559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1158 AM EDT MON OCT 22 2018  
 
VALID 12Z THU OCT 25 2018 - 12Z MON OCT 29 2018  
   
..OVERVIEW
 
 
DEEP EASTERN TROUGHING AND A COASTAL STORM WILL BE THE FOCUS FOR  
THE MEDIUM RANGE PERIOD AS THE REMNANTS OF HURRICANE WILLA IN THE  
PACIFIC COMBINE WITH NORTHERN STREAM ENERGY TO BRING A WIDESPREAD  
RAINFALL TO MUCH OF THE EAST LATER THIS WEEK INTO THIS WEEKEND.  
THIS WILL MAINTAIN A COOL PATTERN FOR THE CENTRAL/EASTERN STATES  
AND MILD TEMPERATURES FOR THE WEST.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
THE SYNOPTIC-SCALE PATTERN AHEAD WILL INITIALLY FEATURE A SPRAWLED  
OUT UPPER TROUGH EXTENDING FROM THE CANADIAN MARITIMES BACK UP  
TOWARD NORTHERN NUNAVUT. THE SOUTHERN EXTENSION OF THESE LOWER  
HEIGHTS SHOULD GRADUALLY GAIN LATITUDE IN RESPONSE TO THE NEXT  
SYSTEM TAKING SHAPE ACROSS THE CENTRAL U.S. A PHASING OF STREAMS  
CONTINUES TO BE ADVERTISED BETWEEN A WAVE OF LOW PRESSURE  
TRAVERSING THE NORTHERN GULF OF MEXICO ON THURSDAY/FRIDAY  
(REMNANTS OF WILLA) BEFORE THE PARENT SHORTWAVE ATTAINS A NEGATIVE  
TILT.  
 
FARTHER UPSTREAM, QUASI-ZONAL FLOW SETS UP OVER THE NORTHEASTERN  
PACIFIC THROUGH FRIDAY WHICH FAVORS A SERIES OF PERTURBATIONS TO  
TRACK INTO THE PACIFIC NORTHWEST AND BRITISH COLUMBIA COASTS. A  
BROAD POSITIVE HEIGHT ANOMALY EMERGES FOR THE WEEKEND ACROSS THE  
WESTERN U.S. WHILE MIGRATORY SHORTWAVES TRAVERSE THE TOP OF THE  
RIDGE WITH A TRACK FROM BRITISH COLUMBIA/ALBERTA TOWARD THE  
NORTH-CENTRAL U.S. MANY SOLUTIONS SUPPORT SIGNIFICANT  
AMPLIFICATION WITH A COUPLE OF THESE WAVES AS THEY MOVE TOWARD THE  
MISSISSIPPI RIVER VALLEY OVER THE WEEKEND. HOWEVER, GIVEN THE  
UNCERTAINTY WITH THE TROPICALLY-INFUSED NOR'EASTER AND A LACK OF  
UPPER LEVEL ANOMALY CENTERS TO GUIDE THE FLOW, OPTED TO CONTINUE  
WITH A MIDDLE GROUND SOLUTION NEAREST THE 00Z ECMWF ENSEMBLE MEAN.  
 
WITH THE DEVELOPING GULF AND WESTERN ATLANTIC SYSTEM, THE 00Z  
ECMWF/UKMET/CANADIAN LIED FARTHER WEST THAN THE ENSEMBLE MEANS AND  
THE RECENT GFS. PREFERRED A SOLUTION JUST WEST OF THE ENSEMBLE  
CONSENSUS GIVEN THE LIKELY AMPLIFIED PATTERN AND THE TENDENCY FOR  
THE GFS TO BE TOO WEAK/EAST WITH EAST COAST SYSTEMS BEYOND  
~100HRS. RECENT FV3-GFS RUNS WERE CLOSER TO THE ECMWF-LED CLUSTER  
BUT THERE REMAINED SIGNIFICANT TIMING DIFFERENCES AS THE LOW LIFTS  
UP THE COAST. THIS ALSO MAINTAINED CONTINUITY FROM THE OVERNIGHT  
FORECAST. BY THE END OF THE PERIOD, WITH SO MUCH SPREAD IN ALL  
DIRECTIONS AS THE LOW HEADS TOWARD/INTO CANADA AND THE SECONDARY  
SYSTEM ROTATES THROUGH THE OHIO VALLEY, RELIED ON THE ENSEMBLES  
(MOSTLY THE ECMWF ENSEMBLE MEAN) TO FORM A CONSENSUS WITH SOME  
ARTISTIC MANIPULATIONS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PRECIPITATION WILL FOCUS ALONG THE CENTRAL/EASTERN GULF COAST AND  
ALONG THE EASTERN SEABOARD WITH SOME MODEST PRECIPITATION OVER THE  
PACIFIC NORTHWEST AS A COUPLE FRONTAL BOUNDARIES WAVER THROUGH  
WA/OR/ID. THE EASTERN COASTAL CYCLONE WILL DEEPEN OVER THE GULF  
STREAM AND SPREAD COMMA-HEAD PRECIPITATION BACK INTO THE COASTAL  
PLAIN AS ITS TROPICAL CONNECTION SUSTAINS ITSELF THROUGH THE  
PERIOD. IMPACTS WILL BE DRIVEN BY THE PROXIMITY OF THE LOW TO THE  
COAST. TO THE NORTH, MARGINALLY COLD AIR WOULD SUPPORT SOME WINTRY  
PRECIPITATION OVER THE NORTHEASTERN U.S. INTERIOR MOUNTAINS  
(CENTRAL/NORTHERN APPALACHIANS). BACK OVER THE NORTHWESTERN U.S.,  
PERSISTENT ONSHORE FLOW WILL ALLOW FOR MULTIPLE DAYS OF  
OROGRAPHICALLY-DRIVEN PRECIPITATION. THE USUAL FOCUS SHOULD BE  
OVER THE UPSLOPE TERRAIN OF THE OLYMPICS AND WASHINGTON CASCADES.  
 
BELOW AVERAGE TEMPERATURES ARE FORECAST FROM THE GREAT PLAINS  
EASTWARD GIVEN THE PERSISTENT TROUGHING. SOME OF THE LARGEST  
DEPARTURES, ON THE ORDER OF 10 TO 15 DEGREES, WILL CONGREGATE OVER  
MUCH OF THE EASTERN SEABOARD GIVEN THE EXTENDED PERIOD OF CLOUD  
COVER/PRECIPITATION AS WELL AS POST-FRONTAL COLD ADVECTION.  
MEANWHILE, MILDER WEATHER WILL PREVAIL OVER SECTIONS OF THE WEST  
COAST INTO THE INTERMOUNTAIN WEST GIVEN THE INFLUENCE OF PACIFIC  
AIR MASSES ON THE FORECAST AND UPPER RIDGING MOVING INTO  
CALIFORNIA, PERHAPS CONTINUING INTO NEXT WEEK.  
 
FRACASSO/RUBIN-OSTER  
 
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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