094  
FXUS02 KWBC 210701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT SUN OCT 21 2018  
 
VALID 12Z WED OCT 24 2018 - 12Z SUN OCT 28 2018  
 
...GUIDANCE/PREDICTABILITY ASSESSMENT AND WEATHER  
HIGHLIGHTS/THREATS...  
 
AT THE START OF THE PERIOD, A STRENGTHENING WAVE OF LOW PRESSURE  
WILL BE IN THE PROCESS OF EXITING NEW ENGLAND TOWARD THE CANADIAN  
MARITIMES. AS THESE HEIGHT FALLS ADJOIN THE PERSISTENT NEGATIVE  
HEIGHT ANOMALY ACROSS NUNAVUT, THE FOCUS SHIFTS TOWARD A PHASING  
PAIR OF STREAMS ACROSS THE MIDDLE OF THE COUNTRY. THIS PROCESS  
WILL ULTIMATELY LEAD TO A DEVELOPMENT OF A SURFACE WAVE WHICH  
TRACKS EASTWARD ACROSS THE NORTHERN GULF OF MEXICO BEFORE MAKING A  
NORTHWARD TURN TOWARD THE GULF STREAM NEXT WEEKEND. CONSIDERING  
THE PATTERN ACROSS THE EASTERN PACIFIC, AFTER ANOTHER POWERFUL  
CYCLONE LIFTS TOWARD THE GULF OF ALASKA, THE MID/UPPER-LEVEL FLOW  
TURNS MORE ZONAL. EVENTUALLY MARKED HEIGHT RISES TAKE SHAPE TOWARD  
THE WESTERN U.S. WHICH ESTABLISHES A HIGHER AMPLITUDE PATTERN INTO  
THE FOLLOWING WEEKEND. NORTHWESTERLY FLOW ON THE EASTERN EXTENT OF  
THIS MENTIONED RIDGE COULD LEAD TO AN IMPRESSIVE CLIPPER-LIKE  
SYSTEM OVER THE MISSISSIPPI RIVER VALLEY BY DAY 6/7, OCTOBER 27/28.  
 
MODEL SPREAD HAS VASTLY IMPROVED WITH THE INITIAL COASTAL LOW  
EXITING NEW ENGLAND MID-WEEK. AS THE PATTERN AMPLIFIES ACROSS THE  
MIDDLE OF THE COUNTRY, THE 12Z UKMET STOOD OUT AS AN OUTLIER AS  
ITS SOLUTION FAVORED A BUILDING RIDGE OVER THE WESTERN ATLANTIC  
WHICH IS NOT SHOWN IN OTHER MODELS. SHIFTING FOCUS TO THE GULF OF  
MEXICO DEVELOPMENT, ENSEMBLE LOW PLOTS STILL EXHIBIT A GREAT DEAL  
OF SPREAD WITH MULTIPLE WAVES POSSIBLE. THE SUGGESTED NEGATIVE  
TILT OF THE SHORTWAVE INDICATES A COASTAL LOW IS LIKELY TO FORM  
OVER THE WESTERN ATLANTIC BUT DETAILS ARE QUITE NEBULOUS. AFTER  
BEING SLOWER IN RECENT MODEL CYCLES, THE QUICKER TREND SHOWN BY  
THE 18Z GFS HAS CONTINUED WITH ITS LATEST 00Z RUN. RUN TO RUN  
CONTINUITY IS LACKING IN MOST OF THE GLOBAL GUIDANCE WHICH  
SUGGESTS DIMINISHED CONFIDENCE WITH ANY FORECAST DETAILS. AS THIS  
WAVE NEARS THE GULF STREAM NEXT WEEKEND, RENEWED HEIGHT FALLS IN  
THE FORM OF A CLIPPER SYSTEM MAY IMPACT THE MIDDLE MISSISSIPPI  
VALLEY AS SHOWN BY THE 18Z GFS AS WELL AS SOME ENSEMBLE MEMBERS OF  
VARIOUS GLOBAL MODELS. OVERALL, THE PREFERENCE FAVORED A CONSENSUS  
OF THE 12Z GFS/ECMWF/CMC THROUGH DAY 4/THURSDAY BEFORE QUICKLY  
ADDING ENSEMBLE MEANS TO THE PICTURE. THE 12Z GFS/ECMWF  
COMBINATION SEEMED TO CLUSTER WELL BUT THE INCONSISTENCY OF  
OPERATIONAL RUNS SUGGESTS ENSEMBLE APPROACHES ARE NECESSARY INTO  
THE LATTER HALF OF THE PERIOD.  
 
THE WETTEST CONDITIONS DURING THE PERIOD SHOULD CONGREGATE ALONG  
THE GULF COAST REGION GIVEN THE EASTWARD MIGRATING WAVE OF LOW  
PRESSURE. RECENT GUIDANCE CONTINUES TO FAVOR THE HEAVIEST RAINFALL  
JUST OFFSHORE WHERE THE BETTER INSTABILITY SHOULD SET UP. HOWEVER,  
IT WOULD NOT TAKE MUCH TO MOVE SOME OF THESE CONVECTIVE ELEMENTS  
INLAND. THE FOCUS SHOULD GRADUALLY MOVE NORTHEASTWARD WITH THE  
COASTAL CAROLINAS GETTING IN ON THE ACTION FRIDAY ONWARD. ACROSS  
THE PACIFIC NORTHWEST, SOMEWHAT PERSISTENT ONSHORE FLOW WILL KEEP  
CONDITIONS WET DURING MUCH OF THE PERIOD. LOCAL OROGRAPHICS WILL  
DOMINATE WITH MOST OF THE ACTIVITY FOCUSING OVER THE OLYMPICS AND  
CASCADES. CONSIDERING WINTRY PRECIPITATION DURING THE FORECAST,  
THE BEST THREAT SHOULD BE OVER NORTHERN MAINE WITH THE INITIAL  
COASTAL LOW MID-WEEK. SOME CHANCES MAY OCCUR NEXT WEEKEND WITH THE  
NEXT COASTAL SYSTEM BUT TOO MUCH UNCERTAINTY EXISTS TO PINPOINT  
MORE PRECISE DETAILS. REGARDING TEMPERATURES, WELL BELOW AVERAGE  
READINGS ARE ANTICIPATED FROM THE MIDDLE OF THE COUNTRY EASTWARD.  
ON WEDNESDAY, DEPARTURES FROM CLIMATOLOGY COULD SIT IN THE 15 TO  
20 DEGREE RANGE OVER THE SOUTHERN PLAINS WITH SIMILAR NUMBERS  
LIKELY ON THURSDAY UP INTO NEW ENGLAND GIVEN PREVAILING  
NORTHWESTERLY FLOW. IN TERMS OF ABOVE AVERAGE TEMPERATURES,  
WEDNESDAY/THURSDAY COULD PROVE TO BE RATHER MILD DAYS OVER THE  
UPPER INTERMOUNTAIN WEST AND NORTHERN ROCKIES AS HIGHS MOVE WELL  
INTO THE 60S.  
 
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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