573  
FXUS02 KWBC 211543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1142 AM EDT SUN OCT 21 2018  
 
VALID 12Z WED OCT 24 2018 - 12Z SUN OCT 28 2018  
 
...GUIDANCE/PREDICTABILITY ASSESSMENT AND WEATHER  
HIGHLIGHTS/THREATS...  
 
A STRENGTHENING LOW WILL EXIT NEW ENGLAND TOWARD THE CANADIAN  
MARITIMES MIDWEEK, WITH LINGERING EARLY SEASON COLD AIR/SNOWS  
WRAPPING BACK INTO NORTHERN NEW ENGLAND. MODEL SPREAD HAS VASTLY  
IMPROVED WITH THIS COASTAL LOW. THE MAIN WEATHER FOCUS THEN SHIFTS  
TO A PHASING PAIR OF STREAMS ACROSS THE MIDDLE OF THE NATION. THIS  
PROCESS ULTIMATELY LEADS 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. THIS OFFERS A  
THREAT OF HEAVY PRECIPITION FOR THE GULF COAST THEN EAST COAST  
WITH COASTAL STORM POTENTIAL. UPSTREAM, ADDITIONAL MID-UPPER LEVEL  
TROUGH AMPLIFICATION MAY ALSO ENHANCE DEVELOPMENT OF A MIDWEST LOW  
WITH SOME SNOW ALONG WITH A REINFORCING TRAILING FRONTAL SURGE  
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. AS THE PATTERN AMPLIFIES OVER  
THE MID-NATION LATE WEEK, THE 00 UTC CANADIAN/UKMET AND TO LESSER  
EXTENT THE 00 UTC ECMWF STAND OUT AS OUTLIERS COMPARED TO THE  
ENSEMBLE ENVELOPE BY LATER WEEK IN BRINGING WARMING WESTERN U.S.  
RIDGING TOO PROGRESSIVELY EASTWARD INTO THE N-CENTRAL U.S.,  
LEADING TO SHARPER DOWNSTREAM TROUGH AMPLIFICATION. THE 06 UTC GFS  
IS MORE IN LINE WITH MULTI-MODEL ENSEMBLES BOTH WITH HOLDING MORE  
AMPLIFIED RIDGING OVER THE WEST AND DOWNSTREAM EAST-CENTRAL U.S.  
TROUGH AMPLITUDE. ACCORDINGLY, WPC PROGS BLENDED BEST CLUSTERED 00  
UTC GEFS/ECMWF ENSEMBLES WITH THE 06 UTC GFS DAYS 3-5, QUICKLY  
TRANSITIONING TO AN ENSEMBLE MEAN BLEND DAYS 6/7 AMID GROWING  
FORECAST SPREAD, NOT COMMITTING YET TO SIGNIFICANT 06 UTC GFS EAST  
COAST LOW GENESIS/QPF.  
 
SHIFTING FOCUS UNDERNEATH TO EXPECTED GULF OF MEXICO DEVELOPMENT,  
ENSEMBLE LOW PLOTS STILL EXHIBIT A GREAT DEAL OF SPREAD WITH  
MULTIPLE WAVES POSSIBLE. THIS AND QPF IS FURTHER COMPLICATED BY  
ANY MOISTURE AND DOWNSTREAM RIPPLES FROM THE REMNANTS OF HURRICANE  
WILLA AND TROPICAL STORM VICENTE THAT ARE BOTH FORECAST TO  
LANDFALL ON THE PACIFIC COAST OF MEXICO MIDWEEK. THEN OVERTOP, THE  
SUGGESTED NEGATIVE TILT/AMPLITUDE OF THE SUPPORTING SHORTWAVE  
ALOFT INDICATES A COASTAL LOW MAY SUBSEQUENTLY FORM DOWNSTREAM  
OVER THE WESTERN ATLANTIC, BUT DETAILS ARE LESS CERTAIN. RUN TO  
RUN CONTINUITY IS LACKING IN THE MODELS WHICH SUGGESTS DIMINISHED  
CONFIDENCE WITH ANY FORECAST DETAILS. EVEN IN CASES WHEN  
PARTICULAR MODEL RUNS HAPPEN TO LOOK SIMILAR AT PARTICULAR TIME  
STEPS, DEVELOPMENT EVOLUITONS HAVE BEEN DIFFERENT. THAT SAID,  
RECENT MODEL RUNS HAVE OVERALL TRENDED TOWARD AT LEAST MODERATE  
SURFACE LOW DEEPENING AND CLOSER TO THE COAST TRACK, AS DO WPC  
PROGS.  
 
THE WETTEST CONDITIONS THIS PERIOD SHOULD TRAIN ALONG THE GULF  
COAST REGION GIVEN THE EASTWARD MIGRATING WAVE OF LOW PRESSURE AND  
TROPICAL MOISTURE INFUSION. RECENT GUIDANCE CONTINUES TO FAVOR THE  
HEAVIEST RAINFALL JUST OFFSHORE WITH BEST INSTABILITY. HOWEVER, IT  
WOULD NOT TAKE MUCH TO MOVE SOME OF THESE CONVECTIVE ELEMENTS  
INLAND. THE FOCUS GRADUALLY MOVES NORTHEASTWARD INTO THE COASTAL  
CAROLINAS GETTING BY FRIDAY, WORKING UP THE COAST NEXT WEEKEND TO  
EFFECT OFFSHORE WATERS AND INLAND TO THE EASTERN MID-ATLANTIC AND  
NEW ENGLAND. COOLED FALL TEMPS OVER THE EASTERN SEABOARD DO NOT  
SEEM SUFFIENTLY COLD TO WARRANT AN EARLY SEASON SNOW THREAT INTO  
NEXT WEEKEND OVERALL. HOWEVER, THERE IS MODEST POTENTIAL FOR  
HIGHER ELEVATIONS OF THE APPALACHIANS AND PORTIONS OF THE  
MIDWEST/GREAT LAKES AND INTERIOR NORTHEAST. FOR THE PACIFIC  
NORTHWEST, PERSISTENT ONSHORE FLOW KEEPS CONDITIONS WET FOR MUCH  
OF THE PERIOD. LOCAL OROGRAPHICS DOMINATE ACTIVITY/ELEVATION SNOWS  
TO FOCUS FROM THE OLYMPICS/CASCADES THAT MODERATES QUICKLY INLAND  
TO THE NORTHERN ROCKIES.  
 
SCHICHTEL  
 
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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