893  
FXUS02 KWBC 190648  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
247 AM EDT FRI OCT 19 2018  
 
VALID 12Z MON OCT 22 2018 - 12Z FRI OCT 26 2018  
 
...GUIDANCE/PREDICTABILITY ASSESSMENT AND WEATHER  
HIGHLIGHTS/THREATS...  
 
SPLIT FLOW WILL INITIALLY SET UP ACROSS NORTH AMERICA WITH A WEAK,  
STRETCHED OUT SOUTHERN STREAM ANCHORING THE DESERT SOUTHWEST. THIS  
FEATURE SHOULD GRADUALLY PRESS EASTWARD IN RESPONSE TO A LOWERING  
OF UPSTREAM HEIGHTS. ACROSS THE HIGHER LATITUDES, HIGHLY AMPLIFIED  
FLOW WILL BE IN PLACE WITH A MEAN UPPER LOW SPINNING OVER THE  
UPPER REACHES OF NUNAVUT. A PAIR OF SHORTWAVES EMBEDDED IN THIS  
FLOW ARE PRIMED TO INTENSIFY AND EVENTUALLY BECOME  
NEGATIVELY-TILTED OVER THE VICINITY OF NEW ENGLAND. MANY SOLUTIONS  
SHOW A RATHER DEEP SURFACE CYCLONE OVER THE CANADIAN MARITIMES BY  
MID-WEEK. BACK TO THE NORTHEASTERN PACIFIC, A VERY ACTIVE PATTERN  
SETS UP CONSISTING OF AT LEAST A PAIR OF POWERFUL SURFACE LOWS  
CRASHING INTO COASTAL BRITISH COLUMBIA. SOME SOLUTIONS DEEPEN SUCH  
WAVES BELOW 965-MB IN SOME CASES WHILE SHEARING INTO THE  
DOWNSTREAM RIDGE. ULTIMATELY THE PATTERN FAVORS THE EASTWARD  
MIGRATION OF A MEAN RIDGE TOWARD CENTRAL CANADA WHILE THE  
SYNOPTIC-SCALE FLOW FLATTENS ACROSS THE NORTHEASTERN PACIFIC.  
SEVERAL PERTURBATIONS IN THE FLOW ARE PRIMED TO INTENSIFY IN THIS  
REGIME WITH UNCERTAINTY LOOMING WITH EACH INSTANCE. LOOKING TO THE  
GULF OF MEXICO, A LINGERING FRONTAL ZONE AND EVENTUAL WAVE OF LOW  
PRESSURE SHOULD IMPACT THE GULF COAST WITH A THREAT FOR HEAVY  
RAINFALL.  
 
MODELS HAVE A REASONABLE GRASP ON THE PATTERN AT HAND THROUGH  
ROUGHLY DAY 4/TUESDAY BEFORE UNCERTAINTY LOOMS ON BOTH COASTS. THE  
12Z UKMET REMAINED SOUTH OF THE CONSENSUS OFF THE NEW ENGLAND  
COAST BY MID-WEEK WITH 12Z ECMWF ENSEMBLE SPREAD IMPROVING  
RELATIVE TO YESTERDAY. ACROSS THE PACIFIC, THE CONVEYOR BELT OF  
SHORTWAVES TRAVERSING THE ACTIVE FLOW WILL REMAIN AN ISSUE GIVEN  
THE DEPTH OF MANY SUCH FEATURES. DECENT CLUSTERING EXISTS WITH THE  
INITIAL WAVE OFF THE BRITISH COLUMBIA COAST ON TUESDAY WITH MORE  
UNCERTAINTY LOOMING OFFSHORE WITH THE NEXT AMPLIFYING SHORTWAVE.  
RUN-TO-RUN SPATIAL DIFFERENCES ARE NOTED WITH GFS SOLUTIONS WITH  
THIS ISSUE NOTED AMONG OTHER GLOBAL MODELS. AT THE VERY LEAST,  
ENSEMBLE MEANS ARE JUXTAPOSED ATOP ONE ANOTHER WHICH WILL BE THE  
PREFERRED ROUTE LATE IN THE PERIOD (DAY 6/7, OCTOBER 25/26).  
BEFORE THEN, A MULTI-MODEL BLEND OF THE 18Z GFS AND 12Z  
ECMWF/UKMET SEEMED REASONABLE EARLY ON.  
 
A HEAVY RAINFALL THREAT WILL REMAIN ALONG THE WESTERN/CENTRAL GULF  
COASTS WITH THE AXIS MOVING EASTWARD IN TIME ACCOMPANYING THE WEAK  
AREA OF LOW PRESSURE. IT REMAINS TO BE SEEN HOW MUCH INSTABILITY  
WILL APPROACH THE COAST BUT HEFTY QPF IS NOTED IN THE GUIDANCE  
JUST OFFSHORE. THE FORECAST ITSELF TENDS TO KEEP THIS AXIS RIGHT  
ALONG THE COAST BUT THE ULTIMATE PLACEMENT WITH BE HEAVILY DRIVEN  
BY WHERE THE INSTABILITY SETS UP. ACROSS THE PACIFIC NORTHWEST,  
SEMI-PERSISTENT ONSHORE FLOW WILL KEEP CONDITIONS WET OVER THE  
OLYMPICS AND CASCADES THROUGHOUT THE PERIOD. PACIFIC DOMINATED AIR  
MASSES WILL PRECLUDE ANY NOTABLE WINTER WEATHER THREATS. ACROSS  
NEW ENGLAND, SOME SNOW MAY FALL OVER NORTHERN MAINE IF THE LOW  
PRESSURE WERE TO DEEPEN ENOUGH TO PULL IN COLDER THICKNESSES DOWN  
FROM EASTERN CANADA. REGARDING TEMPERATURES, BELOW AVERAGE NUMBERS  
WILL PREVAIL OVER MUCH OF THE CENTRAL/EASTERN U.S. DURING THE  
EARLY TO MIDDLE PART OF NEXT WEEK, READINGS COULD BE AROUND 10 TO  
15 DEGREES BELOW CLIMATOLOGY OVER THE SOUTHERN PLAINS WITH  
PERSISTENT NORTHERLY FLOW AND ACTIVE PRECIPITATION. OVER THE  
NORTHWESTERN U.S., AIR MASSES OF PACIFIC ORIGIN WILL KEEP MILD AIR  
IN PLACE THROUGH AT LEAST THE MIDDLE OF NEXT WEEK.  
 
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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