822  
FXUS02 KWBC 170701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT MON SEP 17 2018  
 
VALID 12Z THU SEP 20 2018 - 12Z MON SEP 24 2018  
   
..PATTERN OVERVIEW/GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
WHILE THE EXTRATROPICAL REMAINS OF FLORENCE CONTINUE TO ACCELERATE  
INTO THE CANADIAN MARITIMES, THE FOCUS SHIFTS TOWARD A BAND OF  
HEIGHT FALLS SWEEPING THROUGH THE NORTH-CENTRAL U.S. ON THURSDAY  
MORNING. AN ACCOMPANYING WAVE OF LOW PRESSURE SHOULD LIFT FROM THE  
MIDDLE MISSISSIPPI VALLEY TOWARD THE UPPER GREAT LAKES ON THURSDAY  
EVENING WHERE CONSIDERABLE DEEPENING IS LIKELY TO TAKE PLACE.  
FURTHER STRENGTHENING IS EXPECTED WHILE THE SYSTEM MOVES TOWARD  
WESTERN QUEBEC ON FRIDAY. THE ATTENDANT COLD FRONT SHOULD MAKE  
CONSIDERABLE PROGRESSION EASTWARD BUT SHOULD STRUGGLE TO GET SOUTH  
OF THE MID-ATLANTIC REGION GIVEN THE STOUT UPPER RIDGE TOWARD THE  
SOUTHEASTERN U.S. BACK TOWARD WESTERN NORTH AMERICA, A PAIR OF  
UPPER TROUGHS ARE PRIMED TO AFFECT THE REGION DURING THE PERIOD.  
THIS WAVE TRAIN OF SYSTEMS SHOULD TRAVERSE FROM THE GULF OF ALASKA  
WHILE A BROAD POSITIVE HEIGHT ANOMALY SITS UPSTREAM. THE INITIAL  
CLOSED LOW IS FORECAST TO SWEEP THROUGH THE PACIFIC NORTHWEST  
EARLY FRIDAY BEFORE ADJOINING THE NORTHERN STREAM FLOW ACROSS  
WESTERN CANADA. ON ITS HEELS FOR BE A MORE LARGE-SCALE TROUGH  
WHICH SHOULD JUXTAPOSE ITSELF SOMEWHERE ALONG THE PACIFIC/WESTERN  
U.S. INTERFACE DURING THE DAY 5-7, SEPTEMBER 22-24 PERIOD.  
ELSEWHERE, AN AREA OF DISTURBED WEATHER ACROSS THE SUBTROPICAL  
PACIFIC SHOULD LIFT NORTHWARD TOWARD THE SOUTHERN BAJA CALIFORNIA  
PENINSULA. WHILE TROPICAL DEVELOPMENT REMAINS POSSIBLE, THE BIGGER  
THREAT WILL BE THE ASSOCIATED MOISTURE PLUME PUSHING INTO THE FOUR  
CORNERS REGION. THIS SHOULD AUGMENT THE POTENTIAL FOR HEAVY  
RAINFALL EARLY IN THE FORECAST.  
 
THE MODELS INITIALLY EXHIBIT REASONABLE AGREEMENT AMONGST ONE  
ANOTHER WHILE EVENTUAL DIVERGENCE IN THE SOLUTIONS EXIST BEYOND  
DAY 4/FRIDAY. INITIALLY, THE UNCERTAINTY LOOMS AROUND THE  
DEEPENING WAVE OF LOW PRESSURE EXITING THE GREAT LAKES TOWARD  
EASTERN CANADA. MULTI RUN COMPARISONS HAVE SHOWN A TENDENCY FOR  
THE ECMWF TO BE DECIDEDLY WEAKER THAN THE REST OF THE GUIDANCE.  
GIVEN THE DYNAMIC NATURE OF THE APPROACHING SHORTWAVE, IT DOES  
APPEAR TO BE TOO WEAK AS OTHER MODELS FAVOR DEEPENING TO POSSIBLY  
BELOW 990-MB WHILE MOVING THROUGH QUEBEC. FORTUNATELY, MOST OF THE  
SPREAD DOES NOT SIGNIFICANT IMPACT THE CONTINENTAL U.S. BUT IT IS  
WORTH NOTING. BACK TO THE WESTERN U.S., MODELS HAVE EXHIBITED A  
DEEPENING TREND WITH THE UPPER LOW APPROACHING THE WASHINGTON  
COAST ON FRIDAY MORNING. THIS HAS RESULTED IN QUITE THE UPTICK IN  
PRECIPITATION POTENTIAL OVER WESTERN WASHINGTON UP INTO BRITISH  
COLUMBIA. WHILE THE 18Z GFS/12Z ECMWF INITIALLY AGREED ON TIMING,  
THE 00Z GFS MADE A DECIDED WESTWARD SHIFT WHICH SUGGESTS SOME  
TIMING UNCERTAINTIES ARE IN PLAY. SHIFTING FOCUS TO THE LONGWAVE  
TROUGH DURING THE END OF THE FORECAST, THE 12Z CMC/CMC ENSEMBLE  
MEAN APPEARED TO BE QUICK OUTLIERS. HOWEVER, THE 00Z GFS ACTUALLY  
MADE A LARGE ADJUSTMENT ON DAY 7/SEPTEMBER 24 TOWARD THAT  
PARTICULAR SOLUTION. ENSEMBLE SPAGHETTI PLOTS SHOW PLENTY OF  
SPREAD WITH THIS SCENARIO LYING ON THE FAR EASTERN EDGE OF THE  
PACK. WOULD IMAGINE FUTURE OPERATIONAL MODELS WILL WAFFLE BACK AND  
FORTH THE NEXT COUPLE OF DAYS.  
 
THROUGH DAY 4/FRIDAY, WAS ABLE TO UTILIZE A COMBINATION OF  
OPERATIONAL SOLUTIONS LED BY THE 18Z GFS/12Z ECMWF WITH SOME  
CONTRIBUTIONS FROM THE 12Z UKMET MIXED IN. THEREAFTER, BUILDING  
SPREAD WITH THE EASTERN CANADA WAVE COUPLED WITH ISSUES REGARDING  
THE WESTERN U.S. TROUGH EVOLUTION SUPPORTED INCLUSION OF ENSEMBLE  
MEANS. GRADUALLY RAISED PERCENTAGES FROM THE 12Z ECMWF/NAEFS  
ENSEMBLE MEANS FROM DAY 5/SATURDAY ONWARD WITH CONFIDENCE ROUGHLY  
AROUND AVERAGE THROUGHOUT THIS PERIOD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A PAIR OF HEAVY RAINFALL THREATS ARE IN THE PICTURE THROUGH  
PERHAPS SATURDAY. THE FIRST AREA SHOULD CONCENTRATE NEAR THE  
DYNAMIC AREA OF LOW PRESSURE SWEEPING THROUGH THE GREAT LAKES.  
SECONDLY, TROPICAL MOISTURE PUMPING OUT OF THE PACIFIC SHOULD  
ENHANCE THREAT FOR HEAVY PRECIPITATION OVER THE FOUR CORNERS  
REGION BUT PARTICULARLY INTO THE SOUTHERN HIGH PLAINS. THE LATTER  
AREA IS MORE MODEL DEPENDENT AS THERE ARE PLENTY OF SOLUTIONS  
SHOWING LOWER QPF VALUES. EVENTUALLY THE BOUNDARY STALLING OUT OF  
THE LOWER MID-ATLANTIC COULD PROVE TO KEEP CONDITIONS WET TOWARD  
THE LATTER HALF OF THE WEEKEND INTO THE FOLLOWING WEEK. ACROSS THE  
PACIFIC NORTHWEST, THE COMPACT UPPER LOW SHOULD SPREAD A BURST OF  
SHOWERS TO WESTERN WASHINGTON UP INTO BRITISH COLUMBIA ON FRIDAY  
WITH ADDITIONAL PRECIPITATION LIKELY DURING THE FOLLOWING DAYS IN  
RESPONSE TO THE NEXT TROUGH.  
 
THROUGHOUT THE FORECAST, THE MOST PERSISTENT AREA OF BELOW AVERAGE  
TEMPERATURES WILL RESIDE OVER THE UPPER INTERMOUNTAIN WEST  
EASTWARD ACROSS THE NORTHERN ROCKIES AND INTO THE DAKOTAS. ON  
THURSDAY, SUCH DEPARTURES COULD REACH THE 10 TO 15 DEGREE RANGE AS  
HIGHS STRUGGLE TO ESCAPE THE 50S. TO THE SOUTH OF THE MIGRATORY  
BOUNDARY, WARM CONDITIONS WILL PREVAIL ACROSS THE LOWER/MIDDLE  
MISSISSIPPI VALLEYS INTO THE OHIO/TENNESSEE VALLEYS. TOWARD THE  
WEEKEND, ANOMALIES SHOULD DECREASE A BIT BUT THAT IS ALSO AN  
ARTIFACT OF HEAVY ENSEMBLE USAGE. HOWEVER, IT SHOULD GENERALLY  
REMAIN ON THE COOL SIDE ACROSS THE NORTH-CENTRAL U.S. GIVEN THE  
INFLUENCE OF CANADIAN AIR MASSES.  
 
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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