418  
FXUS02 KWBC 031601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT WED OCT 03 2018  
 
VALID 12Z SAT OCT 06 2018 - 12Z WED OCT 10 2018  
 
...STORMY PATTERN FOR THE WEST/ROCKIES AND A CENTRAL U.S.  
EXCESSIVE RAINFALL THREAT...  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A HIGHLY AMPLIFIED SYNOPTIC REGIME WILL REMAIN A FIXTURE DURING  
THE MEDIUM RANGE PERIOD. POSITIVE HEIGHT ANOMALIES WITH MID-LEVEL  
HEIGHTS OFTEN EXCEEDING 591-DM SHOULD BE COMMONPLACE OVER THE  
EASTERN SEABOARD. MEANWHILE, THE BLOCKING REGIME WILL AFFORD A  
PERSISTENCE OF TROUGHING ACROSS THE SOUTHWESTERN U.S. AS AN UPPER  
LOW PINCHES OFF OVER THE REGION. THIS AXIS OF LOWER HEIGHTS SHOULD  
GRADUALLY DRIFT NORTHEASTWARD NEXT WEEK WITH THE POTENTIAL FOR A  
REINFORCEMENT OF THIS PATTERN AS A DEEP UPPER LOW DESCENDS FROM  
BRITISH COLUMBIA. RELATIVE TO CLIMATOLOGY, MANY OF THESE MENTIONED  
FEATURES ARE IN THE 2 TO 3 SIGMA DEPARTURE RANGE, PARTICULARLY  
WITH THE UPPER LOWS. THE SQUEEZE PLAY BETWEEN THIS PAIR OF  
PERSISTENT FEATURES WILL LEAD TO TREMENDOUS POLEWARD TRANSPORT OF  
GULF MOISTURE INTO THE CENTRAL U.S. ONLY MODEST CHANGES IN THE  
PATTERN ARE ANTICIPATED GIVEN THE STABLE POSITIVE HEIGHT ANOMALIES  
INHABITING THE EASTERN U.S. AND EASTERN PACIFIC, RESPECTIVELY.  
 
SHEARING HEIGHT FALLS WITHIN BROAD MID-LEVEL SOUTHWESTERLY FLOW  
WILL CARRY A DAMPENING WAVE OF LOW PRESSURE ACROSS THE GREAT LAKES  
AND INTO SOUTHERN QUEBEC OVER THE WEEKEND. MODEL AGREEMENT IS MUCH  
STRONGER WITH THIS GUIDANCE PACKAGE RELATIVE TO YESTERDAY.  
SHIFTING FOCUS TO WESTERN NORTH AMERICA, MULTI-DAY ENSEMBLES  
CONTINUE TO FAVOR AN UPPER LOW DROPPING DOWN FROM THE CENTRAL  
GREAT BASIN WHILE CLOSING OFF OVER THE DESERT SOUTHWEST AND FOUR  
CORNERS REGION. THE 00Z ECMWF SITS A BIT TO THE WEST OF THE  
CONSENSUS BY DAY 5/MONDAY ALTHOUGH IT DOES SIT WITHIN THE ENSEMBLE  
CLUSTER. WHERE MODELS DIVERGE IS WITH THE STRENGTH AND TIMING OF A  
WAVE OF LOW PRESSURE POSSIBLY LIFTING INTO THE NORTH-CENTRAL U.S.  
ON MONDAY. THE PAST SEVERAL RUNS OF THE GFS AS WELL AS THE 06Z  
FV3-GFS REMAIN ADAMANT ABOUT A DEEPER SOLUTION ALTHOUGH THIS CAMP  
SITS ON ITS OWN. THE 00Z CMC IS ABOUT A DAY LATER WITH THIS  
EVOLUTION WHILE THE PREVIOUS COUPLE ECMWF CYCLES DEPICT A MORE  
INNOCUOUS PATTERN. FURTHER ISSUES ARISE ACROSS THE HIGHER  
LATITUDES WITH THE AMOUNT OF POLAR ENERGY ABLE TO DRIVE SOUTHWARD  
ACROSS WESTERN CANADA AND INTO THE PACIFIC NORTHWEST. IT APPEARS  
THE COMPLEXITIES LIE WITH A BAND OF SHORTWAVE ENERGY PEELING OFF A  
VORTEX CURRENTLY SITTING OVER NUNAVUT. A COMPACT UPPER LOW SITTING  
UNDERNEATH A CLOSED HIGH ACROSS THE NORTHERN NORTHWEST TERRITORIES  
APPEARS TO BE THE CULPRIT. THIS PARTICULAR SCENARIO IS NOT SHOWN  
BY RECENT RUNS OF THE GFS WHICH ALLOWS SHORTWAVE ENERGY TO FREELY  
DROP DOWN THROUGH BRITISH COLUMBIA AND INTO THE NORTHWESTERN  
STATES. IN CONTRAST, THE 00Z/12Z ECMWF FORM THAT HIGH-LATITUDE  
BLOCK WHICH DELAYS THE ARRIVAL OF LOWER HEIGHTS INTO THE PACIFIC  
NORTHWEST BY ROUGHLY A DAY.  
 
GIVEN THE UNCERTAINTY THAT LOOMS BEYOND DAY 4/SUNDAY, ONLY KEPT  
SUBSTANTIAL AMOUNTS OF OPERATIONAL GUIDANCE IN THE MIX THROUGH THE  
WEEKEND. THIS WAS COMPROMISED PRIMARILY OF THE 00Z GFS/ECMWF WITH  
MINOR CONTRIBUTIONS FROM THE 00Z CMC/UKMET. BY DAY 5/MONDAY  
ONWARD, ADDED MORE SIGNIFICANT CHUNKS OF THE 00Z GEFS/ECMWF  
ENSEMBLE MEANS INTO THE PICTURE. DECIDED AGAINST USING THE 00Z  
NAEFS MEAN AS IT IS BIASED TOWARD THE GULF DEVELOPMENT DEPICTED BY  
THE CMC ENSEMBLES.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WITH THE LEVEL OF AMPLIFICATION ANTICIPATED DURING THE PERIOD, IT  
IS NOT SURPRISING THAT A HEAVY RAINFALL THREAT WILL ENSUE OVER THE  
MIDDLE OF THE COUNTRY WITH IMPRESSIVE TEMPERATURE GRADIENTS  
BOUNDING THE MEANDERING FRONTAL ZONE. MODELS DEPICT PRECIPITABLE  
WATER VALUES EXCEEDING THE 95TH PERCENTILE OVER THE CENTRAL U.S.  
WITH DEEP LAYERED SOUTHERLY FLOW ALLOWING FOR THE POTENTIAL FOR  
TRAINING CONVECTIVE ELEMENTS. THE MULTI-DAY NATURE OF THE RAINFALL  
WILL LIKELY AUGMENT FLASH FLOOD CONCERNS, PARTICULARLY ALONG THE  
RED RIVER NORTHWARD TO THE CENTRAL PLAINS WHERE MODELS ARE MOST  
ROBUST. SEVERAL INCHES OF RAIN ARE LIKELY DURING THE FORECAST  
PERIOD WITH THE THREAT GRADUALLY LIFTING NORTH-NORTHEASTWARD INTO  
THE MIDDLE MISSISSIPPI VALLEY AND UPPER MIDWEST. FARTHER WEST  
WITHIN THE DOME OF COLD AIR, WINTRY PRECIPITATION IS ANTICIPATED  
OVER LARGE SECTIONS OF MONTANA AND WYOMING EXTENDING INTO THE  
WESTERN DAKOTAS.  
 
REGARDING TEMPERATURES, WELL BELOW AVERAGE NUMBERS ARE LIKELY FROM  
THE UPPER MIDWEST BACK TO THE WEST COAST. THE BOWL OF COLDEST AIR  
WILL RESIDE ACROSS THE NORTHERN/CENTRAL ROCKIES INTO THE ADJACENT  
HIGH PLAINS WITH DEPARTURES FROM CLIMATOLOGY ON THE ORDER OF 20 TO  
30 DEGREES IN SOME CASES. SOME HIGHER ELEVATION LOCATIONS WILL  
STRUGGLE TO GET OUT OF THE 30S FOR HIGHS WHILE OVERNIGHT LOWS  
PLUNGE INTO THE 20S. ON THE OTHER SIDE OF THE BOUNDARY, IT WILL  
FEEL MORE LIKE THE END OF SUMMER WITH HIGHS AROUND 10 TO 15  
DEGREES ABOVE AVERAGE, ESPECIALLY AROUND THE GREAT LAKES AND OHIO  
VALLEY. EVEN SOME DAILY RECORDS MAY BE BROKEN GIVEN THE VAST  
OVERNIGHT WARMTH OVER THE REGION.  
 
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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