EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1146 AM EDT SAT SEP 30 2017 VALID 12Z TUE OCT 03 2017 - 12Z SAT OCT 07 2017 ...15Z UPDATE/MIDDAY THOUGHTS... AS MENTIONED BELOW, THE PATTERN OVERALL FOR MUCH OF THE MEDIUM RANGE PERIOD WILL BE HIGH AMPLITUDE WITH SHARP RIDGE OVER THE EASTERN PACIFIC INTO AK, NEGATIVE ANOMALY OVER THE WEST AND LARGE UPPER RIDGE ENCOMPASSING MUCH OF THE EASTERN TWO-THIRDS OF THE COUNTRY. THE MEANS SUGGEST THOUGH BY D+8, THAT THE RIDGE OFF THE WEST COAST PERSISTS BUT CONTINUOUS HEIGHT FALLS FROM PROGRESSIVE SHORT WAVE FEATURES FROM AK/BERING SEA WITH THE UPPER LOW SHIFTING FROM THE WEST INTO THE MIDDLE OF THE COUNTRY MAY INTRODUCE HEIGHTS OF 1 TO 2 STANDARD DEVIATIONS BELOW AVG FROM WEST CENTRAL CANADA TO THE PLAINS/MS VALLEY ALONG WITH A POSSIBLE WEAKNESS EXTENDING FROM OFF THE EAST COAST THROUGH FL INTO THE EASTERN GULF OF MEXICO. HOWEVER, EVEN THE MEANS SUGGEST ENOUGH SPREAD AND UNCERTAINTY WITH THE POSSIBLE SHIFT IN THE PATTERN. THE OPERATIONAL GUIDANCE VARIES QUITE SIGNIFICANTLY BY WED OR THURS WITH THE 00Z ECMWF SHOWING PERHAPS THE BEST CONTINUITY. STARTING ON TUES, A MASSIVE RIDGE OVER THE EAST, INITIAL HEIGHT FALLS SPREADING OUT INTO THE UPPER MIDWEST AND ADDITIONAL UPPER DYNAMICS CLOSING ANOTHER LOW NEAR OR/ID/NORTHERN CA/NV. THE INITIAL SET OF HEIGHT FALLS TRACK ACROSS THE NORTHERN TIER OF THE COUNTRY OR THROUGH THE GREAT LAKES AND NORTHEAST BY NEXT FRI. THE FAST ZONAL UPPER JET THEN UPSTREAM OVER THE NERN PAC KICKS THE UPPER LOW INTO THE NRN PLAINS ON FRI AND INTERACTS WITH ONE ANOTHER OR PHASES OVER THE CENTER OF THE COUNTRY. WPC FAVORED A BLEND OF OPERATIONAL GUIDANCE ON TUES BEFORE SHIFTING ON WED TO A BASE OF THE 00Z ECMWF AND ECMWF MEAN. THEN THURS THROUGH NEXT SAT, A BLEND OF THE 00Z ECMWF/NAEFS AND 06Z GEFS MEAN WITH A SMALL PERCENTAGE OF THE 00Z ECMWF WAS USED. THIS ALLOWS THE MEANS TO FILTER OUT THE SPREAD AND DETAILS TO CAPTURE THE LARGE SCALE UPPER PATTERN AND TAKES ADVANTAGE OF THE OPERATIONAL ECMWF CONTINUITY OVER THE POOR GFS. THE WPC 5 DAY MEAN MAX TEMP ANOMALY FEATURES ABOVE AND BELOW AVG TEMPS. THE BELOW AVG HIGHS WILL BE OUT WEST INTO THE NORTHERN PLAINS/FRONT RANGE OF THE NORTHERN/CENTRAL ROCKIES. THE COLDEST ANOMALIES WITH HIGHS 10 TO 20 DEG BELOW AVG ENCOMPASSES TWO STATES, MT AND WY. MEANWHILE, HIGHS 5 TO 10 DEG ABOVE AVG WILL BE COMMON FROM THE LOWER MS VALLEY/SOUTH THROUGH THE OH/TN VALLEYS INTO THE GREAT LAKES/INTERIOR NORTHEAST/NORTHERN NEW ENGLAND. FOR PRECIPITATION THROUGH WED, THE HIGHLIGHTS INCLUDE MT WITH DEFORMATION ZONE QPF, SCATTERED SHOWERS/STORMS FROM THE GREAT LAKES/UPPER MS VALLEY BACK TO THE CENTRAL/SOUTHERN PLAINS NEAR THE FRONTAL ZONE (PERHAPS MORE ORGANIZED FROM NORTHERN WI/SOUTH AND EASTERN MN/NWRN IA/SERN SD AND EAST CENTRAL NE) AND ERLY/SERLY ONSHORE FLOW FROM SOUTHEAST FL TO THE TX/LA COAST. THE 48 HR PERIOD FROM THURS THROUGH FRI SHOULD FEATURE HEAVIER QPF OVER THE PLAINS/SOUTHERN ROCKIES AND MID TO UPPER MS VALLEY TO PARTS OF THE GREAT LAKES/OH VALLEY INTO NORTHERN NEW ENGLAND AND THE CONTINUED ERLY ONSHORE COMPONENT INTO THE EAST CENTRAL TO SERN FL PENINSULA. MUSHER ...EARLIER DISCUSSION... ...OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... HIGH-AMPLITUDE FLOW WILL CONTINUE TO DOMINATE THE SYNOPTIC-SCALE PATTERN DURING THE PERIOD. OVER THE COURSE OF MUCH OF THE FORECAST...A BROAD NEGATIVE HEIGHT ANOMALY SHOULD ENCOMPASS THE WESTERN U.S. MEANWHILE...AN UPPER RIDGE INITIALLY POSITIONED FROM THE MS RIVER VALLEY TO THE EASTERN SEABOARD IS FORECAST TO SLIDE SOUTH AND WEST AS THE NORTHERN STREAM BEGINS TO AMPLIFY ACROSS NEW ENGLAND BY THURSDAY/FRIDAY. A PERSISTENT WAVE TRAIN IS NOTED IN THE GUIDANCE EMANATING FROM THE BERING SEA AS A SERIES OF SYSTEMS RACE THROUGH THE GULF OF AK AND INTO WESTERN/CENTRAL CANADA. THIS REMAINS THE MOST CONTENTIOUS SECTOR OF THE MAP AS FEATURES TRACK ATOP A STOUT UPPER RIDGE ANCHORING THE EASTERN PACIFIC. LIKEWISE...MODELS SEEM TO BE STRUGGLING WITH HOW LONG TO MAINTAIN A CLOSED LOW ACROSS THE WESTERN STATES GIVEN UPSTREAM DIFFERENCES AND ANY INTERACTIONS WITH AMPLIFIED NORTHERN STREAM FLOW. ON TUESDAY MORNING...AN AREA OF LOWER HEIGHTS SHOULD EXTEND OVER THE CENTRAL GREAT BASIN AND INTERMOUNTAIN WEST. THE PAST SEVERAL RUNS OF THE ECMWF CONTINUE TO DEPICT A SEPARATE CLOSED LOW CENTER EJECTING THROUGH THE DAKOTAS. SUCH A FORECAST IS NOW IN STARK CONTRAST FROM OTHER SOLUTIONS WHICH DEPICT SHORTWAVE RIDGING IN ITS PLACE. EVENTUALLY THIS FEATURE WILL BECOME ABSORBED WITHIN THE ACTIVE NORTHERN STREAM AND BRING A LONGWAVE TROUGH AND THE ASSOCIATED COLD FRONT INTO NEW ENGLAND BY POSSIBLY THURSDAY. THIS SCENARIO IS NOT COMPLETELY AGREED UPON AS THE 12Z CMC/UKMET WERE QUICKER TO EJECT THE WESTERN U.S. CLOSED LOW WITH MORE GENERAL SOUTH-SOUTHWESTERLY FLOW ESTABLISHED DOWNSTREAM ACROSS THE NORTHEASTERN STATES. HOWEVER...IT IS WORTH NOTING THE 00Z CMC/UKMET MADE SOME WESTWARD ADJUSTMENTS TO THE FORECAST WHICH ALLOWS THE NORTHERN STREAM TO DIP DOWN INTO THE GREAT LAKES/NORTHEASTERN U.S. AS SHOWN IN THE GFS/ECMWF FORECASTS. RUN-TO-RUN CONTINUITY IN MANY OPERATIONAL MODELS HAS BEEN RATHER POOR SUGGESTING A BELOW AVERAGE CONFIDENCE FORECAST IN HAND. THIS IS SUPPORTED BY THE CHAOTIC NATURE OF RELEVANT ENSEMBLE SPAGHETTI PLOTS BY AS EARLY AS DAY 5/THURSDAY. MOVING INTO THE DAY 5-7...OCTOBER 5-7 TIMEFRAME...UNCERTAINTY INCREASES MARKEDLY ACROSS THE EASTERN PACIFIC INTO THE WESTERN U.S. IT SEEMS LIKELY A PERSISTENT UPPER RIDGE WILL REMAIN OVER THE EASTERN PACIFIC WITH A RATHER POTENT PACIFIC JET STRETCHING OUT TO ITS NORTH. MUCH OF THE GUIDANCE SHOW AN ENHANCED HEIGHT GRADIENT WITH A NUMBER OF SHARP PERTURBATIONS DASHING TOWARD WESTERN NORTH AMERICA DURING THIS WINDOW OF TIME. ENSEMBLE FORECASTS WILL LIKELY BE THE BEST GUIDE AS THE FORECAST TRULY BREAKS DOWN AS SPREAD RISES MARKEDLY. FOR DAYS 3-5...TUESDAY THROUGH THURSDAY...TOOK A COMBINATION OF THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF ENSEMBLE MEAN...70 PERCENT WEIGHTED TOWARD THE LATTER. CHOSE THIS ROUTE GIVEN A MORE CONSISTENT ECMWF FORECAST INCLUDING ITS DEPICTION OF THE AFOREMENTIONED DAY 3/TUESDAY CLOSED LOW ACROSS THE NORTHERN HIGH PLAINS. GRADUALLY DECREASED THE WEIGHTING OF OPERATIONAL MODELS BEFORE GOING SOLELY ENSEMBLE MEANS...18Z GEFS/12Z ECMWF ENSEMBLE MEANS BY FRIDAY WITH BELOW AVERAGE CONFIDENCE. ...SENSIBLE WEATHER/HIGHLIGHTS... DESPITE THE AMPLIFIED PATTERN IN PLACE...THERE ARE CURRENTLY NO TEMPERATURE RECORDS EXPECTED TO BE BROKEN THROUGH NEXT SATURDAY...OCTOBER 7. THE COLDEST WEATHER WILL BE ON TUESDAY WITH READINGS ON THE ORDER OF 2O TO 25 DEGREES BELOW NORMAL ACROSS THE NORTHERN ROCKIES. EXPECTED HIGH TEMPERATURES MAY NOT ESCAPE THE MID/UPPER 30S OVER NORTHWESTERN WY INTO CENTRAL MT WITH 40S MORE LIKELY ACROSS THE ADJACENT HIGHER TERRAIN. ON THE CONVERSE...ABUNDANT WARMTH IS IN THE FORECAST OVER DOWNSTREAM LOCATIONS GIVEN WELL ABOVE AVERAGE HEIGHTS. THROUGH THE MIDDLE OF NEXT WEEK...EXPECT THE 80 DEGREE MARK TO SURGE AS FAR NORTH AS THE LOWER GREAT LAKES. WHILE EXPECTED TEMPERATURE ANOMALIES SHOULD DECREASE DEEPER INTO THE FORECAST...THIS IS LIKELY AN ARTIFACT OF HEAVIER ENSEMBLE MEAN USE. A SLOW-MOVING FRONTAL ZONE ACROSS THE NORTH-CENTRAL U.S. WILL BE THE INITIAL FOCUS FOR ORGANIZED PRECIPITATION. THIS SHOULD GRADUALLY PUSH EASTWARD WITH HIGHER AMOUNTS NORTH OF THE U.S./CANADIAN BORDER WHERE BETTER FORCING WILL RESIDE. ACROSS THE FOUR CORNERS REGION...MANY OPERATIONAL MODELS SHOW SOME FAIRLY HEFTY AMOUNTS AHEAD OF THE SLOW-MOVING CLOSED LOW. DEPENDING ON HOW LONG THAT SYSTEM REMAINS LOCKED IN PLACE OVER THE WEST...THERE COULD BE MULTIPLE DAYS OF PRECIPITATION GIVEN HIGHLY DIFFLUENT FLOW ALOFT. AND ACROSS FL...IT SHOULD BE A WET PERIOD AHEAD...PARTICULARLY ACROSS THE SOUTHERN HALF OF THE STATE. THE NATIONAL HURRICANE CENTER CONTINUES TO WATCH TWO AREAS OF DISTURBED WEATHER...EACH OF WHICH WILL KEEP HEAVY RAINFALL LIKELY. ADDITIONALLY...IT SHOULD BE QUITE BREEZY WITH A STRONG SURFACE HIGH ACROSS NEW ENGLAND AS NORTHEASTERLY WINDS REACH THE 2O TO 25 KNOT RANGE. RUBIN-OSTER