317
FXUS02 KWBC 190700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT THU OCT 19 2023
VALID 12Z SUN OCT 22 2023 - 12Z THU OCT 26 2023
..OVERVIEW
MODELS AND ENSEMBLES HAVE MAINTAINED A FAIRLY AGREEABLE PATTERN
EVOLUTION OVER THE EAST FROM THIS WEEKEND INTO THE FIRST HALF OF
NEXT WEEK, WITH STRONG CANADIAN MARITIMES LOW PRESSURE PRODUCING
GUSTY WINDS AND SOME LINGERING PRECIPITATION OVER THE NORTHEAST ON
SUNDAY FOLLOWED BY A COOL AND DRIER PATTERN AS THE SUPPORTING
UPPER TROUGH DEPARTS IN FAVOR OF RIDGING AT THE SURFACE AND ALOFT.
IN CONTRAST, RECENT GUIDANCE RUNS HAVE BEEN SHOWING INCREASING
UNCERTAINTY FOR THE EASTERN PACIFIC INTO NORTH AMERICA PATTERN
FROM ABOUT TUESDAY ONWARD AND THE NEW 00Z MODEL RUNS HAVE RAISED
THE UNCERTAINTY TO ANOTHER LEVEL. THE MOST LIKELY SCENARIO WOULD
HAVE A MEAN TROUGH DIGGING INTO THE WEST TO BRING A WETTER AND
COOLER PERIOD TO THE REGION, WITH EXPANDING COVERAGE OF NORTHERN
ROCKIES/PLAINS SNOW BY MIDWEEK, WHILE MOISTURE FROM HURRICANE
NORMA IN THE EAST PACIFIC MAY CONTRIBUTE MOISTURE TO A GENERAL
PATTERN FAVORABLE FOR SOME AREAS OF HEAVY RAINFALL OVER THE
CENTRAL U.S. BY TUESDAY. ALTERNATIVE POSSIBILITIES INCLUDE A MUCH
WARMER AND DRIER PATTERN OVER THE WEST AND LESS PRECIPITATION
COVERAGE OVER THE CENTRAL U.S.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE PRIMARY SOURCE OF THE RECENT WIDENING SPREAD IN THE GUIDANCE
FROM THE FAR EASTERN PACIFIC INTO NORTH AMERICA IS THE EVOLUTION
OF UPPER RIDGING THAT BUILDS INTO ALASKA AND THE PATTERN JUST TO
THE SOUTH. DURING THE FIRST HALF OF THE WEEK, THE NEW 00Z GFS/CMC
AND UKMET (THROUGH THE END OF ITS RUN LATE TUESDAY) HAVE SHIFTED
TOWARD VARYING DEGREES OF ELONGATING SHORTWAVE ENERGY UNDERNEATH
THE ALASKA RIDGE. THIS ULTIMATELY LEADS TO AN UPPER RIDGE BUILDING
INTO THE WEST BY DAYS 6-7 WEDNESDAY-THURSDAY WITH BROAD CYCLONIC
FLOW DOWNSTREAM ACROSS SOUTHERN CANADA AND NORTHERN TIER U.S. ON
THE OTHER HAND, RECENT CONSENSUS HAS BEEN FOR THE HIGHER LATITUDE
SHORTWAVE ENERGY TO DROP INTO THE WESTERN U.S. TO FORM A LARGER
SCALE TROUGH WITH POSSIBLE EMBEDDED UPPER LOW (ALBEIT WITH A FAIR
AMOUNT OF SPREAD FOR SPECIFICS, INCLUDING SOME EARLIER CMC RUNS
BEING QUITE AGGRESSIVE WITH THE TROUGH/UPPER LOW). THE NEW 00Z
ECMWF STILL GOES ALONG WITH THE WESTERN TROUGH, THOUGH IN SLIGHTLY
LESS AMPLIFIED FORM THAN THE 12Z RUN. THUS FAR THERE HAS BEEN A
SUFFICIENT MAJORITY OF ENSEMBLE MEMBERS FAVORING THE WESTERN
TROUGH SOLUTION FOR THE MEANS TO DEPICT THAT IDEA. THIS INCLUDES
THE NEW 00Z GEFS MEAN THAT ENDS UP VERY CLOSE TO THE 12Z ECMWF
MEAN BY DAY 7. THAT SAID, 12Z GEFS/ECENS/CMCENS CLUSTERING
ACTUALLY SHOWED A TOTAL OF ABOUT ONE-THIRD OF MEMBERS GROUPED INTO
A VARIATION OF THE WESTERN RIDGE PATTERN. MEANWHILE
TELECONNECTIONS RELATIVE TO THE STRONG CORE OF POSITIVE HEIGHT
ANOMALIES THAT MOST MODELS/MEANS HAVE BEEN SHOWING OVER THE
SOUTHERN COAST OF ALASKA IN THE D+8 MULTI-DAY MEANS SUPPORT
WESTERN U.S. TROUGHING. BASED ON GUIDANCE THROUGH THE 12Z/18Z
CYCLES AND TELECONNECTION RELATIONS, PREFERENCE SIDED WITH THE
WESTERN TROUGH SOLUTION THAT REPRESENTS CONTINUITY. AHEAD OF THIS
MID-LATE PERIOD WESTERN PATTERN ISSUE, SPECIFICS WILL PLAY A ROLE
IN HOW LONG AN INITIAL TROUGH/UPPER LOW MOVING INTO THE WEST
LINGERS NEAR THE WESTERN U.S.-MEXICO BORDER. IN GENERAL MORE
AMPLIFIED UPSTREAM TROUGHING LEADS TO A FASTER EJECTION OF THE
LEADING FEATURE, THOUGH WITH THE 12Z GFS POSSIBLY BEING TOO FAST.
PLUS THERE IS A WIDE DISPERSION FOR THE TRACK OF NORMA IN THE
EASTERN PACIFIC, WITH LATEST GFS RUNS A CONSIDERABLY FAST EXTREME.
FOR THE SYSTEM AFFECTING THE NORTHEAST, LATEST MODELS HAVE
GRAVITATED TO A MODERATELY PROGRESSIVE SOLUTION OVER THE CANADIAN
MARITIMES AS OF THE START OF THE PERIOD EARLY SUNDAY--CLOSEST TO
WHAT THE ENSEMBLE MEANS HAD BEEN SHOWING IN RECENT RUNS.
THE UPDATED FORECAST, BASED ON INFORMATION FROM THE 12Z/18Z RUNS,
STARTED WITH A MODEL COMPOSITE EARLY IN THE PERIOD (MORE GFS/ECMWF
WEIGHT RELATIVE TO THE UKMET/CMC) AND THEN TRANSITIONED TOWARD
HALF MODELS AND HALF ENSEMBLE MEANS (18Z GEFS/12Z ECENS) BY THE
END OF THE PERIOD TO DOWNPLAY LOWER CONFIDENCE SPECIFICS FROM
INDIVIDUAL MODEL RUNS.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS THAT COVER SUNDAY THROUGH
MONDAY NIGHT DEPICT NO RISK AREAS AT THIS TIME. ON SUNDAY GUIDANCE
SHOWS SIGNALS OF SOME LINGERING NORTHEAST PRECIPITATION THAT COULD
PRODUCE LOCALIZED ENHANCED TOTALS, BUT LACK OF INSTABILITY AND
FAIRLY RAPID DEPARTURE OF ANOMALOUS MOISTURE SHOULD TEMPER
SHORT-TERM RAINFALL RATES. ON MONDAY THE GFS IS THE ONLY MODEL TO
DEPICT THE ONSET OF LOCALLY HEAVY RAINFALL OVER THE SOUTHERN
PLAINS DUE TO ITS FAST PROGRESSION OF NORMA, WITH OTHER MODELS
MUCH DRIER. OTHERWISE MOISTURE WILL SPREAD ACROSS THE WEST WITH
THE CENTRAL WEST COAST AND NORTHERN ROCKIES LIKELY SEEING
RELATIVELY HIGHER BUT NOT EXCEPTIONALLY HEAVY TOTALS.
PROBABILITIES FOR MEANINGFUL SNOW AT HIGH ELEVATIONS OF THE
ROCKIES SHOULD START INCREASING EARLY IN THE WEEK.
THE WESTERN AND CENTRAL U.S. FORECAST DURING TUESDAY-THURSDAY WILL
DEPEND IN PART ON THE INCREASINGLY UNCERTAIN PATTERN ALOFT. BASED
ON THE CURRENT MAJORITY SCENARIO OF A DEEPENING WESTERN U.S. UPPER
TROUGH, MODERATELY COOL HIGHS OVER THE WEST EARLY IN THE WEEK
WOULD TREND SIGNIFICANTLY COLDER OVER THE INTERMOUNTAIN WEST INTO
NORTHERN PLAINS. NORTHERN HIGH PLAINS LOCATIONS COULD SEE HIGHS AT
LEAST 20F BELOW NORMAL. THIS TREND WOULD LEAD TO INCREASING SNOW
COVERAGE INCLUDING AT SOME LOWER ELEVATIONS, WITH SOME HIGHER
ELEVATION SNOW IN THE CASCADES AS WELL. A SMALLER MINORITY
SCENARIO WOULD LEAD TO A WARMER AND DRIER PATTERN OVER THE WEST,
SO CONTINUE TO MONITOR UPCOMING FORECASTS. MEANWHILE THE PATTERN
SHOULD BECOME FAVORABLE FOR INCREASING COVERAGE AND INTENSITY OF
RAINFALL OVER THE CENTRAL U.S. BY TUESDAY OR SO, WITH MOISTURE
FROM HURRICANE NORMA LIKELY CONTRIBUTING TO THIS ACTIVITY. THE
CURRENT MOST LIKELY PATTERN WOULD PRODUCE A FAIRLY BROAD AREA OF
MODERATE TO HEAVY RAINFALL, WHILE THE MINORITY WESTERN UPPER RIDGE
SCENARIO WOULD PROBABLY RESULT IN A MORE CONFINED AREA OF RAINFALL
AHEAD OF THE LEADING SHORTWAVE/UPPER LOW ALONG THE MEXICO BORDER.
EXPECT THE NORTHEAST TO SEE GUSTY WINDS ON SUNDAY BEHIND THE
DEPARTING CANADIAN MARITIMES SYSTEM. COOL TEMPERATURES OVER THE
EAST SUNDAY-MONDAY SHOULD MODERATE TO NEAR OR ABOVE NORMAL BY
MIDWEEK. THE EARLY WEEK COOL SPELL MAY BRING THE FIRST FROST OR
FREEZE OF THE SEASON TO SOME LOCATIONS FROM THE GREAT LAKES INTO
THE APPALACHIANS. WARM TEMPERATURES OVER THE ROCKIES AND THEN
PLAINS EARLY IN THE WEEK, WITH SOME HIGHS AT LEAST 10-15F ABOVE
AVERAGE, SHOULD MODERATE THEREAFTER BUT MORNING LOWS WILL REMAIN
QUITE WARM (10-20F ABOVE NORMAL) ACROSS THE PLAINS INTO THE GREAT
LAKES THROUGH THURSDAY IN THE MOIST FLOW AROUND THE WESTERN SIDE
OF EAST COAST/ATLANTIC HIGH PRESSURE.
RAUSCH
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE 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/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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