135
FXUS02 KWBC 311901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT TUE OCT 31 2023
VALID 12Z FRI NOV 03 2023 - 12Z TUE NOV 07 2023
..SYNOPTIC OVERVIEW
LATEST GUIDANCE AGREES UPON PROGRESSIVE AND LOW AMPLITUDE FLOW
ALOFT FROM FRIDAY THROUGH THE WEEKEND, AND THEN A POTENTIAL
TRANSITION BY NEXT TUESDAY TO FLAT WESTERN U.S. RIDGING AND AN
EASTERN U.S. TROUGH. THIS PATTERN WILL PROMOTE MULTIPLE EPISODES
OF ENHANCED PRECIPITATION FROM THE PACIFIC NORTHWEST INTO THE
NORTHERN ROCKIES. THE DEVELOPING UPPER TROUGH FORECAST TO REACH
THE EAST NEAR THE END OF THE PERIOD MAY SUPPORT AN ORGANIZED LOW
PRESSURE SYSTEM WITH AN ACCOMPANYING BROAD AREA OF PRECIPITATION.
AN EXPANSIVE SURFACE HIGH WILL CONTINUE TO ENCOMPASS MUCH OF THE
EASTERN U.S. FOR THE END OF THE WEEK, WITH THE HIGH SLOWLY MOVING
OFFSHORE OVER THE WEEKEND. THIS AIRMASS WILL SLOWLY MODERATE
AFTER THE WIDESPREAD FROST AND FREEZE CONDITIONS EARLIER IN THE
WEEK. HOWEVER, COLD TEMPERATURES WILL HAVE STAYING POWER FOR THE
NORTHERN PLAINS AND UPPER MIDWEST THROUGH EARLY NEXT WEEK.
LOCATIONS FROM THE SOUTHWEST/SOUTHERN GREAT BASIN INTO PARTS OF
THE CENTRAL-SOUTHERN PLAINS SHOULD SEE THE MOST PERSISTENT ABOVE
NORMAL HIGH TEMPERATURES DURING THE PERIOD.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
WHILE THERE IS GENERAL CONSENSUS FOR THE OVERALL PATTERN
EVOLUTION, THE PROGRESSIVE AND LOW AMPLITUDE NATURE OF THE UPPER
FLOW EXPECTED FOR MOST OF THE PERIOD NORMALLY LEADS TO LOWER
PREDICTABILITY. THIS IN TURN KEEPS CONFIDENCE IN SPECIFICS ON THE
LOWER SIDE OF THE SPECTRUM. A SIGNIFICANT MODEL DIFFERENCE
AFFECTING THE PACIFIC NORTHWEST INVOLVES PACIFIC LOW PRESSURE
WHOSE LEADING FRONTAL SYSTEM SHOULD AFFECT THE REGION DURING THE
WEEKEND. A NUMBER OF ECMWF RUNS, AND ESPECIALLY THE 00Z VERSION,
TRACK THE THE LOW MUCH CLOSER TO THE COAST THAN MOST OTHER
GUIDANCE. THE 00Z ECMWF WAS WITHIN THE FULL ENSEMBLE SPREAD BUT
12Z GUIDANCE THUS FAR MAINTAINS THE OVERWHELMING CONSENSUS TOWARD
A FARTHER OFFSHORE TRACK. THE NEW 12Z ECMWF IS STILL AN EASTERN
EXTREME BUT NOT QUITE TO THE EXTENT OF THE 00Z RUN. FARTHER EAST,
RECENT GFS RUNS HAVE BEEN STRONGER THAN MOST OTHER SOLUTIONS WITH
THE LEADING SHORTWAVE EMERGING FROM THE WEST AFTER FRIDAY. THIS
LEADS TO MUCH STRONGER SURFACE DEVELOPMENT OVER THE GREAT LAKES BY
SUNDAY AND THEN FARTHER EASTWARD PROGRESSION OF THE TRAILING FRONT
BY MONDAY-TUESDAY. THE GEFS MEAN REFLECTS THIS WAVE BUT IN MUCH
WEAKER FORM, AND THE NEW 12Z CMC ALSO HAS A MODEST WAVE ALONG A
SIMILAR TRACK. IT SEEMS PLAUSIBLE FOR THE WESTERN SHORTWAVE
ENERGY TO SUPPORT A FRONTAL WAVE BUT MOST LIKELY A FAIRLY WEAK
ONE. THEN THE GFS HAS BEEN WEAKER WITH TRAILING ENERGY THAT LEADS
TO THE CONSENSUS EASTERN U.S. UPPER TROUGH BY NEXT TUESDAY. AT
THAT TIME THE COMBINATION OF THESE TWO ISSUES IN THE GFS HAVE LED
TO HIGHER SURFACE PRESSURES OVER THE EAST VERSUS THE GUIDANCE
MAJORITY (INCLUDING THE GEFS/ECENS/CMCENS MEANS) DEPICTING LOWER
GREAT LAKES/OHIO VALLEY LOW PRESSURE. THE NEW 12Z GFS HAS MADE A
FAVORABLE TREND CLOSER TO, BUT STILL SOMEWHAT EAST OF CONSENSUS,
FOR THE SURFACE LOW.
A BLEND OF THE 00Z/06Z OPERATIONAL MODEL RUNS PROVIDED A
REASONABLE STARTING POINT THROUGH THE FIRST HALF OF THE FORECAST
PERIOD, LEANING AWAY FROM THE LEAST CONFIDENT ASPECTS OF THE
GFS/ECMWF AS DESCRIBED ABOVE. WITH THE 00Z/06Z GFS STRAYING AWAY
EVEN MORE FROM CONSENSUS LATE IN THE PERIOD, THE BLEND PHASED OUT
THE GFS COMPLETELY BY DAY 7 TUESDAY. OTHERWISE THE FORECAST
RAPIDLY INCREASED 06Z GEFS/00Z ECENS MEAN WEIGHT SO THAT IT
COMPRISED 60 PERCENT OF THE BLEND BY NEXT TUESDAY. THE 00Z
ECMWF/CMC PROVIDED THE REMAINING OPERATIONAL INPUT.
..WEATHER/HAZARDS HIGHLIGHTS
MOST OF THE CONTINENTAL U.S. WILL LIKELY NOT SEE ANY SIGNIFICANT
RAINFALL HAZARDS THROUGH SATURDAY. THE ONLY REGION WHERE THERE
WILL BE A RISK OF EXCESSIVE RAINFALL LEADING TO FLOODING CONCERNS
WILL BE IN THE PACIFIC NORTHWEST WITH A PERIOD OF ENHANCED ONSHORE
FLOW LATE THIS WEEK INTO SATURDAY. DURING THE DAY 4 TIME FRAME
(FRIDAY-FRIDAY NIGHT) THERE IS A WIDE SPREAD OF GUIDANCE FOR
POTENTIAL QPF MAXIMA ESPECIALLY OVER NORTHWESTERN CALIFORNIA/FAR
SOUTHWESTERN OREGON, CORRESPONDING IN PART TO UNCERTAINTY OVER
DETAILS OF OFFSHORE LOW PRESSURE. THE HEAVY SIDE OF THE SPREAD
(00Z ECMWF/12Z CMC) WOULD BE PROBLEMATIC BUT CURRENT PREFERENCE TO
MAINTAIN NO RISK AREA REFLECTS THE LARGER MAJORITY OF GUIDANCE
THAT KEEPS PROJECTED RAINFALL TOTALS LOWER. THE DAY 5 OUTLOOK
(SATURDAY) MAINTAINS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM
NORTHWESTERN CALIFORNIA TO THE OLYMPIC PENINSULA IN WASHINGTON.
LOCALLY HEAVIER RAINFALL ON THE WEST FACING SLOPES MAY JUST BE
ENOUGH TO CAUSE SOME MINOR FLOODING CONCERNS, ESPECIALLY IN URBAN
AND BURN SCAR AREAS. THERE ARE SOME MIXED SIGNALS FROM GUIDANCE
FOR DISTRIBUTION OF HIGHEST TOTALS (THOUGH APPEARING A LITTLE MORE
LIKELY ALONG COASTAL NORTHERN CALIFORNIA) WHILE THERE IS SOME
OVERLAP OF FAVORABLE MOISTURE/INSTABILITY WITHIN THE OUTLOOK AREA
WHICH COULD ENHANCED RAINFALL INTENSITY AT TIMES. LOOKING BEYOND
SATURDAY, ADDITIONAL MODERATE TO HEAVY RAIN IS LIKELY FOR THESE
SAME AREAS AS ANOTHER POTENTIAL ATMOSPHERIC RIVER EVENT DEVELOPS
WITH A FEW INCHES OF ADDITIONAL RAINFALL DURING THE SUNDAY-MONDAY
TIME PERIOD. SOME RAINFALL MAY CONTINUE INTO TUESDAY. MEANWHILE
SOME OF THIS MOISTURE WILL EXTEND INTO THE NORTHERN ROCKIES, WITH
SNOW OVER HIGHER ELEVATIONS.
PARTS OF THE NORTHERN PLAINS INTO GREAT LAKES MAY SEE
PRECIPITATION DURING THE WEEKEND INTO EARLY NEXT WEEK, DEPENDING
ON LOW-CONFIDENCE DETAILS OF ONE OR MORE FRONTAL WAVES. A
STRONGER SURFACE SYSTEM THAT MAY DEVELOP OVER THE EASTERN U.S. BY
NEXT TUESDAY COULD PRODUCE A BROADER AREA OF PRECIPITATION WITH
HIGHER TOTALS. IN BOTH CASES THE MAJORITY OF PRECIPITATION WILL
BE RAIN, BUT WINTRY WEATHER WILL BE POSSIBLE IN THE NORTHERN
FRINGE OF THE MOISTURE SHIELD.
TEMPERATURES WILL START OFF COLD FRIDAY MORNING ACROSS MUCH OF THE
SOUTHEASTERN QUADRANT OF THE LOWER 48 WITH A SLOW WARMING TREND
ENSUING OVER THE WEEKEND AS THE AIRMASS CONTINUES TO MODIFY.
PLEASANTLY MILD CONDITIONS WILL ALSO RETURN TO THE ROCKIES AND
ADJACENT PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS THIS WEEKEND
WITH SOME HIGHS REACHING UP TO 10F OR SO ABOVE NORMAL. WARM
TEMPERATURES SHOULD PERSIST OVER SOUTHERN AREAS INTO EARLY NEXT
WEEK. IN CONTRAST, THE NORTHERN PLAINS AND UPPER MIDWEST SHOULD
REMAIN QUITE CHILLY WITH HIGHS REMAINING 5-15F BELOW NORMAL.
RAUSCH/HAMRICK
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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