474
FXUS02 KWBC 061901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST WED DEC 06 2023
VALID 12Z SAT DEC 09 2023 - 12Z WED DEC 13 2023
..PATTERN OVERVIEW
GUIDANCE SHOWS BROAD CYCLONIC MEAN FLOW ALOFT PERSISTING OVER THE
EASTERN TWO-THIRDS OF THE LOWER 48 THROUGH MOST OF THE MEDIUM
RANGE PERIOD. INITIALLY, A DEEP UPPER-LEVEL SHORTWAVE WILL EMERGE
FROM THE WEST LATE THIS WEEK, SUPPORTING A WAVY SURFACE FRONT THAT
SHOULD CROSS THE EASTERN HALF OF THE COUNTRY DURING THE WEEKEND
INTO EARLY MONDAY. SIGNIFICANT PRECIPITATION IS LIKELY FROM THE
GULF COAST TO THE GREAT LAKES AND INTO THE NORTHEAST AS THIS
SYSTEM PROGRESSES, WHICH COULD CAUSE FLOODING CONCERNS. MEANWHILE,
THE PACIFIC NORTHWEST WILL BE DEALING WITH ANOTHER SURGE OF
PACIFIC MOISTURE ON SATURDAY THAT MAY RESULT IN SCATTERED
INSTANCES OF FLOODING. BY NEXT MONDAY-WEDNESDAY, AN AMPLIFYING
PACIFIC INTO NORTHWEST U.S. RIDGE WILL HELP TO REINFORCE THE MEAN
TROUGH OVER THE EAST BUT WITH A POSITIVE TILT THAT WILL LEAD TO
MORE BENIGN WEATHER OVER MOST AREAS. INCOMING PACIFIC ENERGY MAY
EVENTUALLY FORM A TROUGH OR UPPER LOW OVER THE SOUTHWESTERN STATES.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD GENERAL AGREEMENT
ON THE LARGER SCALE ASPECTS OF THE FORECAST BUT DIFFER ON SOME
IMPORTANT DETAILS. DURING THE WEEKEND, RECENT GUIDANCE RUNS HAVE
BEEN CHANGING THE SPECIFICS OF HOW SHORTWAVE ENERGY IS DISTRIBUTED
WITHIN THE OVERALL UPPER TROUGH CROSSING THE EASTERN HALF OF THE
COUNTRY--LEADING TO DIFFERENCES AT THE SURFACE. INSTEAD OF PRIOR
CONSENSUS THAT HAD DEPICTED A DOMINANT PLAINS THROUGH GREAT LAKES
INTO EASTERN CANADA SYSTEM, THE LATEST CLUSTERING HAS TONED DOWN
THE STRENGTH OF THE LEADING WAVE CROSSING THE GREAT LAKES SOMEWHAT
BUT THEN SHOWS ADDITIONAL FRONTAL WAVE DEVELOPMENT OVER THE
APPALACHIANS INTO NEW ENGLAND AND SOUTHEASTERN CANADA. THE SCALE
OF SHORTWAVE DETAILS INVOLVED IS SMALL ENOUGH TO HAVE LOW
PREDICTABILITY 4-5 DAYS OUT IN TIME SO ADDITIONAL CHANGES ARE
CERTAINLY POSSIBLE. HOWEVER THE OVERALL PATTERN EVOLUTION
MAINTAINS THE IDEA THAT THIS SYSTEM WILL PRODUCE OF A BROAD AREA
OF SIGNIFICANT PRECIPITATION AND POTENTIAL FOR SOME BRISK TO
STRONG WINDS.
FARTHER UPSTREAM, RECENT TRENDS (MINUS THE 12Z CMC) HAVE BEEN LESS
AMPLIFIED AND/OR FARTHER NORTH WITH ENERGY THAT DROPS INTO
SOUTHERN CANADA BY EARLY NEXT WEEK, LEADING TO A BIT OF A SLOWER
TREND WITH SOUTHWARD FRONTAL PROGRESSION OVER THE CENTRAL U.S. BY
NEXT TUESDAY. BACK TOWARD THE WEST COAST, THE MODELS VARY
CONSIDERABLY WITH SMALL-SCALE/LOW-PREDICTABILITY SHORTWAVE DETAILS
THAT WILL AFFECT THE DURATION AND COVERAGE OF SIGNIFICANT
PRECIPITATION OVER THE PACIFIC NORTHWEST DURING THE WEEKEND.
LATEST CMC RUNS HAVE BEEN ON THE DEEP/SLOW SIDE OF THE SPREAD WITH
ITS PRIMARY SHORTWAVE. THIS LEADS INTO THE MIDDLE OF NEXT WEEK
WHEN CMC RUNS ARE LIKEWISE FARTHEST WEST WITH AN UPPER LOW THAT
COULD FORM OVER THE SOUTHWEST/CALIFORNIA. RECENT TRENDS HAVE BEEN
LEANING TOWARD AT LEAST SOME DEGREE OF FLOW SEPARATION, THOUGH THE
CLOSED LOW HAS BEEN NEW ENOUGH IN THE GUIDANCE NOT TO GO
COMPLETELY TO IN THE UPDATED FORECAST BASED ON 00Z/06Z RUNS. NEW
12Z GUIDANCE (INCLUDING THE GFS/GEFS, ALBEIT WITH A SLIGHT TIMING
DELAY) MAINTAINS THE CLOSED LOW POTENTIAL.
A COMPOSITE OF 00Z/06Z OPERATIONAL RUNS FOR THE FIRST HALF OF THE
PERIOD FOLLOWED BY INCORPORATING SOME 06Z GEFS/00Z ECENS MEANS
AGAIN PROVIDES A GOOD REFLECTION OF LATEST GUIDANCE IDEAS WITH A
COMBINATION OF CONTINUITY OR TRENDS DEPENDING ON THE
FEATURE/REGION. THE BLEND EXCLUDED THE 00Z CMC LATE DUE TO GREATER
DIFFERENCES FROM CONSENSUS.
..WEATHER/HAZARDS HIGHLIGHTS
EXPECT ANOTHER SURGE OF PACIFIC MOISTURE TO IMPACT THE PACIFIC
NORTHWEST ON SATURDAY, AND THE REGION WILL BE PARTICULARLY
SENSITIVE TO ANY ADDITIONAL RAINFALL AFTER RECEIVING SIGNIFICANT
RAINFALL EARLIER IN THE WEEK. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK
CONTINUES TO SHOW A MARGINAL RISK OF EXCESSIVE RAINFALL FOR
WESTERN WASHINGTON AND OREGON INTO FAR NORTHWESTERN CALIFORNIA
WITH AN EMBEDDED SLIGHT RISK AREA ALONG THE COAST. IN ADDITION TO
HEAVY RAINFALL, WIDESPREAD MODERATE TO LOCALLY HEAVY SNOW WILL BE
POSSIBLE IN THE HIGHER ELEVATIONS OF THE CASCADES AND NORTHERN
ROCKIES AS MOISTURE PUSHES INLAND. PRECIPITATION WILL LIGHTEN UP
ACROSS THE REGION THEREAFTER BUT WITH CONSIDERABLE UNCERTAINTY
OVER HOW QUICKLY. THE MAJORITY OF GUIDANCE TRENDS LIGHT ENOUGH ON
SUNDAY TO PRECLUDE ANY RISK AREA FOR DAY 5, BUT THE HEAVY EXTREME
(VIA STRONGER AND SLOWER UPPER SHORTWAVE IN THE CMC) WOULD HAVE
LOCALLY SIGNIFICANT TOTALS EXTEND INTO SUNDAY. WILL CONTINUE TO
MONITOR GUIDANCE FOR ANY CHANGE TO THE CURRENT LIGHTER MAJORITY
CLUSTER.
SOME OF THE WAVE DETAILS HAVE BEEN CHANGING FOR THE FRONT CROSSING
THE EASTERN HALF OF THE COUNTRY THIS WEEKEND, BUT THERE CONTINUES
TO BE A GOOD SIGNAL FOR THIS OVERALL SYSTEM TO SPREAD A BROAD AREA
OF MODERATE TO HEAVY RAINFALL FROM THE GULF COAST TO THE GREAT
LAKES AND EASTWARD TOWARD THE EAST COAST. THE DAY 4 EXCESSIVE
RAINFALL OUTLOOK MAINTAINS A MARGINAL RISK AREA FROM THE GULF
COAST TO PARTS OF THE MID-MISSISSIPPI AND LOWER OHIO VALLEYS,
WHILE DAY 5 HOLDS ONTO A MARGINAL RISK AREA FROM THE FLORIDA
PANHANDLE THROUGH THE MID-ATLANTIC AND INTO MUCH OF THE NORTHEAST.
PRIMARY ADJUSTMENT IN THIS AFTERNOON'S UPDATE WAS TO ACCOUNT FOR A
SLIGHTLY FASTER TREND IN THE GUIDANCE FOR BOTH DAYS. THE
PROGRESSIVE NATURE OF THIS SYSTEM WILL HELP TO LIMIT RAINFALL
TOTALS; HOWEVER, ANOMALOUS MOISTURE AND SUFFICIENT INSTABILITY
SHOULD STILL SUPPORT RAIN RATES HIGH ENOUGH TO PRODUCE SOME
FLOODING CONCERNS. CURRENT SNOW COVER OVER PARTS OF THE NORTHEAST
COULD ALSO EXACERBATE ANY RUNOFF ISSUES WHETHER IT MELTS BEFORE OR
DURING THE RAINFALL EVENT. IN ADDITION TO POTENTIAL FLOODING,
THUNDERSTORMS WILL LIKELY THREATEN AREAS ALONG THE GULF COAST AND
THE LOWER MISSISSIPPI VALLEY ON SATURDAY. THE STORM PREDICTION
CENTER HAS HIGHLIGHTED AN AREA CENTERED ON EAST TEXAS AND THE
ARK-LA-MISS WHERE SEVERE THUNDERSTORMS ARE EXPECTED. SOME SNOW
WILL BE POSSIBLE IN THE NORTHWESTERN SIDE OF THE PRECIPITATION
SHIELD FROM THE NORTH-CENTRAL PLAINS/MIDWEST INTO THE GREAT LAKES,
BUT DETAILS REMAIN UNCERTAIN DUE TO GUIDANCE VARIABILITY FOR
SURFACE WAVE SPECIFICS. SOME SNOW MAY ALSO FALL AFTER SYSTEM
PASSES, MAINLY OVER THE GREAT LAKES INTO THE APPALACHIANS.
FINALLY, BRISK TO STRONG WINDS ARE LIKELY TO DEVELOP OVER PORTIONS
OF THE ROCKIES AND PLAINS ON SATURDAY AS THE PRESSURE GRADIENT
TIGHTENS BETWEEN THE WAVY FRONT AND STRONG INTERIOR WEST HIGH
PRESSURE. STRONG WINDS WILL ALSO BE POSSIBLE IN THE WARM SECTOR
ALONG THE MID-ATLANTIC INTO NEW ENGLAND COASTS SUNDAY AND PERHAPS
INTO MONDAY (NORTHERN AREAS).
OTHERWISE, PARTS OF THE SOUTHERN PLAINS AND FAR SOUTHEAST
(ESPECIALLY FLORIDA) MAY SEE INCREASING RAINFALL TOWARD MIDWEEK,
AIDED BY MOISTURE BEGINNING TO INTERACT WITH A FRONT AND
ESTABLISHMENT OF MOIST EASTERLY FLOW RESPECTIVELY. AN APPROACHING
FRONTAL SYSTEM MAY BRING LIGHT PRECIPITATION INTO THE PACIFIC
NORTHWEST BY NEXT WEDNESDAY.
IN TERMS OF TEMPERATURES, MUCH OF THE WEST WILL EXPERIENCE BELOW
NORMAL TEMPERATURES IN THE WAKE OF THE UPPER-LEVEL SHORTWAVE ON
SATURDAY, FOLLOWED BY A REBOUND TO NEAR OR SLIGHTLY ABOVE NORMAL
READINGS LATE WEEKEND INTO EARLY NEXT WORK WEEK AND PERHAPS A
SLIGHT DECLINE BY MIDWEEK. ABOVE TO WELL ABOVE NORMAL TEMPERATURES
WILL PREVAIL IN THE WARM SECTOR AHEAD OF THE WAVY FRONT CROSSING
THE EASTERN HALF OF THE COUNTRY DURING THE WEEKEND, WITH
WIDESPREAD PLUS 10-25 DEGREE ANOMALIES. A FEW LOCATIONS OVER OR
NEAR THE NORTHERN MID-ATLANTIC COULD CHALLENGE DAILY RECORD HIGHS
ON SUNDAY. TEMPERATURES WILL RETURN TO NEAR NORMAL AFTER THE
SYSTEM PASSES, ASIDE FROM SOME BELOW NORMAL HIGHS NEAR THE GULF
COAST AND MODERATELY ABOVE NORMAL READINGS OVER THE
NORTHERN/CENTRAL PLAINS EARLY NEXT WEEK.
RAUSCH/DOLAN
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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