497
FXUS02 KWBC 141900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST WED DEC 14 2022
VALID 12Z SAT DEC 17 2022 - 12Z WED DEC 21 2022
...HEAVY SNOW THREAT WILL LINGER INTO SATURDAY ACROSS NORTHERN NEW
ENGLAND, WITH HEAVY RAIN POSSIBLE ALONG THE COAST...
...BITTERLY COLD TEMPERATURES LIKELY TO SETTLE OVER THE NORTHERN
PLAINS FROM SUNDAY ONWARD...
..OVERVIEW
A COASTAL LOW WILL BE LIFTING ALONG THE NORTHEAST COAST BY THE
START OF THE PERIOD ON SATURDAY, CONTINUING A THREAT FOR HEAVY
SNOWS OVER MAINLY INTERIOR LOCATIONS, AND SOME MODERATE TO HEAVY
RAIN ALONG THE COAST. THE PARENT UPPER LOW WILL BE OVER THE GREAT
LAKES AT THE SAME TIME WITH ITS ENERGY EVENTUALLY LIFTING NORTH
AND WEAKENING OVER CANADA, WHILE SOME ACCUMULATING LAKE EFFECT
SNOWS WILL BE POSSIBLE BEHIND THE SYSTEM AS WELL. THERE REMAINS
CONSIDERABLE UNCERTAINTY REGARDING THE CHARACTER OF A DEEP UPPER
TROUGH (WITH LIKELY EMBEDDED LOW) EXPECTED TO EVOLVE OVER WESTERN
CANADA, AFFECTING FLOW ACROSS THE NORTHEASTERN PACIFIC AND LOWER
48. IN GENERAL THE FORECAST CONFIDENCE IS HIGHEST WITH RESPECT TO
THIS PATTERN SUPPORTING WELL BELOW NORMAL TEMPERATURES ACROSS MUCH
OF THE NORTHERN ROCKIES TO NORTHERN PLAINS, WHILE CONFIDENCE IS
LOWER FOR THE COVERAGE OF COLD AIR BEYOND THIS MOST LIKELY CORE.
THIS COLD PATTERN SHOULD PERSIST WELL BEYOND THE END OF THE MEDIUM
RANGE PERIOD, PER CLIMATE PREDICTION CENTER OUTLOOKS. MEANWHILE A
COUPLE SHORTWAVES INTERACTING WITH A WAVY FRONT FROM THE GULF OF
MEXICO TO JUST SOUTH OF FLORIDA MAY PRODUCE EPISODES OF RAINFALL,
THE FIRST MAINLY OVER FLORIDA DURING THE WEEKEND AND THE SECOND
ACROSS MORE OF THE GULF COAST REGION EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE FORECAST OVER AND NEAR THE NORTHEAST FROM THE WEEKEND INTO
MONDAY STILL HAS SOME COMPLEXITY TO IT ALOFT, WITH EAST-WEST
ELONGATING ENERGY DRIFTING TOWARD NOVA SCOTIA AND THEN FOLLOWED BY
UPSTREAM FLOW (SOME COMBINATION OF THE PARENT UPPER LOW AND/OR
SURROUNDING ENERGY POSSIBLY CLOSING OFF ANOTHER LOW OVER THE
CANADIAN MARITIMES. RECENT CONSENSUS TRENDS FOR THE SURFACE LOW
NEAR THE NEW ENGLAND COAST HAVE BEEN MORE OFFSHORE AFTER EARLY
SATURDAY (A NOD TO EARLIER GFS RUNS) AND THEN THE 12Z
GFS/CMC/ECMWF SUGGEST THE EVOLUTION ALOFT BY MONDAY COULD
ULTIMATELY PULL BACK THE SURFACE LOW A BIT TO A POSITION JUST
NORTHEAST OF MAINE.
MODELS/ENSEMBLES CONTINUE TO VARY WITH THE EVOLUTION OF THE DEEP
UPPER TROUGH DIGGING INTO WESTERN CANADA, WITH INFLUENCE FROM
EXACTLY HOW UPPER RIDGING EVOLVES OVER FAR WESTERN ALASKA AND
CLOSES OFF TO THE NORTHWEST OF THE STATE--AND DEPICTION OF
SHORTWAVE ENERGY TO THE SOUTHEAST OF THE UPPER HIGH--ALONG WITH
WESTERN PACIFIC RIDGING. ONE NOTICEABLE TREND OVER THE PAST DAY IS
FOR LEADING ENERGY THAT REACHES WESTERN CANADA DURING THE WEEKEND
TO BE SOMEWHAT LESS AMPLIFIED AS IT PROGRESSES EASTWARD. WHILE GFS
RUNS HAVE BEEN ERRATIC OVER THE PAST DAYS AND GEFS MEANS HAVE
TENDED TO HAVE HIGHER HEIGHTS OVER SOUTHWESTERN CANADA AND THE
WESTERN U.S. VERSUS OTHER GUIDANCE, THERE HAS BEEN A GENERAL TREND
TOWARD A MORE PERSISTENT UPPER LOW OVER WESTERN CANADA THAT WOULD
SUPPORT FAIRLY LOW HEIGHTS EXTENDING INTO THE NORTHWEST FOR A
TIME. THEN THE OVERALL TROUGH AXIS WOULD BEGIN TO PRESS EASTWARD
BY AROUND MIDWEEK. THE NEW 12Z GFS COMPARES BETTER TO THIS TREND
WHILE THE 12Z CMC HAS FAVORABLY MODIFIED ITS 00Z RUN THAT WAS
FAIRLY EXTREME WITH ITS WESTWARD POSITION OF THE UPPER LOW AND WAS
SIMILAR TO THE 00Z ECMWF IN BEING ON THE AMPLIFIED SIDE OF THE
SPREAD OVER THE WEST. THE NEW 12Z ECMWF HAS ALSO TONED DOWN ITS
WESTERN U.S. TROUGHING. THE 12Z GEFS MEAN HAS TRENDED LOWER WITH
ITS SOUTHWESTERN CANADA/NORTHWESTERN U.S. HEIGHTS, BRINGING IT
CLOSER TO THE 00Z ECMWF MEAN. AS THE GUIDANCE SEEMS TO BE
NARROWING ITS SPREAD FOR THE PATTERN ALOFT SOMEWHAT, THERE IS A
GRADUALLY INCREASING SIGNAL THAT PACIFIC FLOW MAY CARRY A SYSTEM
INTO THE NORTHWEST BY AROUND TUESDAY, WITH A FRONT POSSIBLY
CONTINUING INTO THE WEST THEREAFTER.
THE UPDATED FORECAST STARTED WITH A 00Z/06Z OPERATIONAL MODEL
COMPOSITE FOR ABOUT THE FIRST HALF OF THE PERIOD AND THEN
TRANSITIONED TO A BLEND OF OPERATIONAL MODELS AND 00Z ECENS/CMCENS
AND 06Z GEFS MEANS. THIS APPROACH PROVIDED SOME ELEMENTS OF
CONTINUITY WHILE LEADING TO ADJUSTMENTS WHERE SUGGESTED BY A
SIGNIFICANT MAJORITY OF GUIDANCE. EXAMPLES OF THE LATTER INCLUDE
THE FARTHER EAST TRACK OF THE INITIAL NEW ENGLAND COASTAL SYSTEM,
A MODERATE TREND TOWARD LOWER HEIGHTS OVER THE NORTHWEST AFTER THE
WEEKEND, AND A MORE SUPPRESSED WAVY GULF FRONT TOWARD MIDWEEK.
..WEATHER/HAZARD HIGHLIGHTS
THE LIKELY DEVELOPMENT OF A NOR'EASTER NEAR THE MID-ATLANTIC COAST
AT THE END OF THE SHORT RANGE PERIOD AND SUBSEQUENT
NORTHWARD/NORTHEASTWARD TRACK SHOULD CONTINUE A THREAT FOR WINTRY
WEATHER AND POTENTIALLY HEAVY SNOWFALL ACROSS INTERIOR PORTIONS OF
NORTHERN NEW ENGLAND THROUGH SATURDAY. THERE IS STILL SOME
UNCERTAINTY IN THE DETAILS OF THIS SYSTEM, AND EVEN SMALL SHIFTS
IN THIS LOW TRACK COULD CAUSE SIGNIFICANT DIFFERENCES IN
PRECIPITATION TYPES/AMOUNTS. CLOSER TO THE COAST, RAIN COULD BE
HEAVY AT TIMES DOWN EAST MAINE, BUT WITH A GENERAL LACK OF
INSTABILITY, THIS RAIN IS UNLIKELY TO CREATE ANY SIGNIFICANT
HAZARDS. RECENT GUIDANCE TREND FOR A SLIGHTLY FARTHER OFFSHORE LOW
TRACK DOES OFFER THE POTENTIAL FOR SOME SNOW TO EXTEND A LITTLE
FARTHER SOUTHEASTWARD THAN IN SOME EARLIER FORECASTS. ELSEWHERE,
FAVORABLE WESTERLY FLOW ON THE BACKSIDE OF THE MAIN UPPER LOW WILL
CONTINUE TO SUPPORT SOME LAKE EFFECT SNOWS OVER FAVORABLE DOWNWIND
AREAS. A SHORTWAVE REACHING INTO THE SOUTHWEST THIS WEEKEND MAY
PRODUCE A LITTLE LIGHT/SCATTERED PRECIPITATION. THIS SHORTWAVE
WILL THEN TRACK ACROSS THE SOUTHERN TIER AND COULD THEN INFLUENCE
SPECIFICS OF MOISTURE THAT MAY SPREAD ACROSS AND NORTH FROM THE
GULF COAST LATE WEEKEND INTO NEXT WEEK WITH SOME INDICATIONS FOR
WIDESPREAD MODERATE TO HEAVY RAIN. A LEADING SHORTWAVE BRUSHING
THE SOUTHERN TIER MAY PRODUCE SOME RAINFALL DURING THE WEEKEND,
MOST LIKELY CONFINED TO FLORIDA, BUT THERE IS STILL A FAIR DEGREE
OF UNCERTAINTY WITH AMOUNTS FOR THIS EPISODE. FLOW TO THE SOUTH OF
A DEEPENING WESTERN CANADA TROUGH/UPPER LOW MAY PRODUCE SOME
TERRAIN-ENHANCED PRECIPITATION OVER THE NORTHWEST/NORTHERN ROCKIES
FROM SUNDAY ONWARD. CONFIDENCE IS CURRENTLY FAIRLY LOW FOR AMOUNTS
AND SOUTHWARD EXTENT (WHETHER MOISTURE REACHES AS FAR SOUTH AS
PORTIONS OF CALIFORNIA AND THE GREAT BASIN) DUE TO GUIDANCE SPREAD
FOR IMPORTANT SPECIFICS OF THE UPPER PATTERN. SNOW LEVELS COULD BE
FAIRLY LOW OVER PARTS OF THE NORTHWEST DEPENDING ON PATTERN
SPECIFICS.
BEHIND THE SYSTEM MOVING OFF THE EAST COAST, TEMPERATURES ACROSS
MOST OF THE CONUS SHOULD BE NEAR TO BELOW NORMAL BY THIS WEEKEND.
THE MOST WIDESPREAD AND EXTREME COLD ANOMALIES WILL LIKELY BE
ACROSS THE NORTHERN PLAINS FROM SUNDAY ONWARD, WITH DAYTIME
TEMPERATURES AS COLD AS 30-40 DEGREES BELOW NORMAL, EQUATING TO
WIDESPREAD AND BITTERLY COLD SINGLE DIGIT TEMPERATURES WITH SOME
BELOW ZERO READINGS ACROSS EASTERN MONTANA AND NORTH DAKOTA.
MORNING LOWS COULD FALL TO AT LEAST 10-20 DEGREES BELOW ZERO.
SURROUNDING AREAS SHOULD SEE LESS EXTREME ANOMALIES OF 10-20F
BELOW NORMAL. THE WEST WILL TEND TO SEE READINGS 5-15F BELOW
NORMAL DURING THE WEEKEND FOLLOWED BY SOME MODERATION OVER
CENTRAL/SOUTHERN AREAS. THE EAST SHOULD BE COOLEST OVER THE
WEEKEND WITH SOME HIGHS UP TO 10-12F BELOW NORMAL, WITH SLIGHT
MODERATION TO SINGLE-DIGIT NEGATIVE ANOMALIES EARLY-MID WEEK.
RAUSCH/SANTORELLI
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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES 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
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