834
FXUS02 KWBC 131900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST TUE DEC 13 2022
VALID 12Z FRI DEC 16 2022 - 12Z TUE DEC 20 2022
...HEAVY SNOW THREAT SHIFTING THROUGH THE INTERIOR NORTHEAST
FRIDAY INTO SATURDAY, WITH HEAVY RAIN POSSIBLE ACROSS COASTAL
LOCATIONS...
...VERY COLD TEMPERATURES LIKELY TO SETTLE OVER AND EXPAND FROM
THE NORTHERN PLAINS SUNDAY ONWARD...
..OVERVIEW
A DEEP UPPER LEVEL LOW (THE SAME ONE RESPONSIBLE FOR A NORTHERN
PLAINS WINTER STORM EARLY THIS WEEK) WILL BE IN PLACE OVER THE
UPPER MIDWEST AT THE START OF THE PERIOD ON FRIDAY. SHORTWAVE
ENERGY ROUNDING THE BASE OF THIS TROUGH WILL LIKELY INDUCE COASTAL
LOW DEVELOPMENT BY EARLY FRIDAY NEAR THE MID-ATLANTIC, WITH HEAVY
SNOWFALL FOR PORTIONS OF THE INTERIOR NORTHEAST ONGOING AS IT
LIFTS NORTH. SOME ACCUMULATING LAKE EFFECT SNOWFALL WILL BE
POSSIBLE BEHIND THE SYSTEM AS WELL. THE INITIAL UPPER LOW MAY
WOBBLE OVER THE GREAT LAKES FOR A PERIOD OF TIME BEFORE IT WEAKENS
AND QUICKLY TRACKS EASTWARD ACROSS SOUTHEAST CANADA AS THE NEXT
SYSTEM DROPS IN FROM WESTERN CANADA. THERE IS A LOT OF UNCERTAINTY
REGARDING THE SPECIFICS AND TRACK OF THIS LATTER SYSTEM (A CLOSED
LOW FOR SOME PERIOD OF TIME, AND POSSIBLY FOLLOWED BY ADDITIONAL
ENERGY) BUT IN GENERAL IT SHOULD SEND TROUGHING/HEIGHT FALLS
THROUGH THE WESTERN AND CENTRAL U.S., PROMOTING WELL BELOW NORMAL
TEMPERATURES ACROSS MUCH OF THE NORTHERN ROCKIES TO NORTHERN AND
CENTRAL PLAINS. THIS COLD PATTERN IS LIKELY TO PERSIST WELL BEYOND
THE END OF THE MEDIUM RANGE PERIOD, PER CLIMATE PREDICTION CENTER
OUTLOOKS. MEANWHILE SHORTWAVE ENERGY AND A WAVY FRONT MAY PRODUCE
AN EPISODE OF ENHANCED RAINFALL NEAR THE GULF COAST REGION EARLY
NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE EARLY PART OF THE FORECAST CONTINUES TO FEATURE DECENT
CLUSTERING AMONG MOST GUIDANCE BUT A STRAY RUN OR TWO FOR THE
INITIAL UPPER MIDWEST-GREAT LAKES UPPER LOW AND THE SURFACE SYSTEM
TRACKING ALONG THE NORTHEAST COAST. THE 00Z UKMET WAS THE ODD
MODEL OUT FOR THE UPPER LOW, PULLING IT NORTH/NORTHWEST OF
CONSENSUS AFTER FRIDAY. LATEST GFS RUNS (INCLUDING THE 12Z
VERSION) HAVE BEEN TRACKING THE COASTAL SYSTEM FARTHER OFFSHORE
THAN MOST OTHER GUIDANCE. THE 00Z UKMET TRACKED IT FARTHEST INLAND
SATURDAY ONWARD BUT THE NEW 12Z RUN SHIFTED TO THE EAST.
PREFERENCE BASED ON THE 00Z/06Z GUIDANCE SIDED MORE WITH THE 00Z
ECMWF/CMC, WHILE THE 00Z GFS/06Z GEFS MEAN WERE A LITTLE CLOSER TO
THIS IDEA THAN THE 06Z GFS BY SUNDAY.
GUIDANCE CONTINUES TO HAVE TROUBLE IN RESOLVING IMPORTANT DETAILS
OF TRAILING FLOW ACROSS WESTERN/SOUTHERN CANADA AND VICINITY FROM
ABOUT SUNDAY ONWARD. DURING THE WEEKEND THERE IS AGREEMENT IN
PRINCIPLE UPON DIGGING WESTERN CANADA ENERGY DOWNSTREAM FROM A
STRONG UPPER RIDGE THAT BUILDS OVER FAR WESTERN ALASKA. HOWEVER
DIFFERENCES IN DETAILS FOR EXACTLY HOW QUICKLY AN UPPER HIGH
CLOSES OFF NORTHWEST OF ALASKA (BY SUNDAY-MONDAY) AND THE
CHARACTER OF ENERGY TO ITS SOUTHEAST LEAD TO THE INCREASING SPREAD
THAT MODELS/ENSEMBLES EXHIBIT. GIVEN THE FAIRLY HIGH UNCERTAINTY,
THE UPDATED FORECAST ATTEMPTED TO MAINTAIN A REASONABLE DEGREE OF
CONTINUITY. BY LATE IN THE PERIOD THIS APPROACH REFLECTED A TILT
TOWARD THE ENSEMBLE MEANS WHICH SHOW ENERGY FROM THE UPPER LOW
DROPPING INTO SOUTHWESTERN CANADA ULTIMATELY TRANSLATING EASTWARD
ACROSS FAR SOUTHERN CANADA EARLY NEXT WEEK. LATEST 12Z MODEL RUNS
ARE ALL SHOWING POTENTIAL FOR A FAIRLY DEEP UPPER LOW TO PERSIST
OVER OR NEAR BRITISH COLUMBIA DURING THE FIRST PART OF NEXT WEEK
WHILE LEADING ENERGY COULD BE SOMEWHAT MORE SHEARED THAN DEPICTED
IN THE MEANS. NEEDLESS TO SAY, CONFIDENCE AN ANY PARTICULAR
SOLUTION IS LOW AND THIS IS REINFORCED BY THE HISTORICAL TENDENCY
FOR LOW PREDICTABILITY TO ARISE FROM CASES WHEN THERE IS AN
INFLUENCE FROM A CLOSED UPPER HIGH AT MID-HIGH LATITUDES.
ACROSS THE SOUTHERN TIER, GFS RUNS HAVE BEEN A LITTLE MORE
AMPLIFIED WITH THE PACIFIC UPPER TROUGH (REACHING THE SOUTHWEST
AROUND SUNDAY) ASSOCIATED WITH AN OPENING UPPER LOW AND ALSO
SLOWER TO OPEN UP THE LOW ITSELF. AHEAD OF THIS FEATURE, GFS RUNS
ARE ALSO A BIT ON THE STRONG/SLOW SIDE WITH A WESTERN SHORTWAVE
THAT ULTIMATELY BRUSHES THE SOUTHERN U.S. TO PRODUCE MORE RAINFALL
ALONG THE GULF COAST AND OVER THE FLORIDA PENINSULA. THE 12Z UKMET
HAS A WEAKER SHORTWAVE BUT STILL PRODUCES CONSIDERABLE RAINFALL
THOUGH.
BASED ON THE ABOVE PREFERENCES, THE EARLY PART OF THE FORECAST
EMPLOYED AN OPERATIONAL MODEL COMPOSITE WITH GREATEST WEIGHT ON
THE 00Z ECMWF AND THE LEAST ON THE UKMET (WHICH WAS PHASED OUT
AFTER DAY 3 FRIDAY). THE FORECAST THEN TRENDED TOWARD A
MODEL/ENSEMBLE MEAN MIX (AND SWITCHING THE GFS FROM THE 00Z TO 06Z
RUN LATE BASED ON COMPARISONS TO CONSENSUS) WHILE ALSO INCLUDING
SOME CONTINUITY BY DAY 7 TUESDAY. THIS PROVIDED FAIRLY GOOD
CONSISTENCY WHILE WAITING FOR ANY POTENTIALLY SIGNIFICANT SHIFTS
IN THE BEST CONSENSUS.
..WEATHER/HAZARD HIGHLIGHTS
THE LIKELY DEVELOPMENT OF A NOR'EASTER NEAR THE MID-ATLANTIC COAST
AND SUBSEQUENT NORTHWARD TRACK SHOULD CONTINUE A THREAT FOR WINTRY
WEATHER AND POTENTIALLY HEAVY SNOWFALL ACROSS INTERIOR PORTIONS OF
THE NORTHEAST. THERE REMAINS PLENTY OF 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 OVER EASTERN NEW
ENGLAND, BUT WITH A GENERAL LACK OF INSTABILITY, THIS RAIN IS
UNLIKELY TO CREATE ANY SIGNIFICANT HAZARDS. THE GRADIENT BETWEEN
THIS SYSTEM AND HIGH PRESSURE WELL TO THE NORTH COULD PRODUCE A
PERIOD OF STRONG ONSHORE WINDS THOUGH. ELSEWHERE, LINGERING
WRAP-BACK MOISTURE ON THE BACKSIDE OF THE MAIN UPPER LOW WILL
CONTINUE TO SUPPORT SOME LIGHT SNOWFALL ACCUMULATIONS ACROSS THE
UPPER MIDWEST WITH ALSO 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
EVEN 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 DUE TO
GUIDANCE SPREAD FOR IMPORTANT SPECIFICS OF THE UPPER PATTERN. SNOW
LEVELS COULD BE FAIRLY LOW OVER SOME AREAS.
BEHIND THE SYSTEM MOVING OFF THE EAST COAST, TEMPERATURES ACROSS
MOST OF THE CONUS SHOULD BE NEAR TO BELOW NORMAL BY FRIDAY AND
ESPECIALLY NEXT WEEKEND. THE MOST WIDESPREAD AND EXTREME COLD
ANOMALIES WILL LIKELY BE ACROSS THE WEST INTO THE NORTHERN PLAINS,
TRENDING COLDER WITH TIME. BY SUNDAY AND ESPECIALLY MONDAY AND
TUESDAY, DAYTIME TEMPERATURES ACROSS THE NORTHERN PLAINS COULD BE
AS MUCH AS 30-40 DEGREES BELOW NORMAL, EQUATING TO WIDESPREAD
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. LESS EXTREME ANOMALIES OF 10-20F
BELOW NORMAL SHOULD SPREAD OVER THE CENTRAL PLAINS AND UPPER
MIDWEST. PARTS OF THE NORTHWEST MAY BE 10-25 DEGREES BELOW NORMAL
BY THE START OF NEXT 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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