367
FXUS02 KWBC 140658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM 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 FOR 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, AND WITH SOME ACCUMULATING LAKE EFFECT
SNOWS POSSIBLE BEHIND THE SYSTEM AS WELL. THERE REMAINS
CONSIDERABLE UNCERTAINTY REGARDING THE SPECIFICS AND TRACK OF A
STRONG SHORTWAVE/CLOSED LOW DROPPING THROUGH WESTERN CANADA, 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
ON THE LARGE SCALE, THERE'S GOOD AGREEMENT THE EARLY PART OF THE
PERIOD, BUT STILL QUESTIONS IN THE DETAILS. THE GFS (INCLUDING THE
NEW 00Z RUN TONIGHT) REMAINS A BIT FARTHER OFFSHORE WITH THE
COASTAL LOW IN THE NORTHEAST THAN THE BETTER CONSENSUS. THERE ARE
ALSO DIFFERENCES WITH THE PARENT UPPER LOW... THE GFS AND UKMET
PULL IT NORTH SEPERATE FROM A DEVELOPMING COMPACT LOW OFF THE
CANADIAN MARITIMES WHILE THE ECMWF AND CMC FAVOR MORE OF A
COMBINATION OF THE TWO. EITHER WAY, THE RESULT IS THE SAME OVER
THE EASTERN PORTION OF THE CONUS, RISING HEIGHTS AND MORE ZONAL
FLOW. DESPITE THESE DIFFERENCES, A GENERAL MODEL BLEND SUFFICED
FOR THE FIRST HALF OF THE PERIOD.
GUIDANCE CONTINUES TO HAVE TROUBLE IN RESOLVING IMPORTANT DETAILS
OF TRAILING FLOW ACROSS WESTERN/SOUTHERN CANADA AND VICINITY BY
EARLY NEXT WEEK. THE GFS AND ECMWF ARE WEAKER WITH INITIAL ENERGY
SKIRTING NORTHERN TIER BUT DO SHOW SOME BIT OF AMPLIFICATION OF
THE FLOW OVER THE NORTHWEST AS A SECOND PIECE OF ENERGY DROPS IN.
THE CMC IS MUCH MORE AMPLIFIED (AND WEST) WITH THE INITIAL UPPER
LOW AND ALSO TRACKS THE SECOND ENERGY MORE EAST INTO THE NORTHERN
PLAINS WHILE THE GFS AND ECMWF BRINGS IT SOUTH. A LOOK AT THE
ENSEMBLES SHOW A LOT OF SPREAD AS WELL SO THE FORECAST OUT HERE
REMAINS HIGHLY UNCERTAIN. REGARDLESS, THERE IS A GENERAL TREND
TOWARDS MUCH MORE AMPLIFIED FLOW TOWARDS THE VERY END OF THE
MEDIUM RANGE PERIOD (AND BEYOND).
ACROSS THE SOUTHERN TIER, GFS RUNS ARE MORE AMPLIFIED WITH ENERGY
REACHING THE SOUTHEAST THIS WEEKEND, AND SLOWER TO OPEN IT UP AS
IT SLIDES INLAND. IT BECOMES MORE OF AN OUTLIER AS IT KEEPS SOME
ENERGY FARTHER SOUTH (WEAKENING NORTHERN ENERGY THROUGH THE
SOUTHERN U.S.) WHILE THE CMC, ECMWF, AND UKMET ARE STRONGER WITH
THAT NORTHERN ENERGY. THIS OF COURSE HAS IMPLICATIONS FOR QPF
ALONG THE CENTRAL GULF COAST BY EARLY TO MID NEXT WEEK.
THE WPC FORECAST THIS CYCLE USED A GENERAL MODEL BLEND THROUGH DAY
5 OF THE LATEST DETERMINISTIC MODEL RUNS. AFTER THIS, THE CMC WAS
DROPPED DUE TO ITS DIFFERENCES ACROSS THE NORTHWEST IN FAVOR OF
THE ENSEMBLE MEANS WHICH SEEMED MORE INLINE WITH THE GFS/ECMWF
SOLUTIONS. BY DAY 7, A NEARLY EQUAL DETERMINISTIC/ENSEMBLE MEAN
BLEND WAS UTILIZED WHICH TONED DOWN SOME OF SMALLER SCALE DETAILS,
BUT WITH MORE OVERALL DEFINITION TO THE PATTERN. THIS ALSO
MAINTAINED GOOD AGREEMENT (THROUGH DAY 6) WITH THE PREVIOUS WPC
FORECAST AS WELL.
..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
TRACK SHOULD CONTINUE A THREAT FOR WINTRY WEATHER AND POTENTIALLY
HEAVY SNOWFALL ACROSS INTERIOR PORTIONS OF NORTHERN NEW ENGLAND
THROUGH SATURDAY. 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
DOWN EAST MAINE, 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 AS WELL. 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 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 ALTHOUGH 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 THIS 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 BEYOND, DAYTIME TEMPERATURES ACROSS THE
NORTHERN PLAINS COULD BE AS MUCH 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. 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.
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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