324
FXUS02 KWBC 150652
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
151 AM EST THU DEC 15 2022
VALID 12Z SUN DEC 18 2022 - 12Z THU DEC 22 2022
...BITTERLY COLD TEMPERATURES LIKELY TO SETTLE OVER THE NORTHERN
PLAINS FROM SUNDAY ONWARD...
..OVERVIEW
THE OVERALL UPPER PATTERN LOOKS TO BECOME LESS AMPLIFIED AT THE
START OF THE MEDIUM RANGE PERIOD ON SUNDAY, WITH AN UPPER LOW (THE
SAME ONE RESPONSIBLE FOR THE NORTHERN PLAINS BLIZZARD EARLIER THIS
WEEK) SHEERING EAST TOWARDS THE CANADIAN MARITIMES. BEHIND THAT,
THE FLOW BECOMES MOSTLY ZONAL FOR A FEW DAYS, ASIDE FROM A WEAK
SHORTWAVE MOVING THROUGH THE SOUTHERN TIER STATES. BY MID TO LATE
NEXT WEEK, ANOTHER UPPER LOW DROPS IN FROM WESTERN CANADA AND INTO
THE NORTHERN PLAINS, WHICH COULD KICK OFF AN ACTIVE PERIOD OF
WEATHER, THOUGH WITH STILL QUITE A BIT OF UNCERTAINTY. 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 WITH THIS COLD PATTERN LIKELY
TO PERSIST WELL BEYOND THE END OF THE MEDIUM RANGE PERIOD, PER
CLIMATE PREDICTION CENTER OUTLOOKS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE FORECAST OVER AND NEAR THE NORTHEAST 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. THE BETTER CONSENSUS KEEPS THE LOW CLOSER TO THE COAST
THAN THE UKMET SHOWS, AND SUGGESTS THE EVOLUTION ALOFT BY MONDAY
COULD ULTIMATELY PULL BACK THE SURFACE LOW A BIT TO A POSITION
JUST NORTHEAST OF MAINE.
MODELS AND ENSEMBLES CONTINUE TO VARY WIDELY WITH THE EVOLUTION OF
AN UPPER TROUGH DIGGING INTO WESTERN CANADA EARLY NEXT WEEK, BUT
ARE BEGINNING TO SHOW 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. THE ECMWF AND CMC
ARE STRONGER/DEEPER WITH THIS LOW COMPARED TO THE GFS AND UKMET
(THROUGH DAY 5), BUT THERE IS ENSEMBLE SUPPORT FOR THAT. THE
OVERALL TROUGH AXIS SHOULD BEGIN TO PRESS EASTWARD BY AROUND
MIDWEEK, POSSIBLY BRINGING AN UPPER LOW INTO THE NORTHERN PLAINS.
THE 12Z/DEC 14 CMC WAS MUCH QUICKER THAN THE ECMWF AND THE WEAKER
GFS WITH ITS SOUTHWARD PROGRESSION INTO THE MIDWEST NEXT THURSDAY,
BUT THE NEW 00Z RUN CAME IN SLOWER AND WEAKER (LIKELY IN PART DUE
TO A STRONGER UPPER LOW DEPICTION OVER SOUTHERN CANADA EARLIER IN
THE PERIOD). DESPITE THE UNCERTAINTY, THERE IS SUPPORT FOR SURFACE
CYCLOGENESIS MAYBE AROUND THE NORTH-CENTRAL U.S. NEXT WEEK.
THE WPC FORECAST FOR TONIGHT USED A NON-UKMET (IT JUST SEEMED TOO
FLAT WITH THE FLOW EARLY ON) BLEND FOR DAYS 3-5. FOR 6 AND 7, THE
GFS AND CMC WERE PHASED OUT (DUE TO ISSUES OUTLINED ABOVE)
FAVORING A BLEND OF THE ECMWF WITH THE ENSEMBLE MEANS. THE ECMWF
SEEMED CLOSEST IN LINE WITH THE ENSEMBLE MEANS AND WAS USED FOR
SOME ADDED SYSTEM DEFINITION WITHIN A VERY UNCERTAIN LATE PERIOD
PATTERN. THIS MAINTAINED GOOD CONTINUITY WITH THE PREVIOUS WPC
FORECAST AS WELL.
..WEATHER/HAZARD HIGHLIGHTS
HEAVY SNOWS ACROSS NEW ENGLAND ASSOCIATED WITH A NOR'EASTER MOVING
NORTHWARD SHOULD BE COMING TO AN END BY SUNDAY. FAVORABLE WESTERLY
FLOW ON THE BACKSIDE OF THE MAIN UPPER LOW WILL CONTINUE TO
SUPPORT SOME LAKE EFFECT SNOWS OVER DOWNWIND AREAS. A SHORTWAVE
TRACKING ACROSS THE SOUTHERN TIER COULD 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, MAINLY ALONG THE COAST. 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. POTENTIAL CYCLOGENESIS IN THE CENTRAL U.S. COULD BRING
ANOTHER THREAT OF SNOW TO PARTS OF THE PLAINS AND UPPER MIDWEST,
WITH RAIN FARTHER SOUTH, BUT ANY SORT OF GUESS ON AMOUNTS OR
IMPACTS IS IMPOSSIBLE AT THIS POINT GIVEN THE SPREAD.
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 (LOCALLY HIGHER
ACROSS EASTERN MONTANA), 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 (LOCALLY COLDER IN SPOTS).
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 ON SUNDAY WITH
SOME HIGHS UP TO 10-12F BELOW NORMAL, WITH SLIGHT MODERATION TO
SINGLE-DIGIT NEGATIVE ANOMALIES EARLY-MID 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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