561
FXUS02 KWBC 121949
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 PM EST MON DEC 12 2022
VALID 12Z THU DEC 15 2022 - 12Z MON DEC 19 2022
...HEAVY SNOW THREAT SHIFTS INTO THE INTERIOR NORTHEAST THURSDAY
AND FRIDAY, WITH HEAVY RAIN POSSIBLE ACROSS COASTAL LOCATIONS...
..OVERVIEW
A DEEP UPPER LEVEL LOW (THE SAME ONE RESPONSIBLE FOR A NORTHERN
PLAINS WINTER STORM IN THE SHORT RANGE) WILL BE IN PLACE OVER THE
MIDWEST BY THE START OF THE PERIOD ON THURSDAY WITH A COLD FRONT
AT THE SURFACE SPREADING MODERATE TO HEAVY RAINFALL INTO EASTERN
PORTIONS OF THE U.S.. COASTAL LOW DEVELOPMENT LOOKS LIKELY BY
EARLY FRIDAY NEAR THE MID-ATLANTIC, BRINGING A THREAT FOR HEAVY
SNOWFALL TO PORTIONS OF THE CENTRAL APPALACHIANS AND INTERIOR
NORTHEAST AS IT LIFTS NORTH. SOME ACCUMULATING LAKE EFFECT
SNOWFALL WILL BE POSSIBLE BEHIND THE SYSTEM AS WELL. THE INITIAL
UPPER LOW SHOULD TRACK INTO THE UPPER GREAT LAKES AND THEN
SOUTHEASTERN CANADA/NEW ENGLAND (WITH SOME DETAILS TO BE
DETERMINED BY INTERACTION WITH CANADIAN FLOW). WITHIN THE PATTERN
THAT GUIDANCE SUGGESTS WILL BRING BELOW NORMAL TEMPERATURES TO
MOST OF THE CONUS BY THE WEEKEND AND EARLY NEXT WEEK, THERE HAS
BEEN SOME SHIFTING FOR THE DETAILS OF LATE-PERIOD CANADIAN INTO
NORTHERN TIER U.S. TROUGHING AND THUS FOR SPECIFICS OF THE COLDEST
AIR FORECAST TO REACH THE NORTHERN TIER. SHORTWAVES EMBEDDED
WITHIN THE MEAN FLOW (INCLUDING ENERGY FROM AN OPENING PACIFIC
UPPER LOW) WILL PROVIDE SOME MEDIUM TO SMALLER SCALE UNCERTAINTIES
IN THE FORECAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
IN GENERAL THE GUIDANCE FROM THE 00Z/06Z CYCLES APPEARED TO ADD TO
THE RECENT IMPROVING TREND WITH RESPECT TO CLUSTERING FOR THE
COMPACT COASTAL WAVE EXPECTED TO TRACK FROM NEAR THE MID-ATLANTIC
FROM AROUND THURSDAY NIGHT INTO THE WEEKEND. HOWEVER THE NEW 12Z
RUNS HAVE DIVERGED SOMEWHAT, WITH THE GFS ADJUSTING MORE OFFSHORE
AND THE UKMET/CMC STRAYING FASTER WITH THE NORTHWARD PROGRESSION.
THIS DIVERGENCE MAINTAINS A DECENT AMOUNT OF UNCERTAINTY FOR
COVERAGE/LOCATION OF VARIOUS PRECIPITATION TYPES FOR NEW ENGLAND.
AS FOR THE INITIAL UPPER LOW, THERE IS STILL A QUESTION OF HOW IT
EVOLVES AFTER IT REACHES NEAR THE UPPER GREAT LAKES BY AROUND
EARLY SATURDAY--RANGING BETWEEN CONTINUED EASTWARD PROGRESS OVER
SOUTHEASTERN CANADA AND/OR NEW ENGLAND TO A SHEARING PROCESS THAT
YIELDS A SEPARATE LOW TO THE EAST, OR MAYBE EVEN BOTH. INTERACTION
WITH SURROUNDING CANADIAN FLOW CONTINUES TO PROVIDE SOME
UNCERTAINTY OVER THE COURSE OF THE PERIOD AS WELL. THE FAVORED
BLEND FROM THE 00Z/06Z GUIDANCE TILTED TOWARD THE SHEARING
SCENARIO BUT THE NEW 12Z GUIDANCE LEANS TOWARD ONE BETTER DEFINED
UPPER LOW.
MEANWHILE GUIDANCE HAS BEEN STRUGGLING WITH EXACTLY HOW TO DEPICT
ANOTHER AMPLIFYING UPPER RIDGE OVER AND NEAR ALASKA ALONG WITH
ENERGY ON ITS EASTERN SIDE, LEADING TO SIGNIFICANT DIFFERENCES AND
TRENDS FOR CANADIAN TROUGHING EXPECTED TO AMPLIFY INTO THE
NORTHERN PORTIONS OF THE LOWER 48 AFTER SATURDAY. RECENT GFS RUNS
THROUGH THE 00Z CYCLE HAD BEEN SHOWING MORE HIGH-LATITUDE CLOSING
OF THE UPPER RIDGE (AS SHORTWAVE ENERGY PULLS WESTWARD OVER
ALASKA) TO RESULT IN A CENTRAL NORTH AMERICA TROUGH AXIS. GEFS
MEANS THROUGH 06Z AND THE 00Z CMC MEAN SIDED WITH THIS IDEA AS
WELL. ON THE OTHER HAND, RECENT ECMWF RUNS HAVE BEEN TRENDING
TOWARD A FARTHER WEST AXIS OF DIGGING ENERGY, WITH THE 00Z CMC AND
06Z/12Z GFS RUNS TRENDING THIS WAY AS WELL. THE GEFS MEAN HAS BEEN
SLOWER WITH ITS TREND BUT BY DAY 7 MONDAY THE 12Z VERSION HAS
ADJUSTED TO THE 00Z ECMWF MEAN (AN INTERMEDIATE SOLUTION BETWEEN
LATEST TRENDS AND RECENT CONTINUITY).
FINALLY, THE DETAILS OF THIS EVOLUTION WILL LIKELY HAVE SOME
INFLUENCE ON HOW SHORTWAVE ENERGY FROM AN INITIAL PACIFIC UPPER
LOW MAY ULTIMATELY INFLUENCE THE LOWER 48 AS IT BECOMES EMBEDDED
IN THE MEAN FLOW. THE 00Z-12Z ECMWF AND 12Z GFS/CMC ARE NOT TOO
FAR APART IN PRINCIPLE ASIDE FROM TYPICAL TIMING DIFFERENCES.
THE UPDATED FORECAST STARTED WITH A 00Z/06Z OPERATIONAL MODEL
BLEND EARLY IN THE PERIOD, WITH MORE GFS/ECMWF WEIGHT AND LEAST
WEIGHT ON THE UKMET (WHICH STRAYED FROM CONSENSUS FOR VARIOUS
PARTS OF THE FORECAST, ESPECIALLY TO THE NORTHWEST OF THE CONUS
LATE IN ITS RUN). THEN THE FORECAST TRANSITIONED TOWARD A NEARLY
EVEN BLEND OF THE GFS/ECMWF/CMC AND THE 06Z GEFS/00Z ECMWF MEANS
BY THE END OF THE PERIOD. THIS PROVIDED A MEANINGFUL TREND TO
REFLECT EMERGING GUIDANCE ADJUSTMENTS WHILE NOT YET GOING FULLY TO
THE PURE OPERATIONAL MODEL CLUSTER GIVEN THE NEWNESS OF THE TREND.
..WEATHER/HAZARD HIGHLIGHTS
DEEP/AMPLE MOISTURE WILL CONTINUE TO STREAM NORTHWARD AHEAD OF THE
CENTRAL U.S. UPPER LOW TO FUEL MODERATE TO HEAVY RAINFALL INTO THE
EASTERN U.S. THURSDAY AND FRIDAY. THE LIKELY DEVELOPMENT OF A
NOR'EASTER NEAR THE MID-ATLANTIC COAST AND SUBSEQUENT NORTHWARD
TRACK WOULD BRING A THREAT FOR WINTRY WEATHER AND POTENTIALLY
HEAVY SNOWFALL ACROSS INTERIOR PORTIONS OF THE MID-ATLANTIC AND
NORTHEAST, WITH EVEN SOME ICE/SLEET POTENTIAL CLOSER IN TOWARDS
THE I-95 CORRIDOR EARLY THURSDAY BEFORE THE WARM AIR MOVES IN AND
CHANGES THE PRECIPITATION TO RAIN. 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 ACROSS THE EAST. 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, AND ESPECIALLY DOWNWIND OF THE GREAT LAKES. DETAILS
OF SHORTWAVE ENERGY ORIGINATING FROM A PACIFIC UPPER LOW WILL
DETERMINE IF ANY LIGHT PRECIPITATION REACHES CALIFORNIA AND THE
SOUTHWEST NEXT WEEKEND. THIS SHORTWAVE COULD THEN INFLUENCE
SPECIFICS OF MOISTURE THAT MAY SPREAD ACROSS AND NORTH FROM THE
GULF COAST LATE WEEKEND INTO NEXT WEEK. SOME PRECIPITATION MAY
ALSO REACH THE NORTHWEST/NORTHERN ROCKIES AS UPPER TROUGHING
DESCENDS TOWARD THE REGION. SNOW LEVELS COULD BE FAIRLY LOW OVER
SOME AREAS.
EXPECT ABOVE NORMAL TEMPERATURES AHEAD OF THE COLD FRONT SLIDING
THROUGH THE EAST LATER THIS WEEK, BUT MUCH 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 MONDAY, DAYTIME TEMPERATURES ACROSS THE NORTHERN PLAINS
COULD BE AT LEAST 20-35 DEGREES BELOW NORMAL, EQUATING TO
WIDESPREAD SINGLE DIGIT TEMPERATURES WITH SOME BELOW ZERO READINGS
ACROSS EASTERN MONTANA AND NORTH DAKOTA. RECENT TRENDS SUGGEST
PARTS OF THE NORTHWEST COULD 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
HAZARDS:
- HEAVY SNOW ACROSS PORTIONS OF THE GREAT LAKES AND THE UPPER
MISSISSIPPI VALLEY, THU, DEC 15.
- HEAVY SNOW ACROSS PORTIONS OF THE GREAT LAKES, THE MID-ATLANTIC,
THE NORTHEAST, AND THE CENTRAL
APPALACHIANS, THU-SAT, DEC 15-DEC 17.
- SEVERE WEATHER ACROSS PORTIONS OF THE SOUTHEAST, THU, DEC 15.
- FREEZING RAIN ACROSS PORTIONS OF THE SOUTHERN APPALACHIANS, THE
GREAT LAKES, THE MID-ATLANTIC,
THE CENTRAL APPALACHIANS, AND THE OHIO VALLEY, THU, DEC 15.
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHERN
PLAINS.
- HIGH WINDS ACROSS PORTIONS OF THE NORTHEAST, FRI-SAT, DEC 16-DEC
17.
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL
PLAINS, THE NORTHERN PLAINS, THE
NORTHERN ROCKIES, THE CENTRAL ROCKIES, THE NORTHERN GREAT BASIN,
AND THE UPPER MISSISSIPPI VALLEY,
SUN-MON, DEC 18-DEC 19.
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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