808
FXUS02 KWBC 211859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM EST WED DEC 21 2022
VALID 12Z SAT DEC 24 2022 - 12Z WED DEC 28 2022
***A MAJOR WINTER STORM EXITING INTO EASTERN CANADA WILL LIKELY
CONTINUE TO CAUSE STRONG WINDS ON SATURDAY NEAR THE GREAT LAKES,
ALONG WITH BITTERLY COLD TEMPERATURES FOR CHRISTMAS WEEKEND***
..SYNOPTIC OVERVIEW
AFTER AN EXTREMELY IMPACTFUL WINTER STORM HAMMERS THE EASTERN U.S.
DURING THE SHORT RANGE PERIOD, CONDITIONS WILL START TO IMPROVE
OVER THE CHRISTMAS WEEKEND AS THE CORE OF THE LOW LIFTS NORTHWARD
ACROSS QUEBEC AND THEN HUDSON BAY. HOWEVER, IT WILL STILL BE
QUITE WINDY ACROSS THE GREAT LAKES REGION AND THE NORTHEAST ON
SATURDAY WITH A STRONG PRESSURE GRADIENT STILL IN PLACE, ALONG
WITH SOME LAKE EFFECT SNOW. THE IMPRESSIVELY COLD AIRMASS WILL
BEGIN MODIFYING AS SURFACE HIGH PRESSURE SETTLES ACROSS THE
SOUTHERN TIER STATES. MEANWHILE, THE INITIAL WEST COAST
RIDGE/EASTERN TROUGH UPPER PATTERN WILL PROMOTE RAIN/MOUNTAIN SNOW
OVER THE NORTHWEST AS NORTHEASTERN PACIFIC SYSTEMS BRUSH THE
REGION. FLATTING OF THE MEAN UPPER PATTERN AS AN EASTERN PACIFIC
TROUGH APPROACHES/MOVES INTO THE WEST WILL SPREAD PRECIPITATION
FARTHER SOUTH AND EAST ACROSS THE WEST NEXT WEEK WHILE CONTINUING
A WARMER TREND FOR TEMPERATURES ACROSS MOST OF THE LOWER 48.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AN OPERATIONAL MODEL COMPOSITE DURING THE FIRST HALF OF THE PERIOD
FOLLOWED BY A TRANSITION TOWARD A HALF MODEL/HALF ENSEMBLE MEAN
BLEND BY DAY 7 WEDNESDAY AGAIN PROVIDED A REASONABLE APPROACH TO
DEPICT THE MOST COMMON ELEMENTS OF GUIDANCE THROUGH THE PERIOD
WHILE TONING DOWN THE TYPICAL DETAIL UNCERTAINTIES THAT AROSE FOR
VARIOUS FEATURES.
CONSENSUS LOOKS GOOD FOR THE STORM EASTERN CANADA STORM DURING THE
WEEKEND. BEHIND THIS SYSTEM, THERE IS STILL A GENERAL SIGNAL FOR
A SHORTWAVE AND ASSOCIATED SURFACE WAVE SUNDAY-MONDAY, BUT WITH
CONTINUED SPREAD FOR SPECIFICS. GUIDANCE HAS BEEN HAVING
DIFFICULTY IN RESOLVING WHAT HAPPENS TO THIS SHORTWAVE AS IT
PROGRESSES THROUGH THE BASE OF THE MEAN TROUGH ALOFT, LEADING TO
SPREAD FOR THE SURFACE/MOISTURE PATTERN FROM THE GULF OF MEXICO
INTO WESTERN ATLANTIC AS WELL AS NEARBY PARTS OF THE SOUTH AND
EAST. THE NEW 12Z GFS HAS COME IN WITH THE SLOWEST AND MOST
CLOSED UPPER FEATURE BY TUESDAY-WEDNESDAY (VERSUS THE 06Z GFS THAT
WAS ACTUALLY SOMEWHAT ON THE FAST SIDE). ON THE OTHER HAND THE
NEW 12Z ECMWF IS SOMEWHAT FASTER/LESS AMPLIFIED THAN THE PRIOR
RUN. THIS VARIABILITY FAVORS A BLENDED/MEAN APPROACH UNTIL
CLUSTERING AND CONTINUITY IMPROVE. MEANWHILE THERE IS A LOT OF
SPREAD FOR EXACTLY WHAT HAPPENS FROM THE NORTHERN TIER INTO THE
GREAT LAKES DURING THE LATTER HALF OF THE PERIOD. OVER TIME ECMWF
RUNS HAVE BEEN GRADUALLY BACKING OFF WITH THE AMPLITUDE OF CENTRAL
CANADA ENERGY REACHING THE GREAT LAKES WHILE LATEST GFS RUNS SHOW
MINIMAL INFLUENCE FROM THAT REGION AND INSTEAD BRING PACIFIC
ENERGY INTO THE REGION. A CONSERVATIVE APPROACH CLOSER TO THE
MEANS SEEMS BEST FOR THIS ASPECT OF THE FORECAST.
EASTERN PACIFIC INTO WESTERN NORTH AMERICA SHORTWAVES AND
ASSOCIATED SURFACE SYSTEMS DURING THE WEEKEND GENERALLY APPEAR TO
EXHIBIT TYPICAL DETAIL AND TIMING SPREAD. THEN BY LATE MONDAY THE
06Z AND 12Z GFS RUNS STRAY FARTHER SOUTHEAST THAN OTHER SOLUTIONS
WITH LOW PRESSURE REACHING THE COAST. THE 12Z GFS BECOMES QUITE
PRONOUNCED IN HOW IT FLATTENS THE DOWNSTREAM RIDGE OVER THE WEST
BY EARLY TUESDAY. THE LARGER SCALE TROUGH EXPECTED TO MOVE INTO
THE WEST BY DAY 7 WEDNESDAY, ALONG WITH THE FAIRLY STRONG PACIFIC
SURFACE LOW THE PRIOR DAY, HAVE SOME DETAIL/TIMING SPREAD BUT WITH
A REASONABLY CONSISTENT MULTI-DAY SIGNAL OVERALL.
..WEATHER/HAZARD HIGHLIGHTS
THE DEPARTING WINTER STORM IS STILL LIKELY TO PRODUCE STRONG AND
POTENTIALLY DAMAGING WINDS FROM MICHIGAN TO UPSTATE NEW YORK
THROUGH THE DAY SATURDAY BEFORE GRADUALLY ABATING BY SATURDAY
NIGHT. INTENSE COLD AIR ADVECTION OVER THE WARMER LAKE WATERS
SHOULD PRODUCE BANDS OF LAKE EFFECT/ENHANCED SNOW, SOME OF WHICH
COULD BE LOCALLY HEAVY. FRIGID WIND CHILLS WILL ALSO BE A
SIGNIFICANT HAZARD FROM THE MIDWEST TO THE OHIO/TENNESSEE VALLEY,
AND THEN EXTENDING EASTWARD TO THE EAST COAST (THOUGH NOT QUITE AS
SEVERE AS OVER LOCATIONS WEST OF THE APPALACHIANS). EXPECT THE
WIDESPREAD SNOW FROM EARLIER IN THE EVENT TO PRODUCE A WHITE
CHRISTMAS FOR MANY LOCATIONS.
SATURDAY WILL FEATURE WIDESPREAD TEMPERATURE ANOMALIES OF 20-30
DEGREES BELOW AVERAGE FOR MORNING LOWS AND DAYTIME HIGHS ACROSS
MUCH OF THE CENTRAL AND EASTERN U.S. BEST POTENTIAL FOR DAILY
RECORDS ON THIS DAY WILL BE FOR COLD HIGHS OVER THE EAST. SUNDAY
WILL STILL BE COLD OVER THE EASTERN HALF OF THE COUNTRY WITH A
BROAD AREA OF READINGS 15-25F BELOW NORMAL, BUT DAILY RECORDS
SHOULD BE FAIRLY ISOLATED. THE THEME FOR NEXT WEEK WILL BE A
PRONOUNCED WARMING TREND AS THE UPPER PATTERN BECOMES LESS
AMPLIFIED. MODERATELY ABOVE NORMAL TEMPERATURES WILL BECOME
ESTABLISHED OVER THE WEST (LOWS TENDING TO BE A LITTLE MORE ABOVE
NORMAL THAN DAYTIME HIGHS) AND THEN SPREAD INTO THE HIGH PLAINS BY
NEXT TUESDAY-WEDNESDAY WITH SOME AREAS REACHING 10-20F ABOVE
NORMAL. THE EASTERN HALF OF THE COUNTRY WILL CONTINUE TO
MODERATE, LEADING TO MOSTLY SINGLE-DIGIT NEGATIVE ANOMALIES BY
NEXT WEDNESDAY.
A SERIES OF NORTHEASTERN PACIFIC SYSTEMS SHOULD BRING A MULTI-DAY
PERIOD OF ATMOSPHERIC RIVER EVENTS TO THE WEST COAST, LEADING TO
SIGNIFICANT TOTALS OF RAIN AND HIGHER ELEVATION SNOW. SOME OF
THIS MOISTURE WILL LIKELY EXTEND INTO THE ROCKIES. EXPECT THE
PRIMARY FOCUS DURING THE WEEKEND TO BE OVER THE PACIFIC NORTHWEST
AND FAR NORTHERN ROCKIES. THERE IS STILL SOME UNCERTAINTY OVER
THE PRECISE INTENSITY AND SOUTHWARD EXTENT OF PRECIPITATION AT ANY
PARTICULAR TIME, THOUGH AT THE MOMENT THE BEST SIGNAL FOR HIGHEST
AMOUNTS IS OVER THE OLYMPICS AND WASHINGTON CASCADES. THEN THE
PRECIPITATION SHIELD WILL LIKELY PUSH SOUTHEASTWARD DURING THE
FIRST HALF OF NEXT WEEK AS A MORE AMPLIFIED UPPER TROUGH AND
LEADING WAVY SURFACE FRONT MOVE IN. AS THIS OCCURS, THE HEAVIEST
RAIN/MOUNTAIN SNOW SHOULD FOCUS ALONG THE CENTRAL WEST COAST INTO
THE SIERRA NEVADA WHILE SOME MOISTURE EXTENDS FARTHER SOUTH OVER
CALIFORNIA AND EAST INTO THE GREAT BASIN/ROCKIES, AND MEANINGFUL
PRECIPITATION PERSISTS OVER THE NORTHWEST. THESE SYSTEMS MAY
BRING PERIODS OF BRISK TO STRONG WINDS TO SOME AREAS NEAR THE WEST
COAST.
ELSEWHERE, A WEAK WAVE MAY SPREAD AN AREA OF LIGHT SNOW FROM THE
NORTHERN PLAINS INTO THE MIDWEST DURING SUNDAY-MONDAY. SOUTHERN
TIER SHORTWAVE ENERGY AND ASSOCIATED WAVY SHOULD SPREAD MOISTURE
ACROSS THE GULF OF MEXICO AND WESTERN ATLANTIC EARLY-MID WEEK,
WITH FLORIDA POSSIBLY SEEING SOME RAINFALL IN THE PROCESS. IT
REMAINS A BIG QUESTION MARK HOW MUCH, IF ANY, OF THIS MOISTURE
REACHES THE GULF/SOUTHEAST COAST AREAS THOUGH.
RAUSCH/HAMRICK
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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