077
FXUS02 KWBC 161919
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
218 PM EST FRI DEC 16 2022
VALID 12Z MON DEC 19 2022 - 12Z FRI DEC 23 2022
***BITTERLY COLD TEMPERATURES LIKELY TO SETTLE OVER THE NORTHERN
PLAINS FOR MUCH OF NEXT WEEK, SPREADING SOUTH AND EAST AFTER
WEDNESDAY***
***POTENTIAL FOR A SIGNIFICANT WINTER STORM OVER THE EAST LATE IN
THE WEEK***
..SYNOPTIC OVERVIEW
EXPECT A SIGNIFICANT PATTERN CHANGE TO EVOLVE ACROSS MUCH OF THE
CONTINENTAL U.S. NEXT WEEK AS A PRONOUNCED UPPER TROUGH DIGS INTO
THE EASTERN HALF OF THE NATION, ACCOMPANIED BY AN IMPRESSIVE
ARCTIC AIRMASS SETTLING SOUTHWARD ACROSS THE PLAINS AND THEN
ADVECTING EASTWARD, WITH WIDESPREAD SUBZERO TEMPERATURES LIKELY
FOR THE NORTH-CENTRAL STATES. THIS IS MAINLY IN RESPONSE TO A
BUILDING WEST COAST UPPER RIDGE (+PNA PATTERN) AND A NEGATIVE
ARCTIC OSCILLATION (-AO PATTERN) THAT ALLOWS ARCTIC AIRMASSES TO
DROP SOUTHWARD FROM CANADA. MOST GUIDANCE SUGGESTS THAT THE
AMPLIFYING TROUGH MAY SUPPORT STRONG SURFACE DEVELOPMENT AFTER
MIDWEEK, PRODUCING AREAS OF HEAVY RAIN AND/OR SNOW ALONG WITH
STRONG WIND. DETAILS OF THIS EVOLUTION WILL DEPEND ON THE
RELATIVE EMPHASIS OF FEATURES IN THE NORTHERN AND SOUTHERN STREAMS
AND HOW/WHEN THEY MAY INTERACT.
..GUIDANCE/PREDICTABILITY ASSESSMENT
OVER THE PAST SEVERAL DAYS THE GUIDANCE HAS HAD DIFFICULTY IN
RESOLVING IMPORTANT DETAILS FOR FLOW ACROSS AREAS FROM
NORTHWESTERN NORTH AMERICA AND THE NORTHEASTERN PACIFIC THROUGH
SOUTHERN CANADA AND THE LOWER 48. THIS ORIGINATES FROM THE
TYPICALLY LOW PREDICTABILITY FOR FLOW DOWNSTREAM FROM A CLOSED
UPPER HIGH SUCH AS THE ONE EXPECTED NORTHWEST OF ALASKA BY THE
START OF THE MEDIUM RANGE PERIOD. EVEN IN THE NEW 12Z GUIDANCE,
THERE IS PRONOUNCED SPREAD FOR LEADING ENERGY TRACKING ACROSS
SOUTHERN CANADA AND THE NORTHERN TIER U.S. (GFS/GEFS
DEEPEST/SOUTHWARD WITH THE SOUTHERN CANADA UPPER LOW AND MOST
AMPLIFIED WITH THE TROUGH TO ITS SOUTH) AND THEN THE SHAPE OF
UPSTREAM FLOW. UKMET RUNS THUS FAR HAVE BEEN THE MOST AMPLIFIED
WITH BROAD TROUGHING THAT PASSES THROUGH THE WEST, BUT NOW THE 12Z
CMC/ECMWF HAVE ADJUSTED AWAY FROM CONSENSUS TOWARD MORE
WESTERN-CENTRAL U.S. TROUGH AMPLIFICATION WEDNESDAY-THURSDAY AS
WELL (JUST A LITTLE EAST OF THE UKMET). LATEST GFS RUNS ARE
SOMEWHAT BETWEEN ESTABLISHED CONSENSUS AND THE WESTERN EXTREMES.
THESE DIFFERENCES LEAD INTO THE SIGNIFICANT SPREAD FOR WHERE
POTENTIALLY DEEP LOW PRESSURE MAY DEVELOP/CONSOLIDATE LATE IN THE
WEEK. GFS RUNS DIG THE STRONGEST BUNDLE OF NORTHERN STREAM ENERGY
WELL INTO THE SOUTHEAST BY EARLY FRIDAY, LEADING TO FARTHER
SOUTH/LATER DEVELOPMENT (AND MORE MID-ATLANTIC SNOW THREAT). IN
CONTRAST, THE NEW 12Z CMC/ECMWF EVOLUTION LEADS TO A DEEP UPPER
LOW OVER THE MICHIGAN/OHIO/INDIANA REGION AND A STRONG
MIDWEST/GREAT LAKES SURFACE LOW. THIS WOULD PRODUCE THE GREATEST
SNOW THREAT MUCH FARTHER WEST WHILE SWEEPING A STRONG COLD FRONT
(WITH MORE LIQUID PRECIPITATION) OVER THE EAST. OTHER
ALTERNATIVES INCLUDE PRIOR ECMWF RUNS THAT EMPHASIZED A LEADING
COASTAL WAVE THAT MERGED WITH THE APPROACHING GREAT LAKES SYSTEM.
MEANWHILE, ECENS/CMCENS MEANS HAVE BEEN QUITE CONSISTENT IN
POSITIONING AN UPPER LOW OVER THE EASTERN GREAT LAKES AS OF EARLY
FRIDAY. A LAGGED AVERAGE OF OPERATIONAL ECMWF RUNS HAS TRACKED
THE UPPER LOW JUST A BIT SOUTH OF THOSE MEANS. GEFS MEANS HAVE
BEEN WAFFLING MORE, AT TIMES KEEPING THE UPPER LOW FARTHER NORTH
THOUGH THIS IS PARTIALLY JUST DUE TO BEING ABOUT 12 HOURS SLOWER
WITH OVERALL DEVELOPMENT. EVEN AMONG THE PREFERRED MORE
CONSISTENT ECMWF/ECENS/CMCENS CLUSTER (ADDING THE 00Z CMC
OPERATIONAL AS WELL), THERE IS MEANINGFUL SPREAD FOR WHETHER THE
MAIN SURFACE LOW TAKES A COASTAL OR MORE INLAND TRACK--RELATED TO
THE UNCERTAINTY OF RELATIVE EMPHASIS OF THE SYSTEM REACHING THE
GREAT LAKES AND POTENTIAL LEADING WAVE NEAR THE EAST COAST. A
BLEND EMPHASIZING THAT PAST COUPLE ECMWF RUNS, THE 00Z ECMWF MEAN,
AND 00Z CMC BEST ACCOUNTED FOR THE MOST STABLE GUIDANCE AVAILABLE
THROUGH THE 06Z CYCLE. NEEDLESS TO SAY, THE AFOREMENTIONED 12Z
CMC/ECMWF OPEN THE DOOR TO YET ANOTHER SCENARIO THAT WOULD PRODUCE
A BROAD AREA OF SIGNIFICANT WEATHER BUT DISTRIBUTED VERY
DIFFERENTLY.
ELSEWHERE, FROM LATE MONDAY ONWARD THE GFS/GEFS MEAN BECOME
SOMEWHAT SLOWER/DEEPER THAN MOST OTHER GUIDANCE WITH THE SOUTHERN
STREAM SHORTWAVE THAT SUPPORTS A SURFACE WAVE CROSSING THE GULF OF
MEXICO/FLORIDA PENINSULA. THERE CONTINUES TO BE A VAGUE SIGNAL
FOR A WEAK SURFACE WAVE/FRONT TO REACH THE NORTHWEST AROUND
EARLY-MID WEEK. HOWEVER DETAILS ARE VERY UNCERTAIN, WITH A
DEPENDENCE ON UPPER LEVEL FLOW DETAILS THAT THE MODELS HAVE NOT
YET FIGURED OUT. THERE IS SOMEWHAT BETTER GENERAL AGREEMENT THAT
THE SURFACE FRONT DRAPED OVER THE NORTHERN PACIFIC NORTHWEST
SHOULD LIFT NORTHWARD BY LATE IN THE WEEK, AHEAD OF A SYSTEM
SUPPORTED BY A SHORTWAVE HEADING INTO THE MEAN RIDGE ALOFT.
THE EARLY PART OF THE UPDATED FORECAST STARTED WITH A 06Z/00Z
MODEL COMPOSITE WITH MOST WEIGHT ON THE 00Z ECMWF AND LEAST ON THE
UKMET. THEN THE BLEND TRANSITIONED TO GREATER EMPHASIS ON THE 00Z
ECMWF MEAN, LAST TWO ECMWF RUNS, AND 00Z CMC/CMC MEAN WITH ONLY
MODEST INCORPORATION OF THE 06Z GFS/GEFS MEAN. THIS SOLUTION LED
TO THE BEST CONTINUITY POSSIBLE GIVEN THE GUIDANCE SPREAD. THE
NEW 12Z RUNS SUGGEST THAT THE "MOST LIKELY" SCENARIO COULD WELL
CHANGE, DEPENDING ON HOW THE REMAINDER OF THE 12Z ENSEMBLE SUITE
TURNS OUT.
..WEATHER/HAZARD HIGHLIGHTS
THE MAIN WEATHER STORY THAT WILL MAKE WEATHER HEADLINES NEXT WEEK
WILL BE THE MASSIVE EXPANSE OF FRIGID TEMPERATURES FROM
MONTANA/NORTHERN PLAINS TO THE MIDWEST AND THEN FARTHER SOUTH AND
EAST BY NEXT THURSDAY-FRIDAY. A COUPLE OF REINFORCING COLD FRONTS
WILL USHER IN THE COLDEST AIR OF THE SEASON BY A CONSIDERABLE
MARGIN WITH THE EXPECTATION OF WIDESPREAD SUBZERO READINGS FOR
OVERNIGHT LOWS FROM THE NORTHERN ROCKIES TO THE CENTRAL/NORTHERN
PLAINS AND THE UPPER MIDWEST, WITH SOME LOWS APPROACHING -30
DEGREES ACROSS PORTIONS OF MONTANA AND NORTH DAKOTA. ALTHOUGH THE
FRIGID AIRMASS WILL MODIFY AS IT REACHES THE OHIO VALLEY AND
MID-SOUTH, IT WILL STILL BE QUITE COLD WITH WIDESPREAD 0S AND 10S
FOR OVERNIGHT LOWS BY THE END OF THE WEEK, AND THIS VERY COLD
WEATHER IS EXPECTED TO HAVE SOME STAYING POWER AS THE CLIMATE
PREDICTION CENTER OUTLOOK INDICATES HIGH PROBABILITIES OF BELOW
AVERAGE TEMPERATURES PERSISTING OVER MUCH OF THE EASTERN HALF OF
THE COUNTRY FOR THE WEEK BEYOND NEXT FRIDAY. AS COLD AS IT WILL
LIKELY BE, IT WILL BE DIFFICULT TO CHALLENGE DAILY RECORD
LOWS/COLD HIGHS DUE TO THIS EVENT OVERLAPPING WITH PORTIONS OF THE
HISTORIC 1983 AND 1989 COLD WAVES. AT THE VERY LEAST PORTIONS OF
THE EAST MAY END UP SEEING THE COLDEST TEMPERATURES FOR ANY
DECEMBER SINCE 2017.
AS FOR THE POTENTIAL STORM AFFECTING THE EASTERN U.S. LATE NEXT
WEEK, THE MOST COMMON THEME IN THE CURRENT GUIDANCE IS THAT THE
STRONG UPPER TROUGH AMPLIFICATION AND POTENTIALLY DEEP UPPER LOW
WOULD PRODUCE A VIGOROUS SURFACE SYSTEM AND A BROAD AREA OF STRONG
WINDS. BEYOND THAT, A POTENTIAL STORM TRACK ANYWHERE BETWEEN THE
GREAT LAKES AND EAST COAST MAKE IT VERY DIFFICULT TO OUTLINE WHERE
THE HEAVIEST SNOW IS MOST LIKELY. THE COMPOSITE OF ENSEMBLE-BASED
GUIDANCE AND MODELS BEFORE ARRIVAL OF THE 12Z RUNS HIGHLIGHTED THE
CENTRAL APPALACHIANS/NORTHERN MID-ATLANTIC INTO NEW ENGLAND AS
HAVING THE HIGHEST PROBABILITY OF SIGNIFICANT SNOWFALL,
CORRESPONDING TO A NEAR-COASTAL SURFACE LOW TRACK. HOWEVER IF
TRENDS END UP GOING SOMEWHAT FARTHER WESTWARD, THEN THE GREATEST
SNOW THREAT COULD SHIFT AS FAR BACK AS THE MIDWEST WITH MORE
LIQUID PRECIPITATION FARTHER EAST ALONG A STRONG COLD FRONT.
CONTINUE TO MONITOR FORECASTS FOR UPDATED INFORMATION AS GUIDANCE
HOPEFULLY BETTER RESOLVES THIS DEVELOPMENT IN COMING RUNS.
ELSEWHERE, A SHORTWAVE TRACKING NEAR THE GULF COAST ALONG WITH AN
ASSOCIATED FRONTAL WAVE OVER THE NORTHERN GULF OF MEXICO COULD
PRODUCE A PERIOD OF ENHANCED RAINFALL ACROSS PORTIONS OF SOUTHERN
TEXAS AND LOUISIANA ON MONDAY, AND THEN INTO FLORIDA ON TUESDAY.
THE NORTHWEST SHOULD SEE MULTIPLE EPISODES OF RAIN AND HIGHER
ELEVATION SNOW WITH HIGHEST TOTALS OVER FAVORED TERRAIN.
CONFIDENCE IS MODERATE AT BEST FOR DAY-TO-DAY DETAILS REGARDING
AMOUNTS AND COVERAGE GIVEN THE DEPENDENCE ON EXACT SHAPE OF THE
UPPER FLOW OVER THE REGION. THE GRADUAL WARMING TREND OVER THE
WEST AS UPPER RIDGING BUILDS TOWARD THE COAST SHOULD LEAD TO
SLOWLY RISING SNOW LEVELS WITH TIME.
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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