381
FXUS02 KWBC 150659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST WED JAN 15 2025
VALID 12Z SAT JAN 18 2025 - 12Z WED JAN 22 2025
...STRONG ARCTIC SURGE PUSHING INTO THE CENTRAL U.S. BY THE WEEKEND
AND THEN EXPANDING EASTWARD...
..OVERVIEW
THE LATEST MODELS AND ENSEMBLES STILL SHOW A VERY AMPLIFIED PATTERN
FEATURING AN EASTERN PACIFIC/FAR NORTHWESTERN NORTH AMERICA RIDGE
AND A DEEP CANADA THROUGH LOWER 48 TROUGH. IN THE EARLY STAGES OF
THIS PATTERN, VERY COLD AIR WILL DROP INTO THE CENTRAL U.S. THEN
WELL BELOW NORMAL ANOMALIES WILL SPREAD INTO THE SOUTH AND EAST BY
EARLY NEXT WEEK AS THE UPPER TROUGH LIKELY TAKES ON A MORE
POSITIVE TILT, WHILE THE NORTHERN/CENTRAL PLAINS EVENTUALLY
MODERATE. WITHIN THE HIGHLY CONFIDENT OVERALL TROUGH, GUIDANCE IS
COMING CLOSER TO CONVERGING FOR DETAILS OF A COUPLE LOWER-
PREDICTABILITY LEADING SHORTWAVES--ONE AIDING IN POTENTIALLY HEAVY
RAIN OVER PARTS OF THE SOUTHEAST ON SATURDAY AND ANOTHER THAT MAY
PRODUCE SOME SNOW OVER THE ROCKIES ON SATURDAY AND THEN A
PROGRESSIVE WAVE WITH MOSTLY LIGHT PRECIPITATION OF VARIOUS TYPES
NEAR THE EAST COAST ON SUNDAY. GUIDANCE IS NOW STARTING TO FOCUS IN
ON ANOTHER SHORTWAVE DIGGING INTO THE WEST AROUND NEXT MONDAY, WITH
CONSIDERABLE UNCERTAINTY FOR ITS EVOLUTION/SURFACE PATTERN THAT MAY
RESULT IN POTENTIAL FOR WINTER WEATHER THREATS ACROSS THE SOUTHERN
TIER DURING THE FIRST HALF OF NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
EARLY IN THE PERIOD, GUIDANCE HAS FINALLY IMPROVED ITS CLUSTERING
FOR A LEADING SHEARED SHORTWAVE THAT ORIGINATES FROM AN UPPER LOW
CURRENTLY TO THE WEST OF BAJA CALIFORNIA, AND WHICH SHOULD SUPPORT
WEAK WAVINESS ACROSS THE SOUTHEAST ON SATURDAY. THE SAME IS ALSO
THE CASE FOR TRAILING ENERGY DIGGING INTO THE ROCKIES ON EARLY
SATURDAY AND CONTINUING RAPIDLY EASTWARD AFTER THEN. RECENT
GUIDANCE HAS BEEN TRENDING TOWARD WHAT HAD BEEN THE FASTER HALF OF
THE SPREAD, WITH THE 12Z CMC STILL IN CATCH-UP MODE AFTER
CONSISTENTLY BEING A SLOW EXTREME. HOWEVER THERE IS STILL SOME
SPREAD FOR THE ASSOCIATED SURFACE WAVE THAT SHOULD TRACK FROM THE
MID-ATLANTIC/SOUTHEAST TO A POSITION SOUTH OR EAST OF NOVA SCOTIA
BY EARLY MONDAY.
THE REMAINDER OF THE FORECAST IS MUCH MORE CONTENTIOUS THOUGH.
THERE IS A DECENT SIGNAL FOR ANOTHER WELL-DEFINED SHORTWAVE TO DROP
INTO THE WESTERN HALF OF THE COUNTRY BY AROUND NEXT MONDAY, WITH A
LOT OF MODEL DIVERGENCE AND RUN-TO-RUN VARIABILITY FOR HOW THIS
FEATURE WILL EVOLVE THEREAFTER. SOME OF THIS IS ATTRIBUTED TO
VARIOUS IDEAS FOR YET ANOTHER SHORTWAVE THAT COULD REACH WESTERN
NORTH AMERICA BY MIDWEEK (WITH A LOT OF SPREAD FOR HOW IT MAY PASS
AROUND/THROUGH THE MEAN RIDGE TO THE WEST). OVERALL THE 12Z MACHINE
LEARNING (ML) MODELS SUGGEST FASTER PROGRESSION OF THE MONDAY
SHORTWAVE THAN MOST DYNAMICAL GUIDANCE, ALBEIT WITH A LOT OF DETAIL
DIFFERENCES, DUE TO ARRIVAL OF THE TRAILING MIDWEEK ENERGY.
MEANWHILE THE VERY ELONGATED 00Z GFS/ECMWF ARE NOT EXACTLY A
COMMON THEME EITHER. THE CHARACTER OF THIS ENERGY WILL PLAY AN
IMPORTANT ROLE IN THE SURFACE PATTERN ACROSS LOWER LATITUDES ALONG
WITH THE COVERAGE AND AMOUNTS OF WHAT COULD BE MEANINGFUL WINTRY
PRECIPITATION OVER PARTS OF THE SOUTH.
DURING THE FIRST HALF OF THE PERIOD A COMPOSITE OF THE 12Z ECMWF,
12Z/18Z GFS, AND 12Z UKMET PROVIDED A REASONABLE STARTING POINT FOR
THE FORECAST TO ACCOUNT FOR COMMON THEMES OR LINGERING SPREAD
DEPENDING ON THE FEATURE. DAY 5 MONDAY STARTED TO INTRODUCE SOME
18Z GEFS/12Z ECENS MEANS. RAPIDLY WIDENING SPREAD FOR INDIVIDUAL
MODEL RUNS FAVORED INCREASED ENSEMBLE MEAN WEIGHT BY DAYS 6-7
TUESDAY-WEDNESDAY, REACHING 60-70 PERCENT TOTAL WEIGHT AT THAT
TIME.
..WEATHER/HAZARDS HIGHLIGHTS
THE WELL-ADVERTISED SURGE OF ARCTIC AIR DROPPING SOUTH OVER THE
EASTERN ROCKIES INTO THE PLAINS BY THIS WEEKEND WILL BRING
INCREASING COVERAGE OF TEMPERATURES THAT ARE 15-35F OR SO BELOW
NORMAL, WITH EXPANSION OF THE COLD AIR BRINGING HIGHS DOWN TO
15-25F BELOW NORMAL ACROSS MUCH OF THE EAST BY MONDAY. THE MOST
EXTREME ANOMALIES ARE LIKELY TO BE OVER THE CENTRAL HIGH PLAINS AND
VICINITY DURING SUNDAY-MONDAY AND OVER THE FAR SOUTHERN PLAINS BY
TUESDAY. COLDEST TEMPERATURES ON AN ABSOLUTE BASIS SHOULD BE OVER
NORTHERN TIER AREAS WHERE SOME SUBZERO HIGHS AND LOWS IN THE -20S
ARE EXPECTED. WIND CHILLS SHOULD BE EVEN COLDER AND MORE DANGEROUS.
WPC HAS BEGUN POSTING KEY MESSAGES REGARDING THIS COLD OUTBREAK.
THE NORTHERN-CENTRAL ROCKIES/PLAINS SHOULD MODERATE CONSIDERABLY BY
TUESDAY-WEDNESDAY WHILE COLD WEATHER WILL PERSIST INTO WEDNESDAY
ACROSS THE SOUTH AND EAST. THE WEST WILL SEE A PERIOD OF COOL BUT
NOT EXTREME TEMPERATURES FROM THE WEEKEND INTO THE START OF NEXT
WEEK, WITH NEAR NORMAL READINGS SLOWLY BEGINNING TO PUSH EASTWARD
FROM THE WEST COAST. AHEAD OF THE COLD FRONT PROGRESSING INTO THE
EAST BY SATURDAY, EXPECT AN AREA OF ABOVE NORMAL TEMPERATURES THAT
WILL EXTEND TO THE EAST COAST INTO EARLY SUNDAY.
APPROACHING SHORTWAVE ENERGY AND A COMBINATION OF FRONTS SHOULD
START TO GENERATE SOME ENHANCED RAINFALL OVER THE LOWER
MISSISSIPPI VALLEY AND SOUTHEAST AROUND FRIDAY NIGHT (JUST BEFORE
THE EXTENDED FORECAST PERIOD STARTS). AS THIS ACTIVITY CONTINUES
ACROSS THE SOUTHEAST DURING SATURDAY AND SATURDAY NIGHT, THE
SUPPORTING WAVY SURFACE FRONT MAY BECOME MORE EAST-WEST ORIENTED,
POSSIBLY SUPPORTING SOME TRAINING. THUS THERE IS SOME POTENTIAL FOR
LOCALLY HEAVIER RAINFALL DURING THE DAY 4/SATURDAY ERO PERIOD IN
THE SOUTHEAST AND FLORIDA PANHANDLE. AS RAIN OCCURRED IN THAT
GENERAL VICINITY YESTERDAY, SOIL MOISTURE PERCENTILES ARE ACTUALLY
SHOWING SOME ABOVE AVERAGE ANOMALIES THERE CURRENTLY, BUT SOILS
SHOULD DRY OUT BY LATE WEEK. HOWEVER LATEST GUIDANCE HAS IMPROVED
ITS CLUSTERING, MAINLY FOCUSING A POTENTIAL BAND OF RELATIVELY
HEAVIER RAINFALL ACROSS NORTHERN FLORIDA AND SOUTHERN GEORGIA. ALSO
SOME FIRST-GUESS GUIDANCE NOW INDICATES A MARGINAL RISK LEVEL OF
THREAT, SO THE NEW DAY 4 OUTLOOK HAS INTRODUCED SUCH AN AREA OVER
THE AFOREMENTIONED REGION. SOME RAINFALL SHOULD PERSIST OVER PARTS
OF THE SOUTHEAST, ESPECIALLY PORTIONS OF FLORIDA, INTO SUNDAY BUT
WITH LESSER RATES.
MUCH LIGHTER PRECIPITATION OF VARYING TYPES WILL BE POSSIBLE
FARTHER NORTH OVER THE EAST DURING THE WEEKEND. GUIDANCE IS SLOWLY
IN THE PROCESS OF CONVERGING TOWARD THE IDEA OF A WEAK AND
PROGRESSIVE EAST COAST INTO WESTERN ATLANTIC WAVE ON SUNDAY WHICH
COULD PRODUCE SOME LIGHT PRECIPITATION OF VARYING TYPES, WITH
WINTRY TYPES POSSIBLE FROM THE MID- ATLANTIC/APPALACHIANS
NORTHWARD. PROBABILITIES OF 0.25 INCH LIQUID IN THE FORM OF SNOW
CONTINUE TO REFLECT THE FAIRLY LIGHT NATURE OF THIS EVENT, WITH
LOW COVERAGE OF 10 PERCENT OR GREATER VALUES, THOUGH HIGHER
ELEVATIONS LIKE THE ADIRONDACKS HAVE HIGHER PROBABILITIES. SOME
LOCALLY HEAVIER SNOW WILL BE POSSIBLE TO THE LEE OF THE GREAT LAKES
FROM THE WEEKEND INTO EARLY NEXT WEEK, AS WELL AS ACROSS THE
APPALACHIANS WITH UPSLOPE FLOW.
FARTHER WEST, EXPECT SNOW TO MOVE SOUTHWARD ACROSS THE
ROCKIES/HIGH PLAINS INTO SATURDAY IN ASSOCIATION WITH THE ARCTIC
SURGE AND DIGGING SHORTWAVE ENERGY. UPSLOPE FLOW SHOULD HELP TO
ENHANCE ACCUMULATIONS OVER SOME AREAS. ANOTHER SHORTWAVE MAY
PRODUCE SOME SNOW OVER PARTS OF THE ROCKIES EARLY NEXT WEEK. WHAT
THIS SHORTWAVE DOES WILL DETERMINE HOW MUCH PRECIPITATION EMERGES
OVER THE SOUTHERN TIER FROM TEXAS EASTWARD ON THE SOUTHERN
PERIPHERY OF THE OVERALL UPPER TROUGH AND SURFACE HIGH PRESSURE.
HOWEVER, THE SOUTHERN EXTENT OF THE COLD AIR AT THE SURFACE MEANS
THAT IF THERE IS PRECIPITATION IT WOULD MAINLY BE IN THE FORM OF
WINTRY PRECIP, PERHAPS ALL THE WAY TO THE GULF COAST, AND FREEZING
RAIN/ICE IS AN INCREASING CONCERN FOR PARTS OF THE SOUTH GIVEN THE
POSSIBILITY OF A WARM NOSE ATOP ARCTIC COLD AIR. SOUTHERN TIER SNOW
POTENTIAL SHOWS UP IN THE WINTER WEATHER PROBABILITIES FOR 0.25
INCH LIQUID IN THE FORM OF SNOW, NOW 10-15 PERCENT OVER AN AREA
JUST NORTH OF THE WESTERN HALF OF THE GULF COAST FOR THE EARLY
MONDAY-EARLY TUESDAY PERIOD AND ABOUT 10-35 PERCENT ACROSS A MUCH
BROADER SWATH OF THE SOUTH INTO SOUTHERN MID-ATLANTIC EARLY TUESDAY
INTO EARLY WEDNESDAY.
UNCERTAINTIES IN THE SETUP ALOFT OVER THE SOUTHWEST INTO NEXT WEEK
MAKE THE PATTERN AT THE SURFACE SOMEWHAT UNCERTAIN AS WELL.
HOWEVER, THERE IS A GENERAL TREND FOR WEAKER WINDS OVER SOUTHERN
CALIFORNIA INTO THE WEEKEND, BUT OFFSHORE FLOW MAY STRENGTHEN
AGAIN BY MONDAY AS 850MB WINDS INCREASE (DRAMATICALLY IN SOME
MODELS). KEEP AN EYE ON SPC FIRE WEATHER FORECASTS FOR FURTHER
INFORMATION.
RAUSCH/TATE
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 FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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