958
FXUS02 KWBC 270658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM EST WED NOV 27 2024
VALID 12Z SAT NOV 30 2024 - 12Z WED DEC 04 2024
...ARCTIC COLD WILL SPREAD ACROSS THE NORTHERN PLAINS WITH BELOW
NORMAL TEMPERATURES REACHING MOST AREAS EAST OF THE ROCKIES...
...MULTIPLE DAYS OF HEAVY LAKE EFFECT SNOW THIS WEEKEND INTO NEXT
WEEK...
..OVERVIEW
A LARGE SCALE UPPER TROUGH IS EXPECTED TO ENCOMPASS MUCH OF THE
CENTRAL TO EASTERN UNITED STATES OVER THE WEEKEND, WITH SOME VERY
GRADUAL SHIFT EAST AS NEXT WEEK PROGRESSES. THIS PATTERN WITH
WEST/NORTHWEST WINDS WILL BECOME FAVORABLE FOR LONG DURATION LAKE
EFFECT SNOW. SOME OF THE SNOWFALL ACCUMULATIONS COULD BE
SIGNIFICANT IN FAVORED LOCATIONS. THE TROUGH ALOFT AND COLD SURFACE
HIGH PRESSURE STEMMING FROM THE ARCTIC WILL CAUSE TEMPERATURES TO
DROP WELL BELOW AVERAGE ACROSS MUCH OF THE COUNTRY EAST OF THE
ROCKIES. THIS WILL BE THE COLDEST AIR OF THE SEASON SO FAR, WITH
TEMPERATURES FALLING INTO THE NEGATIVE TEENS AND WIND CHILLS IN THE
-20S IN NORTH DAKOTA SATURDAY MORNING. MEANWHILE BENIGN WEATHER
AND MORE MODERATE TEMPERATURES ARE FORECAST FOR THE WEST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LARGE SCALE PATTERN OVER THE WEEKEND IS FORTUNATELY REASONABLY
PREDICTABLE WITH THE BROAD TROUGHING ENCOMPASSING MUCH OF THE
CENTRAL AND EASTERN UNITED STATES. SMALLER SCALE FEATURES EMBEDDED
WITHIN THE MEAN FLOW COULD HAVE SENSIBLE WEATHER IMPACTS THOUGH.
THIS INCLUDES MULTIPLE SHORTWAVES TRACKING THROUGH THE WESTERN AND
SOUTHERN SIDES OF THE TROUGH THAT COULD PROVIDE FORCING FOR LIGHT
SNOW EVENTS AND HELP DETERMINE WHERE THE FRONTAL POSITION WILL BE.
BY TUESDAY-WEDNESDAY THE MODELS DIFFER WITH THE STRENGTH OF A
SHORTWAVE EXITING INTO THE WESTERN ATLANTIC AND ALSO WITH HOW MUCH
ENERGY COMES SOUTH FROM CENTRAL CANADA. THE 12Z GFS FOR EXAMPLE WAS
QUITE STRONG WITH THE LEADING SHORTWAVE, AND ALONG WITH THE 12Z
CMC ENDED UP SHOWING TWO TROUGH AXES WITHIN THE BROADER ONE BY
WEDNESDAY. GEFS MEANS SHOWED HINTS OF THIS AS WELL. THE 12Z ECMWF
ON THE OTHER HAND WAS STRONG AND DEEP WITH THE CANADIAN ENERGY,
WHICH DOMINATED TO SHOW ONE DEEPER TROUGH. THE 18Z AND NEWER 00Z
GFS INDICATES ONE TROUGH BUT SHALLOWER. GENERALLY PREFERRED A BROAD
TROUGH AT THIS POINT TO TRY TO AVOID PICKING OUT INDIVIDUAL
SHORTWAVE TIMING.
SPLIT FLOW WILL PREVAIL ACROSS THE EASTERN PACIFIC TO WESTERN
U.S., AS NORTHERN STREAM RIDGING COMES ACROSS THE PACIFIC
NORTHWEST, WHILE A COUPLE ROUNDS OF SOUTHERN STREAM TROUGHING
NUDGES TOWARD CALIFORNIA. MODELS ARE PRETTY AGREEABLE ON THE
NORTHERN STREAM RIDGE, OTHER THAN SOME CMC RUNS THAT HAVE SHOWN
SHORTWAVES MOVING THROUGH THE NORTHERN SIDE OF THE RIDGE AND
SUPPRESSING IT IF NOT OUTRIGHT BECOMING OUT OF PHASE. THERE ARE
SOME TIMING DIFFERENCES WITH THE SOUTHERN STREAM TROUGHING, BOTH
THE FIRST WEAK ENERGY OVER THE WEEKEND AND THE NEXT ROUND OF
POSITIVELY TILTED TROUGHING APPROACHING BY TUESDAY OR SO, BUT
NOTHING TOO EGREGIOUS FOR A PACIFIC FORECAST IN THE MEDIUM RANGE
TIME FRAME, SO A MODEL/MEAN BLEND WORKED FINE.
WITH BROAD AGREEMENT ON THE LARGE SCALE, THE WPC FORECAST USED A
BLEND OF THE DETERMINISTIC MODELS EARLY IN THE FORECAST PERIOD, AND
GRADUALLY REDUCED THE DETERMINISTICS WHILE INCREASING THE
PROPORTION OF ENSEMBLE MEANS TO HALF BY DAY 7 TO SMOOTH OUT
DIFFERENCES FROM THE INDIVIDUAL MODELS.
..WEATHER/HAZARDS HIGHLIGHTS
AS A PRONOUNCED UPPER TROUGH SETTLES OVER THE CENTRAL AND EASTERN
U.S., A SIGNIFICANT ARCTIC AIR INTRUSION IN THE FORM OF A COLD
SURFACE HIGH WILL SPREAD FROM THE NORTHERN PLAINS SOUTHEASTWARD
INTO THE WEEKEND. THIS EVENT DOES NOT LOOK TO BREAK MANY
TEMPERATURE RECORDS NOR IS IT PARTICULARLY EARLY IN THE SEASON FOR
THE FIRST ARCTIC AIRMASS. HOWEVER, THE COLD WILL BE A NOTICEABLE
CHANGE IN MANY AREAS, AND FOR SOME ACROSS THE NORTHERN PLAINS,
POTENTIALLY DANGEROUS COLD TEMPERATURES ARE EXPECTED. OVERNIGHT
LOWS IN THE NEGATIVE TEENS COULD COMBINE WITH WIND TO BRING WIND
CHILLS DOWN INTO THE -20S, APPROACHING -30F, IN NORTH DAKOTA IN
PARTICULAR FOR THE WEEKEND. THIS WILL POSE AN INCREASED RISK OF
HYPOTHERMIA AND FROSTBITE ON EXPOSED SKIN, SO TAKE APPROPRIATE
PREPAREDNESS ACTIONS. HIGHS ARE FORECAST TO REMAIN IN THE SINGLE
DIGITS AND TEENS. TEMPERATURES ARE LIKELY TO GRADUALLY MODERATE
ACROSS THE NORTH-CENTRAL U.S. AND REACH AVERAGE VALUES BY MIDWEEK.
MEANWHILE THE COLD WILL SPREAD FARTHER SOUTHEAST IN MODERATED
FORM, REACHING MOST AREAS EAST OF THE ROCKIES ACROSS THE
MISSISSIPPI VALLEY, OHIO/TENNESSEE VALLEYS, AND TOWARD THE
APPALACHIANS AND EASTERN SEABOARD. A HANDFUL OF RECORD LOW MAX AND
MIN TEMPERATURES ARE POSSIBLE ACROSS THE OHIO VALLEY INTO THE
APPALACHIANS, WHERE TEMPERATURES ARE FORECAST TO BE AROUND 15-20F
BELOW AVERAGE AND STAY BELOW FREEZING FOR HIGHS IN MANY AREAS. BY
TUESDAY-WEDNESDAY THE COLDEST AIR WILL MIGRATE OVER THE EAST WITH
TEMPERATURES GENERALLY 10-15 DEGREES BELOW NORMAL AS THE UPPER
TROUGH AXIS SLOWLY SHIFTS EAST.
HEAVY LAKE-EFFECT SNOW IS EXPECTED THROUGH MUCH OF THE PERIOD, AS
WESTERLY FLOW DEVELOPS SNOW EAST OF THE LAKES THROUGH SATURDAY,
AND WINDS MAY SHIFT TO A MORE NORTHWESTERLY DIRECTION INTO EARLY
NEXT WEEK FOR A SNOW FOCUS SOUTHEAST OF THE LAKES SUNDAY-TUESDAY.
SIGNIFICANT ACCUMULATIONS ARE POSSIBLE FOR SOME OF THE FAVORED LAKE
EFFECT LOCATIONS. WHILE SPECIFIC ACCUMULATIONS WILL BECOME CLEARER
OVER THE NEXT FEW DAYS, PEAK AMOUNTS ARE LIKELY DOWNWIND OF LAKE
ERIE AND LAKE ONTARIO. TRAVEL DISRUPTIONS ARE LIKELY ESPECIALLY ON
I-90 BETWEEN CLEVELAND AND BUFFALO AND I-81 NORTH OF SYRACUSE.
ELSEWHERE, A QUIETER PERIOD IS LIKELY FOR THE WEST PRECIPITATION-
WISE. LIGHT TO LOCALLY MODEST PRECIPITATION IS POSSIBLE AT TIMES IN
THE PACIFIC NORTHWEST INTO THE NORTHERN ROCKIES. EXPECT
TEMPERATURES IN THE ROCKIES WESTWARD TO BE NEAR AVERAGE LATE THIS
WEEK AND WARM TO A FEW DEGREES ABOVE NORMAL INTO EARLY NEXT WEEK.
MODELS CONTINUE TO SHOW A GENERAL SIGNAL FOR A ROUND OF LIGHT TO
MODERATE SNOW ACROSS PARTS OF THE NORTH-CENTRAL PLAINS TO MID-
MISSISSIPPI AND OHIO VALLEYS SATURDAY, THOUGH THE AXIS AND TIMING
ARE SOMEWHAT UNCERTAIN DEPENDING ON A SHORTWAVE ALOFT. RETURN FLOW
OF MOISTURE COULD START TO COME INTO THE WESTERN GULF COAST REGION
OVER THE WEEKEND FOR SOME SHOWER ACTIVITY, WHICH MAY SPREAD INLAND
INTO THE SOUTH-CENTRAL U.S. BY MONDAY AND TUESDAY.
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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