635
FXUS02 KWBC 241811
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
110 PM EST SUN DEC 24 2023
VALID 12Z WED DEC 27 2023 - 12Z SUN DEC 31 2023
...SOME AREAS OF ENHANCED RAINFALL POSSIBLE FOR THE NORTHERN
MID-ATLANTIC/NORTHEAST WEDNESDAY...
...TWO EPISODES OF LOCALLY HEAVY PRECIPITATION POSSIBLE FOR THE
CENTRAL WEST COAST MID-LATE WEEK...
..OVERVIEW
AN AMPLIFIED AND BLOCKY LARGE SCALE PATTERN IS FORECAST TO SETTLE
IN LATER THIS WEEK WITH MEAN RIDGING OVER THE WESTERN U.S. AND
WESTERN CANADA, WHICH WILL WEAKEN INCOMING PACIFIC SHORTWAVES, AND
TROUGHING OVER THE EASTERN HALF OF NORTH AMERICA. AN INITIAL UPPER
LOW OVER THE PLAINS/MIDWEST WILL ESTABLISH THE UPPER TROUGH AS IT
MOVES INTO THE EAST. MODELS SUGGEST WAVES OF ENERGY WILL SWING
SOUTH FROM CANADA INTO THE EASTERN HALF OF THE NATION, BUT IT IS
STILL UNCERTAIN HOW THESE WAVES WILL INTERACT WITH THE INITIAL
LOW. A LEADING WAVE NEAR THE EAST COAST SHOULD SPREAD RAINFALL
NORTHWARD INTO THE MID-ATLANTIC AND NORTHEAST MID-TO-LATE WEEK
WHILE THE UPPER LOW MAY PRODUCE LINGERING LIGHT SNOW OVER THE
NORTH-CENTRAL PLAINS AND THEN A MIX OF PRECIPITATION TYPES TO THE
EAST. OVER THE WEST, MOST PRECIPITATION SHOULD FOCUS NEAR THE WEST
COAST WITH A COUPLE FRONTAL SYSTEMS TENDING TO PRODUCE THE HIGHEST
TOTALS OVER/NEAR NORTHERN CALIFORNIA.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MOST UNCERTAINTY IN THE FORECAST SURROUNDS THE UPPER LEVEL
STRUCTURE IN THE EAST AS THE INITIAL LOW AND WAVES OF ENERGY FROM
CANADA INTERACT THURSDAY THROUGH THE WEEKEND. THE LATEST RUNS OF
THE GFS/GEFS (06/12Z) CONTINUE TO SHOW THE INITIAL LOW MOVING
SLOWER AND MORE SOUTH THAN OTHER GUIDANCE AND PROGRESSES NORTHERN
STREAM ENERGY A BIT FASTER AND FURTHER NORTH. THIS SOLUTION LIMITS
INTERACTION BETWEEN THE TWO, WHICH IS NOT CONSISTENT WITH THE
GENERAL MODEL CONSENSUS, PREVIOUS RUNS OF THE GFS, OR WPC'S
OVERNIGHT FORECAST. MOST OTHER GUIDANCE SHOWS THE INITIAL LOW AND
NORTHERN STREAM ENERGY INTERACTING IN THE VICINITY OF THE GREAT
LAKES REGION AND NORTHEAST. FOR THIS REASON, THE WEIGHT OF THE 06Z
GFS WAS LIMITED IN WPCS MODEL BLEND AND MORE EMPHASIS WAS PLACED
ON THE 00Z GFS. THE ECMWF STILL SHOWS A FASTER AND MORE NORTHERN
TRACK WITH THE INITIAL LOW, AND THE CMC FALLS IN BETWEEN THE ECMWF
AND GFS SOLUTIONS BUT CLOSER TO THE ECMWF SOLUTION.
MODEL AGREEMENT IS MUCH HIGHER FOR THE WEST, MAINTAINING RIDGING
OVER THE INTERMOUNTAIN WEST AS A FEW PACIFIC SHORTWAVES APPROACH
THE COAST. MODELS ARE STARTING TO SHOW AN UPPER LEVEL WAVE SLIDING
UNDER THE RIDGE INTO THE SOUTHWEST OVER THE WEEKEND, BUT THERE
REMAINS A SIGNIFICANT AMOUNT OF UNCERTAINTY AND SPREAD WITH THIS
FEATURE AMONG MODEL SOLUTIONS. FOR NOW PREFERENCE IS TO LEAN
TOWARD A MODEST AMOUNT OF UNDERCUTTING ENERGY.
FOR WPC'S AFTERNOON FORECAST, A DETERMINISTIC MODEL BLEND OF THE
00Z CMC/UKMET/ECMWF AND THE 00/06Z GFS WAS USED FOR WEDNESDAY AND
THURSDAY, THEN ENSEMBLE MEANS WERE ADDED AND TO SMOOTH OUT MODEL
DIFFERENCES FRIDAY THROUGH THE WEEKEND.
..WEATHER/HAZARDS HIGHLIGHTS
THE SYSTEM FORECAST TO AFFECT THE CENTRAL AND EASTERN U.S.
MID-LATE WEEK WILL SPREAD RAINFALL FROM THE MID-ATLANTIC INTO
NORTHEAST AND BACK INTO THE GREAT LAKES/UPPER MIDWEST, WITH SNOW
TRENDING LIGHTER AND MORE SCATTERED OVER THE NORTH-CENTRAL PLAINS
AND PARTS OF THE UPPER MIDWEST. RAINFALL MAY BE LOCALLY HEAVY AT
TIMES ALONG THE EAST COAST, WITH THE DAY 4/WEDNESDAY EXCESSIVE
RAINFALL OUTLOOK DEPICTING A MARGINAL RISK AREA FROM PARTS OF THE
MID-ATLANTIC INTO NEW ENGLAND. DIFFERENCES IN UPPER LEVEL AND
SURFACE DETAILS HAVE GENERALLY DECREASED CONFIDENCE IN THIS AREA,
BUT THERE IS STILL SOME AGREEMENT TOWARD AN AXIS OF ANOMALOUS
MOISTURE CROSSING THE REGION WHILE THE 12Z ECMWF INDICATED
POTENTIAL FOR SOME INSTABILITY WITHIN THE MOISTURE SHIELD.
PRECIPITATION SPECIFICS BECOME EVEN MORE UNCERTAIN AFTER
WEDNESDAY. AT THE VERY LEAST THERE SHOULD BE SOME LAKE
EFFECT/TERRAIN ENHANCED SNOW LATER IN THE WEEK AND POSSIBLY INTO
THE WEEKEND AS TEMPERATURES GRADUALLY DECLINE AND ONE OR MORE
UPPER FEATURES MOVE OVERHEAD.
OVER THE WEST, THE FORECAST REMAINS ON TRACK FOR TWO PRIMARY
PRECIPITATION EVENTS TO BE FOCUSED OVER THE CENTRAL WEST COAST
AROUND WEDNESDAY AND FRIDAY WITH THE ARRIVAL OF UPPER SHORTWAVES
AND ASSOCIATED SURFACE FRONTS. THE DAY 4/WEDNESDAY EXCESSIVE
RAINFALL OUTLOOK MAINTAINS THE MARGINAL RISK AREA DEPICTED IN
YESTERDAY'S DAY 5 OUTLOOK, INCLUDING PARTS OF NORTHERN CALIFORNIA
AND FAR SOUTHWESTERN OREGON. MODEL/ENSEMBLE GUIDANCE MAINTAINS
REASONABLE AGREEMENT TOWARD A BRIEF EPISODE OF LOCALLY HEAVY
RAINFALL WITHIN THE OUTLOOK AREA. CURRENTLY THE FRIDAY EVENT LOOKS
LIKE IT MAY PRODUCE SIMILAR TOTALS. IN BOTH EVENTS EXPECT SNOW
OVER HIGHER ELEVATIONS. THE PACIFIC NORTHWEST WILL LIKELY SEE SOME
PRECIPITATION FROM THESE EVENTS BUT WITH LIGHTER AMOUNTS, WHILE
THE FRIDAY EVENT (PERHAPS LINGERING INTO SATURDAY WITH LIGHTER
AMOUNTS) MAY SPREAD A LITTLE MOISTURE INTO THE SOUTHERN HALF OF
CALIFORNIA BUT WITH LOWER CONFIDENCE. THERE IS ALSO NOT MUCH
CONFIDENCE IN COVERAGE OF ANY MOSTLY LIGHT PRECIPITATION THAT
COULD REACH FARTHER INLAND DURING THE WEEKEND.
TEMPERATURES WILL BE UNSEASONABLY WARM ON WEDNESDAY FROM THE UPPER
MIDWEST INTO NORTHEAST AND SOUTH INTO THE MID-ATLANTIC, WITH
MORNING LOWS UP TO 20-25 DEGREES ABOVE NORMAL AND DAYTIME HIGHS UP
TO 10-15 DEGREES ABOVE NORMAL. LATER IN THE WEEK, THE GREATEST
ANOMALIES WILL BECOME MORE CONFINED TO NEW ENGLAND AND GRADUALLY
DECLINE. SOME DAILY RECORDS FOR WARM LOWS WILL BE POSSIBLE
WEDNESDAY INTO THURSDAY FROM THE GREAT LAKES EASTWARD.
ESTABLISHMENT OF UPPER TROUGHING OVER THE EAST AFTER MIDWEEK WILL
FAVOR PERSISTENT CHILLY CONDITIONS WITH COOLEST ANOMALIES OF 5-10
DEGREES BELOW NORMAL OVER THE SOUTHEAST AND INTO THE CENTRAL
APPALACHIANS. CONTINUE TO EXPECT MODERATELY ABOVE NORMAL
TEMPERATURES OVER THE WEST FOR MOST OF THE PERIOD, THOUGH PERHAPS
TRENDING CLOSER TO NORMAL BY NEXT SUNDAY.
DOLAN/RAUSCH
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES 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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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