687
FXUS02 KWBC 120710
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
210 AM EST THU DEC 12 2024
VALID 12Z SUN DEC 15 2024 - 12Z THU DEC 19 2024
..OVERVIEW
TODAY'S MODELS AND ENSEMBLE MEANS STILL SHOW A CHANGE IN PATTERN
FROM A SEQUENCE OF PROGRESSIVE SHORTWAVES SPACED A COUPLE DAYS OR
SO APART DURING THE WEEKEND AND FIRST HALF OF NEXT WEEK TOWARD A
LARGER SCALE MEAN CONFIGURATION CONSISTING OF A WEST COAST UPPER
RIDGE AND EAST-CENTRAL U.S. TROUGH. CURRENTLY THE GUIDANCE HAS
DIVERSE IDEAS FOR SOME DETAILS OF THIS TRANSITION AROUND
WEDNESDAY-THURSDAY BEFORE CLUSTERING ACTUALLY SEEMS TO IMPROVE A
DAY OR TWO LATER. INDIVIDUAL SHORTWAVES AND ASSOCIATED SURFACE
SYSTEMS WILL PRODUCE RAIN AND HIGHER ELEVATION SNOW OVER PARTS OF
THE WEST AND THEN DEVELOP PRECIPITATION OVER THE SOUTHERN PLAINS
AND EXPANDING ACROSS THE EAST-CENTRAL U.S. THEN CONTINUING INTO
PORTIONS OF THE EAST. MOST OF THE WEST WILL TREND DRIER MID-LATE
NEXT WEEK AS THE UPPER RIDGE BUILDS IN. MUCH OF THE COUNTRY SHOULD
SEE NEAR TO ABOVE NORMAL TEMPERATURES DURING THE PERIOD WITH A FEW
POCKETS OF MODERATELY BELOW NORMAL READINGS. HOWEVER TEMPERATURE
AND PRECIPITATION DETAILS BECOME MORE UNCERTAIN OVER THE EASTERN
HALF OF THE COUNTRY LATE IN THE PERIOD.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE COMPACT SHORTWAVE/UPPER LOW CROSSING THE GREAT LAKES SUNDAY
INTO MONDAY STILL EXHIBITS DECENT CLUSTERING WITH A SMALL NUMBER OF
STRAY SOLUTIONS. LATEST GFS/CMC RUNS HAVE GENERALLY BEEN BEST, WITH
ECMWF RUNS BECOMING TOO FAR SOUTH BY MONDAY AND 12Z UKMET DIVERTING
THE FEATURE TOO FAR NORTH. BOTH MODELS TREND TO CONSENSUS IN THE
NEW 00Z RUN.
FOR THE TRAILING UPPER TROUGH REACHING THE PLAINS BY MONDAY,
GUIDANCE SEEMS TO BE ATTEMPTING TO CONVERGE TOWARD AN INTERMEDIATE
SOLUTION BY WAY OF PREVIOUSLY SLOW ECMWF RUNS TRENDING FASTER AND
PRIOR FAST GFS RUNS PLUS ECMWF-INITIALIZED MACHINE LEARNING (ML)
MODELS NUDGING BACK A LITTLE. HOWEVER THERE ARE STILL FINE SCALE
DIFFERENCES THAT AFFECT SURFACE LOW DEVELOPMENT MONDAY INTO
TUESDAY, WITH A GENERAL THEME OF ONE POTENTIAL LOW NEAR THE
CANADIAN BORDER AND A SEPARATE WAVE FARTHER SOUTH NEAR THE MIDWEST.
BOTH DYNAMICAL AND ML MODELS DIFFER OVER WHICH LOW ULTIMATELY
DEVELOPS MORE STRONGLY, OR PERHAPS WITH SOME MERGING OF THE TWO
WAVES. THUS THE CONFIDENCE IN THIS DETAIL OF THE FORECAST IS
FAIRLY LOW, BUT WILL AFFECT SENSIBLE WEATHER OVER PARTS OF THE
MIDWEST/WESTERN GREAT LAKES. NOTE THAT THE 00Z CMC IS A SOUTHERN
EXTREME WITH THE SURFACE LOW BY TUESDAY, WHILE THE 00Z UKMET IS
SLOW WITH THE UPPER TROUGH.
MINUS THE 00Z UKMET, THERE IS NOW DECENT CLUSTERING FOR THE UPPER
TROUGH AND SURFACE SYSTEM OFF THE WEST COAST AS OF EARLY MONDAY AND
MOVING THROUGH THE WESTERN HALF OF THE COUNTRY TUESDAY-WEDNESDAY.
BY WEDNESDAY SOME DIFFERENCES WITH NORTHERN STREAM FLOW ARISE, WITH
RECENT ECMWF/ECENS MEANS IN PARTICULAR LEADING TO STRONG HIGH
PRESSURE OVER THE PLAINS VERSUS SOME DEGREE OF SURFACE
WAVINESS/FRONTAL SYSTEM IN THE GFS/GEFS AND CMC. THUS FAR THE ML
MODELS HAVE FAVORED AT LEAST SOME PLAINS WAVINESS (THOUGH WEAKER
THAN SOME EARLIER GFS RUNS) AND INDEED THE NEW 00Z ECMWF IS FINALLY
TRENDING THAT WAY. FROM WEDNESDAY INTO THURSDAY, ECMWF RUNS
FURTHER DIVERGE FROM MOST OTHER GUIDANCE DUE TO A STRONGER PACIFIC
SHORTWAVE PUSHING THROUGH THE WEST COAST MEAN RIDGE. HOWEVER EVEN
AMONG OTHER DYNAMICAL/ML GUIDANCE THERE IS A FAIR AMOUNT OF SPREAD
FOR WHAT STREAM INTERACTION MAY OCCUR. THERE IS A SLIGHTLY BETTER
ML THEME (INCLUDING THE ECMWF-AIFS ENSEMBLE MEAN) FOR THE
WEDNESDAY PLAINS WAVINESS TO TRACK INTO THE GREAT LAKES PER LATEST
GFS RUNS GIVEN SUFFICIENT INCLUSION OF NORTHERN STREAM FLOW. THERE
ARE OTHER POSSIBILITIES DEPENDING ON UPPER TROUGH TIMING AND ENERGY
DISTRIBUTION.
BASED ON GUIDANCE COMPARISONS, THE FIRST HALF OF THE FORECAST
CONSISTED OF HALF GFS (12Z/18Z SPLIT) AND THE REST DIVIDED BETWEEN
THE 12Z ECMWF AND CMC. THEN THE BLEND STEADILY SHIFTED GFS INPUT
MORE TOWARD THE 18Z RUN (12Z RUN POSSIBLY STRAYING A LITTLE FAST
WITH THE EASTERN UPPER TROUGH) WHILE INCLUDING MORE 18Z GEFS MEAN
THAN 12Z ECENS MEAN AS WELL AS MAINTAINING SOME 12Z CMC.
..WEATHER/HAZARDS HIGHLIGHTS
A SHORTWAVE WITH AN EMBEDDED COMPACT UPPER LOW WILL LIKELY TRACK
FROM THE MIDWEST/WESTERN GREAT LAKES THROUGH OR NEAR THE EASTERN
GREAT LAKES DURING SUNDAY-MONDAY, PRODUCING AN AREA OF
PRECIPITATION FROM THE GREAT LAKES SOUTHWARD AND THEN CONTINUING
EASTWARD. PARTS OF THE APPALACHIANS/NORTHEAST MAY SEE SOME WINTRY
PRECIPITATION BUT IT MOST SHOULD BE LIGHT, AS PROBABILITIES FOR
0.25 INCH LIQUID IN THE FORM OF SNOW REMAIN BELOW 30 PERCENT.
THE NEXT SYSTEM WILL CROSS THE INTERIOR WEST AND ROCKIES ON SUNDAY,
PRODUCING SNOW OVER THE NORTHERN-CENTRAL ROCKIES BUT WITH TOTALS
LOWER THAN FORECAST OVER THE NORTHERN ROCKIES/NORTHWEST ON
SATURDAY. EXPECT CONTINUED PROGRESSION OF THE UPPER TROUGH WITH
LEADING COLD FRONT AND INCORPORATION OF SOME GULF MOISTURE TO
DEVELOP AN AREA OF PRECIPITATION FROM THE SOUTHERN HALF OF THE
PLAINS NORTHEASTWARD BY MONDAY-MONDAY NIGHT. MOST OF THIS ACTIVITY
WILL BE IN THE FORM OF RAIN EXCEPT FOR SOME SCATTERED POCKETS OF
WINTRY WEATHER IN THE FAR NORTHERN TIER. GUIDANCE CONTINUES TO
SUGGEST GREATER INSTABILITY WITH THIS SYSTEM RELATIVE TO ITS
PREDECESSOR, LEADING TO HIGHER RAINFALL RATES WITHIN PORTIONS OF AN
AREA EXTENDING FROM THE SOUTHERN HALF OF THE EASTERN PLAINS INTO
THE LOWER OHIO VALLEY. FOR THE TIME BEING THE DAY 5 EXCESSIVE
RAINFALL OUTLOOK DEPICTS NO RISK AREAS, GIVEN A COMBINATION OF
CURRENT GUIDANCE SPREAD FOR HEAVIEST RAINFALL MAGNITUDE/LOCATION AS
WELL AS GENERALLY DRY ANTECEDENT CONDITIONS. HOWEVER SOME RAINFALL
EXPECTED IN THE SHORT TERM MAY TEMPER THE INITIAL DRYNESS AND
IMPROVED CLUSTERING TOWARD HEAVIER RAINFALL WOULD INCREASE THE
POTENTIAL FOR RUNOFF ISSUES. FASTER MOVEMENT OF THE COLD FRONT
SHOULD LEAD TO LOWER RAINFALL TOTALS OVER THE EAST ON TUESDAY.
THE NORTHERN HALF OF THE WEST COAST SHOULD SEE ANOTHER EPISODE OF
RAIN AND HIGHER ELEVATION SNOW ON MONDAY, WITH MOISTURE EXTENDING
EAST THROUGH THE ROCKIES BY TUESDAY. DETAILS OF THE SURFACE PATTERN
AND CORRESPONDING PRECIPITATION COVERAGE AND AMOUNTS OVER THE
CENTRAL/EASTERN U.S. BECOME INCREASINGLY UNCERTAIN BY NEXT
WEDNESDAY-THURSDAY.
WITHIN A GENERALLY MILD PATTERN ACROSS THE LOWER 48, WARMEST
ANOMALIES SHOULD EXTEND FROM THE EASTERN SLOPES OF THE ROCKIES TO
JUST WEST OF THE APPALACHIANS SUNDAY-MONDAY AND REACH THE EAST ON
TUESDAY AHEAD OF A PROGRESSIVE COLD FRONT. EXPECT A DECENT AREA OF
HIGHS 10-15F ABOVE NORMAL AND ANOMALIES UP TO PLUS 15-25F IN PLACES
FOR MORNING LOWS. LOCALIZED AREAS WITH SLIGHTLY BELOW NORMAL HIGHS
SHOULD INCLUDE THE CENTRAL WEST COAST SUNDAY-MONDAY, THE FAR
NORTHERN TIER TUESDAY-WEDNESDAY, AND POSSIBLY PARTS OF THE SOUTH BY
NEXT THURSDAY.
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 (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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