112
FXUS02 KWBC 140700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST SAT DEC 14 2024
VALID 12Z TUE DEC 17 2024 - 12Z SAT DEC 21 2024
..OVERVIEW
THE MEDIUM RANGE PERIOD BEGINS TUESDAY WITH A SHORTWAVE SHIFTING
THROUGH THE NORTHEAST AS ANOTHER ENTERS THE WEST COAST. THE LATTER
WILL BE RESPONSIBLE FOR A SIGNIFICANT PATTERN SHIFT AS IT AMPLIFIES
ACROSS THE CENTRAL-EASTERN U.S. MID TO LATE WEEK AND RIDGING
DEVELOPS UPSTREAM OVER THE WEST. THIS SHOULD PROMOTE WARMER AND
DRIER CONDITIONS ACROSS THE WEST, AND COOLING TEMPERATURES AND
PRECIPITATION FOR THE EAST, WITH A POTENTIAL EMERGING NOR'EASTER
THREAT LATE WEEK, BUT WITH LOW CONFIDENCE.
..GUIDANCE/PREDICTABILITY ASSESSMENT
BESIDES SOME RELATIVELY MINOR TIMING/SURFACE DETAIL DIFFERENCES,
THE MODELS ARE STILL IN GOOD AGREEMENT ON TUESDAY AS A SHORTWAVE
MOVES THROUGH THE MIDWEST INTO THE NORTHEAST, AND ANOTHER SHORTWAVE
ENTERS THE WEST. THE PATTERN GROWS MORE AMPLIFIED AFTER TUESDAY
BUT MUCH MORE UNCERTAIN WITH THE DETAILS AS TROUGHING BECOMES
ESTABLISHED OVER THE EAST. THERE IS UNCERTAINTY IN THE TIMING OF
THE TROUGH/SHORTWAVE BUT ALSO WITH SPLITTING OF STREAMS AND A
POSSIBLE LOW PRESSURE SYSTEM NEAR THE MID-ATLANTIC LATER IN THE
WEEK. THE GFS WAS NOTICEABLY SLOWER WITH THE TROUGH THROUGH ITS 18Z
RUN TONIGHT, BUT THE NEW 00Z RUN WAS SLIGHTLY FASTER. THE ECMWF
HAD THE BEST SUPPORT FROM OTHER GUIDANCE (INCLUDING AI/ML MODELS)
BUT WAS STRONGEST WITH A SOUTHERN STREAM UPPER LOW OVER THE
SOUTHEAST AND A MORE CLASSIC NOR'EASTER TYPE FEATURE UP THE EAST
COAST. HOWEVER, THE NEW 00Z RUN TRENDED MUCH WEAKER WITH SOUTHERN
STREAM ENERGY AND OVERALL MORE PROGRESSIVE WITH THE FRONT AND A
MORE NORTHERN SURFACE LOW. SO IT REMAINS TO BE SEEN WHAT KIND OF
TREND MAY BE EMERGING, SO LATE WEEK DETAILS ARE VERY UNCERTAIN.
THERE HAS BEEN GENERAL AGREEMENT THOUGH FOR AN OVERALL MORE
AMPLIFIED PATTERN AS THIS FIRST SYSTEM EXITS AND ADDITIONAL
SHORTWAVE ENERGY RELOADS OVER THE EAST INTO NEXT WEEKEND.
THE WPC BLEND WAS ABLE TO UTILIZE A GENERAL MODEL BLEND THE FIRST
DAY OR TWO OF THE PERIOD, BUT SHIFTED QUICKLY TOWARDS MORE ENSEMBLE
MEANS (WITH MINOR CONTRIBUTIONS FROM THE ECMWF AND CMC) TO ACCOUNT
TO INCREASING SPREAD AND VERY LOW CONFIDENCE IN ANY SOLUTION AT
THIS TIME. GENERALLY MAINTAINED GOOD CONTINUITY WITH THE PREVIOUS
WPC FORECAST, BUT WAS OVERALL FASTER WITH THE SYSTEM THROUGH THE
EAST.
AS FOR THE QPF FORECAST, DIVERGED COMPLETELY FROM THE NBM FOR THE
FIRST COUPLE OF DAYS AS THE 18Z GFS WAS AN OUTLIER IN THE FRONT AND
PRECIPITATION SETUP FARTHER NORTH AND THE NBM APPEARED VERY
INFLUENCED BY THAT SOLUTION. THE QPF FOR DAYS 4 AND 5 WAS BASED ON
THE ECMWF, THE EC MEAN, AND THE EC AI MODEL. WAS ABLE TO LEAN MORE
HEAVILY ON THE NBM LATER IN THE WEEK.
..WEATHER/HAZARDS HIGHLIGHTS
A LINGERING SURFACE FRONT ACROSS THE MID-SOUTH SHOULD BE
ACCOMPANIED BY SOME MODEST PRECIPITATION, BUT DOESN'T AT THIS TIME
APPEAR HAZARDOUS. DETAILS OF THE SURFACE PATTERN AND CORRESPONDING
PRECIPITATION COVERAGE, AMOUNTS, AND TIMING OVER THE
CENTRAL/EASTERN U.S. BECOME INCREASINGLY UNCERTAIN BY WEDNESDAY
THOUGH, BUT MODELS DO CONTINUE TO INDICATE ANOTHER ROUND OF
MODERATE TO HEAVY RAINFALL EMERGING IN THE SOUTH-CENTRAL U.S. TO
MID-SOUTH. THERE REMAINS ENOUGH UNCERTAINTY TO PRECLUDE ANY SORT OF
RISK AREAS ON THE DAY 5/WEDNESDAY EXCESSIVE RAINFALL OUTLOOK AT
THIS TIME, BUT THERE IS SOME GENERAL AGREEMENT ON ENHANCED RAINFALL
ACROSS PORTIONS OF THE SOUTHERN-CENTRAL APPALACHIANS WHERE SOIL
MOISTURE IS ABOVE NORMAL. PRECIPITATION MAY SPREAD ACROSS THE
EASTERN SEABOARD ON THURSDAY- FRIDAY, WITH SOME WINTRY WEATHER
POSSIBLE ON THE NORTHWESTERN SIDE OF THE LOW TRACK. THIS FORECAST
INDICATES SOME 10-30 PERCENT PROBABILITIES OF 0.25" QPF IN THE FORM
OF SNOW IN THE INTERIOR NORTHEAST ESPECIALLY IN HIGHER ELEVATIONS,
BUT CONTINUE TO MONITOR FORECASTS BECAUSE SNOW AMOUNTS AND
PLACEMENT WILL BE QUITE SENSITIVE TO THE UNCERTAIN EVENTUAL LOW
TRACK.
THE NORTHERN HALF OF THE WEST COAST AND INTO THE NORTHWEST SHOULD
SEE ANOTHER EPISODE OF RAIN AND HIGHER ELEVATION SNOW LASTING INTO
TUESDAY. AFTER THAT, THE WEST SHOULD SEE DRIER CONDITIONS
UNDERNEATH RIDGING, OTHER THAN PERHAPS SOME MOISTURE INTO WESTERN
WASHINGTON ROUNDING THE RIDGE.
WITHIN AN INITIAL GENERALLY MILD PATTERN ACROSS THE LOWER 48,
WARMEST ANOMALIES SHOULD EXTEND FROM THE SOUTHWEST/ROCKIES INTO TO
THE EAST ON TUESDAY. EXPECT A DECENT AREA OF HIGHS 10-15 DEGREES
ABOVE NORMAL AND ANOMALIES UP TO PLUS 15-25 DEGREES IN PLACES FOR
MORNING LOWS. AFTER WEDNESDAY, TEMPERATURES SHOULD TREND BACK TO
NORMAL ACROSS THE CENTRAL- EASTERN U.S. WITH A WARMING TREND OUT
WEST. AS TROUGHING GETS ESTABLISHED ACROSS THE EAST LATE WEEK, AN
ARCTIC HIGH WILL SPREAD BELOW NORMAL TEMPERATURES FROM THE NORTHERN
PLAINS/UPPER MIDWEST INTO MUCH OF THE EAST BY NEXT WEEKEND.
SANTORELLI
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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