218
FXUS02 KWBC 290652
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
152 AM EST FRI DEC 29 2023
VALID 12Z MON JAN 01 2024 - 12Z FRI JAN 05 2024
..OVERVIEW
THE LATEST MODELS AND ENSEMBLES SHOW A PATTERN TRANSITION FOR NEXT
WEEK TOWARDS A SERIES OF PACIFIC SHORTWAVES (SOME WITH AN EMBEDDED
UPPER LOW) TO AFFECT THE WEST COAST, AND THEN TRACK ACROSS THE
SOUTHERN TIER. IN THE EAST, A LEADING TROUGH WILL PASS THROUGH
MONDAY-TUESDAY, WITH SECONDARY TROUGHING TO FOLLOW MID TO LATTER
NEXT WEEK AS A RIDGE BUILDS OVER WESTERN CANADA. THIS PATTERN
KEEPS THE BULK OF THE PRECIPITATION CONFINED TO THE WEST COAST AND
THE GULF COAST/SOUTHEAST WITH LOCALLY HEAVY RAIN POSSIBLE IN SOME
SPOTS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
OVERALL, THE GUIDANCE SHOWS GOOD AGREEMENT ON THE LARGE SCALE
PATTERN, BUT A LOT OF QUESTIONS REMAIN WITH THE DETAILS AND TIMING
OF INDIVIDUAL SYSTEMS. THERE WAS ENOUGH AGREEMENT EARLY IN THE
WEEK BETWEEN THE 12Z/18Z (DEC 28) GUIDANCE TO USE A GENERAL MODEL
BLEND AS THE STARTING POINT FOR THE OVERNIGHT WPC MEDIUM RANGE
PACKAGE. BY WEDNESDAY, AND ESPECIALLY BEYOND, THE GREATEST AREA OF
UNCERTAINTY SURROUNDS THE TIMING OF THE SECOND SHORTWAVE/UPPER LOW
DROPPING DOWN THE WEST COAST WED-THURS AND INTO THE SOUTHWEST ON
FRIDAY. THE LAST FEW RUNS OF THE GFS (INCLUDING THE NEW 00Z RUN
TONIGHT) ARE NOTABLY FASTER WITH THIS SYSTEM COMPARED TO THE 12Z
ECMWF AND CMC, BUT IT SHOULD BE NOTED THAT THE NEW 00Z RUN OF THE
ECMWF THIS MORNING (AVAILABLE AFTER FORECAST GENERATION TIME) DID
TREND SIGNIFICANTLY FASTER WITH THE UPPER LOW AND IS IN BETTER
AGREEMENT WITH THE GFS RUNS. ENSEMBLE MEAN PLOTS SUGGEST THOUGH
THERE IS STILL A LOT OF UNCERTAINTY. DOWNSTREAM, THERE ARE ALSO
QUESTIONS WITH AMPLITUDE/SHARPNESS OF SHORTWAVE ENERGY INTO THE
SOUTHEAST AND ALSO DETAILS AROUND ADDITIONAL TROUGHING/ENERGY
ACROSS THE NORTHEAST. FOR THE SECOND HALF OF THE PERIOD, THE WPC
FORECAST FOR TONIGHT RELIED CLOSER TO THE ENSEMBLE MEANS WITH MORE
EMPHASIS TOWARDS THE SLOWER WESTERN U.S. EVOLUTION AS ADVERTISED
BY THE 12Z (DEC 28) ECMWF AND CMC DETERMINISTIC MODELS.
..WEATHER/HAZARDS HIGHLIGHTS
A COUPLE OF WEAK FRONTS THROUGH THE EAST ON MONDAY MAY PRODUCE
MAINLY LIGHT PRECIPITATION ACROSS PARTS OF THE LOWER GREAT
LAKES/CENTRAL APPALACHIANS AS WELL AS DOWN ACROSS THE
SOUTHEAST/CENTRAL GULF COAST. THE INITIAL UPPER LOW/SHORTWAVE INTO
THE SOUTHWEST MAY BRING SOME LIGHT PRECIPITATION TO PARTS OF THE
REGION ON MONDAY, BUT SHOULD BEGIN TO TAP GULF OF MEXICO MOISTURE
TO FUEL MORE WIDESPREAD SHOWERS ACROSS PARTS OF THE SOUTHERN
PLAINS/GULF COAST/SOUTHEAST TUESDAY-WEDNESDAY. THE LATEST MODEL
GUIDANCE IS INDICATING JUST MODEST MOISTURE AND INSTABILITY
AVAILABILITY, AND NOT ENOUGH AT THIS POINT TO WARRANT ANY KIND OF
RISK AREA ON THE DAY 5/TUESDAY EXCESSIVE RAINFALL OUTLOOK. THE
SECOND LOW FORECAST TO APPROACH THE WEST COAST BY WEDNESDAY AND
MOVE INTO THE SOUTHWEST BY THURSDAY MAY BRING ORGANIZED
PRECIPITATION TO THE WEST COAST AND SOME AREAS FARTHER INLAND,
WITH RELATIVELY HIGHER TOTALS MOST LIKELY OVER FAVORED TERRAIN IN
PORTIONS OF CALIFORNIA.
THE GREATEST TEMPERATURE ANOMALIES DURING THE PERIOD SHOULD FOCUS
ACROSS THE FAR NORTHERN TIER NEAR THE CANADIAN BORDER WITH SOME
LOCATIONS SEEING MULTIPLE DAYS WITH READINGS 10-20F OR SO ABOVE
NORMAL, WITH MORNING LOWS TENDING TO BE FARTHER ABOVE NORMAL THAN
DAYTIME HIGHS. MOST AREAS FROM CALIFORNIA EASTWARD ACROSS THE
SOUTHERN TIER SHOULD SEE NEAR TO BELOW NORMAL HIGH TEMPERATURES
DURING THE PERIOD AS A SERIES OF UPPER LEVEL SYSTEMS MOVE THROUGH.
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, 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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