708
FXUS02 KWBC 280658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST THU DEC 28 2023
VALID 12Z SUN DEC 31 2023 - 12Z THU JAN 04 2024
..OVERVIEW
THE LATEST MODELS AND ENSEMBLES STILL ADVERTISE A PATTERN
TRANSITION TOWARD A SERIES OF PACIFIC SHORTWAVES (SOME WITH AN
EMBEDDED UPPER LOW) AFFECTING SOME LOCATIONS NEAR THE WEST COAST
AND THEN TRACKING ACROSS THE SOUTHERN TIER. AFTER A LEADING
TROUGH PASSES THROUGH THE EASTERN U.S. EARLY NEXT WEEK, WESTERN
CANADA MEAN RIDGING SHOULD BE PERSISTENT ENOUGH TO SUPPORT A
DOWNSTREAM TROUGH EXTENDING INTO THE NORTHEASTERN QUADRANT OF THE
LOWER 48 BY MID-LATE WEEK. THIS PATTERN SHOULD CONFINE THE
MAJORITY OF PRECIPITATION AND SOMEWHAT BELOW NORMAL HIGH
TEMPERATURES TO PARTS OF THE WEST COAST AND THE GULF COAST
STATES/SOUTHEAST. THE GREAT LAKES COULD SEE SOME LIGHT SNOW
AROUND SUNDAY AND WEDNESDAY-THURSDAY. WARMEST TEMPERATURE
ANOMALIES WILL LIKELY BE NEAR THE CANADIAN BORDER.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AS HAS GENERALLY BEEN THE CASE LATELY, MOST GUIDANCE REFLECTS
SIMILAR IDEAS FOR THE GENERAL PATTERN BUT SHOWS SOME MEANINGFUL
DETAIL DIFFERENCES WITH SIGNIFICANT FEATURES. THESE INCLUDE THE
UPPER MIDWEST TROUGH PROGRESSING THROUGH THE EAST EARLY NEXT WEEK,
A SYSTEM REACHING NEAR SOUTHERN CALIFORNIA BY MONDAY AND
CONTINUING EASTWARD THEREAFTER, THEN FOLLOWED BY ANOTHER SYSTEM
REACHING THE WEST COAST AROUND MIDWEEK.
GUIDANCE HAS YET TO DECIDE ON THE DETAILS OF THE LEADING UPPER
TROUGH CROSSING THE EAST. RECENT GFS RUNS HAVE GENERALLY BEEN ON
THE SLOW AND CLOSED SIDE OF THE SPECTRUM, THOUGH UP THROUGH THE
18Z RUN THERE HAD BEEN ATTEMPTS AT CONVERGENCE BY SOME SOLUTIONS,
WITH THAT GFS A LITTLE FASTER AND DELAYING CLOSURE OF AN UPPER LOW
WHILE THE 12Z UKMET/ECMWF WERE CLOSE IN PRINCIPLE. THE NEW 00Z
GFS HAS REVERTED BACK TO PULLING OFF A SLOWER AND MORE CLOSED
FEATURE FROM THE TRAILING PART OF THE INITIAL POSITIVELY TILTED
TROUGH. ONE 00Z GFS ADJUSTMENT CLOSER TO CONSENSUS IS TOWARD
WEAKER SURFACE LOW DEVELOPMENT JUST OFF THE EAST COAST VERSUS
PRIOR RUNS. MAJORITY CLUSTERING ALSO SUPPORTS THE MORE SOUTHERN
GFS LATITUDE VERSUS THE 12Z ECMWF. ENSEMBLE SPREAD HAS KEPT THE
MEANS OPEN AND POSITIVELY TILTED WITH THE UPPER TROUGH, LEADING TO
LESS DEFINED WESTERN ATLANTIC LOW PRESSURE THAN MOST OPERATIONAL
SOLUTIONS. A COMPROMISE AMONG THE 18Z GFS AND 12Z MODELS PROVIDED
A REASONABLE STARTING POINT GIVEN ONGOING DIFFERENCES.
SOUTHERN STREAM ENERGY EMERGING FROM THE SOUTHERN ROCKIES ON
SUNDAY SHOULD SHEAR OUT UNDERNEATH THE LEADING EASTERN TROUGH.
THE BEST CLUSTERING WITH THE NEXT SYSTEM IN THE PROGRESSION IS FOR
AN UPPER LOW TRACK JUST OFFSHORE SOUTHERN CALIFORNIA AS OF MONDAY
FOLLOWED BY A MORE OPEN APPEARANCE AS THE SHORTWAVE CONTINUES
EASTWARD ACROSS THE SOUTHERN TIER AND POSSIBLY INTERACTS WITH
NORTHERN STREAM TROUGHING. CURRENTLY THE MAJORITY OF GUIDANCE HAS
A PHASED APPEARANCE WITH OVERALL EASTERN U.S. TROUGHING BY DAY 7
THURSDAY, LEAVING THE MORE SEPARATED 12Z ECMWF WITH A SLOWER
SOUTHERN CLOSED LOW IN THE MINORITY. THE UNCERTAINTY OVER EASTERN
U.S. TROUGH DETAILS AT THAT TIME RESULTS IN A BROAD SPREAD FOR ONE
OR MORE SURFACE LOWS--AT LEAST FROM THE EAST-CENTRAL GULF COAST TO
NEAR THE SOUTHEAST/MID-ATLANTIC COAST.
THE 12Z ECMWF/GFS, LATEST GEFS/ECENS/CMCENS MEANS, AND TO SOME
DEGREE CMC RUNS PROVIDE A SIZABLE MAJORITY CLUSTER FOR THE NEXT
SYSTEM OF INTEREST. THIS EVOLUTION WOULD HAVE AN UPPER LOW
REACHING CLOSE TO THE CENTRAL WEST COAST BY EARLY WEDNESDAY AND
THE SOUTHWESTERN U.S. BY THURSDAY. THE EXPERIMENTAL 12Z
ECMWF-INITIALIZED MACHINE LEARNING MODELS DIFFER SOMEWHAT FOR
WEDNESDAY BUT MOST CONVERGE WITH THE AFOREMENTIONED MAJORITY BY
NEXT THURSDAY. THE 18Z/00Z GFS RUNS STRAYED FASTER WITH
NORTHEASTERN PACIFIC FLOW AND FASTER/SOUTHEAST WITH THE WEST
COAST/SOUTHWEST SYSTEM AND THUS WERE NOT FAVORED.
THE FIRST HALF OF THE UPDATED FORECAST STARTED WITH THE 18Z
GFS/12Z ECMWF WITH LESSER WEIGHT OF THE 12Z CMC/UKMET. BY DAYS
6-7 WEDNESDAY-THURSDAY, GFS INPUT SWITCHED TO THE 12Z RUN TO
REFLECT PREFERENCES OVER THE WEST WHILE MAINTAINING SOME WEIGHT OF
THE ECMWF/CMC. THOSE THREE RUNS ALREADY COMPARED FAIRLY WELL TO
THE ENSEMBLE MEANS SO THE LATTER PART OF THE FORECAST NEEDED ONLY
A MODEST INCLUSION OF THE 18Z GEFS/12Z ECENS TO SMOOTH THE
PROVERBIAL ROUGH EDGES.
..WEATHER/HAZARDS HIGHLIGHTS
THE WEAK SYSTEM CROSSING THE GREAT LAKES THROUGH SUNDAY MAY
PRODUCE A LITTLE LIGHT SNOW NEAR ITS PATH. THE TRAILING FRONT MAY
PRODUCE AREAS OF RAIN ACROSS THE GULF COAST/SOUTHEAST
SUNDAY-MONDAY BUT WITH CONTINUED UNCERTAINTY OVER COVERAGE/AMOUNTS
DUE TO DETAIL DIFFERENCES IN THE GUIDANCE FOR SOUTHERN STREAM
ENERGY EMERGING FROM THE SOUTHWEST AND THE ORIENTATION/TIMING OF
NORTHERN STREAM ENERGY. AT THE VERY LEAST, THIS ACTIVITY SHOULD
BE PROGRESSIVE ENOUGH TO KEEP MOST TOTALS IN THE LIGHT TO MODERATE
RANGE. ANOTHER WEAK WAVE AND TRAILING CYCLONIC FLOW MAY PRODUCE A
LITTLE LAKE EFFECT SNOW BY MID-LATE WEEK.
IT WILL BE A CLOSE CALL AS TO WHETHER A SYSTEM BRUSHING SOUTHERN
CALIFORNIA WILL BRING ANY MOISTURE TO THAT PART OF THE STATE. AS
THE UPPER TROUGH CONTINUES EASTWARD, EXPECT SOME RAIN TO SPREAD
ALONG AND NEAR THE GULF COAST BY MIDWEEK, WITH UNCERTAIN NORTHWARD
COVERAGE BY THE TIME THE MOISTURE REACHES THE EAST COAST BY
THURSDAY. CURRENTLY EXPECT MOST OF THIS MOISTURE TO STAY FAR
ENOUGH SOUTH TO MAINTAIN RAIN AS THE PRIMARY PRECIPITATION TYPE.
A BETTER DEFINED SYSTEM 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 BE OF
THE ABOVE NORMAL VARIETY ACROSS THE FAR NORTHERN TIER NEAR THE
CANADIAN BORDER WITH SOME LOCATIONS SEEING MULTIPLE DAYS WITH
READINGS 10-20F OR SO ABOVE NORMAL. MORNING LOWS WILL TEND TO BE
FARTHER ABOVE NORMAL THAN DAYTIME HIGHS. ON THE OTHER HAND, ASIDE
FROM THE SOUTHERN PLAINS BEING ABOVE NORMAL ON SUNDAY AHEAD OF A
FRONT, MOST AREAS FROM CALIFORNIA EASTWARD ACROSS THE SOUTHERN
TIER SHOULD SEE NEAR TO BELOW NORMAL HIGH TEMPERATURES DURING THE
PERIOD. LOW TEMPERATURES WILL ALSO BE BELOW NORMAL ON SOME DAYS,
SUCH AS SUNDAY MORNING WHEN READINGS IN THE LOW TO MID 30S MAY
LEAD TO FROST OR FREEZES OVER PORTIONS OF NORTHERN FLORIDA.
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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