559
FXUS02 KWBC 270700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST WED DEC 27 2023
VALID 12Z SAT DEC 30 2023 - 12Z WED JAN 03 2024
...LOCALLY HEAVY PRECIPITATION POSSIBLE FOR NORTHERN CALIFORNIA
LATE THIS WEEK TRENDING LIGHTER SATURDAY...
..OVERVIEW
GUIDANCE CONTINUES TO SHOW MEAN TROUGHING OVER THE EASTERN U.S.
INTO EARLY NEXT WEEK IN RESPONSE TO UPSTREAM RIDGING. WITHIN THIS
TROUGH, SOME COMBINATION OF SEPARATE ENERGIES AS OF SATURDAY
SHOULD DEVELOP LOW PRESSURE OVER/OFFSHORE THE NEW ENGLAND COAST,
FOLLOWED BY A SHORTWAVE DROPPING SOUTHEASTWARD FROM THE UPPER
MIDWEST AND LIKELY CROSSING THE EASTERN U.S. BY EARLY NEXT WEEK.
BEHIND THIS LATTER SYSTEM, AN EMERGING SEQUENCE OF PACIFIC
FEATURES SHOULD LEAD INTO WHAT APPEARS TO BE A MORE PROGRESSIVE
PATTERN OVER AN INCREASING PORTION OF THE LOWER 48. A DECENT
PORTION OF THE COUNTRY SHOULD BE DRY DURING THE PERIOD, WITH BEST
PRECIPITATION FOCUS ALONG THE WEST COAST ON SATURDAY AND NEXT
WEDNESDAY WHILE COVERAGE/TOTALS ARE UNCERTAIN OVER THE EASTERN
U.S. EARLY NEXT WEEK. THE SOUTHEAST WILL TEND TO SEE VARYING
COVERAGE OF BELOW NORMAL TEMPERATURES OVER THE COURSE OF THE
PERIOD WHILE MOST OTHER AREAS OF THE LOWER 48 SHOULD SEE NEAR TO
ABOVE NORMAL READINGS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST GUIDANCE CONTINUES TO DIFFER FOR THE DETAILS OF BOTH
PRIMARY FEATURES WITHIN THE EASTERN U.S. UPPER TROUGH. IDEAS
STILL VARY FOR THE COMBINATION OF EASTERN CANADA/NEW ENGLAND
ENERGY AND FARTHER SOUTH FOR SPECIFICS OF EJECTING ENERGY FROM THE
SHORT-TERM UPPER LOW OVER THE EAST-CENTRAL U.S. FORTUNATELY, EVEN
WITH THIS DETAIL SPREAD THERE IS A LOOSE CONSENSUS TOWARD SURFACE
LOW PRESSURE FORMING ALONG/OFFSHORE THE NEW ENGLAND COAST AS OF
THE START OF THE PERIOD EARLY SATURDAY AND CONTINUING INTO THE
CANADIAN MARITIMES THEREAFTER. FOR THE NEXT TROUGH PROGRESSING
FROM THE NORTHERN TIER INTO THE EAST, RECENT GFS RUNS HAVE BEEN
MORE CLOSED AND SLOWER THAN MOST OTHER GUIDANCE INCLUDING
OPERATIONAL MODELS, ENSEMBLE MEANS, AND ECMWF-INITIALIZED MACHINE
LEARNING MODELS (WHICH HAVE LEANED ON THE FASTEST SIDE OF THE FULL
GUIDANCE SPREAD). SOME CMC RUNS HAD BEEN ON THE FAST SIDE TOO BUT
THE NEW 00Z VERSION HAS COME BACK TO MATCH THE MAJORITY CLUSTER
WITH A TROUGH AXIS NEAR THE EAST COAST AS OF EARLY TUESDAY.
PREFERENCE CONTINUES TO SIDE WITH THE NON-GFS GROUP AS LONG AS
SUCH A PRONOUNCED CONSENSUS EXISTS.
FARTHER WEST, GUIDANCE HAS STABILIZED FAIRLY WELL FOR THE
WEAKENING SHORTWAVE REACHING THE WEST COAST BY SATURDAY. MOST OF
THE TROUGH'S ENERGY THAT PROGRESSES EASTWARD SHOULD CROSS THE
SOUTHWEST AND THEN STREAM UNDERNEATH THE AMPLIFYING NORTHERN
STREAM TROUGH OVER THE CENTRAL-EASTERN U.S. THE NEXT UPPER TROUGH
IN THE SERIES SHOULD REACH THE SOUTHWESTERN U.S. BY AROUND MONDAY
AND THE SOUTHERN PLAINS BY WEDNESDAY, WITH THE LATEST GFS/CMC RUNS
AND THE ENSEMBLE MEANS PROVIDING THE BEST STARTING POINT AS A
MODERATELY PROGRESSIVE SCENARIO HAS HELD UP BEST. THE 12Z ECMWF
STRAYS SLIGHTLY SLOWER AT TIMES WHILE THE 00Z UKMET REMAINS A
LITTLE SLOW TOO, ALBEIT FASTER THAN ITS 12Z RUN. YET ANOTHER
UPPER TROUGH, THIS ONE WITH A BETTER DEFINED SURFACE SYSTEM SHOULD
APPROACH BY EARLY WEDNESDAY. RECENT ECMWF RUNS/12Z ECENS MEAN,
THE 12Z/18Z GFS, AND CMC RUNS (THOUGH MORE AMPLIFIED WITH THE
UPPER TROUGH) WOULD HAVE A CLOSED UPPER LOW OFF THE
CENTRAL/NORTH-CENTRAL WEST COAST AT THAT TIME. 12Z
ECMWF-INITIALIZED MACHINE LEARNING MODELS FAVOR AN UPPER LOW OFF
THE COAST A LITTLE NORTH OF 40N LATITUDE. THE NEW 00Z GFS STRAYS
NORTH OF MOST SOLUTIONS.
GUIDANCE COMPARISONS AMONG THE 12Z/18Z CYCLES FAVORED AN
OPERATIONAL MODEL COMPOSITE TO START THE FORECAST ON DAY 3
SATURDAY. THEN THE BLEND FIRST PHASED OUT THE UKMET DUE TO
DIVERGING FROM CONSENSUS OVER MOST AREAS, AND THEN THE GFS DUE TO
ITS SLOW EASTERN U.S. SYSTEM. THIS LED TO A MID-LATE PERIOD
WEIGHTING OF ABOUT HALF 18Z GEFS/12Z ECENS MEANS AND THE REST
CONSISTING OF THE 12Z ECMWF/CMC.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER TROUGH NEAR THE EAST COAST ON SATURDAY MAY PRODUCE SOME
LINGERING AREAS OF LIGHT PRECIPITATION FROM THE APPALACHIANS INTO
NEW ENGLAND, WITH BEST POTENTIAL FOR SNOW IN THE APPALACHIANS AND
NORTHERN NEW ENGLAND. THE WEAK SYSTEM CROSSING THE GREAT LAKES
THIS WEEKEND MAY PRODUCE A LITTLE LIGHT SNOW ALONG ITS PATH AND
PERHAPS A LITTLE RAIN JUST TO THE SOUTH. RAINFALL TOTALS ARE MORE
UNCERTAIN OVER THE GULF COAST AND SOUTHEAST, WITH SENSITIVITY TO
DETAILS OF SOUTHERN STREAM ENERGY EMERGING FROM THE SOUTHWEST AND
THE ORIENTATION/TIMING OF THE NORTHERN STREAM TROUGH. EVEN AMONG
THE FAVORED GUIDANCE CLUSTER FOR MASS FIELDS, AMOUNTS THERE COULD
RANGE ANYWHERE FROM FAIRLY LIGHT TO LOCALLY HEAVY. THIS SCENARIO
WOULD KEEP THE MIDDLE LATITUDES OF THE EAST DRIER THAN RECENT GFS
RUNS THOUGH.
THE WEST COAST STATES WILL LIKELY SEE RAIN AND HIGH ELEVATION SNOW
CONTINUE INTO SATURDAY WITH THE WEAKENING SHORTWAVE/SURFACE FRONT
REACHING THE REGION, BUT WITH TOTALS TRENDING LIGHTER COMPARED TO
FRIDAY. BEST FOCUS ON SATURDAY SHOULD BE OVER THE SIERRA NEVADA
AND SOUTHERN CALIFORNIA COASTAL RANGES. MOST OF ANY PRECIPITATION
MAKING ITS WAY FARTHER INTO THE WEST SHOULD BE ONLY LIGHT AND
SCATTERED. ANOTHER UPPER TROUGH MAY BRING SOME MOSTLY LIGHT
PRECIPITATION INTO CALIFORNIA BUT CONFIDENCE IS LOWER IN
SPECIFICS, WITH SOME GUIDANCE KEEPING MOST MOISTURE FARTHER
SOUTHWEST. EASTWARD PROGRESSION OF THIS TROUGH MAY HELP TO
DEVELOP RAINFALL NEAR THE GULF COAST BY NEXT WEDNESDAY. ALSO AT
THAT TIME A POTENTIALLY BETTER DEFINED SYSTEM MAY BRING ORGANIZED
PRECIPITATION TO THE WEST COAST, WITH BEST FOCUS CURRENTLY
EXPECTED TO BE OVER CENTRAL AND SOUTHERN AREAS.
CHILLY TEMPERATURES WILL LIKELY CONTINUE INTO THE WEEKEND OVER THE
SOUTHEAST, WITH FLORIDA SEEING THE BEST POTENTIAL FOR SOME
READINGS AT LEAST 10F BELOW NORMAL. LOW TEMPERATURES IN THE LOW
TO MID 30S MAY LEAD TO FROST OR FREEZES OVER PORTIONS OF NORTHERN
FLORIDA SATURDAY AND SUNDAY MORNING. A MODERATING TREND SHOULD
CONFINE NEGATIVE ANOMALIES JUST TO FLORIDA BY SUNDAY-MONDAY. A
FRONTAL PASSAGE EARLY NEXT WEEK MAY LOWER TEMPERATURES AGAIN OVER
THE SOUTH FOR NEXT TUESDAY INTO EARLY WEDNESDAY. IN CONTRAST,
PARTS OF THE FAR NORTHERN PLAINS AND UPPER MIDWEST (EXCLUDING
NORTH-CENTRAL PLAINS LOCATIONS THAT MAY STILL HAVE SNOW COVER)
WILL BE CONSISTENTLY ABOVE NORMAL WITH MULTIPLE DAYS OF PLUS 10F
OR GREATER ANOMALIES. ALSO THE NORTHEAST SHOULD SEE ABOVE NORMAL
MORNING LOWS THROUGH THE PERIOD. MOST OF THE WEST WILL SEE
MODERATELY ABOVE NORMAL READINGS DURING THE PERIOD BUT WITH A
GRADUAL COOLING TREND AS THE WESTERN U.S. MEAN RIDGE ALOFT WEAKENS
IN FAVOR OF PACIFIC SHORTWAVE TROUGHS. PARTS OF CALIFORNIA MAY
SEE SLIGHTLY BELOW NORMAL HIGHS. THE SOUTHERN PLAINS WILL BE ON
THE WARM SIDE DURING THE WEEKEND AND THEN TREND TO NEAR OR
SLIGHTLY BELOW NORMAL TEMPERATURES AFTER A FRONTAL PASSAGE.
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page