932
FXUS02 KWBC 101859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST SUN DEC 10 2023
VALID 12Z WED DEC 13 2023 - 12Z SUN DEC 17 2023
..AN EMERGING WET PATTERN OVER THE SOUTH/SOUTHEAST
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
MULTIPLE ASPECTS OF THE FORECAST CONTINUE TO BE IN A STATE OF
FLUX, WITH GUIDANCE VARYING CONSIDERABLY FOR PARTICULAR
FEATURES/EVOLUTIONS OR RAPIDLY SHIFTING THEIR CLUSTERING. AHEAD OF
THE SYSTEMS LEADING TO THE ONGOING UNCERTAINTIES, A FAIRLY STRONG
UPPER TROUGH CROSSING EASTERN CANADA AND THE NORTHEASTERN QUADRANT
OF THE LOWER 48 WEDNESDAY-THURSDAY WILL PUSH A LEADING COLD FRONT
INTO THE ATLANTIC. THEN A PROMINENT FEATURE OF INTEREST THROUGH
THE PERIOD WILL BE A CLOSED LOW EXPECTED TO FORM OVER THE
SOUTHWEST BY THE START OF THE PERIOD EARLY WEDNESDAY AND MOST
LIKELY TRACK INTO THE SOUTHERN HIGH PLAINS BY FRIDAY. ALTHOUGH
CLUSTERING HAS BEEN IMPROVING OVER THE PAST 24-36 HOURS, THERE IS
STILL QUITE A BIT OF SPREAD BY EARLY SATURDAY WITH THE 12Z UKMET
QUITE SLOW AND ECMWF RUNS ALTERNATING BETWEEN THE SOUTHERN (00Z
RUN) AND NORTHERN (NEW 12Z RUN) SIDE OF THE SPREAD. IMPLIED
ENSEMBLE MEAN UPPER LOW TRACKS WOULD ALSO BE ON THE NORTHERN SIDE.
MEANWHILE FARTHER UPSTREAM THE GUIDANCE HAS RAPIDLY CONSOLIDATED
TOWARD SEPARATION OF EASTERN PACIFIC UPPER TROUGHING THAT WAS
FIRST ADVERTISED BY THE OPERATIONAL GFS. THIS HAS LED TO AN
EASTERN PACIFIC UPPER LOW FAR ENOUGH OFFSHORE TO BRING MINIMAL
PRECIPITATION TO CALIFORNIA, WHILE LATEST TRENDS WITH MORE
PROGRESSIVE NORTHERN STREAM FLOW ARE FOR THE SHORTWAVE ENERGY TO
PUT LESS OF A DENT IN THE NORTHERN PART OF THE WESTERN U.S. MEAN
RIDGE THAN THE FIRST SOLUTIONS THAT DEPICTED THE SPLITTING TROUGH.
SHEARED SHORTWAVE ENERGY REACHING THE PACIFIC NORTHWEST AND
SOUTHWESTERN CANADA BY THURSDAY, WHICH HAS BEEN A PROBLEM IN THE
FORECAST FOR DAYS, CONTINUES TO PLAY A POTENTIAL ROLE IN A
MINORITY OF GUIDANCE. NOW IT'S THE GFS/GEFS THAT ARE GENERALLY ON
THEIR OWN WITH THE DEGREE TO WHICH THIS ENERGY AMPLIFIES OVER/EAST
OF THE MISSISSIPPI VALLEY BY NEXT WEEKEND. THE GFS/GEFS EVOLUTION
LEADS TO NORTHERN STREAM DYNAMICS PULLING GULF OF MEXICO/FLORIDA
LOW PRESSURE WELL NORTHWARD NEAR THE EAST COAST COMPARED TO THE
REMAINING MAJORITY THAT IS MORE SUPPRESSED DUE TO THE SLOWER
SOUTHERN STREAM FEATURE PROVIDING PRIMARY DYNAMIC SUPPORT FOR LOW
PRESSURE. 00Z ECMWF-INITIALIZED MACHINE LEARNING MODELS ALSO FAVOR
A FAIRLY SUPPRESSED SOLUTION, WITH POTENTIAL LOW PRESSURE
POSITIONS AS OF EARLY NEXT SUNDAY RANGING BETWEEN THE CENTRAL GULF
AND JUST EAST OF THE FLORIDA PENINSULA. GFS TRENDS OVER THE PAST
COUPLE DAYS HAVE BEEN SLOWER WITH NORTHWARD PROGRESSION OF THE
SURFACE LOW THOUGH.
BASED ON THE LATEST ARRAY OF GUIDANCE AND TRENDS, AN OPERATIONAL
BLEND OF 00Z/12Z MODEL GUIDANCE PROVIDED A REASONABLE STARTING
POINT EARLY IN THE PERIOD. THEN PREFERENCE ADJUSTED TO A
MODEL/MEAN BLEND THAT TILTED CONSIDERABLY MORE TO THE ECMWF/CMC
AND THEIR ENSEMBLE MEANS RELATIVE TO THE GFS/GEFS (WITH THE 00Z
GFS A LITTLE LESS EXTREME THAN THE 06Z RUN). NOTE THAT NBM QPF
TOTALS WERE CONSIDERABLY REDUCED LATE IN THE PERIOD ALONG PARTS OF
THE EAST COAST, THOUGH NORTHERN EXTENT OF LIGHT MEASURABLE TOTALS
IS LIKELY STILL TOO GREAT. MEANWHILE ADJUSTMENTS WERE ALSO MADE
TO SLOW THE ADVANCE OF RAINFALL TOWARD CALIFORNIA, IN A PARTIAL
TREND TO OPERATIONAL MODEL CLUSTERING.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER LOW EXPECTED TO FORM OVER THE SOUTHWEST BY EARLY
WEDNESDAY AND THEN TRACK INTO THE SOUTHERN HIGH PLAINS, COMBINED
WITH THE INLAND RETURN OF GULF MOISTURE INTERACTING WITH A WAVY
LEAD FRONT, WILL LIKELY GENERATE AN AREA OF HEAVY PRECIPITATION
OVER THE SOUTHERN ROCKIES/HIGH PLAINS MID-LATE WEEK. CURRENTLY THE
BEST POTENTIAL FOR HEAVIEST SNOW EXTENDS FROM NORTHERN-EASTERN NEW
MEXICO/SOUTHEASTERN COLORADO INTO FAR NORTHWESTERN TEXAS. WITHIN
THE AREA FORECAST TO SEE RAIN, MOST OF THE SOUTHERN HIGH PLAINS
REGION HAS FAIRLY DRY ANTECEDENT CONDITIONS AND SHORT-TERM
RAINFALL RATES DO NOT APPEAR TO BE HIGH ENOUGH TO MERIT
SIGNIFICANT RUNOFF CONCERNS THROUGH THE WEDNESDAY. THUS THE DAY
4/WEDNESDAY EXCESSIVE RAINFALL OUTLOOK CONTINUES TO SHOW NO RISK
AREAS. HOWEVER, BY DAY 5/THURSDAY THE GUIDANCE SIGNALS FOR HEAVY
RAINFALL POTENTIAL HAVE BECOME MORE PRONOUNCED WITH IMPROVED
(THOUGH NOT YET IDEAL) CLUSTERING FOR UPPER LOW TRACK. THE
COMBINATION OF POTENTIALLY WETTER GROUND CONDITIONS TO START DAY 5
ALONG WITH THE 00Z ECMWF SHOWING SOME INSTABILITY AND GUIDANCE
OVERALL SHOWING SOME LONGEVITY OF LOW LEVEL UPSLOPE FLOW, THE DAY
5 ERO UPDATE PROPOSES AN EXPANDED MARGINAL RISK AREA CENTERED OVER
WEST-CENTRAL TEXAS. DEPENDING ON THE PATH OF THE UPPER LOW, HEAVY
RAINFALL COULD EXTEND FARTHER EAST OVER TEXAS INTO FRIDAY-FRIDAY
NIGHT.
ACROSS FLORIDA, EXPECT EASTERLY FLOW TO SET UP SOUTH OF STRONG
EASTERN U.S. HIGH PRESSURE WHILE MOISTURE AND INSTABILITY INCREASE
FOR MIDWEEK AND BEYOND. THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS
FOR WEDNESDAY-THURSDAY MAINTAIN A MARGINAL RISK AREA FOR
SOUTHEASTERN FLORIDA GIVEN THESE INGREDIENTS FOR HEAVY RAINFALL
ARE IN PLACE ACROSS THE SENSITIVE URBAN CORRIDOR. THERE IS STILL
UNCERTAINTY OVER THE MAGNITUDE AND NORTHWARD EXTENT OF
INSTABILITY/DEEP MOISTURE THOUGH. THE SENSIBLE WEATHER FORECAST
OVER A BROADER PORTION OF THE SOUTH/EAST BECOMES INCREASINGLY
UNCERTAIN TOWARD FRIDAY AND NEXT WEEKEND, GIVEN THE COMBINED
SPREAD FOR THE ULTIMATE PATH OF THE UPPER LOW OVER THE SOUTHERN
HIGH PLAINS ON FRIDAY AND EVOLUTION OF NORTHERN STREAM FLOW. THE
MOST LIKELY SCENARIO WOULD HAVE MOISTURE SPREADING FARTHER NORTH
OVER THE EASTERN U.S. BUT WITH LOW CONFIDENCE REGARDING NORTHWARD
EXTENT AND TOTALS. AT THE VERY LEAST, TRENDS CONTINUE TO FAVOR A
SLOWER NORTHWARD PROGRESSION OF MOISTURE THAN RECENT GFS/GEFS RUNS.
OVER THE WEST, A WEAKENING FRONT MAY BRING SOME LIGHT
PRECIPITATION TO THE PACIFIC NORTHWEST AROUND MIDWEEK OR SO, WITH
SOME LIGHT/SCATTERED ACTIVITY POSSIBLY EXTENDING INTO THURSDAY.
THEREAFTER, NORTHERN PARTS OF THE PACIFIC NORTHWEST COULD SEE
ADDITIONAL PRECIPITATION WITH ANOTHER FRONT WHILE LATEST TRENDS
ARE GRADUALLY INCREASING CONFIDENCE IN THE IDEA THAT THE UPPER LOW
OVER THE EASTERN PACIFIC SHOULD REMAIN FAR ENOUGH OFFSHORE TO KEEP
MOST OF CALIFORNIA DRY THROUGH NEXT SUNDAY.
MUCH OF THE NORTHERN TIER OF THE LOWER 48 EAST OF THE ROCKIES
SHOULD SEE ABOVE NORMAL TEMPERATURES DURING THE PERIOD. EXPECT
FAIRLY BROAD COVERAGE OF PLUS 10F OR GREATER ANOMALIES WHICH COULD
SPREAD GRADUALLY FARTHER SOUTH BY THE WEEKEND, WITH WARMEST
READINGS (15F OR MORE ABOVE NORMAL) TENDING TO BE CENTERED OVER
THE DAKOTAS INTO UPPER MIDWEST THURSDAY-SATURDAY. ALSO MEAN
RIDGING ALOFT OVER THE WEST WILL FAVOR MODERATELY ABOVE NORMAL
TEMPERATURES AND A VERY GRADUAL WARMING TREND WITH TIME. IN
CONTRAST, CLOUDS/PRECIPITATION DURING THE LATTER HALF OF THE WEEK
WILL PRODUCE CHILLY HIGHS OVER THE SOUTHERN HIGH PLAINS DURING THE
LATTER HALF OF THE WEEK. TEMPERATURES THERE SHOULD REBOUND CLOSER
TO NORMAL NEXT WEEKEND. AREAS NEAR THE EAST COAST SHOULD SEE A DAY
OR SO OF BELOW NORMAL HIGHS LATE THIS WEEK AS STRONG AND COOL HIGH
PRESSURE BUILDS IN BEHIND A COLD FRONT. HIGHS OVER NORTHERN PARTS
OF THE EAST SHOULD TREND ABOVE NORMAL FRIDAY INTO THE WEEKEND
WHILE THE SOUTH STAYS NEAR NORMAL (BUT WITH FLORIDA ABOVE NORMAL
FOR LOWS DUE TO PERSISTENT CLOUDS/RAINFALL).
RAUSCH/SCHICHTEL
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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