926
FXUS02 KWBC 091900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST SAT DEC 09 2023
VALID 12Z TUE DEC 12 2023 - 12Z SAT DEC 16 2023
..OVERVIEW
A DRY PATTERN WILL BE IN PLACE AT THE START OF THE MEDIUM RANGE
PERIOD TUESDAY. THEN BY WEDNESDAY-THURSDAY, RETURN FLOW FROM THE
GULF OF MEXICO STREAMING INTO THE SOUTHERN HIGH PLAINS AHEAD OF A
SOUTHWEST/FOUR CORNERS UPPER LOW SHOULD LEAD TO INCREASING
PRECIPITATION THERE. THE UPPER TROUGH PUSHING EASTWARD INTO THE
CENTRAL U.S. LATE IN THE WEEK MAY SPREAD SOME PRECIPITATION
FARTHER EAST WHILE HIGH PRESSURE BUILDS IN BEHIND. ADDITIONALLY,
MOIST EASTERLY FLOW COULD SPREAD CONVECTION INTO THE FLORIDA
PENINSULA WITH A TENDENCY TOWARD HIGHER RAINFALL TOTALS LATE IN
THE WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
A COMPOSITE OF OPERATIONAL MODEL GUIDANCE FROM THE 00Z/06Z CYCLES
PROVIDED A REASONABLE DEPICTION OF IMPORTANT FEATURES FOR ABOUT
THE FIRST HALF OF THE PERIOD (TUESDAY INTO EARLY THURSDAY). THESE
INCLUDE AN UPPER TROUGH CROSSING THE NORTHEAST QUADRANT OF THE
LOWER 48 WITH LEADING COLD FRONT, THE UPPER LOW CLOSING OFF OVER
THE SOUTHWEST/FOUR CORNERS, AND A WEAKENING FRONT THAT REACHES THE
PACIFIC NORTHWEST. SOME NORTH-SOUTH SPREAD DEVELOPS FOR THE
WESTERN UPPER LOW, WITH THE GFS/UKMET LEANING SOUTH AND CMC/ECMWF
NORTH WITH THE ENSEMBLE MEANS GENERALLY IN-BETWEEN. NEW 12Z
CMC/ECMWF RUNS LEAD TO MORE CLUSTERING ON THE SOUTHERN SIDE.
MULTIPLE FORECAST UNCERTAINTIES ARISE DURING THE LATTER HALF OF
THE PERIOD. IN ADDITION TO THE PATH/EVOLUTION OF THE WESTERN UPPER
LOW, THERE HAVE BEEN VARIED SOLUTIONS FOR THE SHORTWAVE ENERGY
REACHING THE NORTHWEST AND VICINITY BY THURSDAY AND HOW IT MAY
AFFECT THE LEADING LOW/TROUGH. THESE ISSUES PLUS DIFFERENCES IN
MORE SUBTLE DETAILS OF SOUTHERN STREAM ENERGY LEAD TO WIDENING
SPREAD FOR SURFACE EVOLUTION FROM THE GULF OF MEXICO
NORTHEASTWARD. MEANWHILE SOLUTIONS ALSO DIFFER WITHIN THE
AMPLIFYING UPPER TROUGH OVER THE EASTERN PACIFIC, AFFECTING THE
WEST COAST PATTERN BY NEXT SATURDAY.
REGARDING THE CENTRAL-EASTERN U.S. PATTERN, THE 00Z/06Z GUIDANCE
ESSENTIALLY FLIPPED FROM 24 HOURS AGO IN THAT THE GFS (INSTEAD OF
THE ECMWF/CMC) SHOWED GREATER INFLUENCE OF NORTHWESTERN
U.S./SOUTHWESTERN CANADA ENERGY ON THE CENTRAL U.S. UPPER
LOW/TROUGH LATER IN THE PERIOD. THIS LIKELY PLAYS A ROLE IN THE
GFS BEING THE MOST AGGRESSIVE WITH DEVELOPING GULF LOW PRESSURE
AND TRACKING IT NORTHEASTWARD. 00Z GUIDANCE SHOWED THE BEST
CLUSTERING AMONG THE ECMWF/CMC AND THE ENSEMBLE MEANS, WITH THE
CORE OF UPPER TROUGHING REACHING THE CENTRAL PLAINS AND A
SUPPRESSED SURFACE PATTERN WITH ONLY A HINT OF WEAK LOW PRESSURE
OVER THE GULF BY NEXT SATURDAY. MOST OF THE 00Z ECMWF-INITIALIZED
MACHINE LEARNING MODELS FELL IN LINE WITH THE 00Z
ECMWF/CMC/ENSEMBLE MEAN SCENARIO BUT WITH UNCERTAINTY OVER WHETHER
THE CENTRAL U.S. TROUGH WOULD CONSIST OF JUST THE EMERGING WESTERN
UPPER LOW OR INCLUDE ENERGY APPROACHING FROM THE NORTHWEST. OVER
THE EASTERN PACIFIC, GFS RUNS WERE IN THE MINORITY FOR PULLING OFF
A CLOSED LOW FARTHER OFFSHORE THAN CONSENSUS. THIS LED TO NORTHERN
STREAM FLOW DAMPENING THE UPPER RIDGE OVER THE NORTHWESTERN
U.S./SOUTHWESTERN CANADA MUCH MORE THAN OTHER GUIDANCE.
BASED ON THE ABOVE LATE-PERIOD COMPARISONS, THE FORECAST BLEND
BASED ON 00Z/06Z GUIDANCE QUICKLY ADJUSTED TO A BLEND OF ECMWF/CMC
MODELS WITH ENSEMBLE MEAN INPUT SPLIT AMONG THE 06Z GEFS AND 00Z
ECENS/CMCENS BY DAYS 6-7 FRIDAY-SATURDAY. THE REMAINDER OF
ARRIVING 12Z GUIDANCE HAS LOWERED CONFIDENCE CONSIDERABLY. THE
DEVELOPING SOUTHERN CLUSTERING FOR THE WESTERN UPPER LOW CONTINUES
INTO THE PLAINS TO VARYING DEGREES WHILE THE 12Z ECMWF NOW DIGS
NORTHWESTERN ENERGY INTO THE CENTRAL U.S. TROUGH. FARTHER WEST THE
ECMWF HAS SWITCHED TO THE GFS CONFIGURATION BUT THE 12Z
GEFS/CMCENS MEANS ARE NOT VERY SUPPORTIVE OF THAT IDEA YET. ON THE
OTHER HAND, THE 12Z CMC IS ON THE OPPOSITE EXTREME WITH HOW MUCH
TROUGHING EXTENDS INTO CALIFORNIA BY NEXT SATURDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE LOWER 48 WILL BE MOSTLY DRY ON TUESDAY, THOUGH WITH PERHAPS
SOME LIGHT LAKE EFFECT AND INTERIOR NORTHEAST SNOW. BY LATER
TUESDAY NIGHT INTO WEDNESDAY-THURSDAY, CHANCES FOR PRECIPITATION
SHOULD INCREASE ACROSS THE SOUTHERN HIGH PLAINS AS AN UPPER LOW
SETTLES OVER THE SOUTHWEST/FOUR CORNERS AND MOISTURE INTERACTS
WITH A FRONT. SOME OF THIS PRECIPITATION MAY BE SNOW FROM THE
SOUTHERN ROCKIES EASTWARD, WITH TYPICAL ELEVATION/COVERAGE
UNCERTAINTY. DETAILS OF PRECIPITATION COVERAGE/AMOUNTS/TYPE WILL
DEPEND ON THE UPPER LOW TRACK, WHICH AT THIS TIME GUIDANCE
SUGGESTS COULD BE ANYWHERE OVER THE CENTRAL OR SOUTHERN PLAINS BY
FRIDAY. IN THE CASE OF BOTH SNOW AND RAIN AREAS, SOME TOTALS COULD
BE LOCALLY HEAVY. 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.
THE SOUTHERN ROCKIES/HIGH PLAINS SHOULD TREND DRIER WHILE RAINFALL
BRIEFLY INCREASES NEAR THE WESTERN GULF COAST TOWARD THE END OF
THE WEEK AS THE SYSTEM'S MOISTURE ADVANCES EASTWARD.
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 DAY 5/THURSDAY EXCESSIVE RAINFALL
OUTLOOK MAINTAINS 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 NEXT
FRIDAY-SATURDAY, GIVEN THE COMBINED SPREAD FOR UPPER LEVEL
TROUGHING EXPECTED TO REACH THE CENTRAL U.S. (ANCHORED BY THE
EMERGING WESTERN UPPER LOW BUT POSSIBLY CONTAINING ENERGY
APPROACHING FROM THE NORTHWEST) AND SOUTHERN STREAM IMPULSES ALOFT
ALONG WITH HOW THESE FEATURES MAY INFLUENCE THE SURFACE PATTERN.
THE MOST LIKELY SCENARIO WOULD HAVE SOME RAINFALL EXTENDING EAST
THROUGH THE MISSISSIPPI VALLEY AND BY THE WEEKEND LIFTING
NORTHWARD OVER THE SOUTHEAST BUT WITH LOWER TOTALS/COVERAGE THAN
DEPICTED IN SOME RECENT GFS/GEFS RUNS. THE FAVORED PATTERN WOULD
MAINTAIN A POTENTIAL FOR AREAS OF HEAVY RAINFALL NEAR THE EASTERN
COAST OF FLORIDA BEYOND WEDNESDAY.
OVER THE WEST, A WEAKENING FRONT MAY BRING SOME LIGHT
PRECIPITATION TO THE PACIFIC NORTHWEST AROUND MIDWEEK OR SO, WITH
ONLY LIGHT/SCATTERED ACTIVITY EXTENDING FARTHER INLAND BY THURSDAY
IF AT ALL. INCREASING PRECIPITATION CHANCES ARE THEN POSSIBLE FOR
CALIFORNIA BY SATURDAY AS AN EASTERN PACIFIC LOW PRESSURE SYSTEM
EDGES CLOSER TO THE COAST. A MINORITY SCENARIO WOULD KEEP THE
PACIFIC SYSTEM FAR ENOUGH WEST OF CALIFORNIA TO KEEP THAT REGION
DRY WHILE SOME MOISTURE REACHES THE PACIFIC NORTHWEST.
THE MOST ANOMALOUS TEMPERATURES DURING THE PERIOD WILL BE OF THE
WARM VARIETY ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST FROM
WEDNESDAY ONWARD. MORE MODERATE WARMTH SHOULD ALSO EXTEND INTO THE
NORTHERN HALF TO TWO-THIRDS OF THE EAST LATER IN THE WEEK. EXPECT
DECENT COVERAGE OF TEMPERATURES 10F OR MORE ABOVE NORMAL AND
PORTIONS OF THE DAKOTAS INTO UPPER MIDWEST MAY BE 15-25F ABOVE
NORMAL THURSDAY INTO EARLY FRIDAY. MEANWHILE CLOUDS/PRECIPITATION
OVER AND NEAR THE SOUTHERN HIGH PLAINS MID-LATE WEEK WILL SUPPORT
A FAIRLY NARROW TEMPERATURE RANGE (ABOVE NORMAL LOWS, BELOW NORMAL
DAYTIME HIGHS). MUCH OF THE WEST WILL LIKELY SEE NEAR TO SLIGHTLY
ABOVE NORMAL TEMPERATURES DURING THE PERIOD.
RAUSCH/TATE
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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