134
FXUS02 KWBC 090701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM 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. BUT BY WEDNESDAY-THURSDAY, RETURN FLOW FROM THE
GULF OF MEXICO STREAMING INTO THE SOUTHERN HIGH PLAINS AHEAD OF A
FOUR CORNERS UPPER LOW SHOULD LEAD TO INCREASING PRECIPITATION
CHANCES THERE. THE UPPER TROUGH PUSHING EASTWARD INTO LATE 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
MODEL AND ENSEMBLE AGREEMENT REMAINS GOOD FOR THE EARLY PART OF
THE FORECAST PERIOD, SHOWING ENERGY DIGGING THROUGH THE WEST
TUESDAY SERVING TO CREATE A CLOSED LOW OVER THE GREAT BASIN TO
FOUR CORNERS WEDNESDAY-THURSDAY. MEANWHILE A RELATIVELY SHALLOW
TROUGH IS FORECAST TO PUSH THROUGH THE NORTH-CENTRAL TO
NORTHEASTERN U.S. WHILE UPPER RIDGING BUILDS IN BETWEEN THESE
FEATURES AND OVER THE WEST COAST. GIVEN THE GENERAL MODEL
AGREEMENT, A MULTI-MODEL DETERMINISTIC BLEND WAS USED FOR THE
EARLY PART OF THE PERIOD.
BY THE LATTER PART OF THE WEEK, MODEL SPREAD NOTABLY INCREASES,
STARTING WITH A SHORTWAVE THAT IS SET TO COME INTO THE PACIFIC
NORTHWEST BY THURSDAY. THIS FEATURE HAS SHOWN CONSIDERABLE
VARIABILITY IN ITS STRENGTH AND THE POTENTIAL INTERACTION WITH THE
DOWNSTREAM UPPER LOW OVER THE LAST COUPLE OF DAYS. THE 12Z ECMWF
WAS A LITTLE LESS AGGRESSIVE WITH THE SHORTWAVE THAN ITS PREVIOUS
RUNS BUT STILL DIGS THE ENERGY FARTHER WEST THAN THE GFS RUNS INTO
THE NORTHERN ROCKIES BY FRIDAY, THOUGH THE CMC SHOWS SOME HINTS OF
THIS AS WELL. SOME EC ENSEMBLE MEMBERS REFLECT SOME ASPECTS OF THE
OPERATIONAL RUN BUT THE RESULTING MEAN DOES TONE DOWN THE MORE
EXTREME ASPECTS. THE BULK OF OTHER MODEL AND ENSEMBLE GUIDANCE
(INCLUDING THE ECMWF-INITIALIZED MACHINE LEARNING MODELS) AS WELL
AS THE WPC PREFERENCE HAS BEEN LEANING AWAY FROM A SHORTWAVE THAT
IS STRONG ENOUGH TO SIGNIFICANTLY IMPACT THE TRACK OF THE
DOWNSTREAM LOW. HOWEVER, NOW THE 00Z GFS KEEPS THE SHORTWAVE AND
THE DOWNSTREAM LOW (IN WEAKER FORM) AS SEPARATE FEATURES WITHIN
THE BROADER TROUGH. THIS ALSO IMPACTS THE OTHER MAIN UNCERTAIN
ASPECT OF THE FORECAST--THE POTENTIAL FOR A SURFACE LOW TO SPIN UP
IN THE GULF OF MEXICO LATE WEEK IF IT HAS ENOUGH SUPPORT FROM
ENERGY ALOFT. THE 12Z GFS AND ECMWF RUNS WERE REASONABLY AGREEABLE
IN SHOWING THIS SURFACE LOW, SO EVEN THOUGH THIS IS A SOMEWHAT NEW
TREND, THE WPC FORECAST TRENDED TOWARD IT SLIGHTLY. BUT THE 18Z
AND 00Z GFS RUNS SHOW AN INCREASINGLY NORTHEAST POSITION OVER THE
SOUTHEASTERN U.S. WITH THE SURFACE LOW, WHICH SEEMS QUITE
AGGRESSIVE. MEANWHILE THE ENSEMBLE MEANS FROM THE VARIOUS MODEL
SUITES INDICATE A SUPPRESSED SURFACE PATTERN, BUT THE ENSEMBLE
MEMBERS ARE ALL OVER THE PLACE WITH A POTENTIAL SURFACE LOW.
CLEARLY THE MODEL DIFFERENCES ARE NOT YET RESOLVED. THE LATTER
PART OF THE WPC FORECAST QUICKLY TRANSITIONED TOWARD A MORE
ENSEMBLE MEAN-HEAVY BLEND, ESPECIALLY FROM THE GEFS MEAN, WHILE
MAINTAINING A BIT OF 12Z GFS AND CMC IN THE BLEND.
..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, 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.
PRECIPITATION SHOULD CONTINUE INTO THURSDAY THERE, BUT INCREASING
SPREAD FOR SPECIFICS OF WESTERN-CENTRAL U.S. FLOW ALOFT BY THE END
OF THE WEEK LOWERS CONFIDENCE IN HOW PRECIPITATION MAY PROGRESS
EASTWARD OVER THE PLAINS/MISSISSIPPI VALLEY ON FRIDAY AND PERHAPS
INTO THE SOUTHEAST ON SATURDAY. 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. A
MARGINAL RISK IS PLANNED FOR THE DAY 5/THURSDAY EXCESSIVE RAINFALL
OUTLOOK FOR SOUTHEASTERN FLORIDA GIVEN THESE INGREDIENTS FOR HEAVY
RAINFALL ARE IN PLACE ATOP THE SENSITIVE URBAN CORRIDOR. MORE
WIDESPREAD HEAVY RAINFALL MAY BE A CONCERN FOR LATE WEEK FOR
FLORIDA, BUT THIS IS DEPENDENT ON LOW-PREDICTABILITY IMPULSES IN
SOUTHERN STREAM FLOW AND WHETHER THEY ARE STRONG ENOUGH TO SUPPORT
A SURFACE FEATURE. FARTHER 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.
SOME COOL TEMPERATURES A FEW DEGREES BELOW NORMAL ARE FORECAST
ACROSS THE SOUTHEAST ON TUESDAY BEHIND A COLD FRONT, WITH NEAR
NORMAL TEMPERATURES FARTHER NORTH. ABOVE AVERAGE TEMPERATURES WILL
BUILD ATOP THE NORTH-CENTRAL U.S. UNDERNEATH UPPER RIDGING THERE,
LIKELY PEAKING THURSDAY ACROSS THE DAKOTAS AND THE UPPER MIDWEST
WITH TEMPERATURE ANOMALIES 15-25F ABOVE AVERAGE. SOME MILDER THAN
AVERAGE TEMPERATURES SHOULD PUSH INTO THE OHIO/TENNESSEE VALLEYS
AND THE NORTHEAST FOR THE LATTER PART OF THE WEEK BUT IN MODERATED
FORM. MOST OF THE WEST WILL TEND TO SEE NEAR TO SOMEWHAT ABOVE
NORMAL TEMPERATURES. 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).
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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