040
FXUS02 KWBC 110711
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
211 AM EST MON DEC 11 2023
VALID 12Z THU DEC 14 2023 - 12Z MON DEC 18 2023
..AN EMERGING WET PATTERN OVER THE SOUTH/SOUTHEAST
..OVERVIEW
A SOUTHERN STREAM UPPER LOW TRAVERSING THE SOUTHERN U.S. DURING
THE MEDIUM RANGE PERIOD WILL BE A PRIMARY DRIVER OF THE SENSIBLE
WEATHER, INCLUDING RAIN AND SNOW IN THE SOUTHERN PLAINS LATE THIS
WEEK, WITH RAIN POTENTIALLY SPREADING INTO THE SOUTHEAST INTO THE
WEEKEND AND EARLY NEXT WEEK THOUGH WITH MORE UNCERTAINTY AS MODELS
STRUGGLE TO AGREE ON A POSSIBLE GULF SURFACE LOW FORMING. AHEAD OF
THIS POSSIBILITY, MOIST EASTERLY FLOW INTO SOUTHERN FLORIDA COULD
CAUSE LOCALIZED HEAVY RAIN. FAIRLY PROGRESSIVE MID-UPPER FLOW IN
THE NORTHERN STREAM IS LIKELY WITH A SERIES OF SHALLOW TROUGHS
MOVING THROUGH THE NORTH-CENTRAL AND NORTHEASTERN U.S., WHILE MEAN
RIDGING BUILDS IN THE WEST AHEAD OF EASTERN PACIFIC TROUGHING.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE FIRST SYSTEM OF NOTE FOR MODEL DIAGNOSTIC PURPOSES IS AN UPPER
LOW LIKELY NEAR THE ARIZONA/NEW MEXICO BORDER AS THE PERIOD BEGINS
EARLY THURSDAY. RECENT MODELS START OUT WITH REASONABLY GOOD
AGREEMENT WITH ITS POSITION BUT SHOW INCREASING SPREAD WITH TIME.
THE 12Z UKMET WAS THE FIRST TO DIVERGE FROM CONSENSUS WITH A
SLOWER UPPER LOW TRACK THAN OTHER GUIDANCE BY FRIDAY, AND THUS WAS
NOT PREFERRED IN THIS FORECAST. A COMPLICATION WITH THE WAY THIS
UPPER LOW EVOLVES IS WITH A NORTHERN STREAM SHORTWAVE UPSTREAM
THAT SHOULD MOVE NEAR THE U.S./CANADIAN BORDER THURSDAY-FRIDAY AND
THEN POTENTIALLY DIVE SOUTHEASTWARD INTO THE EAST-CENTRAL U.S.
OVER THE WEEKEND. THE STRENGTH AND TRACK OF THIS SHORTWAVE WILL
HAVE IMPLICATIONS DOWNSTREAM. NAMELY, THE GFS/GEFS SUITE LATELY
HAS BEEN INTERACTING/COMBINING THE PREEXISTING SOUTHERN STREAM
UPPER LOW AND THIS SHORTWAVE, WHICH FORMS AN EAST-CENTRAL U.S.
TROUGH THAT SUPPORTS A STRONG AND FASTER MOVING SURFACE LOW IN THE
GULF AND PROGRESSING NORTHEASTWARD. THIS IS OPPOSED TO THE
REMAINING MAJORITY THAT KEEPS THE NORTHERN STREAM SHORTWAVE
PROGRESSIVE AND NORTH OF THE SOUTHERN STREAM UPPER LOW. WITHOUT
THE PHASING OF THESE FEATURES, THE SOUTHERN STREAM UPPER LOW WILL
BE SLOWER AND THE ONLY DYNAMICAL SUPPORT FOR SURFACE LOW PRESSURE,
SO THE SURFACE LOW FORMS/STRENGTHENS LATER AND STAYS MORE
SUPPRESSED. WPC HAS BEEN GENERALLY FAVORING THE MORE SUPPRESSED
AND SLOWER TRACK GIVEN IT IS THE LARGER CLUSTER WITH MULTIPLE
MODEL SUITES (THE ECMWF AND CMC). ENSEMBLE MEMBERS TEND TO FAVOR
WITH THEIR OPERATIONAL RUNS BUT DO SHOW A LOT OF SPREAD.
ELSEWHERE, UPPER RIDGING SEEMS WELL PREDICTED IN THE WEST BEHIND
THESE FEATURES AND AHEAD OF LARGE SCALE EASTERN PACIFIC TROUGHING.
THE TIMING OF THE TROUGHING EDGING TOWARD THE WEST COAST EARLY
NEXT WEEK STILL SHOWS SOME SPREAD BUT NOTHING TOO EGREGIOUS FOR
THE DAYS 6-7 TIMEFRAME.
THUS THE WPC FORECAST STARTED WITH A BLEND OF THE 18Z GFS, 12Z
ECMWF, AND 12Z CMC EARLY IN THE FORECAST PERIOD, BUT QUICKLY
REDUCED THE AMOUNT OF THE GFS IN FAVOR OF THE ENSEMBLE MEAN
GUIDANCE. THE 12Z CMC ACTUALLY SEEMED TO BE A REASONABLE MIDDLE
GROUND FOR THE TRACK AND TIMING OF THE GULF LOW COMPARED TO THE
FAST GFS AND THE ECMWF THAT BECAME PRETTY SLOW WITH LINGERING ITS
UPPER AND SURFACE LOW FEATURES. TRANSITIONED TO USING HALF THE EC
AND GEFS ENSEMBLE MEANS BY DAYS 6-7 GIVEN THE INCREASING SPREAD.
REGARDING QPF, THE NBM SEEMED TO FAVOR A MORE GFS/GEFS-LIKE LOW
TRACK WITH ITS PRECIPITATION FORECAST. SINCE THE PREFERENCE WAS
FOR A MORE SUPPRESSED AND SLOWER SOLUTION WITH THE LOW, TONED DOWN
THE HIGH TOTALS IT SHOWED OVER FLORIDA LATE WEEK.
..WEATHER/HAZARDS HIGHLIGHTS
THE SOUTHERN STREAM UPPER LOW TRACKING ACROSS THE SOUTHWEST TO THE
SOUTH-CENTRAL U.S. LATE WEEK COMBINED WITH AMPLE MOISTURE WILL
PROVIDE SUPPORT FOR WIDESPREAD RAIN IN TEXAS. MARGINAL RISKS ARE
IN PLACE IN THE EXCESSIVE RAINFALL OUTLOOK FOR THURSDAY AND FRIDAY
AS UPPER-LEVEL DYNAMICAL SUPPORT WILL BE GOOD AND PRECIPITABLE
WATER ANOMALIES ARE LIKELY TO BE NEAR/OVER THE 95TH PERCENTILE.
INSTABILITY MAY BE SOMEWHAT MEAGER, BUT WITH THESE FACTORS AND
POTENTIALLY WETTER GROUND FROM RAIN ON WEDNESDAY, SOME FLOODING
RISK SEEMS POSSIBLE. ON THE WESTERN SIDE OF THE PRECIPITATION
SHIELD, SNOW IS LIKELY FROM THE SOUTHERN ROCKIES INTO THE SOUTHERN
HIGH PLAINS INCLUDING MUCH OF THE EASTERN HALF OF NEW MEXICO AND
INTO THE TEXAS AND OKLAHOMA PANHANDLES. MEANWHILE ON THURSDAY,
MOIST EASTERLY FLOW WILL LIKELY PERSIST INTO SOUTHEASTERN FLORIDA
WITH SOME INSTABILITY IN PLACE, SO A MARGINAL RISK PERSISTS FOR
THAT AREA FROM WEDNESDAY/DAY 3, ESPECIALLY FOR THE SENSITIVE URBAN
CORRIDOR. BY FRIDAY AND BEYOND, CONFIDENCE WITH THE EXTENT OF
PRECIPITATION RAPIDLY LOWERS AS MODEL GUIDANCE DIVERGES. SOME
MODEL GUIDANCE SHOWS WIDESPREAD HEAVY RAIN OVER FLORIDA AND THE
SOUTHEAST BY LATE WEEK, BUT THE MORE FAVORED/LARGER CLUSTER OF
GUIDANCE MAINTAINS A FOCUS FOR HEAVY RAINFALL IN THE SOUTHERN PART
OF THE STATE (OR EVEN PERHAPS FULLY SOUTH OF FLORIDA) TO THE SOUTH
OF HIGH PRESSURE AT THE SURFACE. THE WPC FORECAST SHOWS A MARGINAL
RISK FOR FAR SOUTHERN FLORIDA INTO THE KEYS ON FRIDAY AS THERE IS
POTENTIAL FOR AMPLE MOISTURE TO POOL THERE, BUT THIS WILL HAVE TO
BE REFINED IN FUTURE FORECASTS DEPENDING ON MODEL TRENDS. FOR THE
WEEKEND, SOME RAIN LOOKS TO SPREAD INTO THE SOUTHEAST, BUT THERE
IS HIGH UNCERTAINTY PARTICULARLY WITH HOW FAR NORTH THE
PRECIPITATION COULD SPREAD. AT THE VERY LEAST, TRENDS CONTINUE TO
FAVOR A SLOWER NORTHWARD PROGRESSION OF MOISTURE THAN RECENT
GFS/GEFS RUNS.
ELSEWHERE, MAINLY DRY CONDITIONS ARE FORECAST THROUGH LATE WEEK,
OTHER THAN SOME LIGHT SHOWERS IN THE UPPER MIDWEST/UPPER GREAT
LAKES AND SOME PACIFIC NORTHWEST PRECIPITATION. BY EARLY NEXT
WEEK, EASTERN PACIFIC UPPER TROUGHING COULD SNEAK ENERGY INTO
CALIFORNIA AND SPREAD PRECIPITATION THERE, BUT THERE REMAINS
UNCERTAINTY IN TIMING WHEN THIS WILL OCCUR WHICH AFFECTS THE
PRECIPITATION AMOUNTS. THE INTERIOR WEST IS FORECAST TO STAY DRY.
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 (15-25F ABOVE NORMAL) TENDING TO BE CENTERED OVER THE
DAKOTAS INTO UPPER MIDWEST THURSDAY-SATURDAY. 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 PART 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 THURSDAY 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).
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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