715
FXUS02 KWBC 120715
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
214 AM EST TUE DEC 12 2023
VALID 12Z FRI DEC 15 2023 - 12Z TUE DEC 19 2023
..A WET PATTERN IS LIKELY OVER THE SOUTH/SOUTHEAST
..OVERVIEW
A SOUTHERN STREAM UPPER LOW TRAVERSING THE SOUTH-CENTRAL TO
SOUTHEASTERN 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. RAIN WILL POTENTIALLY SPREAD 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, IN PART DUE TO DIFFERENCES WITH POSSIBLE
INTERACTION OF A NORTHERN STREAM SHORTWAVE. AHEAD OF THIS
POSSIBILITY, MOIST EASTERLY FLOW INTO SOUTHERN FLORIDA COULD CAUSE
LOCALIZED HEAVY RAIN INTO FRIDAY. 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
MODEL GUIDANCE CONTINUES TO BE AGREEABLE WITH THE SOUTHERN STREAM
UPPER LOW SYSTEM INTO FRIDAY, BUT MODEL SPREAD RAPIDLY INCREASES
INTO THE WEEKEND. MODELS HAVE BEEN WAFFLING OVER THE LAST FEW DAYS
WITH THE STRENGTH AND TRACK OF A NORTHERN STREAM SHORTWAVE LIKELY
MOVING SOUTHEAST THROUGH THE NORTHERN PLAINS FRIDAY. THIS FEATURE
MAY COMBINE WITH THE SOUTHERN STREAM UPPER LOW IN THE EAST-CENTRAL
U.S. IN SOME WAY, AND MOST MODELS FROM THE 12/18Z MODEL SUITE AND
NOW THE INCOMING 00Z HAVE TRENDED TOWARD SOME SORT OF COMBINATION.
GFS RUNS INDICATE THE NORTHERN STREAM ENERGY DROPPING SOUTH TO
BECOME PART OF THE UPPER LOW, WHILE THE ECMWF AND CMC CREATE MORE
OF A PHASED TROUGH FOR A TIME BEFORE ADDITIONAL SOUTHERN STREAM
ENERGY COMES IN TO HELP CLOSE OFF A LOW (WITH DIFFERENT TIMING
BETWEEN THE TWO). THESE DIFFERENCES ALOFT CREATE AMPLE DIFFERENCES
IN MODELS AND ENSEMBLE MEMBERS FOR A RESULTANT SURFACE LOW TRACK
AND STRENGTH. A DAY AGO, GFS RUNS (WITH SOME GEFS MEMBER SUPPORT)
WERE THE FASTEST AND FARTHEST NORTH TO BRING THE LOW TRACK ACROSS
FLORIDA. NOW THE GFS RUNS HAVE SLOWED BUT STILL LOOK TO BE ON THE
WEST SIDE OF THE SPREAD (RESULTING IN MORE QPF ONSHORE IN THE
SOUTHEAST). MEANWHILE THE 12Z AND NEWER 00Z CMC SWITCHED TO THE
FASTER SIDE OF THE SPREAD IN BRINGING A LOW NORTH. THE 12Z ECMWF
SEEMED TO BE A GOOD MIDDLE GROUND IN TRACK/TIMING BUT THE 00Z
ECMWF IS EVEN STRONGER AND FASTER IN BRINGING THE LOW NORTH AS A
NOR'EASTER. CONFIDENCE IS VERY LOW IN THE LOW TRACK AND QPF GIVEN
THE MODELS ARE IN SUCH FLUX. THE NORTHERN STREAM SHORTWAVE IS
CURRENTLY OVER THE NORTHEASTERN PACIFIC, A CLASSIC DATA-DEVOID
REGION THAT LEADS TO UNCERTAINTIES IN THE MODELS DOWNSTREAM.
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 WITH AN INITIAL SHORTWAVE PERHAPS ENTERING THE LOWER 48
MONDAY-TUESDAY STILL SHOWS SOME SPREAD BUT NOTHING TOO EGREGIOUS
FOR THE DAYS 5-7 TIMEFRAME.
THE WPC FORECAST STARTED WITH A BLEND OF DETERMINISTIC GUIDANCE
FAVORING THE ECMWF AND GFS, BUT WITH LESSENING AMOUNTS OF THE
DETERMINISTIC MODELS IN FAVOR OF THE EC AND GEFS ENSEMBLE MEANS,
REACHING OVER HALF ENSEMBLE MEANS BY DAYS 6-7. EXPECT CHANGES TO
THE FORECAST THAT COULD BE SIGNIFICANT IN FUTURE CYCLES.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER LOW TRAVERSING THE SOUTHERN U.S. COMBINED WITH AMPLE
MOISTURE ADVECTING NORTHWARD WILL LEAD TO WIDESPREAD RAIN ACROSS
TEXAS AS THE MEDIUM RANGE PERIOD BEGINS FRIDAY. INSTABILITY MAY BE
SOMEWHAT LIMITED EXCEPT RIGHT NEAR THE GULF COAST, BUT THE
DYNAMICAL SUPPORT THE UPPER LOW PROVIDES COULD LEAD TO HEAVY
ENOUGH RAIN TO CAUSE SOME FLOODING ISSUES, SO A MARGINAL RISK IS
IN PLACE IN THE EXCESSIVE RAINFALL OUTLOOK FOR THE EASTERN HALF OF
TEXAS AND INTO WESTERN LOUISIANA ON FRIDAY. THERE MAY BE SOME
LINGERING SNOW ON THE NORTHWESTERN SIDE OF THE PRECIPITATION
SHIELD IN THE TEXAS PANHANDLE, BUT MOST OF THE SNOW CHANCE HAS
PHASED INTO THE SHORT RANGE/THURSDAY NOW. BY SATURDAY THERE IS
INCREASING UNCERTAINTY WITH HOW MUCH RAIN FALLS ONSHORE VERSUS
OFFSHORE IN THE CENTRAL GULF. THIS ITERATION OF THE WPC FORECAST
TRENDED DOWN IN RAIN TOTALS INTO THE LOWER MISSISSIPPI VALLEY AND
SOUTHEAST AS MODEL GUIDANCE IS REDUCED WITH AMOUNTS OTHER THAN
RIGHT ALONG THE COAST. A MARGINAL RISK IS PLANNED FOR SATURDAY FOR
COASTAL AREAS OF LOUISIANA WHERE THE BEST CHANCE OF HEAVY RAIN IS
(COINCIDENT WITH SOME INSTABILITY) BUT THIS MAY NEED TO BE
EXPANDED IF MODELS TREND NORTH, OR REMOVED IF THEY TREND SOUTH.
THE SAME TYPE OF FORECAST ISSUE IS SEEN ACROSS FLORIDA, WHERE
THERE IS UNCERTAINTY WITH PLACEMENT OF HEAVY RAIN ON THE SOUTHERN
SIDE OF A STRONG SURFACE HIGH IN EASTERLY FLOW. FOR FRIDAY A
MARGINAL RISK REMAINS IN PLACE ACROSS SOUTH FLORIDA TO THE KEYS
WHERE THE BEST MODEL COMPROMISE FOR HEAVY RAIN IS, BUT THERE IS
POTENTIAL THAT PRECIPITATION COULD FOCUS NORTH OR SOUTH. THIS LOW
CONFIDENCE CONTINUES FURTHER INTO SATURDAY AS THE UPPER LEVELS AND
SURFACE LOW TRACK STILL SHOW MODEL SPREAD. THE GENERAL CONSENSUS
IS THAT SOME HEAVY RAIN SHOULD BE SPREADING NORTH BY SATURDAY, SO
A MARGINAL RISK REFLECTS THIS. PRECIPITATION REACHING FARTHER
NORTH ALONG THE EASTERN SEABOARD INTO EARLY NEXT WEEK IS QUITE
UNCERTAIN AT THIS TIME.
ELSEWHERE, A CLIPPER SYSTEM SHOULD SPREAD RAIN AND SNOW TO THE
UPPER MIDWEST/UPPER GREAT LAKES ON FRIDAY AND PERHAPS LINGERING
INTO SATURDAY. LIGHT PACIFIC NORTHWEST PRECIPITATION IS ALSO
POSSIBLE. BY EARLY NEXT WEEK, EASTERN PACIFIC UPPER TROUGHING
SHOULD SNEAK ENERGY AND PERHAPS AN ATMOSPHERIC RIVER 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.
THE NORTHERN TWO-THIRDS OF THE LOWER 48 SHOULD SEE NEAR TO ABOVE
NORMAL TEMPERATURES LATE WEEK INTO EARLY NEXT WEEK, WITH
TEMPERATURES 10-20F ABOVE AVERAGE PERIODICALLY AFFECTING THE
NORTHERN/CENTRAL HIGH PLAINS INTO THE UPPER MIDWEST. THE WEST CAN
ALSO EXPECT GENERALLY ABOVE NORMAL TEMPERATURES UNDERNEATH UPPER
RIDGING. COOLER THAN NORMAL TEMPERATURES WILL BE LIMITED TO THE
SOUTHERN PLAINS (IN TERMS OF HIGHS) EASTWARD INTO THE SOUTHEAST
LATE WEEK, MODERATING CLOSER TO NORMAL BY EARLY NEXT WEEK, THOUGH
FLORIDA COULD STILL SEE BELOW NORMAL HIGHS BEHIND MULTIPLE FRONTAL
PASSAGES.
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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