085
FXUS02 KWBC 121859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM 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 AND
POTENTIALLY UP THE EAST COAST THIS WEEKEND INTO NEXT WEEK...
..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 AND POSSIBLY LIFTING UP THE EAST COAST, 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
THROUGH THE 00Z/06Z GUIDANCE, THERE CONTINUES TO BE GOOD AGREEMENT
ON THE SOUTHERN STREAM UPPER LOW SYSTEM INTO FRIDAY, BUT A LOT OF
MODEL SPREAD INTO THE WEEKEND AS THIS SYSTEM MAY BEGIN TO INTERACT
WITH THE NORTHERN STREAM SHORTWAVE. THE GFS IS MORE SUPPORTIVE OF
KEEPING THE TWO SYSTEMS SEPARATE WITH THE SOUTHERN STREAM ONE
BECOMING MORE DOMINANT EVENTUALLY, WHILE THE 00Z RUN OF THE ECMWF
WAS SHOWING THE OPPOSITE - WEAKENING THE SOUTHERN LOW RAPIDLY AS
THE NORTHERN STREAM SYSTEM SWINGS THROUGH THE SOUTHEAST. THIS
CREATES DRASTICALLY DIFFERENT INTERPRETATIONS AT THE SURFACE WITH
THE ECMWF SUGGESTING A RAPIDLY DEEPENING CYCLONE MOVING VERY
QUICKLY UP THE COAST, WHILE THE GFS IS CONSIDERABLY SLOWER (BUT
STILL SUPPORTIVE OF LOW PRESSURE). HOWEVER, THE NEW 12Z RUN OF THE
ECMWF (WHICH WAS AVAILABLE AFTER FORECAST GENERATION TIME) CAME IN
VERY DIFFERENT FROM IT'S PREVIOUS RUN AND MORE IN LINE WITH THE
GFS WITH A WEAKER AREA OF LOW PRESSURE MOVING OFF THE EAST COAST.
THE CMC IS GENERALLY IN LINE WITH THIS IDEA TOO, BUT THE UKMET IS
THE STRONGEST WITH A VERY DEEP LOW OFF THE SOUTHEAST COAST ON DAY
6/MONDAY. GIVEN THIS IS STILL 5 OR MORE DAYS OUT, CONFIDENCE IS
NOT HIGH WITH REGARDS TO THIS SYSTEM, AND THERE IS STILL A LOT OF
TIME FOR THE GUIDANCE TO CHANGE STILL. THE WPC BLEND FROM THIS
MORNING WAS BASED ON THE LATEST 00Z/06Z GUIDANCE AND GENERALLY
USED A MODEL COMPROMISE THROUGH DAY 4. DAY 5 AND BEYOND, INCREASED
CONTRIBUTIONS FROM THE ENSEMBLE MEANS AMIDST GROWING SPREAD, ALONG
WITH WPC CONTINUITY.
ELSEWHERE, TROUGHING MOVING INTO THE WEST COAST EARLY NEXT WEEK
SHOWS GOOD AGREEMENT, BUT PLENTY OF UNCERTAINTY IN THE DETAILS. A
GENERAL BLEND OF THE ENSEMBLE MEANS IS A GOOD STARTING POINT.
..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 LOCALIZED FLOODING ISSUES, SO A MARGINAL
RISK REMAINS IN PLACE IN THE EXCESSIVE RAINFALL OUTLOOK FOR PARTS
OF EAST 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
ACTUALLY TRENDED UP IN RAIN TOTALS ACROSS FLORIDA AND THE
SOUTHEAST BASED ON LATEST MODEL TRENDS. 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 ADJUSTED FURTHER DEPENDING ON LATEST MODEL TRENDS. 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 BUT THERE IS DEFINITELY POTENTIAL FOR HEAVY RAINFALL.
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.
SANTORELLI/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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