749
FXUS02 KWBC 150700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT SUN SEP 15 2024
VALID 12Z WED SEP 18 2024 - 12Z SUN SEP 22 2024
...POSSIBLE SUBTROPICAL OR TROPICAL LOW COULD SPREAD HEAVY RAIN TO
THE MID-ATLANTIC INTO WEDNESDAY...
..OVERVIEW
RELOADING UPPER TROUGHING WILL BE IN PLACE ACROSS THE WESTERN U.S.
THROUGH THIS WEEK, PROMOTING COOLER WEATHER AND RAIN PLUS HIGHER
ELEVATION SNOW. SOME RAIN WILL SPREAD INTO THE PLAINS AS WELL,
BECOMING MODERATE TO HEAVY AT SOME LOCATIONS INTO LATE WEEK, BUT
WITH WARMER THAN AVERAGE TEMPERATURES. MEANWHILE IN THE EAST, A
PERSISTENT UPPER LOW WILL MEANDER WHILE POTENTIALLY ACCOMPANIED BY
SURFACE LOW PRESSURE SYSTEMS. IN PARTICULAR, NHC AND WPC ARE
MONITORING POSSIBLE SURFACE LOW DEVELOPMENT IN THE SHORT TERM OFF
THE SOUTHEAST COAST (MAY BECOME TROPICAL OR SUBTROPICAL) THAT COULD
BE INLAND OVER THE MID-ATLANTIC AT THE START OF THE PERIOD
WEDNESDAY. HOWEVER THERE HAS BEEN CONSIDERABLE UNCERTAINTY WITH
THIS SYSTEM'S EVOLUTION AND PATH ALONG WITH THE COVERAGE/INTENSITY
OF ASSOCIATED RAINFALL.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE EASTERN U.S. IS THE AREA OF GREATEST UNCERTAINTY DURING THE
EARLY PART OF THE MEDIUM RANGE PERIOD, DESPITE GENERAL AGREEMENT IN
AN UPPER LOW MEANDERING THERE, LIKELY SINKING SOUTH BY NEXT
WEEKEND. HOWEVER, DETAILS OF ENERGY WITHIN THE UPPER LOW AND TROUGH
WILL IMPACT THE PATTERN AT THE SURFACE, INCLUDING A LOW PRESSURE
SYSTEM THAT COULD SHOW TROPICAL OR SUBTROPICAL DEVELOPMENT EARLY TO
MIDWEEK. BY THE START OF THE PERIOD WEDNESDAY, THIS SURFACE LOW
SEEMS MOST LIKELY TO BE ONSHORE WITH A MIDDLE GROUND CONSENSUS
POSSIBLY OVER VIRGINIA. WHETHER THE LOW WILL BE
TROPICAL/SUBTROPICAL OR EXTRATROPICAL REMAINS IN QUESTION. BY
WEDNESDAY IT MAY BE ACQUIRING FRONTS EVEN IF IT APPROACHES THE U.S.
AS TROPICAL OR SUBTROPICAL, WHICH IS CERTAINLY A POSSIBILITY PER
THE NHC OUTLOOK PROBABILITIES AND A LOOK AT THE CYCLONE PHASE SPACE
DIAGRAMS FROM VARIOUS MODELS. THE STRUCTURE/TYPE OF THE LOW WILL
ALSO IMPACT QPF. THE 12Z ECMWF SEEMED LIKE A SOUTHWESTERN OUTLIER
IN ITS QPF FOCUS COMPARED TO THE 12Z EC MEAN AND OTHER GUIDANCE
LIKE THE CMC AND UKMET, SO FOR QPF DID BLEND IN SOME OF THESE
SOLUTIONS TO THE NBM FOR A TREND NORTH. THE FORECAST WILL LIKELY
CONTINUE TO FLUCTUATE GIVEN THE CONTINUED MODEL SPREAD. LATER IN
THE WEEK, WEAK LOWS THAT MAY STAY FRONTAL MAY FORM OVER THE WESTERN
ATLANTIC SUPPORTED BY THE TROUGHING ALOFT.
IN THE WEST, THERE IS GENERALLY BETTER MODEL AGREEMENT FOR AT
LEAST THE EARLY PART OF THE PERIOD, AS A FIRST UPPER LOW AND ITS
ASSOCIATED SURFACE LOW PIVOT NORTHEAST FROM MONTANA INTO SOUTHERN
CANADA WEDNESDAY-THURSDAY WHILE TRAILING ENERGY DIGGING ACROSS
CALIFORNIA MAY CREATE ANOTHER EMBEDDED UPPER LOW. BY FRIDAY AND
BEYOND THOUGH, MODELS SHOW INCREASING SPREAD REGARDING NORTH
PACIFIC FLOW ENTERING THE WESTERN NORTH AMERICA MEAN TROUGH AND ITS
INFLUENCE ON THURSDAY'S CALIFORNIA/NEVADA TROUGH OR UPPER LOW. THE
FLOW PATTERN IN THE 18Z GFS LED TO AN INTERIOR WEST TO CENTRAL
U.S. TROUGH EJECTING EAST MUCH MORE QUICKLY THAN CONSENSUS. THE 12Z
GFS SHOWED BETTER POSITIONING WITH THIS TROUGH BUT WAS
FASTER/FLATTER THAN FAVORED WITH THE NORTHERN STREAM. THE NEWER 00Z
GFS FORTUNATELY NOW SEEMS TO HAVE BETTER AGREEMENT WITH THE EC/CMC
AND ENSEMBLE MEANS.
THE WPC FORECAST BEGAN WITH A BLEND OF THE DETERMINISTIC MODELS
(12/18Z GFS AND 12Z ECMWF/CMC/UKMET). GRADUALLY DECREASED THE
PROPORTION OF THE DETERMINISTIC MODELS, PARTICULARLY THE GFS RUNS
(AND THE TIME-LIMITED UKMET), AS THE PERIOD PROGRESSED IN FAVOR OF
THE GEFS AND EC ENSEMBLE MEANS. THE ENSEMBLE MEAN PROPORTION
REACHED HALF BY DAY 6 AND MORE BY DAY 7 GIVEN THE INCREASING
SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
THE SURFACE LOW THAT MAY BE TROPICAL OR SUBTROPICAL PER NHC/WPC
COORDINATION REMAINS MORE UNCERTAIN THAN DESIRED, BUT FOR NOW IS
MOST LIKELY TO BE LOCATED IN VIRGINIA OR VICINITY BY WEDNESDAY,
SPREADING RAIN INTO THE MID-ATLANTIC AND APPALACHIANS. FOR
WEDNESDAY/DAY 4, A MARGINAL RISK AREA REMAINS IN PLACE ACROSS MUCH
OF THE MID-ATLANTIC REGION AND STRETCHES INTO SOUTHERN PARTS OF THE
NORTHEAST GIVEN A SLIGHTLY FASTER TREND IN THE RAIN SPREADING
NORTH AND EAST. ABOVE NORMAL MOISTURE LOOKS TO LINGER INTO
THURSDAY/DAY 5, PROMPTING A CONTINUED MARGINAL RISK IN THE MID-
ATLANTIC ESPECIALLY AFTER A WET PREVIOUS COUPLE OF DAYS. SHOWERS
ARE LIKELY TO PERSIST INTO LATE WEEK ALONG PARTS OF THE EASTERN
SEABOARD AS UPPER TROUGHING REMAINS OVERHEAD. THE FLORIDA PENINSULA
COULD ALSO SEE SOME FOCUSED CONVECTION DUE TO A FRONTAL BOUNDARY
IN A MOIST ENVIRONMENT. CURRENTLY THIS THUNDERSTORM ACTIVITY SEEMS
TO BE SUB-MARGINAL RISK FOR FLASH FLOODING, BUT IF GUIDANCE IS
AGREEABLE IN TRENDING UPWARD WITH AMOUNTS IN FUTURE CYCLES, FUTURE
MARGINALS CANNOT BE RULED OUT, ESPECIALLY FOR THE TYPICALLY
SENSITIVE URBAN CORRIDOR OF SOUTHEAST FLORIDA.
THE MEAN TROUGH ALOFT OVER THE WEST WILL PROVIDE AMPLE
LIFT/FORCING TO SUPPORT ROUNDS OF PRECIPITATION THERE THIS WEEK.
HEAVY RAIN MAY LINGER ACROSS PARTS OF MONTANA ON DAY 4/WEDNESDAY
FOR A CONTINUED MARGINAL RISK, AS THE REGION COULD SEE HEAVY RAIN
RATES FOR MULTIPLE HOURS NEAR THE TRACK OF A LEADING UPPER LOW
EJECTING THROUGH THE NORTHERN ROCKIES, WITH POTENTIAL FOR SOME
INSTABILITY TO BE IN PLACE. THE SECONDARY UPPER LOW SHOULD THEN
SPREAD PRECIPITATION FROM THE WEST COAST STATES MIDWEEK INTO THE
GREAT BASIN AND ROCKIES. THE HIGHER ELEVATIONS CAN EXPECT SNOW WITH
BOTH OF THESE SYSTEMS GIVEN THE DEPTH OF THE UPPER LOW. FARTHER
EAST, SCATTERED RAIN AND THUNDERSTORMS ARE LIKELY IN THE PLAINS FOR
MULTIPLE DAYS THIS WEEK NEAR AND AHEAD OF FRONTAL SYSTEMS.
RAINFALL AMOUNTS MAY INCREASE IN THE NORTH- CENTRAL U.S. BY LATER
WEEK, AS THE FRONT TRAILING FROM THE INITIAL MONTANA INTO CANADA
SYSTEM STALLS FOR A PERIOD OF TIME WHILE AWAITING SOME COMBINATION
OF NORTHERN STREAM AND WESTERN U.S. DYNAMICS (WITH POSSIBLE SURFACE
LOW DEVELOPMENT).
UNDERNEATH THE UPPER TROUGH IN THE WEST, MUCH COOLER THAN NORMAL
TEMPERATURES ARE EXPECTED. HIGH TEMPERATURES WILL BE 15-25F BELOW
NORMAL ACROSS MUCH OF THE WEST, GRADUALLY MODERATING CLOSER TO
NORMAL ACROSS THE WEST COAST STATES BUT WITH SOME MINUS 10-15F
ANOMALIES PERSISTING IN THE INTERIOR WEST AND REACHING THE NORTHERN
HIGH PLAINS OVER THE WEEKEND. SOME AREAS MAY REACH THE FREEZING
MARK IN THE HIGHER ELEVATIONS OF THE GREAT BASIN/ROCKIES.
MEANWHILE, THE CENTRAL U.S. SHOULD SEE WARMER THAN NORMAL HIGHS BY
10-15F AND LOWS BY 15-20F, INCLUDING HIGHS IN THE 90S IN THE
SOUTHERN HALF OF THE PLAINS, THROUGH THE LATTER HALF OF THE WEEK.
THE SCOPE OF ABOVE NORMAL TEMPERATURES WILL DECREASE INTO THE
WEEKEND AND MAINLY FOCUS ACROSS THE UPPER GREAT LAKES TO THE OHIO
VALLEY. THE NORTHEAST CAN EXPECT WARMER THAN AVERAGE TEMPERATURES
UNTIL A COLD FRONTAL PASSAGE LATE WEEK, WHILE TEMPERATURES SOUTH OF
THERE ARE FORECAST TO BE NEAR TO BELOW NORMAL.
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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