288
FXUS02 KWBC 181918
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
318 PM EDT THU SEP 18 2025
VALID 12Z SUN SEP 21 2025 - 12Z THU SEP 25 2025
..DESERT SOUTHWEST HEAT WAVE NEXT WEEK
..OVERVIEW
A POTENT TROUGH REMAINS ON TRACK TO ENTER THE PACIFIC NORTHWEST
SUNDAY, DIG AND AMPLIFY OVER THE PLAINS ON MONDAY, CLOSE OFF INTO A
DEEP LOW OVER THE CENTRAL PLAINS TUESDAY, THEN LINGER OVER THE
CENTRAL STATES THROUGH MIDWEEK. TROUGHING/UPPER LOW PRESSURE
LINGERS ALONG/OFF THE CALIFORNIA COAST AFTER BECOMING CUTOFF ON
SUNDAY. OTHERWISE, DEEP RIDGING OVER THE DESERT SOUTHWEST IS
EXPECTED MONDAY THROUGH MIDWEEK SETTING UP A HEAT WAVE. PERSISTENT
TROUGHING OVER THE CENTRAL U.S. AHEAD OF THE APPROACHING POTENT
TROUGH SHOULD ALLOW LOCALLY HEAVY RAIN TO LINGER SUNDAY AND MONDAY
OVER THE SOUTHERN PLAINS AND MID-SOUTH. COOL AIR DAMMING LINGERS
OVER THE EASTERN SEABOARD THROUGH THE CAROLINA PIEDMONT SUNDAY AND
MONDAY.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MORNING WPC FORECAST WAS COMPRISED OF A BLEND THAT FEATURED
THE DETERMINISTIC 00Z ECMWF/GFS AND THE 06Z GFS IN ADDITION TO THE
THE 00Z EC/06Z GEFS ENSEMBLE MEANS. THE WEIGHT OF ENSEMBLE MEANS
GRADUALLY INCREASED TO 60 PERCENT BY DAY 6 AND 7 FOR BETTER
ANCHORING AND STABILITY. THE ECMWF AND TO SOME DEGREE THE GFS
CONTINUE TO BE FAVORED FOR THEIR DEPICTIONS OF AN INCREASINGLY
AMPLIFIED LARGE SCALE PATTERN, PARTICULARLY WITH HOW THEY HANDLE
THE DEEPENING TROUGH AND EVENTUAL CLOSED LOW OVER THE CENTRAL CONUS
BY THE LATTER HALF OF THE PERIOD. THE 00Z RUNS OF THE CMC AND
UKMET WERE NOT UTILIZED WITH THIS PACKAGE DUE TO DISCREPANCIES WITH
OTHER FEATURES THAT CAUSED NEGATIVE DOWNSTREAM EFFECTS (THE CMC'S
POSITIONING OF THE SOCAL UPPER LOW BEING TOO FAR INLAND/NEAR THE
COAST AS AN EXAMPLE). THE ENSEMBLE MEANS AND PARTICULARLY THE
MACHINE LEARNING GUIDANCE HEADED BY THE EC-AIFS AND GRAPHCAST GFS
CONTINUE TO STRONGLY SUPPORT THE DEEP CLOSED LOW SOLUTION AND HAVE
BARELY WAVERED WITH THEIR DEPICTIONS IN RECENT RUNS. EVEN THE
DETERMINISTIC GFS, WHICH SEEMED TO HAVE A STRONG HANDLE ON THE
PATTERN AT 00Z AND 06Z, MOVED QUITE A BIT AWAY FROM CONSENSUS AT
12Z... WHICH WAS EVEN MORE THE CASE FOR THE 12Z CMC. FORTUNATELY,
THE 12Z UKMET CAME INTO MUCH BETTER ALIGNMENT WITH THE ECMWF AND
MACHINE LEARNING MODELS. DESPITE THE VOLATILITY WITH INDIVIDUAL
DETERMINISTIC MODELS, THE ROCK-SOLID CLUSTERING BETWEEN THE EC/EC
ENSEMBLE MEAN, THE GEFS ENSEMBLE MEAN, AND THE EC-AIFS AND
GRAPHCAST GFS SUGGEST A HIGH CONFIDENCE OUTCOME, WITH HEAVY
RAINFALL CONCERNS ACROSS PARTS OF THE CENTRAL U.S. AS A RESULT.
..WEATHER/HAZARDS HIGHLIGHTS
CONVERGENCE OF GULF MOISTURE STREAMING NORTH TOWARD LINGERING
TROUGHING OVER THE NORTH-CENTRAL U.S. ALLOWS FOR SOME SLOWER MOVING
CLUSTERS OF LOCALLY HEAVY THUNDERSTORMS OVER PORTIONS OF THE
CENTRAL/SOUTHERN PLAINS AND MID/LOWER MISSISSIPPI VALLEY SUNDAY AND
MONDAY. THE INHERITED DAY 4 MARGINAL ERO WAS EXPANDED NORTHWARD
INTO SOUTHEAST IOWA AND NORTHERN ILLINOIS GIVEN A GREATER QPF
SIGNAL IN THOSE AREAS. ON DAY 5, THE MARGINAL ERO WAS SHIFTED TO
THE NORTH AND WEST TO INCLUDE MORE OF EASTERN KANSAS AND NORTHWEST
MISSOURI, AGAIN CITING A NOTABLE CLUSTERING OF QPF GUIDANCE THERE.
SHOWERS AND THUNDERSTORMS WILL BE SUPPORTED BY INCREASINGLY
ANOMALOUS MOISTURE, WITH PWS OF 1.5" INCREASING TO AROUND 1.75"
SUNDAY INTO MONDAY. THIS MOISTURE WHEN COMBINED WITH ADEQUATE
INSTABILITY AND FORCING IS SUFFICIENT FOR EXCESSIVE RAINFALL
CONCERNS WITH SLOW MOVING ACTIVITY/FLOW PARALLEL TO ANY BOUNDARIES.
ADDITIONAL BOUTS OF POTENTIALLY HEAVY RAIN AHEAD OF THE UPPER LOW
THAT CUTS OFF COULD OCCUR OVER SIMILAR AREAS LATER IN THE WEEK, SO
FURTHER ATTENTION WILL NEED TO BE GIVEN TO THIS THREAT.
RIDGING BUILDS NORTHEASTWARD FROM THE MIDWEST, ALLOWING MAX TEMPS
OF 10-15 DEGREES ABOVE NORMAL TO SHIFT TO THE INTERIOR NORTHEAST.
MEANWHILE A STRONG SURFACE HIGH LINGERING ALONG THE NORTHEASTERN
SEABOARD SUNDAY AND MONDAY SHOULD MAINTAIN COOL AIR DAMMING EAST OF
THE APPALACHIANS AND SOUTH THROUGH THE CAROLINA PIEDMONT. ONSHORE
FLOW, SCATTERED CLOUDS, AND POSSIBLE SHOWERS WILL RESULT IN BELOW
NORMAL TEMPERATURES EAST OF THE APPALACHIANS.
AMPLIFYING RIDGING OVER THE DESERT SOUTHWEST WILL LEAD TO A HEAT
WAVE ACROSS THE DESERT SOUTHWEST EARLY THROUGH MID NEXT WEEK. THIS
HEAT MAY CHALLENGE HIGH TEMPERATURE RECORDS WITH HIGHS SOARING INTO
THE MID TO UPPER 100S FOR SOME LOCATIONS. DOWNSTREAM OF THE RIDGE,
A SERIES OF SHORTWAVES WILL LIKELY COMBINE TO FORM A DEEP, CUTOFF
MID-LEVEL LOW PRESSURE SYSTEM ACROSS THE CENTRAL U.S. BY TUESDAY
AND WEDNESDAY, LINGERING INTO THURSDAY. THIS WILL LIKELY LEAD TO
FAIRLY WIDESPREAD STORMY AND UNSETTLED WEATHER ACROSS THE REGION,
RESULTING IN TEMPERATURES TRENDING NEAR TO BELOW NORMAL.
MILLER/JACKSON
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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