865
FXUS02 KWBC 290700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT FRI SEP 29 2023
VALID 12Z MON OCT 02 2023 - 12Z FRI OCT 06 2023
..OVERVIEW
AN AMPLIFIED PATTERN IS IN STORE NEXT WEEK, AS AN UPPER TROUGH
TRACKS ACROSS THE INTERIOR WEST EAST INTO THE CENTRAL AND
EAST-CENTRAL U.S., WITH AMPLIFIED UPPER RIDGES ON EITHER SIDE.
THIS WILL LEAD TO COOLER THAN NORMAL TEMPERATURES IN THE WEST AND
WARMER THAN NORMAL TEMPERATURES IN THE EASTERN HALF OF THE U.S.
THAT MAY MODERATE CLOSER TO NORMAL AS THE WEEK PROGRESSES. MOIST
AND UNSTABLE INFLOW JUST AHEAD OF THE TROUGH AND AN ASSOCIATED
COLD FRONT SHOULD LEAD TO AN EMERGING CONVECTIVE RAINFALL PATTERN
FOR MUCH OF THE PLAINS MONDAY-TUESDAY AND LIKELY FOCUSED IN THE
SOUTHERN PLAINS BY WEDNESDAY.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL DIFFERENCES IN THE FIRST HALF OF THE WEEK MAINLY HAVE TO DO
WITH THE TIMING OF THE TROUGH SHIFTING EASTWARD. THE 12Z UKMET WAS
THE MAIN INITIAL OUTLIER WITH A SLOWER TROUGH THAN THE BETTER
CLUSTERED GFS/ECMWF/CMC RUNS, SO LEANED TOWARD THE LATTER GROUP
FOR THE EARLY PART OF THE FORECAST. BEGINNING WEDNESDAY AND
ESPECIALLY BY THURSDAY-FRIDAY, MODELS DIVERGE FURTHER WITH THE
TIMING AND ALSO THE DEPTH OF THE TROUGH. THIS IS AT LEAST IN PART
DUE TO THE POTENTIAL FOR ADDITIONAL ENERGY TO DIVE INTO THE
NORTHWESTERN SIDE OF THE TROUGH, WHICH THE ECMWF AND CMC RUNS HAVE
BEEN PERSISTENTLY SHOWING. ON THE OTHER HAND, GFS RUNS SHOW ANY
ENERGY THERE IN WEST-CENTRAL CANADA TO BE WEAKER AND TRACK MORE
EASTWARD RATHER THAN DROPPING SOUTHEAST. THIS KEEPS A MORE
AMPLIFIED RIDGE TO THE WEST OF THE TROUGH AND ALLOWS FOR THE
TROUGH TO STAY DEEP AS IT TRACKS EAST. THE NEW 00Z GFS EVEN TENDS
TO BULGE THE BASE OF THE TROUGH IN RESPONSE TO THE NORTHERN SIDE
OF THE RIDGE AND SHOW THE TROUGH'S AXIS AS SLIGHTLY POSITIVELY
TILTED EVEN INTO LATE NEXT WEEK, WHILE OTHER GUIDANCE IS NEUTRAL
TO NEGATIVELY TILTED BY THAT POINT. THE VARIABILITY WITH THE
SHORTWAVE MEANS THE PATTERN IS OUT OF PHASE ACROSS THE NORTHERN
PLAINS AMONG DETERMINISTIC MODELS AND VARIOUS ENSEMBLE MEMBERS BY
THE LATTER HALF OF NEXT WEEK. WITH THIS POSSIBLE/QUESTIONABLE
SHORTWAVE ENERGY LOOKING TO EMERGE FROM THE ARCTIC, THIS WILL
LIKELY REMAIN UNCERTAIN IN FUTURE MODEL CYCLES. ADDITIONALLY,
WHILE NOT THE MAJORITY, A HANDFUL OF RUNS FROM MULTIPLE MODELS
HANG ENERGY BACK INTO THE SOUTHWEST FROM THE TROUGH, PROVIDING
FURTHER UNCERTAINTY. THE 12Z CMC DID THIS AND THE EC ENSEMBLE MEAN
HINTS AT IT AS WELL. GIVEN ALL THE UNCERTAINTY IN THE DETAILS OF
THE PATTERN, THE INITIALLY MAINLY DETERMINISTIC MODEL BLEND
TRANSITIONED TO FAVOR MORE ENSEMBLE MEANS WITH INCREASING LEAD
TIME, TO JUST OVER HALF BY DAY 6 AND 70 PERCENT BY DAY 7 IN THE
MASS FIELDS.
..WEATHER/HAZARDS HIGHLIGHTS
ON MONDAY THERE MAY BE SOME LINGERING SNOW IN HIGHER ELEVATIONS OF
THE WEST WITH THE UPPER TROUGH AXIS OVERHEAD. MEANWHILE FARTHER
SOUTH, MOISTURE AND INSTABILITY LOOK TO POOL IN THE SOUTHERN HIGH
PLAINS JUST AHEAD OF THE COLD FRONT. THE RIGHT ENTRANCE REGION OF
THE JET WILL PROVIDE AMPLE LIFT WHICH COMBINED WITH THE UNSTABLE
CONDITIONS COULD PRODUCE HIGH RAIN RATES, POSSIBLY OVERCOMING THE
ONGOING DRY CONDITIONS THERE TO PRODUCE SOME LOCALIZED FLASH
FLOODING. THUS A MARGINAL RISK HAS BEEN INTRODUCED FOR MONDAY FOR
THAT REGION. RAINFALL CHANCES ARE LIKELY TO INCREASE IN THE
NORTH-CENTRAL U.S. AS WELL, BUT UNCERTAINTY IN THE AMOUNTS AND
PLACEMENT OF ENHANCED CONVECTION PRECLUDES INTRODUCING ANY
EXCESSIVE RAINFALL RISK AT THIS POINT. AS THE TROUGH AND COLD
FRONT EMERGE INTO THE PLAINS ON TUESDAY, RAIN AND THUNDERSTORMS
SHOULD BECOME WIDESPREAD IN THE CENTRAL U.S., WITH MODEL GUIDANCE
SHOWING EMBEDDED HEAVIER AMOUNTS THROUGH ALL THE PLAINS STATES
INTO THE UPPER MIDWEST, BUT WITH LITTLE AGREEMENT IN PLACEMENT AT
THIS TIME. A LARGE MARGINAL RISK IS IN PLACE FOR TUESDAY AS A
STARTING POINT, BUT IT IS LIKELY THAT ONE OR MORE EMBEDDED SLIGHT
RISKS ARE INTRODUCED IN FUTURE FORECASTS IF MODELS START TO
CONVERGE ON LOCATIONS OF HEAVY AMOUNTS. THEN BY WEDNESDAY THE
HEAVIEST CONVECTION LOOKS TO FOCUS ACROSS THE SOUTHERN PLAINS,
WITH SOME RAIN AMOUNTS OF OVER AN INCH ALREADY WIDESPREAD IN THE
DETERMINISTIC FORECAST. SOME RAIN AND STORMS SHOULD SPREAD INTO
THE OHIO/TENNESSEE VALLEYS BY THURSDAY OR SO, BUT WITH UNCERTAINTY
IN TIMING.
ELSEWHERE, SOME LINGERING SHOWERS AND STORMS ARE POSSIBLE OVER
SOUTHERN FLORIDA ESPECIALLY EARLY NEXT WEEK AS A FRONT PUSHES
THROUGH, BUT WITH LIGHTER AMOUNTS THAN IN THE SHORT RANGE. THE
PACIFIC NORTHWEST MAY SEE A ROUND OF LIGHT TO MODERATE
PRECIPITATION MONDAY-TUESDAY BEFORE LIKELY DRYING OUT BY MIDWEEK.
IN TERMS OF TEMPERATURES, THE EARLY WEEK TROUGH ATOP THE WEST WILL
LEAD TO COOLER TEMPERATURES THAN AVERAGE, WITH LOWS 5-10F BELOW
NORMAL BUT HIGHS AROUND 10-20F BELOW AVERAGE. THIS PUTS HIGH
TEMPERATURES GENERALLY IN THE 80S FOR THE SOUTHWEST AND THE 50S
AND 60S FOR THE GREAT BASIN. MEANWHILE, TEMPERATURES FROM THE
PLAINS EASTWARD WILL BE NEAR TO ABOVE AVERAGE ON MONDAY, WITH THE
LARGEST ANOMALIES OF 15-20F ABOVE NORMAL (FOR HIGHS, EVEN MORE
ANOMALOUS FOR LOWS) FOCUSED ON THE UPPER MIDWEST TO GREAT LAKES
AND THE INTERIOR NORTHEAST, WITH HIGHS IN THE UPPER 70S TO LOW
80S. THE PLAINS SHOULD MODERATE TO NEAR NORMAL TO PERHAPS A FEW
DEGREES BELOW NORMAL TEMPERATURES AS THE WEEK PROGRESSES, WITH THE
AREA OF ABOVE NORMAL TEMPERATURES BECOMING MORE LIMITED TO THE
NORTHEAST BY LATER IN THE WEEK AS THE COLD FRONT OVERTAKES THE
EAST-CENTRAL CONUS.
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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