407
FXUS02 KWBC 071901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SUN MAY 07 2023
VALID 12Z WED MAY 10 2023 - 12Z SUN MAY 14 2023
***PERIODS OF HEAVY RAIN EXPECTED FOR PORTIONS OF THE SOUTHERN
PLAINS AND ALSO THE NORTHERN HIGH PLAINS AND VICINITY***
..OVERVIEW
LATEST GUIDANCE CONTINUES TO HIGHLIGHT THE CENTRAL U.S. FOR
HEAVIEST RAINFALL FROM MIDWEEK THROUGH THE WEEKEND, AS A SHARP
UPPER TROUGH OVER THE INTERMOUNTAIN WEST ON WEDNESDAY CLOSES OFF A
LOW THAT SHOULD TRACK THROUGH THE CENTRAL ROCKIES INTO NORTHERN
PLAINS BY FRIDAY BEFORE LIKELY OPENING UP, WHILE FARTHER SOUTH
SHORTWAVE IMPULSES MAY INTERACT WITH A PERSISTENT INFLOW OF
WESTERN GULF MOISTURE AND A STALLED SURFACE FRONT TO PRODUCE HEAVY
RAINFALL OVER THE SOUTHERN PLAINS. SOME OF THE CENTRAL U.S.
MOISTURE MAY EVENTUALLY EXTEND INTO CENTRAL LATITUDES OF THE EAST
BY NEXT WEEKEND. THE ROCKIES/PLAINS SYSTEM SHOULD BUILD A
DOWNSTREAM UPPER RIDGE FROM THE GULF COAST TO THE MIDWEST, WITH
THE NORTHERN PART ERODING AS THE NORTHERN PLAINS SYSTEM OPENS UP.
MEANWHILE EASTERN CANADA INTO NORTHWEST ATLANTIC MEAN TROUGHING
MAY START TO ADJUST FARTHER WEST BY NEXT SUNDAY AND BEYOND IN
RESPONSE TO THE STRONG RIDGE BUILDING INTO THE PACIFIC NORTHWEST
AND WESTERN CANADA.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS AND ENSEMBLES CONTINUE TO SHOW DETAIL AND LATITUDE/TRACK
DIFFERENCES FOR THE VIGOROUS UPPER SYSTEM EXPECTED TO TRACK FROM
THE INTERMOUNTAIN WEST INTO THE PLAINS. THE 00Z UKMET HAD THE
MOST DIFFUSE EVOLUTION AND WAS NOT INCLUDED IN THE UPDATED
FORECAST BLEND DUE TO MOST OTHER SOLUTIONS STILL HAVING A BETTER
DEFINED/CLOSED SYSTEM. WITH OTHER MODELS CLOSER IN PRINCIPLE BUT
SHUFFLING AROUND FOR TRACK, A MULTI-MODEL/MULTI-RUN BLEND SEEMED
TO PROVIDE THE BEST APPROACH TO REPRESENT THE SYSTEM'S OVERALL
EVOLUTION AND TEMPER ADJUSTMENTS IN SUCCESSIVE RUNS THAT MAY OR
MAY NOT TURN OUT TO BE RELIABLE TRENDS. MEANWHILE, THE LATEST
CYCLES OF MOST GUIDANCE HAVE TRENDED NOTICEABLY STRONGER WITH THE
UPPER RIDGE BUILDING OVER THE PACIFIC NORTHWEST AND WESTERN CANADA
BY LATE WEEK AND NEXT WEEKEND. CURRENTLY THE ECMWF/CMC ARE
STRONGEST WITH THE RIDGE WHILE THE 00Z/06Z GEFS MEANS WERE
SUSPICIOUSLY WEAK WITH THE RIDGE AND MOST AGGRESSIVE WITH BRINGING
LEADING HEIGHT FALLS WITH THE UPSTREAM PACIFIC TROUGH TOWARD THE
NORTHWEST. THE 12Z GEFS MEAN HAS TRENDED IN THE RIGHT DIRECTION
THOUGH IT COULD STILL BE A LITTLE FAST WITH THE LEADING SIDE OF
THE PACIFIC TROUGH. THERE IS A GENERAL THEME OF A LINGERING
WEAKNESS OVER THE SOUTHERN HALF OR TWO-THIRDS OF THE WEST BY NEXT
WEEKEND BUT MINIMAL CONFIDENCE IN SPECIFIC UPPER LOW(S) THAT COULD
EXIST WITHIN THIS AREA. FINALLY, A GENERAL MODEL/ENSEMBLE BLEND
BEST RESOLVES EASTERN CANADA/NORTHEASTERN U.S. FLOW WHOSE
SHORTWAVE SPECIFICS CONTINUE TO DECLINE IN PREDICTABILITY AND
CONFIDENCE BY MID-LATE PERIOD.
BASED ON THE ABOVE CONSIDERATIONS, THE FIRST HALF OF THE PERIOD
STARTED WITH A COMPOSITE OF THE 00Z/06Z GFS, 12Z/06 AND 00Z ECMWF,
AND 00Z CMC. THEN THE FORECAST INCORPORATED SOME 00Z ECENS/CMCENS
MEANS IN ADDITION TO THOSE MODEL RUNS, AGAIN EXCLUDING THE GEFS
DUE TO ITS QUESTIONABLE EASTERN PACIFIC/WESTERN NORTH AMERICA
SOLUTION.
..WEATHER/HAZARDS HIGHLIGHTS
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL LIKELY
DEVELOP ACROSS MUCH OF THE PLAINS STATES MID-LATE WEEK, AS
MOISTURE ADVECTION FROM THE GULF OF MEXICO INCREASES AHEAD OF A
SLOWING FRONTAL SYSTEM OVER THE WESTERN HIGH PLAINS AND THE UPPER
TROUGH/LOW APPROACHES FROM THE WEST. THE DAY 4 (12Z WEDNESDAY-12Z
THURSDAY) EXCESSIVE RAINFALL OUTLOOK WILL MAINTAIN A MODESTLY
ADJUSTED SLIGHT RISK AREA OVER PARTS OF THE ARKLATEX REGION WHERE
THERE ARE CONTINUED MODEL SIGNALS FOR LOCALLY HEAVY RAINFALL
TOTALS AS MULTIPLE MCSS DEVELOP IN THE WARM SECTOR OF THE EMERGING
WESTERN LOW, WITH THE AID OF ONE OR MORE SOUTHERN STREAM SHORTWAVE
IMPULSES. SOME OF THIS ACTIVITY MAY EXTEND INTO THE DAY 5 (12Z
THURSDAY-12Z FRIDAY) PERIOD BUT WITH A MORE DIFFUSE SIGNAL FOR
SPECIFICS ANYWHERE FROM THE MIDDLE MISSISSIPPI VALLEY TO THE
WESTERN HALF OF THE GULF COAST--THUS THE DEPICTION OF ONLY A
MARGINAL RISK AREA. THE MIDDLE MISSISSIPPI VALLEY MAY SEE A
COMBINATION OF THE SHORTWAVE ENERGY AND MOISTURE FROM DAY 4 LIFT
NORTHEASTWARD AND INTERACT WITH A SURFACE FRONT NEARING THE
REGION, WHILE PERSISTENT MOISTURE AND POSSIBLY ADDITIONAL UPPER
IMPULSES COULD PRODUCE LOCALLY HEAVY RAIN FARTHER SOUTH.
MEANWHILE THE NORTHERN HIGH PLAINS AND VICINITY MAY SEE A
MULTI-DAY HEAVY RAIN THREAT WITH THE UPPER LOW/SURFACE SYSTEM
TRACKING OVER OR NEAR THE REGION. THE DAY 4 ERO MAINTAINS THE
MARGINAL RISK AREA OVER THE HIGH PLAINS WHILE FOR DAY 5 THE PLAN
IS FOR A SOUTHWARD EXPANSION OF THE SLIGHT RISK AREA OVER PARTS OF
MONTANA INTO NORTHEASTERN WYOMING AND FAR NORTHWEST SOUTH DAKOTA.
THIS IS BASED ON THE BEST OVERLAP OF ENSEMBLE GUIDANCE FOR HIGHEST
EXPECTED RAINFALL ALONG WITH WET GROUND CONDITIONS LIKELY TO EXIST
DUE TO MODERATE/HEAVY RAINFALL THE DAY BEFORE. IT WILL TAKE
ADDITIONAL TIME TO RESOLVE SPECIFICS THAT WILL DEPEND ON THE EXACT
TRACK OF THE UPPER LOW AND IF/HOW LONG IT POTENTIALLY STALLS. BY
LATE WEEK INTO NEXT WEEKEND, PORTIONS OF THE SOUTHERN PLAINS MAY
GET ADDITIONAL ROUNDS OF HEAVIER RAINFALL, WITH SEVERAL INCHES OF
RAIN POSSIBLE IN SOME AREAS OF TEXAS. THE COMBINATION OF
PERSISTENT LOW LEVEL FLOW FROM THE WESTERN GULF, A STALLED SURFACE
FRONT, AND ONE OR MORE SOUTHERN STREAM IMPULSES ALOFT SHOULD
CONTRIBUTE TO THIS POTENTIALLY HEAVY RAINFALL. AS THE UPPER LOW
CROSSES THE GREAT BASIN AND NORTHERN ROCKIES, THERE SHOULD BE
SUFFICIENT COLD AIR ALOFT TO SUPPORT LATE SEASON HIGHER ELEVATION
SNOW, ESPECIALLY OVER THE HIGHER MOUNTAIN RANGES OF NORTHERN
WYOMING AND WESTERN MONTANA. SOME OF THE CENTRAL U.S. MOISTURE
MAY EXTEND INTO CENTRAL LATITUDES OF THE EAST BY THE LATTER HALF
OF THE PERIOD AS THE PLAINS SYSTEM AND LEADING FRONT GRADUALLY
PUSH EASTWARD.
IN TERMS OF TEMPERATURES, HIGHS WILL LIKELY BE RUNNING 5 TO 15
DEGREES ABOVE EARLY MAY AVERAGES FROM THE CENTRAL/NORTHERN PLAINS
TO THE DEEP SOUTH FOR THE MIDDLE OF THE WEEK, AND THEN REACHING
INTO THE MIDWEST STATES AND THE GREAT LAKES/NORTHEAST REGION GOING
INTO THE END OF THE WEEK. NORTHERN PARTS OF THE EAST MAY TREND
COOLER BY NEXT SUNDAY. WARMTH WILL BUILD ACROSS THE PACIFIC
NORTHWEST BY FRIDAY INTO THE WEEKEND WITH HIGHS POTENTIALLY UP TO
20-25 DEGREES ABOVE MID-MAY AVERAGES ACROSS WESTERN WASHINGTON AND
OREGON. LOWS MAY BE A LITTLE LESS EXTREME BUT COULD STILL
CHALLENGE DAILY RECORDS FOR WARM LOWS BY THE WEEKEND (ESPECIALLY
SUNDAY). HIGHS WILL LIKELY BE SLIGHTLY BELOW AVERAGE ACROSS
PORTIONS OF THE WEST COAST STATES AND THE INTERMOUNTAIN WEST IN
ASSOCIATION WITH THE UPPER TROUGH THROUGH THURSDAY BEFORE A
MODERATING TREND COMMENCES. OVERNIGHT LOWS OVER THOSE AREAS EARLY
IN THE PERIOD SHOULD BE CLOSER TO AVERAGE WHILE AREAS EAST OF THE
ROCKIES SHOULD SEE MOSTLY ABOVE AVERAGE LOWS WITH INCREASED CLOUD
COVER AND HUMIDITY.
RAUSCH/HAMRICK
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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES 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
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