731
FXUS02 KWBC 281900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SUN APR 28 2024
VALID 12Z WED MAY 01 2024 - 12Z SUN MAY 05 2024
...ROUNDS OF CONVECTION ARE LIKELY IN THE CENTRAL U.S. FOR MUCH OF
THE WEEK...
..OVERVIEW
INTO MIDWEEK, MEAN TROUGHING WILL AMPLIFY OVER THE INTERMOUNTAIN
WEST AND PROVIDE SUPPORT FOR WIDESPREAD CONVECTION OVER MUCH OF THE
CENTRAL U.S., WITH HEAVY RAIN AND FLASH FLOODING LIKELY WEDNESDAY
AND THURSDAY ACROSS MUCH OF THE PLAINS AND MISSISSIPPI VALLEY.
UPPER TROUGHING AND A SURFACE COLD FRONT PRESSING EAST INTO LATE
WEEK SHOULD PUSH RAIN CHANCES INTO THE EAST-CENTRAL AND EASTERN
U.S., THOUGH SOME MAY REMAIN OVER THE SOUTHERN PLAINS NEAR A
LINGERING FRONTAL BOUNDARY. ADDITIONALLY, PRECIPITATION INCLUDING
SOME HIGHER ELEVATION SNOW IS POSSIBLE AT TIMES ACROSS THE
NORTHWEST INTO THE NORTHERN/CENTRAL ROCKIES, BUT CONFIDENCE IN THE
DETAILS IS STILL LOWER THAN DESIRED. THERE IS ALSO CONSIDERABLE
SPREAD FOR SPECIFICS OF AN AMPLIFYING EASTERN PACIFIC UPPER TROUGH
THAT MAY AFFECT PARTS OF THE WEST BY NEXT WEEKEND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GUIDANCE IS SHOWING SIMILARITIES FOR SOME ASPECTS OF THE GENERAL
PATTERN BUT THERE IS STILL CONSIDERABLE SPREAD FOR IMPORTANT
FEATURES. THE MOST PROMINENT DIFFERENCES INVOLVE THE UPPER TROUGH
INITIALLY AMPLIFYING OVER AND EJECTING FROM THE WEST TO PRODUCE
PLAINS THROUGH GREAT LAKES LOW PRESSURE, A COMPACT UPPER LOW
REACHING WESTERN NORTH AMERICA LATE THIS WEEK, AND EASTERN PACIFIC
UPPER TROUGHING THAT SHOULD AFFECT THE WEST BY LATE WEEK/NEXT
WEEKEND.
THE FORECAST REMAINS SOMEWHAT COMPLEX FOR THE WESTERN-CENTRAL U.S.
DURING THE MID-LATE WEEK TIME FRAME, WITH DYNAMICAL/MACHINE
LEARNING (ML) MODELS AND ENSEMBLES SHOWING VARIOUS IDEAS FOR THE
COMBINATION OF THE UPPER TROUGH EJECTING FROM THE WEST AND UPPER
LOW CIRCULATION CENTERED OVER THE NORTHERN PLAINS/SOUTHERN CANADA.
THE 00Z ECMWF-INITIALIZED MLS AT LEAST GENERALLY FAVOR AN
INTERMEDIATE SOLUTION FOR THE WESTERN U.S./PLAINS FEATURE, BETWEEN
THE FASTER ECMWF AND SLOW/DEEP/CLOSED 06Z GFS--WHILE SHOWING
PERHAPS A LITTLE DEEPER CLOSED LOW ALONG THE CANADIAN BORDER THAN
THE AVERAGE OF DYNAMICAL GUIDANCE. MEANWHILE AS OF EARLY FRIDAY
THE MLS FAVOR ASSOCIATED SURFACE LOW PRESSURE OVER THE WESTERN
GREAT LAKES.
THERE IS NOT YET ANYTHING CLOSE TO A RESOLUTION FOR THE TRACK OF
THE COMPACT UPPER LOW REACHING BRITISH COLUMBIA OR THE PACIFIC
NORTHWEST BY THURSDAY, WITH CONTINUED SIGNIFICANT SPREAD AND MODEL
VARIABILITY. ECMWF AND GFS RUNS HAVE SWAPPED PLACES OVER THE PAST
DAY WITH THE GFS NOW HAVING IT TRACK INTO BRITISH COLUMBIA AND
ECMWF (PLUS THE CMC) FARTHER SOUTH. ENSEMBLE MEANS AND THE MLS
FAVOR BRITISH COLUMBIA, SO PREFERENCE REMAINS ALONG THAT TRACK. AS
WAS THE CASE YESTERDAY THERE IS BETTER CONVERGENCE OVER THE FAR
NORTHERN ROCKIES/CANADIAN BORDER BY FRIDAY.
OVER THE EASTERN PACIFIC/WEST COAST BY LATE WEEK AND NEXT WEEKEND,
THE AVERAGE OF MLS RECOMMEND AN UPPER TROUGH THAT COULD BE DEEPER
THAN SOME OF THE DYNAMICAL GUIDANCE OFFSHORE THE NORTHWEST AS OF
EARLY SATURDAY AND THEN CLOSEST IN PRINCIPLE TO LATEST CMC RUNS/00Z
ECMWF AND THE ENSEMBLE MEANS BY SUNDAY WITH A BROAD OPEN TROUGH
ALIGNED NEAR THE WEST COAST. THERE IS A MINORITY ML SOLUTION HAS A
CLOSED LOW NEAR THE WASHINGTON COAST, WHICH IS ONE DAY SLOWER THAN
THE NEW 12Z UKMET. RECENT GFS RUNS HAVE BEEN PERSISTENT IN PULLING
AN UPPER LOW FARTHER SOUTH, CLOSER TO CALIFORNIA. SOME GEFS MEMBERS
OFFER THIS SCENARIO BUT OTHERWISE THERE IS MINIMAL SUPPORT FOR THE
GFS EVOLUTION. THE NEW 12Z ECMWF INTRODUCES ANOTHER POSSIBILITY,
CONTINUED INLAND PROGRESSION OF AN UPPER LOW THROUGH THE NORTHWEST
DURING THE WEEKEND.
THE EARLY-MID PART OF THE UPDATED FORECAST INCORPORATED A MULTI-
MODEL/MULTI-RUN BLEND OF THE GFS/ECMWF/CMC TO ACHIEVE THE DESIRED
INTERMEDIATE SOLUTION FOR THE WESTERN-CENTRAL U.S. EVOLUTION AND
FAVORED TRACK FOR THE COMPACT UPPER LOW TRACKING INTO WESTERN NORTH
AMERICA. THEN THE BLEND PHASES OUT THE 06Z GFS AND THEN THE 00Z
GFS, ALONG WITH THE 12Z/27 ECMWF, DUE TO ISSUES ALONG THE WEST
COAST WHILE STEADILY INCREASING 06Z GEFS/00Z ECENS INPUT TO 60
PERCENT BY SUNDAY.
..WEATHER/HAZARDS HIGHLIGHTS
ON WEDNESDAY, INGREDIENTS FOR WIDESPREAD RAIN AND THUNDERSTORMS
POTENTIALLY CAUSING FLASH FLOODING WILL BE IN PLACE ACROSS CENTRAL
PARTS OF THE COUNTRY. LOWERING HEIGHTS AS UPPER TROUGHING DIGS IN
THE INTERMOUNTAIN WEST WILL PROVIDE LIFT AND COMBINE WITH AMPLE
MOISTURE AND INSTABILITY. ON WEDNESDAY MODEL GUIDANCE ALREADY SHOWS
HEAVY RAIN TOTALS OF 3-5 INCHES IN SOME AREAS AND HIGH RAIN RATES
ARE LIKELY AS WELL. A LARGE SLIGHT RISK REMAINS IN PLACE FOR THE
DAY 4/WEDNESDAY ERO FOR THE CENTRAL/SOUTHERN PLAINS AND STRETCHING
INTO THE MID-MISSISSIPPI VALLEY. WITHIN THIS RISK AREA, THERE MAY
BE SOMEWHAT GREATER POTENTIAL FROM EASTERN KANSAS/SOUTHEAST
NEBRASKA INTO NORTHWEST MISSOURI AND MUCH OF IOWA GIVEN PROXIMITY
TO THE STALLING SURFACE FRONT AND CENTRAL PLAINS WAVE DEVELOPMENT
BY WEDNESDAY NIGHT. THERE REMAINS A LITTLE MORE SPREAD FARTHER
SOUTH. THEN FOR THURSDAY, ANOTHER LARGE SLIGHT RISK SERVES AS A
STARTING POINT FOR THE DAY 5 ERO JUST SHIFTED A BIT EASTWARD FROM
WEDNESDAY AS THE SURFACE FRONT BEGINS TO PRESS EASTWARD. MANY
LOCATIONS WITHIN THESE AREAS WILL BE SENSITIVE TO ADDITIONAL RAIN
DUE TO WET ANTECEDENT CONDITIONS, AND POTENTIAL FOR EMBEDDED
UPGRADES IS POSSIBLE DEPENDING ON DETAILS OF PRIOR CONVECTION AND
HOW GUIDANCE CONSOLIDATES FOR AMOUNTS/LOCATION OF HEAVIEST
RAINFALL. ONE REGION OF INTEREST IN THAT REGARD MAY EXTEND FROM
EASTERN OKLAHOMA/NORTHEASTERN TEXAS INTO WESTERN ARKANSAS, AS
GUIDANCE SUGGESTS SOMEWHAT SLOWER FRONTAL MOTION THAN TO THE NORTH
AND THERE MAY BE RELATIVELY GREATER SENSITIVITY DUE TO WET GROUND.
BY FRIDAY, THE COLD FRONT PROGRESSING EAST IN THE NORTHERN TWO-
THIRDS OF THE COUNTRY OR SO WILL TRANSITION RAIN CHANCES MORE INTO
THE GREAT LAKES AND OHIO/TENNESSEE VALLEYS, AND TOWARD THE
APPALACHIANS TO EASTERN SEABOARD ON SATURDAY. HOWEVER, THE FRONTAL
BOUNDARY LOOKS TO LINGER LONGER ACROSS THE THE SOUTHERN PLAINS
(POSSIBLY BEGINNING TO LIFT NORTH AS A WARM FRONT BY NEXT
SUNDAY) AND PRODUCE ADDITIONAL ROUNDS OF RAIN AND STORMS THROUGH
THE WEEKEND.
MEANWHILE, AREAS FROM THE NORTHWEST INTO THE NORTHERN AND CENTRAL
ROCKIES MAY SEE ROUNDS OF PRECIPITATION INCLUDING SNOW IN HIGHER
ELEVATIONS THROUGH THE LATTER HALF OF THE WEEK INTO THE WEEKEND. ON
WEDNESDAY-THURSDAY, THE COOLING ASSOCIATED WITH THE UPPER TROUGH
COULD BRING SOME EARLY MAY SNOW INTO THE NORTHERN AND CENTRAL
ROCKIES. THERE IS SOME CHANCE THAT SNOW MAY MAKE IT INTO THE FRONT
RANGE OF COLORADO ON THURSDAY. THERE IS STILL QUITE A BIT OF MODEL
SPREAD INTO LATE WEEK, WHICH AFFECTS THE PRECIPITATION FORECAST.
CURRENTLY THE BEST GUIDANCE CLUSTER SHOWS ANOTHER ROUND OF
PRECIPITATION IN THE NORTHERN ROCKIES FRIDAY WITH A COMPACT UPPER
SYSTEM CROSSING THE AREA, WHILE AN UPSTREAM PACIFIC SYSTEM RENEWS
PRECIPITATION OF VARYING INTENSITY ACROSS THE NORTHWEST FROM LATE
FRIDAY OR SATURDAY THROUGH SUNDAY.
TEMPERATURES ACROSS THE SOUTH-CENTRAL U.S. ALL THE WAY TO THE
EASTERN SEABOARD WILL BE WARMER THAN AVERAGE BY GENERALLY 10-15F
ON WEDNESDAY, WHILE COOLER THAN NORMAL TEMPERATURES (WITH MORE
LOCALIZED COVERAGE OF MINUS 10-15F ANOMALIES) ARE LIKELY FOR THE
NORTHWESTERN QUADRANT OF THE LOWER 48. THEN UPPER TROUGHING AND A
COLD FRONT PUSHING FROM THE NORTHWEST SHOULD COOL TEMPERATURES TO
NEAR OR BELOW NORMAL ACROSS THE CENTRAL U.S. ON THURSDAY OR FRIDAY
AND REACHING THE EAST-CENTRAL U.S. FRIDAY AND SATURDAY.
TEMPERATURES OVER THE WEST MAY REBOUND TO NEAR NORMAL OR MODERATELY
ABOVE NORMAL LEVELS BY LATE WEEK INTO NEXT WEEKEND. CONFIDENCE IN
THIS ASPECT OF THE FORECAST IS STILL NOT GREAT DUE TO GUIDANCE
SPREAD FOR UPPER FLOW DETAILS, BUT A DECENT RANGE OF SOLUTIONS
WOULD AT LEAST SUPPORT SOME WARMUP FOR THE SOUTHERN HALF OF THE
WEST.
RAUSCH/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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