436
FXUS02 KWBC 010701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT MON MAY 01 2023
VALID 12Z THU MAY 04 2023 - 12Z MON MAY 08 2023
...PROGRESSIVELY COOL AND WET PATTERN FOR THE WEST COAST/GREAT
BASIN/ROCKIES THIS WEEK...
..OVERVIEW
WHILE A STUBBORN OMEGA BLOCK UPPER-LEVEL PATTERN EARLY THIS WEEK
WILL FINALLY START TO EVOLVE LATE THIS WEEK/WEEKEND, MEAN
TROUGHING OVER THE WEST WILL REMAIN IN PLACE THROUGH THE WEEKEND
LEADING TO STEADILY INCREASING AND WIDESPREAD PRECIPITATION
CHANCES. SHOWERS AND STORMS ALSO LOOK TO EXPAND ACROSS THE CENTER
OF THE COUNTRY AS INCREASING MOISTURE AND CLASHING FRONTAL SYSTEMS
LEAD TO DAILY ROUNDS OF CONVECTION. TEMPERATURES AND CONDITIONS
SHOULD FINALLY START TO MODERATE THIS WEEKEND OVER THE GREAT
LAKES/NORTHEAST AFTER A COOL AND WET SEVERAL DAYS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE SHOWS THAT THE STUBBORN OMEGA BLOCK UPPER-LEVEL
PATTERN THAT HAS BEEN FORECAST TO CONTINUE THROUGH AT LEAST
MID-WEEK BEGINS TO EVOLVE BY THE WEEKEND INTO EARLY NEXT WEEK,
WITH A LOT OF COMPLEX SMALLER-SCALE DETAILS THAT WILL LIKELY
REMAIN HARDER TO PIN DOWN, ESPECIALLY AT THIS TIME RANGE. THE
PATTERN BEGINS TO SHIFT FROM THE GREAT LAKES/NORTHEAST UPPER-LOW,
RIDGE, WEST COAST UPPER-LOW PHASING THAT HAS BEEN IN PLACE BY THE
START OF THE CURRENT UPDATED FORECAST PERIOD ON THURSDAY. WHILE
GENERAL MEAN TROUGHING/RIDGING/TROUGHING LOOKS TO REMAIN IN PLACE,
ESPECIALLY WITH RESPECT TO THE RIDGE AND WESTERN TROUGH, THE TWO
CLOSED LOWS ARE FORECAST TO WEAKEN AND SHIFT EASTWARD. THE RIDGE
AXIS IS ALSO EXPECTED TO SHIFT EASTWARD, THOUGH THE GUIDANCE
DIFFERS ON EXACTLY HOW FAST AND HOW FAR THIS OCCURS DURING THE
CURRENT FORECAST PERIOD.
THE DETERMINISTIC GUIDANCE SHOWS GENERAL AGREEMENT IN THE EARLY TO
MID-FORECAST PERIOD DEPICTING THE CLOSED LOW OVER THE GREAT
LAKES/NORTHEAST WEAKENING AND SHIFTING EASTWARD OVER THE ATLANTIC,
WITH A WAVE OF UPPER-LEVEL ENERGY SHIFTING SOUTHWARD OVER THE
UPPER MIDWEST AND RETROGRESSING WESTWARD AS A CLOSED HIGH DEVELOPS
OVER CANADA. THE RIDGE OVER THE INTERMOUNTAIN WEST/PLAINS SHIFTS
EASTWARD OVER THE PLAINS/MISSISSIPPI VALLEY AS THE CLOSED LOW OVER
THE WEST COAST BEGINS TO WEAKEN AND SHIFT NORTHEASTWARD OVER THE
GREAT BASIN AND EVENTUALLY TOWARDS CANADA AS AN OPEN WAVE IN THE
MEAN FLOW, WITH RAMIFICATIONS ON PRECIPITATION COVERAGE OVER THE
WEST DURING THE PERIOD. THE ONE NOTABLE DIFFERENCE BETWEEN THE
DETERMINISTIC GUIDANCE MID-PERIOD IS GREATER AMPLIFICATION OF THE
RIDGE AND WESTERN TROUGH IN THE 12Z CMC, AND TO SOME EXTENT THE
12Z UKMET, COMPARED TO BOTH THE MOST RECENT GFS AND ECMWF AS WELL
AS THE ENSEMBLE MEANS. THE GUIDANCE SHOWS MEAN RIDGING BECOMING
DOMINANT OVER THE CENTRAL/EASTERN CONUS, FINALLY BRINGING AN END
TO THE OMEGA BLOCK PATTERN CENTERED OVER THE CONUS, WITH MEAN
TROUGHING ALONG THE WEST COAST. A RECENT TREND RUN-TO-RUN IN THE
ECMWF AS WELL AS THE MORE RECENT 12Z/18Z RUNS OF THE GFS,
ESPECIALLY THE 12Z, IS FOR ONE PARTICULAR SHORTWAVE TO MOVE
EASTWARD OVER THE PLAINS AND INTO THE OHIO VALLEY, BREAKING DOWN
THE RIDGE. THE 12Z CMC CONTINUES TO SHOW MORE HIGHLY AMPLIFIED
MEAN RIDGING/TROUGHING WITHOUT MUCH OF A SIGNAL FOR THIS FEATURE.
ONE LAST NOTABLE DETAIL IS A ANOTHER WAVE OF UPPER-LEVEL ENERGY
DROPPING SOUTH IN THE MEAN FLOW FROM THE NORTHEASTERN PACIFIC,
POTENTIALLY AS A CLOSED LOW AS SHOWN IN THE 12Z ECMWF AND ECENS
MEAN, AND TO A LESSER EXTENT WEAKER BUT HINTED AT IN THE 12Z AND
18Z GFS. THE PATTERN REMAINS MORE AMPLIFIED IN THE 12Z CMC DURING
THIS TIME FRAME. THE UPDATED 00Z GFS CONTINUED DECENT AGREEMENT
WITH THE 12Z ECMWF AND THE ENSEMBLE MEANS IN HANDLING THE
POTENTIAL SHORTWAVE OVER THE PLAINS AND THE CLOSED LOW UPSTREAM
OFF THE PACIFIC COAST.
THE UPDATED WPC FORECAST PACKAGE BEGINS WITH A GENERAL MODEL BLEND
GIVEN OVERALL AGREEMENT ACROSS THE GUIDANCE ON THE LARGE-SCALE FOR
THE EARLY FORECAST PERIOD. CONTRIBUTIONS FROM THE 12Z CMC AND
UKMET ARE STEADILY REDUCED BY MID-PERIOD AS THEIR HANDLING OF THE
EVOLUTION OF SUBSEQUENT TROUGHING OVER THE WEST COAST FOLLOWING
THE DEPARTURE OF AN INITIAL CLOSED LOW DIFFERS FROM THE OTHER
DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE. CONTRIBUTIONS FROM THE
12Z ECENS MEAN AND 18Z GEFS MEAN ARE INCREASED MID- TO LATE PERIOD
IN REPLACEMENT OF THE CMC/UKMET AS ALREADY CUMBERSOME
SMALLER-SCALE DETAILS IN A COMPLEX PATTERN EVOLUTION REMAIN HARD
TO RESOLVE. THE CONTINUED INCLUSION OF THE GFS/ECMWF DETERMINISTIC
GUIDANCE KEEPS MORE OF THE DETAIL OF THE ONE SHORTWAVE MOVING
EASTWARD OVER THE PLAINS WHICH MAY HAVE IMPACTS ON THE SURFACE
FRONTAL STRUCTURE/ORGANIZATION IN THE LATE PERIOD. THE RESULTANT
BLEND HELPS TO RETAIN SOME INFLUENCE FROM THIS FEATURE, WHICH IS
EVEN INDICATED BY THE ECENS MEAN.
..WEATHER/HAZARDS HIGHLIGHTS
PRECIPITATION CHANCES WILL STEADILY INCREASE FROM THE WEST COAST
THROUGH THE GREAT BASIN AND NORTHERN ROCKIES MID- TO LATE WEEK AS
A CLOSED LOW OVER THE WEST EJECTS NORTHEASTWARD OVER THE GREAT
BASIN AND EVENTUALLY NORTHWARD INTO CANADA. THE PRECIPITATION MAY
BE LOCALLY MODERATE TO HEAVY AT TIMES, WITH SOME SCATTERED
THUNDERSTORM CHANCES. SNOW LEVELS WILL BEGIN HIGH BUT ARE FORECAST
TO FALL THROUGH THE LATTER PART OF THE WEEK AS THE
UPPER-LOW/TROUGH PROGRESSES INLAND AND PROVIDES COOL AIR ALOFT.
THE HIGHEST SNOW CHANCES WILL BE FOR THE SIERRA THURSDAY. THE
RAINFALL ON TOP OF ONGOING SNOW MELT MAY LEAD TO SOME FLOODING
CONCERNS IN TERRAIN SENSITIVE AREAS AROUND THE FOOTHILLS OF LOCAL
MOUNTAIN RANGES AS WELL AS ALONG LOCAL STREAMS/RIVERS
THURSDAY-FRIDAY, BUT THIS WILL LIKELY BE DRIVEN MORE BY LOCAL
HYDROLOGICAL CONDITIONS HARDER TO ANTICIPATE AT THIS RANGE. MOIST,
UPSLOPE FLOW SETTLING IN ALONG PORTIONS OF THE NORTHERN/CENTRAL
HIGH PLAINS ALSO LOOK TO LEAD TO SOME LOCALLY MODERATE TO HEAVY
RAINFALL LATE THIS WEEK INTO THE WEEKEND. HIGH TEMPERATURES IN
CALIFORNIA AND ADJACENT PORTIONS OF THE GREAT BASIN/SOUTHWEST WILL
BE RUNNING AS MUCH AS 10-15 DEGREES BELOW AVERAGE THURSDAY-FRIDAY
UNDER THE INFLUENCE OF THE UPPER-LEVEL LOW. IN CONTRAST,
ANOMALOUSLY WARM TEMPERATURES THAT HAVE BEEN IN PLACE OVER THE
NORTHERN GREAT BASIN/ROCKIES WILL AGAIN BE RUNNING 15-20 DEGREES
ABOVE AVERAGE AT LEAST ONE MORE DAY ON THURSDAY BEFORE THE
APPROACHING UPPER-LEVEL WAVE AND PROGRESSIVE FRONTAL SYSTEMS COOL
TEMPERATURES DOWN THROUGH THE WEEKEND.
TO THE EAST, GULF MOISTURE FLOWING NORTHWARD AS WELL AS ADDITIONAL
MOISTURE SOURCED FROM THE PACIFIC WILL SPREAD NORTHEASTWARD ACROSS
THE CENTRAL/SOUTHERN PLAINS AND MIDDLE/LOWER MISSISSIPPI VALLEY
INTO THE SOUTHEAST MID- TO LATE WEEK LEADING TO A BROAD REGION OF
SHOWERS AND THUNDERSTORMS. THE EVOLUTION OF THE PATTERN HAS
REMAINED RATHER NEBULOUS THE LAST FEW DAYS AS THE GUIDANCE HAS
VARIED ON THE TIMING AND EVOLUTION OF A 'BACKDOOR' COLD FRONT
SINKING SOUTHWESTWARD AS WELL AS EMBEDDED SHORTWAVE ENERGY ALOFT,
LESSENING CONFIDENCE ON THE SPECIFICS OF REGIONS OF HEAVIER
RAINFALL AND POTENTIAL FLASH FLOODING CONCERNS. HOWEVER, THE
GUIDANCE HAS SHOWN THE HIGHEST CONFIDENCE IN WIDESPREAD CONVECTION
AND HIGH QPF AMOUNTS OVER THE CENTRAL PLAINS/MIDDLE MISSISSIPPI
VALLEY ON THURSDAY, WHERE A DAY 4 "MARGINAL RISK" EXCESSIVE
RAINFALL OUTLOOK IS IN PLACE. THIS CONVECTION MAY TREND DOWNSTREAM
FRIDAY INTO THE TENNESSEE VALLEY, AND ADDITIONAL HEAVY RAINFALL
CHANCES LOOK POSSIBLE OVER THE WEEKEND MORE BROADLY ACROSS THE
CENTRAL/SOUTHEASTERN U.S., BUT CONFIDENCE REMAINS LOW.
THE COOL/WET PATTERN THIS PAST WEEK/WEEKEND OVER THE GREAT LAKES
AND NORTHEAST UNDER THE INFLUENCE OF A STUBBORN UPPER-LEVEL LOW
WILL FINALLY START TO MODERATE LATER THIS WEEK AS THE LOW DEPARTS
OVER THE ATLANTIC. PERIODS OF LIGHT TO MODERATE PRECIPITATION
SHOULD LINGER WEDNESDAY ACROSS THE NORTHEAST BEFORE LIGHTENING
THURSDAY-FRIDAY AS HIGH PRESSURE OVER CANADA BUILDS SOUTHWARD.
TEMPERATURES WILL REMAIN BELOW AVERAGE BY AROUND 5-10 DEGREES
WEDNESDAY-THURSDAY BEFORE SETTLING IN RIGHT AROUND AVERAGE FOR
EARLY MAY BY THE WEEKEND.
PUTNAM
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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