627
FXUS02 KWBC 310704
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 AM EDT WED MAY 31 2023
VALID 12Z SAT JUN 03 2023 - 12Z WED JUN 07 2023
...BROAD MULTI-DAY THUNDERSTORM AND LOCAL HEAVY RAIN THREAT
CONTINUES FOR PARTS OF THE PLAINS/ROCKIES...
..OVERVIEW
DURING THE WEEKEND INTO NEXT WEEK, MODELS AND ENSEMBLES CONTINUES
TO SHOW A GRADUAL RETROGRESSION OF A STRONG UPPER RIDGE/HIGH OVER
THE UPPER MIDWEST AND SOUTHERN CANADA AS OF EARLY SATURDAY,
SUPPORTING A PERSISTENT AREA OF ABOVE NORMAL TEMPERATURES FROM THE
NORTHERN PLAINS INTO MIDDLE/UPPER MISSISSIPPI VALLEY. WEAK
SHORTWAVE/UPPER LOW ENERGY INITIALLY OVER THE WEST THROUGH
CENTRAL/SOUTHERN HIGH PLAINS WILL WEAKEN DURING THE WEEKEND BUT
THEN A WELL-ADVERTISED SOUTHERN STREAM PACIFIC LOW SHOULD TRACK
INTO CALIFORNIA EARLY NEXT WEEK. THESE FEATURES WILL MAINTAIN THE
POTENTIAL FOR PERIODS OF CONVECTION OVER THE ROCKIES AND PARTS OF
THE CENTRAL-SOUTHERN PLAINS, ALONG WITH GRADUALLY INCREASING
COVERAGE FROM THE SIERRA NEVADA INTO THE GREAT BASIN, BUT LIKELY
WITH SOMEWHAT LOWER RAINFALL TOTALS THAN EXPECTED IN THE SHORTER
TERM. EXPECT ABOVE NORMAL TEMPERATURES OVER THE NORTHWESTERN
STATES, THOUGH A FRONT AHEAD AN EJECTING PACIFIC UPPER LOW MAY
TEMPER ANOMALIES OVER THE PACIFIC NORTHWEST DURING THE WEEKEND.
DOWNSTREAM FROM THE UPPER RIDGE, GUIDANCE CONTINUES TO VARY WITH
TWO MAIN BUNDLES OF ENERGY FORECAST TO DROP SOUTHWARD WITHIN A
GENERAL EVOLUTION FAVORING A STEADILY AMPLIFYING AND DEEPENING
UPPER TROUGH OVER THE EAST. DETAILS REMAIN QUITE UNCERTAIN BUT
EXPECT ASSOCIATED FRONTS TO PRODUCE RAINFALL OF VARYING INTENSITY
AND THE DEEPENING TROUGH SHOULD INCREASE POTENTIAL FOR EPISODES OF
LATE DAY SHOWERS AS WELL.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GUIDANCE IS STILL SHOWING CONSIDERABLE DIFFICULTY IN RESOLVING
DETAILS OVER THE EASTERN U.S. WITHIN THE OVERALL PATTERN FAVORING
INCREASED TROUGHING TO THE EAST OF THE CENTRAL NORTH AMERICA
RIDGE. AS WAS THE CASE YESTERDAY, INDIVIDUAL SOLUTIONS START
DIVERGING EARLY IN THE PERIOD FOR WHERE THE UPPER LOW ASSOCIATED
WITH THE FIRST SHORTWAVE MAY TRACK DURING THE WEEKEND AND FIRST
HALF OF NEXT WEEK. THE 00Z GFS HAS COME IN WITH THE FARTHEST WEST
OPTION THUS FAR, OVER THE LOWER GREAT LAKES/OHIO VALLEY, WHILE THE
12Z ECMWF MEAN GOT NO FARTHER WEST THAN NOVA SCOTIA. 12Z/18Z
GUIDANCE ON WHICH THE UPDATED MANUAL FORECAST WAS BASED GENERALLY
RANGED BETWEEN CAPE COD AND NOVA SCOTIA FOR THE WESTERNMOST POINT
OF THE UPPER LOW'S TRACK (THE 18Z GFS BEING THE WESTERN EXTREME
BEFORE THE ARRIVAL OF THE 00Z RUN). THE 00Z UKMET HAS SHIFTED A
BIT WESTWARD BUT THE 00Z CMC/ECMWF KEEP CAPE COD AS THE FARTHEST
WEST POINT OF THE TRACK. THE SECOND AND LIKELY STRONGER SHORTWAVE
DROPPING THROUGH EASTERN CANADA INTO THE GREAT LAKES/NORTHEAST HAS
PLENTY OF SPREAD AS WELL, WITH A VARIETY OF IDEAS BETWEEN A DEEP
CLOSED LOW AND MORE OPEN TROUGH. THE 00Z GFS HAS ONE OF THE MORE
ORIGINAL EVOLUTIONS SEEN THUS FAR, PULLING A SMALL PIECE OF THE
SECOND SHORTWAVE OFF INTO THE GREAT LAKES TO MERGE WITH THE FIRST
UPPER LOW. BY NEXT TUESDAY-WEDNESDAY, RECENT ENSEMBLE MEANS ARE
SUGGESTING THE DEEPENING TROUGH WILL BE ANCHORED BY AN UPPER LOW
OVER OR NEAR MAINE, WHICH SEEMS TO BE A REASONABLE STARTING POINT
GIVEN THE UNCERTAINTY. MEANWHILE, THE NATIONAL HURRICANE CENTER
CONTINUES TO MONITOR A DISTURBANCE OVER THE GULF OF MEXICO
ASSOCIATED WITH A SOUTHERN TIER UPPER TROUGH. THESE FEATURES
SHOULD CROSS FLORIDA DURING THE WEEKEND, THOUGH WITH SOME
UNCERTAINTY IN SHORTWAVE DETAILS FOR THE UPPER TROUGH. WHAT
HAPPENS TO THE TROUGH AFTER SUNDAY WILL DEPEND ON DETAILS TO THE
NORTH.
THE FORECAST FARTHER WESTWARD IS LESS CHAOTIC IN MOST RESPECTS.
GUIDANCE HAS BEEN CONSISTENT IN WEAKENING INITIAL SHORTWAVE
ENERGY/ONE OR MORE UPPER LOWS OVER THE WEST OVER THE COURSE OF THE
WEEKEND. THERE HAVE BEEN SOME RECENT TRENDS TOWARD A LITTLE
FARTHER EASTWARD EXTENT OF HEIGHT FALLS AHEAD OF THE UPPER LOW
EJECTING FROM THE NORTHEASTERN PACIFIC, WITH CORRESPONDING EFFECTS
ON THE LEADING COLD FRONT AND PACIFIC NORTHWEST TEMPERATURES FOR A
TIME. NEW 00Z RUNS SHOW MORE DIVERGENCE REGARDING WHETHER SOME
ENERGY COULD LINGER OVER NORTHWESTERN WASHINGTON/VANCOUVER ISLAND.
THE MAJORITY CLUSTER OF GUIDANCE HAS HELD UP WELL FOR THE UPPER
LOW EXPECTED TO TRACK INTO CALIFORNIA NEXT WEEK, WITH MOST RECENT
CYCLES HAVING ONE OR TWO STRAY SOLUTIONS THAT ULTIMATELY TREND
CLOSER TO THE MEAN. IN THIS CASE THE 00Z GFS HAS RETURNED TO
CONSENSUS AFTER THE 12Z/18Z RUNS WERE ON THE FAST SIDE BY
TUESDAY-WEDNESDAY.
BASED ON THE ARRAY OF 12Z/18Z GUIDANCE, THE UPDATED FORECAST USED
A COMPOSITE OF 12Z OPERATIONAL MODELS ON DAY 3 SATURDAY AND THEN
GRADUALLY INCREASED 18Z GEFS/12Z ECMWF MEAN COMPONENTS SO THAT
THERE WAS AN EVEN MODEL/MEAN MIX BY DAYS 6-7 TUESDAY-WEDNESDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE PATTERN EVOLUTION ALOFT OVER THE WESTERN HALF OF THE COUNTRY
SHOULD KEEP A PERSISTENT THREAT FOR SHOWERS AND THUNDERSTORMS OVER
PARTS OF THE ROCKIES AND PLAINS DURING THE PERIOD, WITH DIURNALLY
FAVORED SHOWERS/STORMS ALSO LIKELY TO BECOME MORE COMMON WITH TIME
FROM THE SIERRA NEVADA-SOUTHERN CASCADES INTO THE GREAT BASIN AS
THE SOUTHERN STREAM PACIFIC UPPER LOW APPROACHES/REACHES
CALIFORNIA. GRADUAL WEAKENING OF UPPER IMPULSES/LOWS INITIALLY
OVER THE WEST/HIGH PLAINS SHOULD LEAD TO SOMEWHAT LOWER MAXIMUM
RAINFALL TOTALS THAN EXPECTED IN THE SHORTER TERM, BUT LOCALIZED
FLASH FLOOD POTENTIAL WILL LIKELY CONTINUE GIVEN WET GROUND
CONDITIONS FROM RECENT OR SOON TO OCCUR RAINFALL. THE DAYS 4-5
EXCESSIVE RAINFALL OUTLOOKS (COVERING SATURDAY THROUGH SUNDAY
NIGHT) DEPICT MARGINAL RISK AREAS ACROSS LOCATIONS FROM THE
NORTHERN ROCKIES INTO PARTS OF THE SOUTHERN PLAINS TO REFLECT THE
PERSISTENT MOISTURE/OVERALL PATTERN THAT COULD STILL PRODUCE SOME
AREAS OF LOCALLY HEAVY RAINFALL. IT WILL LIKELY TAKE INTO THE
SHORTER RANGE TIME FRAME TO RESOLVE AREAS OF MORE PRONOUNCED
FOCUS/CONFIDENCE FOR ANY EMBEDDED SLIGHT RISK AREAS. THE DAY 5
OUTLOOK INCORPORATES THE SIERRA NEVADA/GREAT BASIN INTO THE
OVERALL MARGINAL RISK AREA WITH MOISTURE EXPECTED TO INCREASE
ACROSS THE REGION. MEANWHILE THE DAY 4 OUTLOOK HAS ADDED A
MARGINAL RISK AREA TO REFLECT THE POTENTIAL FOR SOME LINGERING
HEAVY RAINFALL ACROSS THE FLORIDA PENINSULA. GUIDANCE IS SPLIT ON
TIMING OF THE UPPER TROUGH/ASSOCIATED SURFACE REFLECTION BUT
RAINFALL FROM PREVIOUS DAYS WOULD LOWER THE THRESHOLD FOR ANY
POTENTIAL ISSUES. ELSEWHERE, THERE IS CONSIDERABLE UNCERTAINTY
OVER WHAT A LOW-PREDICTABILITY SHORTWAVE TRACKING UNDERNEATH THE
UPPER RIDGE MAY DO TO PROMOTE CONVECTION OVER PARTS OF THE
NORTHERN TIER DURING THE WEEKEND, WHILE RAINFALL DETAILS OVER THE
EAST CONTINUE TO DEPEND ON UNCERTAIN SPECIFICS ALOFT. IN GENERAL
EXPECT THE POTENTIAL FOR SOME SHOWERS/STORMS WITH A COUPLE COLD
FRONTS DROPPING SOUTH DURING THE PERIOD, WITH A POTENTIAL TREND
TOWARD DIURNALLY FAVORED POCKETS OF RAINFALL IN THE NORTHEAST AS
UPPER TROUGHING DEEPENS NEXT WEEK. MOST OF ANY MOISTURE STILL
CROSSING FLORIDA DURING THE WEEKEND SHOULD PROGRESS OFFSHORE
THEREAFTER, THOUGH STILL WITH SOME LINGERING UNCERTAINTY GIVEN THE
GUIDANCE SPREAD FOR THE PATTERN FARTHER NORTH.
THE AREA OF WELL ABOVE NORMAL TEMPERATURES FROM THE NORTHERN
PLAINS THROUGH UPPER/MIDDLE MISSISSIPPI VALLEY INTO THE CENTRAL
APPALACHIANS AS OF SATURDAY SHOULD GET STEADILY TRIMMED AWAY ON
THE EASTERN SIDE AS UPPER TROUGHING DEVELOPS OVER THE EAST WITH
INCREASING COVERAGE OF NEAR TO SOMEWHAT BELOW NORMAL READINGS.
THE WEEKEND SHOULD START WITH DECENT COVERAGE OF HIGHS 10-20F
ABOVE NORMAL, WITH SOME DAILY RECORDS POSSIBLE, FOLLOWED BY MOST
PLUS 10F OR GREATER ANOMALIES GETTING CONFINED TO THE NORTHERN
PLAINS BY NEXT WEDNESDAY. THE NORTHWEST WILL BE THE OTHER FOCUS
FOR VERY WARM TEMPERATURES, WITH HIGHS 10-20F OR SO ABOVE NORMAL
ESPECIALLY MONDAY-WEDNESDAY. AN EJECTING PACIFIC UPPER
LOW/LEADING FRONT SHOULD TEMPER THE WARMTH DURING THE WEEKEND OVER
WESTERN OREGON/WASHINGTON THOUGH. CLOUDS AND PERIODS OF RAINFALL
SHOULD KEEP HIGHS 5-15F BELOW NORMAL OVER THE SOUTHERN HALF OF THE
ROCKIES/HIGH PLAINS FOR MOST OF THE PERIOD AND THE UPPER LOW
REACHING CALIFORNIA WILL ALSO BRING A COOLING TREND THERE LATER IN
THE PERIOD.
RAUSCH
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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