426
FXUS02 KWBC 200703
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 AM EDT THU JUN 20 2024
VALID 12Z SUN JUN 23 2024 - 12Z THU JUN 27 2024
...HEAT WAVE PERSISTS FOR THE MID-ATLANTIC SUNDAY WITH ELEVATED
HEAT OVER THE SOUTH/PLAINS THROUGH MUCH OF NEXT WEEK...
..HEAVY RAINFALL THREAT OVER PARTS OF NEW ENGLAND INTO SUNDAY
..OVERVIEW
GUIDANCE SHOWS THE UPPER PATTERN SETTLING INTO A STABLE REGIME
FEATURING A PERSISTENT UPPER HIGH ANCHORED OVER THE SOUTHERN
ROCKIES AND VICINITY, WITH SOME RIDGING EXTENDING TO THE NORTH,
WHILE THE NORTHEAST PACIFIC/WEST COAST AND EASTERN NORTH AMERICA
SEE VARYING DEGREES OF MEAN TROUGHING. INDIVIDUAL SHORTWAVES WITHIN
THE WESTERLIES CROSSING THE SOUTHERN CANADA/NORTHERN TIER U.S. WILL
SUPPORT A SERIES OF PROGRESSIVE SOUTHERN CANADA SURFACE SYSTEMS AND
TRAILING FRONTS EXTENDING INTO THE NORTHERN HALF TO TWO-THIRDS OF
THE LOWER 48. A LEADING SYSTEM AFFECTING THE NORTHEAST ON
SUNDAY MAY PRODUCE SOME AREAS OF HEAVY RAINFALL, WHILE THE
TRAILING FRONT AND ANOTHER ONE BEHIND IT MAY PRODUCE EPISODES OF
CONVECTION OVER THE CENTRAL/EASTERN U.S. EXPECT HAZARDOUS HEAT TO
EXTEND INTO SUNDAY OVER THE MID-ATLANTIC, WITH HIGH HEAT INDICES
LIKELY ACROSS THE SOUTH/PLAINS AND SOUTHWEST FOR MOST OF NEXT WEEK.
THERE MAY BE A REBOUND OF SOME HEAT INTO THE MID-ATLANTIC AROUND
MIDWEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST DYNAMICAL AND MACHINE LEARNING (ML) GUIDANCE AGREES FAIRLY
WELL FOR THE OVERALL PATTERN BUT DISPLAYS TYPICAL SPREAD AND RUN-
TO-RUN VARIABILITY FOR THE DETAILS. FOR THE LEADING SYSTEM OVER
EASTERN NORTH AMERICA, THE 18Z GFS STRAYED TO THE STRONG/NORTHWEST
SIDE FOR THE SURFACE LOW WHILE THE NEW 00Z UKMET HAS COME IN ON THE
SLOW SIDE FOR THE UPPER TROUGH BY TUESDAY. MODELS ARE STILL HAVING
A TOUGH TIME DETERMINING WHEN THERE MAY BE A LOW EMBEDDED WITHIN
THE UPPER TROUGH, AFFECTING THE SURFACE EVOLUTION. UPSTREAM, LATEST
GUIDANCE IS HOLDING ONTO RECENT TRENDS TOWARD PROGRESSIVE PACIFIC
ENERGY EVOLVING INTO A SOUTHERN CANADA/NORTHERN TIER U.S. SHORTWAVE
THAT FEEDS INTO THE EASTERN NORTH AMERICA MEAN TROUGH, WHILE
REMAINING UPPER LOW ENERGY NEAR/OFFSHORE BRITISH COLUMBIA MAY
COMBINE WITH INCOMING NORTH PACIFIC ENERGY TO PRODUCE A WEST COAST
UPPER TROUGH BY NEXT THURSDAY WITH AN INCREASINGLY BETTER DEFINED
LEADING SURFACE SYSTEM. FARTHER EAST BY NEXT THURSDAY, IT WILL BE
WORTH WATCHING DYNAMICAL VERSUS ML GUIDANCE AS THE 12Z ECMWF AND
LATEST GFS RUNS ARE SOMEWHAT WEST OF MOST 12Z ECMWF-INITIALIZED ML
RUNS FOR THE UPPER WEAKNESS THAT DEVELOPS OVER AND SOUTH OF THE
TENNESSEE VALLEY.
BASED ON 12Z/18Z GUIDANCE COMPARISONS, THE UPDATED FORECAST STARTED
WITH A COMPOSITE OF 12Z MODELS (18Z GFS LOOKING TOO SHALLOW WITH
THE EASTERN TROUGH BY MONDAY) FOR ABOUT THE FIRST HALF OF THE
PERIOD. THE THE BLEND INCORPORATED SOME 18Z GEFS/12Z ECENS MEANS
WHILE ULTIMATELY TRANSITIONING THE GFS COMPONENT TOWARD THE 18Z RUN
BY NEXT THURSDAY (12Z GFS TRENDING FLAT WITH THE LATE-PERIOD
EASTERN TROUGH).
..WEATHER/HAZARDS HIGHLIGHTS
THE SOUTHEASTERN CANADA SURFACE LOW AND TRAILING FRONT AFFECTING
THE NORTHEAST ON SUNDAY AND PERHAPS TO A LESSER DEGREE INTO MONDAY
WILL HAVE THE POTENTIAL TO PRODUCE SOME AREAS OF LOCALLY HEAVY
RAINFALL ACROSS THE REGION. THERE IS CONTINUED SPREAD IN THE
GUIDANCE FOR EXACTLY HOW THIS SYSTEM WILL EVOLVE AND THUS FOR
RAINFALL DETAILS. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK COVERING
SUNDAY-SUNDAY NIGHT MAINTAINS A SLIGHT RISK AREA OVER PARTS OF
NORTHERN-CENTRAL NEW ENGLAND WITH MODEST ADJUSTMENTS BASED ON
LATEST MODEL/ENSEMBLE GUIDANCE AND FIRST-GUESS FIELDS. THE
SURROUNDING MARGINAL RISK THAT EXTENDS AS FAR SOUTH AS THE CENTRAL
APPALACHIANS IS ALSO CLOSE TO CONTINUITY AND REFLECTS THE
COMBINATION OF POTENTIAL FOR LOCALLY HEAVY ACTIVITY AND TERRAIN
SENSITIVITY. THE DAY 4 ERO MAINTAINS A MARGINAL RISK AREA OVER FAR
SOUTHERN TEXAS WHERE MOIST FLOW TO THE NORTH OF A SECOND POSSIBLE
WESTERN GULF SURFACE LOW MAY PRODUCE SOME ENHANCED RAINFALL. IN
THE DAY 5 ERO PERIOD (MONDAY- MONDAY NIGHT), GUIDANCE IS STARTING
TO SUGGEST AN AREA OF POSSIBLE CONVECTION OVER THE UPPER
MIDWEST/WESTERN GREAT LAKES AHEAD OF AN ADVANCING WARM FRONT. WET
GROUND CONDITIONS DUE TO PRIOR RAINFALL FAVOR STARTING WITH A
MARGINAL RISK AREA, THOUGH CONFIDENCE IN DETAILS IS SOMEWHAT BELOW
AVERAGE FOR NOW.
ANOMALOUS MOISTURE SHOULD PERSIST OVER THE FOUR CORNERS STATES,
AND ARIZONA IN PARTICULAR, DURING THE DAYS 4-5 PERIOD AND
POTENTIALLY LEAD TO SOME LOCALLY HEAVY RAINFALL. HOWEVER WILL AWAIT
A MORE COHERENT SIGNAL IN THE GUIDANCE BEFORE DEPICTING ANY RISK
AREAS. PARTS OF THE SOUTHEAST/FLORIDA MAY ALSO SEE EPISODES OF
DIURNALLY ENHANCED CONVECTION BUT GROUND CONDITIONS ARE ON THE
DRIER SIDE WITH FFG VALUES QUITE HIGH, SO AGAIN FAVORED NO RISK
AREAS FOR THE TIME BEING. BEYOND EARLY TUESDAY, A CENTRAL INTO
EASTERN U.S. FRONT WILL HELP TO FOCUS AREAS OF SHOWERS AND
THUNDERSTORMS WHILE PORTIONS OF THE SOUTH AND THE ROCKIES SHOULD
CONTINUE TO SEE DIURNALLY FAVORED CONVECTION. A LATE-PERIOD FRONT
REACHING THE NORTHWEST MAY PRODUCE SOME LIGHT RAINFALL (AS MAY A
FRONT ON SUNDAY).
THE MID-ATLANTIC SHOULD CONTINUE TO HAVE TEMPERATURES 10-15F ABOVE
NORMAL INTO SUNDAY WITH SOME DAILY RECORDS POSSIBLE FOR MAX/WARM
MIN TEMPERATURES. THE FRONT PROGRESSING INTO THE EAST WILL BRIEFLY
SUPPRESS THE HEAT SOUTHWARD NEXT MONDAY BUT THE HOT TEMPERATURES
SHOULD RETURN NORTHWARD TOWARD MIDWEEK AHEAD OF ANOTHER FRONT.
MEANWHILE A BROAD AREA OF TEMPERATURES GENERALLY 5-15F ABOVE NORMAL
WILL EXTEND FROM THE SOUTH INTO THE PLAINS AS WELL AS CALIFORNIA
INTO THE INTERIOR WEST. NEAR TO SLIGHTLY BELOW NORMAL HIGHS OVER
THE PACIFIC NORTHWEST SHOULD BEGIN TO EXPAND SOUTHEASTWARD BY NEXT
THURSDAY. BEST POTENTIAL FOR MAXIMUM HEAT INDEX VALUES OF 105-110F
OR SO WILL BE FROM THE SOUTH INTO THE SOUTHERN HALF OR SO OF THE
PLAINS AS WELL AS OVER THE DESERT SOUTHWEST.
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 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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