486
FXUS02 KWBC 201906
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
306 PM 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 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 LOW PRESSURE 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. A COUPLE OF SHORTWAVES NEAR THE
CANADIAN SIDE OF THE GREAT LAKES OR SO TO START THE PERIOD MAY
COMBINE AS THEY MOVE TOWARD THE NORTHEAST, BUT WITH DETAIL
UNCERTAINTY. MODELS ARE STILL HAVING A TOUGH TIME DETERMINING WHEN
THERE MAY BE A LOW OR TWO EMBEDDED WITHIN THE UPPER TROUGH,
AFFECTING THE SURFACE EVOLUTION. THE NEW 12Z ECMWF BECOMES SLOW
COMPARED TO OTHER MODELS WITH THIS TROUGH BY MONDAY-TUESDAY.
THEN UPSTREAM, LATEST GUIDANCE IS HOLDING ONTO RECENT TRENDS
TOWARD PROGRESSIVE PACIFIC ENERGY EVOLVING INTO A SOUTHERN
CANADA/NORTHERN TIER U.S. SHORTWAVE THAT EVENTUALLY FEEDS INTO THE
EASTERN NORTH AMERICA MEAN TROUGH. REMAINING UPPER LOW ENERGY
NEAR/OFFSHORE BRITISH COLUMBIA MAY COMBINE WITH INCOMING NORTH
PACIFIC ENERGY. THIS LOOKS TO FORM TROUGHING AND AN EMBEDDED UPPER
LOW NEARING THE NORTHWEST BY NEXT THURSDAY WITH AN INCREASINGLY
BETTER DEFINED LEADING SURFACE SYSTEM. 00/06Z MODEL GUIDANCE WAS
REASONABLY AGREEABLE IN THE TIMING OF THIS, BUT NEWER 12Z MODELS
SPLIT WITH GFS/GEFS FARTHER EAST THAN THE CONSENSUS WITH THE
ECMWF/CMC FARTHER WEST.
THE UPDATED WPC FORECAST WAS BASED ON A BLEND OF THE 00/06Z GFS AND
00Z ECMWF WITH LESSER PROPORTIONS OF THE 00Z UKMET AND CMC EARLY IN
THE PERIOD. LESSENED THE PERCENTAGE OF THE DETERMINISTIC RUNS IN
FAVOR OF THE GEFS AND EC MEANS AS THE PERIOD PROGRESSED, REACHING
HALF MEANS IN THE BLEND BY DAYS 6-7.
..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. IT IS
MORE LIKELY THAN NOT THAT IMPACTFUL RAIN WILL STAY SOUTH OUT OF
TEXAS, BUT A LOW-END CHANCE MAKES MAINTAINING A MARGINAL FOR FAR
SOUTH TEXAS REASONABLE. IN THE DAY 5 ERO PERIOD (MONDAY-MONDAY
NIGHT), GUIDANCE CONTINUES TO SUGGEST AN AREA OF POSSIBLE
CONVECTION DIVING SOUTHEAST OVER THE UPPER MIDWEST/WESTERN GREAT
LAKES AHEAD OF AN ADVANCING WARM FRONT. A MARGINAL RISK REMAINS A
GOOD STARTING POINT, AS DOWNSIDES FOR CONFIDENCE IN FLASH FLOODING
ARE RELATIVELY FAST MOVEMENT AND MODEL SPREAD WITH PLACEMENT, BUT
UPSIDES ARE HIGH RAINFALL RATES AND WET GROUND CONDITIONS.
ANOMALOUS MOISTURE AND SUFFICIENT INSTABILITY SHOULD PERSIST OVER
THE FOUR CORNERS STATES DURING THE DAYS 4-5 PERIOD AND POTENTIALLY
LEAD TO SOME LOCALLY HEAVY RAINFALL. WILL INTRODUCE MARGINAL RISKS
ACROSS MUCH OF ARIZONA THROUGH THE MOGOLLON RIM INTO WESTERN NEW
MEXICO WHERE THE BEST MOISTURE TRANSPORT IS FOR SLOW-MOVING
CONVECTION. ANY BURN SCARS IN THE REGION WOULD BE PARTICULARLY
VULNERABLE. 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 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 FRONT ON SUNDAY AND THEN A LATE-
PERIOD FRONT REACHING THE NORTHWEST MAY PRODUCE SOME LIGHT
RAINFALL EACH TIME.
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. BEST POTENTIAL FOR MAXIMUM HEAT INDEX VALUES OF 105-110F
OR SO WILL BE FROM THE SOUTHEASTERN U.S. INTO THE SOUTHERN HALF OR
SO OF THE PLAINS AS WELL AS OVER THE DESERT SOUTHWEST. NEAR TO
SLIGHTLY BELOW NORMAL HIGHS OVER THE PACIFIC NORTHWEST SHOULD BEGIN
TO EXPAND SOUTHEASTWARD BY NEXT THURSDAY.
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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