730
FXUS02 KWBC 231859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT SUN JUL 23 2023
VALID 12Z WED JUL 26 2023 - 12Z SUN JUL 30 2023
...DANGEROUS HEAT WILL CONTINUE ACROSS THE SOUTHWEST WHILE HOT
TEMPERATURES ARE ALSO LIKELY FROM THE MIDWEST TO THE EAST COAST...
..OVERVIEW
RECENT GUIDANCE HAS BEEN MOST CONSISTENT AND AGREEABLE WITH THE
STRONG UPPER RIDGE/HIGH ANCHORED OVER THE SOUTHERN ROCKIES,
KEEPING EXTREMELY HOT TEMPERATURES ACROSS THE DESERT SOUTHWEST,
AND AN ATLANTIC UPPER RIDGE BUILDING WEST ACROSS FLORIDA AND THE
SOUTHEAST BY LATE WEEK. SOME RIDGING WILL LIKELY TO STRETCH INTO
THE NORTHERN HALF OF THE PLAINS AND MISSISSIPPI VALLEY FOR A TIME,
PROMOTING MUCH ABOVE NORMAL TEMPERATURES THERE AND EAST TOWARD THE
OHIO VALLEY/GREAT LAKES AND THE MID-ATLANTIC/NORTHEAST ESPECIALLY
WEDNESDAY-FRIDAY. MEAN UPPER TROUGHING NEAR THE PACIFIC NORTHWEST
SHOULD PROMOTE THE ONLY AREA OF PERSISTENT BELOW NORMAL
TEMPERATURES OVER THE LOWER 48. THIS TROUGH WILL BE PART OF A
LARGER SCALE REX BLOCK TYPE OF PATTERN COVERING NORTHWESTERN NORTH
AMERICA AND THE NORTHEASTERN PACIFIC, WITH GUIDANCE HAVING
CONSIDERABLE DIFFICULTY OVER DAY-TO-DAY DETAILS OF GENERALLY LOW
AMPLITUDE FLOW ALONG THE U.S.-CANADIAN BORDER. THE OVERALL
TENDENCY SHOULD BE FOR NORTHERN TIER FLOW TO REFLECT A RIDGE AXIS
OVER THE HIGH PLAINS AND BROAD TROUGHING OVER THE GREAT
LAKES/NORTHEAST BY NEXT WEEKEND. THIS PATTERN/EVOLUTION, WHICH
WILL BRING ONE OR MORE FRONTS INTO THE NORTHERN PLAINS THROUGH
NORTHEAST, WILL FAVOR EPISODES OF POTENTIALLY HEAVY CONVECTION BUT
CONFIDENCE REMAINS LOW IN THE DETAILS. DIURNALLY DRIVEN SHOWERS
AND THUNDERSTORMS WILL BE POSSIBLE OVER AND NEAR THE FOUR CORNERS
STATES THROUGH LATE WEEK AND THE WEEKEND WITH MONSOONAL MOISTURE
INCOMING UNDER THE RIDGE.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
OPERATIONAL MODELS AND ENSEMBLE MEMBERS CONTINUE TO BE VERY
CHAOTIC WITH IMPORTANT BUT LOW PREDICTABILITY MEDIUM/SMALL-SCALE
DETAILS FROM THE NORTHEASTERN PACIFIC ACROSS MUCH OF SOUTHERN
CANADA AND THE NORTHERN TIER OF THE U.S. ONE ELEMENT THAT SEEMS TO
HAVE PERSISTED OVER THE PAST 12-24 HOURS IS THE IDEA THAT THE
UPPER LOW FORECAST TO REACH ALBERTA BY EARLY DAY 3 WEDNESDAY
SHOULD SHEAR OUT FAIRLY SOON THEREAFTER. THIS HAS TEMPERED SOME OF
THE OSCILLATIONS WITH EASTWARD PROGRESSION OF THE FRONT INTO THE
GREAT LAKES/NORTHEAST TOWARD THE END OF THE WEEK. HOWEVER IN THE
PAST DAY GUIDANCE HAS DIVERGED A LOT MORE REGARDING WHAT WILL
HAPPEN WITH ONE OR MORE PIECES OF UPSTREAM ENERGY IN THE VANCOUVER
ISLAND/PACIFIC NORTHWEST MEAN TROUGH AND THE OVERALL AMPLITUDE OF
SHORTWAVES CROSSING THE NORTHERN TIER. AMONG THE MORE EXTREME
DETAILS AMONG RECENT OPERATIONAL MODEL RUNS, THE 00Z GFS BECOMES
MORE AMPLIFIED WITH GREAT LAKES TROUGHING LATE IN THE WEEK (WITH
VERY MINIMAL ENSEMBLE SUPPORT, WITH THE 06Z GFS CONSIDERABLY
IMPROVED) AND THE 00Z ECMWF'S EVOLUTION ALOFT JUST NORTH OF THE
PLAINS BY NEXT WEEKEND LEADING TO SURFACE LOW PRESSURE TRACKING
INTO THE GREAT LAKES BY DAY 7 SUNDAY (WHEREAS MOST OTHER
MODEL/ENSEMBLE MEANS SHOW VARYING DEGREES OF HIGH PRESSURE FROM
SOUTH-CENTRAL CANADA INTO THE GREAT LAKES AT THAT TIME).
NEEDLESS TO SAY, THE WIDE RANGE OF POSSIBILITIES FOR SHORTWAVE
DETAILS IN THE GENERALLY LOW AMPLITUDE FLOW ALONG THE CANADIAN
BORDER AND ASSOCIATED SURFACE LOW/FRONTAL DETAILS OVER THE
NORTHERN HALF OF THE LOWER 48 LEAD TO SOME SIGNIFICANT QPF
DIFFERENCES AMONG OPERATIONAL MODEL RUNS AND TO SOME DEGREE IN
ENSEMBLE-BASED PROBABILITIES. THE WPC QPF CONTINUES TO SHOW
SOMEWHAT BROADER COVERAGE OF LIGHT AMOUNTS IN THE NORTH-CENTRAL
U.S. AND EASTERN U.S. COMPARED TO THE NBM USING A MULTI-MODEL AND
ENSEMBLE BLEND TO PROVIDE SOME DEPICTION OF THE RAINFALL
POTENTIAL, BUT EXPECT THESE RAINFALL FORECASTS TO CHANGE
CONSIDERABLY WITH TIME WHEN THE MODELS EVENTUALLY START TO
CONVERGE. FOR THE MASS FIELDS THE WPC FORECAST STARTED WITH A
BLEND OF THE 00Z/06Z OPERATIONAL MODELS FOR THE FIRST PART OF THE
PERIOD. THEN THE FORECAST RAPIDLY TRANSITIONED TO MORE 06Z GEFS
AND 00Z ECENS/CMCENS MEAN EMPHASIS AS CONFIDENCE DECREASED FOR
INDIVIDUAL MODEL SOLUTIONS. BY NEXT WEEKEND, GUIDANCE COMPARISONS
FAVORED 50-75 PERCENT ENSEMBLE WEIGHT WITH AN EVENTUAL PHASING OUT
OF THE 00Z ECMWF.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND THUNDERSTORMS LOOK LIKELY ACROSS PARTS OF THE CENTRAL
AND EASTERN U.S. THROUGH THE LATTER PART OF THE WORKWEEK, WITH ONE
OR MORE WAVY FRONTAL SYSTEMS REACHING THE NORTHERN TIER PROVIDING
THE BEST FOCUS BUT WITH LESS DEFINED FORCING ELSEWHERE. EVEN WITH
THE NORTHERN TIER FRONTAL SYSTEMS, THERE IS A LOT OF UNCERTAINTY
OVER THE PRECISE EAST-WEST TIMING AND SOUTHWARD EXTENT--KEEPING
CONFIDENCE LOW. FOR THE DAY 4 EXCESSIVE RAINFALL OUTLOOK COVERING
WEDNESDAY/WEDNESDAY NIGHT, LOW PRESSURE AND ASSOCIATED FRONTAL
SYSTEM INITIALLY HAVE ENOUGH CLUSTERING IN GUIDANCE ALONG WITH
FAVORABLE MOISTURE AND INSTABILITY TO PRODUCE A THREAT FOR LOCALLY
INTENSE RAINFALL MAINLY OVER EASTERN NORTH DAKOTA AND NORTHERN
MINNESOTA, SO A MARGINAL RISK REMAINS IN PLACE FOR THIS ACTIVITY.
AREAS FARTHER SOUTH ACROSS THE MID-MISSISSIPPI AND OHIO VALLEYS
INTO THE APPALACHIANS COULD ALSO SEE CONVECTION WITH HEAVY
RAINFALL AMOUNTS GIVEN THE INSTABILITY IN PLACE, BUT EROS CONTINUE
TO DEPICT NO EXCESSIVE RAINFALL AREAS AT THIS TIME DUE TO THE
UNCERTAINTY IN THE PLACEMENT OF STORMS. FOR DAY 5/THURSDAY,
LIKEWISE THERE IS CONSIDERABLE UNCERTAINTY WITH PLACEMENT OF HEAVY
RAINFALL WITHIN THE GENERAL AREA OF THE NORTHERN TWO-THIRDS OF THE
PLAINS AND MISSISSIPPI VALLEY INTO THE APPALACHIANS AND
MID-ATLANTIC/NORTHEAST AHEAD OF FRONTAL BOUNDARIES. SOME OF THE
NEW 12Z GUIDANCE IS STARTING TO SIGNAL A RELATIVELY BETTER
POTENTIAL ALONG AND JUST SOUTH OF THE CANADIAN BORDER, INCLUDING
OVER/WEST OF THE UPPER GREAT LAKES AND PARTS OF THE NORTHEAST. FOR
NOW WILL KEEP ANY RISK AREAS OUT OF THE ERO GIVEN THE POOR
GUIDANCE BEHAVIOR UP TO THIS POINT. DEPENDING ON REMAINING 12Z
GUIDANCE AND UPCOMING 18Z GFS/GEFS RUNS, THOSE AFOREMENTIONED
AREAS MAY BE THE BEST CANDIDATE FOR INTRODUCTION OF AN AREA UPON
FURTHER EVALUATION IN THE NEXT ISSUANCE CYCLE. BY LATER IN THE
WEEK THE MAIN COLD FRONT AND LOW PRESSURE SYSTEM PUSHING SOUTHEAST
LOOK TO PROVIDE A CLEARER FOCUS FOR CONVECTION GENERALLY FROM THE
MIDWEST TO GREAT LAKES REGION AND THE NORTHEAST. TRAINING/REPEAT
ACTIVITY COULD EVENTUALLY BECOME MORE OF A FACTOR FOR A TIME IF
FLATTENING FLOW ALOFT ALLOWS SOME OF THE FRONT TO TAKE ON A MORE
EAST-WEST ORIENTATION. THE TRAILING END OF THE FRONT AND POSSIBLY
ANOTHER WAVE COULD LEAD TO LIGHTER/LESS EXPANSIVE CONVECTION OVER
THE NORTHERN HALF OF THE PLAINS NEXT WEEKEND. MEANWHILE OVER THE
WEST, THE FOUR CORNERS STATES ARE LIKELY TO SEE DIURNALLY DRIVEN
CONVECTION EACH DAY THIS WEEK AS MONSOONAL MOISTURE COMES IN
UNDERNEATH THE SOUTHERN ROCKIES UPPER RIDGE. THE EROS DEPICT A
MARGINAL RISK AREA ON BOTH DAYS. FLORIDA AND PARTS OF THE GULF
COAST SHOULD SEE SOME SCATTERED SHOWERS AND THUNDERSTORMS AS WELL,
AND A MARGINAL RISK OF FLASH FLOODING IS IN PLACE FOR SOUTH
FLORIDA FOR THURSDAY AS PRECIPITABLE WATER VALUES WILL BE WELL
ABOVE AN ALREADY HIGH AVERAGE AND CONVECTION COULD PRODUCE HEAVY
RAINFALL RATES, WITH URBAN AREAS PARTICULARLY SENSITIVE. LATEST
GUIDANCE CONTINUES TO SHOW THIS INCREASE OF MOISTURE BY THURSDAY.
THE RELENTLESS UPPER HIGH CENTERED OVER THE SOUTHERN ROCKIES WILL
KEEP TEMPERATURES IN THE DESERT SOUTHWEST AROUND 5-10F ABOVE
AVERAGE FOR HIGHS IN THE 110S. THOUGH THESE ANOMALIES ARE A LITTLE
LESS EXTREME THAN IN THE SHORTER TERM, THE HEAT COULD CONTINUE TO
BE HAZARDOUS THROUGH THE WEEK, BUT WITH PERHAPS SOME POSSIBLE
EASING BY NEXT WEEKEND. HIGHS AROUND 100F ARE FORECAST TO STRETCH
INTO MUCH OF TEXAS AND FARTHER NORTH INTO PARTS OF THE PLAINS AND
MISSISSIPPI VALLEY. THE LARGEST ANOMALIES FOR HIGHS OF 10-20F
ABOVE NORMAL WILL BE LOCATED ACROSS THE NORTHERN PLAINS TO UPPER
MIDWEST/GREAT LAKES FOR WEDNESDAY-THURSDAY, WHILE PLUS 10-15F
ANOMALIES ARE LIKELY ACROSS THE OHIO VALLEY AND
MID-ATLANTIC/NORTHEAST PEAKING THURSDAY-FRIDAY. A HANDFUL OF
RECORD HIGHS MAY BE POSSIBLE IN THE SOUTH-CENTRAL U.S. INTO
WEDNESDAY AND OVER FLORIDA UNDERNEATH RIDGING, BUT THE HEAT IN THE
EAST MAY TIE/SET MORE WIDESPREAD RECORDS FOR HIGHS AND WARM
MINIMUM TEMPERATURES. THE COLD FRONT FORECAST TO PROGRESS FROM THE
NORTHERN PLAINS AROUND MIDWEEK INTO THE NORTHEAST BY SATURDAY
SHOULD ERODE THE NORTHERN TIER HEAT AREA FROM WEST TO EAST,
KEEPING THE EVENT FAIRLY SHORT-LIVED OVER THE NORTHERN THIRD OF
THE COUNTRY. THE HEAT MAY PERSIST LONGER TO THE SOUTH, MOST LIKELY
OVER THE CENTRAL PLAINS/MIDDLE MISSISSIPPI VALLEY AND PARTS OF THE
SOUTHEAST. DETAILS OF CONVECTION AND RESIDUAL CLOUD COVER WILL ADD
AN EXTRA ELEMENT OF UNCERTAINTY TO THE FORECAST OF DAYTIME HIGHS
OVER SOME AREAS. MEAN TROUGHING NEAR THE PACIFIC NORTHWEST WILL
GIVE THAT REGION THE ONLY NOTABLE AREA OF POTENTIALLY BELOW NORMAL
TEMPERATURES, THOUGH STILL WITH HIGHS NO COOLER THAN A FEW DEGREES
BELOW AVERAGE.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES 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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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