839
FXUS02 KWBC 182031
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
431 PM EDT TUE JUL 18 2023
VALID 12Z FRI JUL 21 2023 - 12Z TUE JUL 25 2023
***DANGEROUS HEAT WAVE PERSISTS ACROSS THE SOUTHWESTERN U.S.
THROUGH EARLY NEXT WEEK, AND HEAT BUILDS FARTHER NORTH ACROSS THE
NORTHERN HIGH PLAINS***
..GENERAL OVERVIEW
FROM FRIDAY INTO EARLY NEXT WEEK EXPECT THE ANOMALOUSLY STRONG
UPPER HIGH INITIALLY OVER THE DESERT SOUTHWEST TO SHIFT INTO THE
FOUR CORNERS REGION WHILE THE RIDGE EXTENDING NORTHWARD INTO THE
INTERIOR WEST/NORTHERN ROCKIES AMPLIFIES A BIT AS IT SHIFTS TOWARD
THE NORTHERN HIGH PLAINS. IN RESPONSE, THE DOWNSTREAM UPPER
TROUGH OVER THE EASTERN HALF OF THE LOWER 48 SHOULD AMPLIFY AND
SHARPEN INTO THE WEEKEND. THIS PATTERN WILL MAINTAIN THE DESERT
SOUTHWEST HEAT WAVE AS A SIGNIFICANT WEATHER STORY THROUGH THE
PERIOD WHILE EXPANDING THE HEAT FARTHER NORTH/NORTHEAST THROUGH
THE INTERIOR WEST AND INTO THE NORTHERN ROCKIES/PLAINS. AT THE
SAME TIME, THE AMPLIFYING EASTERN UPPER TROUGH WILL HELP TO
SUPPRESS THE UPPER RIDGING AND ASSOCIATED VERY HOT AND HUMID
CONDITIONS FORECAST TO EXTEND ALONG AND JUST NORTH OF THE GULF
COAST REGION THROUGH LATE WEEK. THE SURFACE FRONT AHEAD OF THE
UPPER TROUGH WILL SUPPORT EPISODES OF POTENTIALLY HEAVY RAINFALL
AND STRONG STORMS OVER PORTIONS OF THE SOUTH. FINALLY, AN UPPER
TROUGH SHOULD APPROACH THE PACIFIC NORTHWEST EARLY NEXT WEEK AS
THE ANCHORING PACIFIC UPPER LOW DRIFTS TOWARD NORTHERN BRITISH
COLUMBIA.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD AGREEMENT
WITH THE LARGE SCALE PATTERN EVOLUTION WITH SOME EMBEDDED DETAIL
UNCERTAINTIES. WITHIN THE EASTERN UPPER TROUGH, SHORTWAVE
SPECIFICS ARE SMALL ENOUGH IN SCALE TO HAVE LOW PREDICTABILITY IN
THE MEDIUM RANGE TIME FRAME BUT WILL BE MEANINGFUL ON A MORE
LOCALIZED BASIS, IN PARTICULAR FOR THE MID-ATLANTIC/NORTHEAST LOW
PRESSURE AND FRONTAL SYSTEM FRIDAY INTO THE WEEKEND AND THEN A
FRONT THAT MAY DROP INTO THE MIDWEST/GREAT LAKES DURING THE FIRST
HALF OF NEXT WEEK. A BLEND/MEAN APPROACH PROVIDES A REASONABLE
INTERMEDIATE SOLUTION WHERE DIFFERENCES EXIST. MEANWHILE GUIDANCE
HAS BEEN A LITTLE MORE ERRATIC WITH DETAILS OVER THE NORTH PACIFIC
INTO THE PACIFIC NORTHWEST AND WESTERN CANADA. STRAY SOLUTIONS
(00Z GFS, 12Z UKMET MOST RECENTLY) HAVE BEEN PROGRESSIVE ENOUGH
WITH UPSTREAM FLOW TO RESULT IN EJECTION OF THE UPPER LOW DRIFTING
OVER THE NORTHEASTERN PACIFIC, WHILE THE 00Z CMC WAS ON THE
AMPLIFIED/FAST SIDE WITH ITS TROUGH CROSSING THE NORTHWEST BY DAY
7 TUESDAY. THE REMAINING MAJORITY CLUSTER HAS FAVORED A SLOWER
UPPER LOW/TROUGH. A 00Z/06Z OPERATIONAL MODEL BLEND EARLY
FOLLOWED BY A TRANSITION TO A COMBINATION OF MODELS AND ENSEMBLE
MEANS (06Z GEFS/00Z ECENS) PROVIDED A GOOD REPRESENTATION OF THE
MOST LIKELY SCENARIO FOR SIGNIFICANT FEATURES.
..WEATHER/HAZARDS HIGHLIGHTS
THE FRONTAL BOUNDARY THAT DROPS SOUTHWARD FROM A SOUTHERN
MID-ATLANTIC THROUGH SOUTHERN PLAINS ORIENTATION INTO THE SOUTH
LATE WEEK INTO THE WEEKEND SHOULD BE A FOCUS FOR SCATTERED TO
PERHAPS NUMEROUS SHOWERS AND STORMS, WITH ANY WAVES PROVIDING
ADDITIONAL ENHANCEMENT. ALIGNMENT OF THE FRONT MAY PROMOTE
WEST-EAST TRAINING OF CONVECTION OVER SOME AREAS AT TIMES. THE
UPDATED DAY 4 EXCESSIVE RAINFALL OUTLOOK COVERING THE
FRIDAY-FRIDAY NIGHT PERIOD PLANS TO INTRODUCE A SLIGHT RISK AREA
CENTERED OVER THE EASTERN HALF OF TENNESSEE, WHERE THERE IS A
FAVORABLE OVERLAP OF SIGNIFICANT RAINFALL EXPECTED IN THE SHORT
TERM AND BEST GUIDANCE SIGNALS FOR ADDITIONAL HEAVY RAINFALL
POTENTIAL DURING DAY 4. THE SURROUNDING MARGINAL RISK AREA
EXTENDS FROM THE GENERAL VICINITY OF THE OZARKS TO PARTS OF THE
INTERIOR SOUTHEAST. THE AXIS FOR POTENTIALLY HEAVY RAINFALL
SHOULD REACH THE CENTRAL GULF COAST REGION THROUGH NORTHERN
FLORIDA AND PARTS OF GEORGIA/SOUTH CAROLINA ON DAY 5
(SATURDAY-SATURDAY NIGHT), WITH ONLY A MARGINAL RISK MAINTAINED IN
LIGHT OF HIGH FLASH FLOOD GUIDANCE VALUES AND SOME LACK OF
CLUSTERING FOR QPF DETAILS. SHORTER TERM GUIDANCE MAY ULTIMATELY
HELP TO RESOLVE AREAS WITH GREATER RAINFALL POTENTIAL. ACROSS THE
WESTERN U.S., A MARGINAL RISK AREA IS VALID FOR BOTH DAYS 4 AND 5
FROM SOUTHEASTERN COLORADO ACROSS MUCH OF NEW MEXICO AND INTO
EASTERN PORTIONS OF ARIZONA. WITH THE CORE OF THE UPPER HIGH
EXPECTED TO SHIFT INTO THE FOUR CORNER REGION, SOME MONSOONAL
MOISTURE MAY ADVECT FROM WESTERN MEXICO NORTHWARD ACROSS THE
SOUTHERN ROCKIES, RESULTING IN ISOLATED TO PERHAPS SCATTERED
COVERAGE OF SLOW MOVING THUNDERSTORMS THAT CAN BE EXPECTED MAINLY
DURING THE AFTERNOON AND EVENING HOURS. MEANWHILE LOW PRESSURE
AND AN ASSOCIATED FRONTAL SYSTEM MAY BRING SOME SHOWERS AND
THUNDERSTORMS TO THE NORTHEAST IN THE DAY 4 TIME FRAME, WITH WET
GROUND CONDITIONS BUT SOME MODEL/ENSEMBLE SCATTER FOR DETAILS
LEADING TO A MARGINAL RISK AREA ACROSS THAT REGION. LOOKING AHEAD
TO SUNDAY AND BEYOND, SHOWERS AND STORMS WILL LIKELY LINGER ACROSS
THE GULF COAST REGION, AND THERE ARE SIGNS THAT SOME THUNDERSTORM
COMPLEXES COULD DEVELOP FROM THE MIDWEST EASTWARD AS A
DECELERATING WARM FRONT CROSSES THE CENTRAL PLAINS AND A COLD
FRONT DROPS SOUTH FROM CANADA INTO THE MIDWEST/GREAT LAKES.
ISOLATED MONSOONAL SHOWERS/STORMS ARE LIKELY TO CONTINUE ACROSS
PARTS OF ARIZONA AND NEW MEXICO.
THE STUBBORN UPPER RIDGE ACROSS THE INTERMOUNTAIN WEST/DESERT
SOUTHWEST WILL RESULT IN A CONTINUATION OF THE ONGOING EXTREME
HEAT THROUGH THIS WEEKEND AND EARLY NEXT WEEK, WITH HIGHS WELL
INTO THE 110S FOR THE LOWER ELEVATIONS OF EASTERN CALIFORNIA,
SOUTHERN ARIZONA, AND SOUTHERN NEVADA, WITH THE POTENTIAL FOR
ADDITIONAL RECORD HIGH TEMPERATURES BEING ESTABLISHED ESPECIALLY
FRIDAY-SATURDAY. TEMPERATURES GENERALLY 5 TO 15 DEGREES ABOVE
AVERAGE FOR BOTH HIGHS AND LOWS WILL LIKELY BUILD FARTHER NORTH
THROUGH THE GREAT BASIN INTO NORTHERN ROCKIES, AND THEN EXTEND
INTO THE NORTHERN PLAINS BY EARLY NEXT WEEK WHILE HIGHS OVER THE
GREAT BASIN COULD START TO MODERATE A BIT NEXT TUESDAY IF AN UPPER
TROUGH GETS CLOSE ENOUGH TO THE PACIFIC NORTHWEST. MEANWHILE THE
FRONT DROPPING INTO THE SOUTHERN TIER SHOULD HELP TO SUPPRESS THE
INTENSE HEAT FROM THE SOUTHERN PLAINS INTO THE GULF COAST REGION
BY THE WEEKEND. IT WILL STILL BE QUITE HOT AND HUMID OVER
SOUTHERN TEXAS AND PARTS OF THE FLORIDA PENINSULA WHERE HEAT
INDICES MAY APPROACH OR EXCEED 110 DEGREES DURING PEAK AFTERNOON
HEATING. ON THE COOL SIDE OF THE FRONT, HIGHS MAY BE UP 10-15
DEGREES BELOW NORMAL OVER THE CENTRAL PLAINS ON FRIDAY. MUCH OF
THE EASTERN U.S. SHOULD GENERALLY BE WITHIN A FEW DEGREES OF LATE
JULY AVERAGES WITH NO MAJOR HEAT WAVES EXPECTED EAST OF THE
MISSISSIPPI RIVER.
RAUSCH/HAMRICK
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
HAZARDS:
- HEAVY RAIN ACROSS PORTIONS OF THE GREAT LAKES, THE NORTHEAST,
THE SOUTHERN APPALACHIANS, AND THE
OHIO/TENNESSEE VALLEY, FRI, JUL 21.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY,
THE SOUTHERN PLAINS, THE NORTHERN
ROCKIES/GREAT BASIN, CALIFORNIA, THE SOUTHEAST, AND THE PACIFIC
NORTHWEST, FRI-SAT, JUL 21-JUL 22.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY,
THE NORTHERN/CENTRAL GREAT BASIN,
THE NORTHERN/SOUTHERN PLAINS, THE NORTHERN/CENTRAL ROCKIES,
CALIFORNIA, THE SOUTHEAST, AND THE
SOUTHWEST, FRI-TUE, JUL 21-JUL 25.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE SOUTHEAST, THE SOUTHERN
APPALACHIANS/PLAINS, THE LOWER
MISSISSIPPI VALLEY, AND THE TENNESSEE VALLEY, FRI, JUL 21.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE NORTHERN PLAINS/ROCKIES,
SAT-TUE, JUL 22-JUL 25.
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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