551
FXUS02 KWBC 150700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT THU AUG 15 2024
VALID 12Z SUN AUG 18 2024 - 12Z THU AUG 22 2024
...HAZARDOUS HEAT LIKELY ACROSS THE SOUTHERN PLAINS AND CENTRAL
GULF COAST STATES INTO NEXT WEEK...
..OVERVIEW
A SLOW-MOVING UPPER PATTERN WILL BE IN PLACE INTO NEXT WEEK. AN
UPPER HIGH ATOP THE SOUTHERN HIGH PLAINS WILL PROMOTE HAZARDOUS
HEAT OVER THE SOUTHERN PLAINS TO THE CENTRAL GULF COAST. AN UPPER
LOW IS FORECAST TO MEANDER OFF THE WEST COAST, LEADING TO A COOLER
AND WETTER PATTERN FOR THE NORTHWEST. MEANWHILE, AN UPPER TROUGH
WILL TAKE HOLD FROM THE GREAT LAKES AND SOUTHWARD AND SLOWLY MOVE
EAST AS THE WEEK PROGRESSES. THIS TROUGH AND A SURFACE LOW
PRESSURE/FRONTAL SYSTEM WILL PROMOTE SCATTERED TO WIDESPREAD
THUNDERSTORMS WITH THE POTENTIAL FOR HEAVY RAIN ACROSS THE
NORTHEAST U.S. INTO EARLY NEXT WEEK. HURRICANE ERNESTO IS FORECAST
TO TRACK NORTH OVER THE WESTERN ATLANTIC AND REMAIN OFFSHORE OF THE
U.S. EAST COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE SCALE
PATTERN DESCRIBED ABOVE, WITH SOME MODEL SPREAD/UNCERTAINTY IN THE
DETAILS. BY WEDNESDAY-THURSDAY, THE EASTERN PACIFIC UPPER LOW
SHOWS SOME MODEL VARIATIONS IN LARGE PART DUE TO UNCERTAIN UPSTREAM
ENERGY FROM ALASKA AND HOW IT INTERACTS WITH THE EXISTING UPPER
LOW--MERGING OR PUSHING THE ORIGINAL LOW EAST. THE 12Z GFS SEEMED
TO BE AN OUTLIER IN DOING THE LATTER, TAKING ENERGY INTO SOUTH-
CENTRAL CANADA. OTHER 12/18Z GUIDANCE AND AND THE NEWER 00Z
GUIDANCE IS MORE IN FAVOR OF KEEPING ONE UPPER LOW JUST OFFSHORE OF
THE PACIFIC NORTHWEST.
FARTHER EAST, THERE IS ALSO PATTERN UNCERTAINTY WITH A COMPACT
UPPER LOW IN CENTRAL CANADA AND ITS TIMING/PLACEMENT AS IT COULD
ERODE THE NORTHERN PART OF THE CENTRAL U.S. RIDGE. MOST 12Z
GUIDANCE SHOWED THE UPPER LOW MOVING EAST INTO THE HUDSON BAY BY
THURSDAY, BUT THE 18Z GFS WAS BULLISH IN HAVING IT FASTER/FARTHER
SOUTHEAST. DID NOT FAVOR THE UPPER LOW TRACK IN THE 18Z GFS, BUT
NOW THE 00Z ECMWF AND GFS HAVE COME IN REASONABLY SIMILAR TO EACH
OTHER AND NOT TOO DIFFERENT FROM THE 18Z GFS IN SHOWING A BIT OF A
SOUTHEASTWARD DIVE BRINGING THE UPPER LOW TOWARD THE GREAT LAKES.
SO THERE IS STILL CONSIDERABLE UNCERTAINTY. THE EVOLUTION OF THIS
FEATURE ALSO IN PART AFFECTS THE TIMING OF THE WEAKENING OF THE
TROUGH IN THE EAST. 12/18Z AND NOW 00Z GUIDANCE VARIES ON THAT,
WITH THE 00Z CMC THROWING A WRENCH INTO THE PATTERN BY SHOWING A
CLOSED LOW ANCHORED IN QUEBEC LATE PERIOD.
THE WPC FORECAST USED A MULTI-MODEL DETERMINISTIC BLEND EARLY IN
THE PERIOD, REDUCING THE PROPORTION OF THE GFS RUNS IN PARTICULAR
IN FAVOR OF THE MORE AGREEABLE ENSEMBLE MEANS, REACHING HALF MEANS
BY DAY 7.
..WEATHER/HAZARDS HIGHLIGHTS
GREAT LAKES UPPER TROUGHING AND A SURFACE LOW PRESSURE/FRONTAL
SYSTEM WILL PROVIDE FORCING FOR RAIN AND THUNDERSTORMS ACROSS THE
EASTERN U.S. INTO EARLY NEXT WEEK. INSTABILITY AND ABOVE NORMAL
MOISTURE WILL BE IN PLACE AND COULD LEAD TO LOCALLY HEAVY RAIN. A
MARGINAL RISK OF EXCESSIVE RAINFALL IS IN PLACE DAY 4/SUNDAY FOR
THE NORTHERN MID-ATLANTIC INTO THE NORTHEAST, AND MORE LIMITED TO
THE NORTHEAST BY DAY 5/MONDAY. WHILE WIDESPREAD HEAVY RAIN IS NOT
PARTICULARLY LIKELY, EMBEDDED HEAVY TOTALS COULD CAUSE FLOODING
CONCERNS ESPECIALLY CONSIDERING WET ANTECEDENT CONDITIONS IN MUCH
OF THE REGION AND THE MULTIPLE DAYS IN A ROW OF RAIN. RAIN IS
EXPECTED TO TAPER OFF IN THE NORTHEAST TUESDAY-WEDNESDAY AS SURFACE
HIGH PRESSURE BUILDS IN BEHIND THE COLD FRONT(S). HOWEVER, THE
FRONT STALLING FARTHER SOUTH ACROSS FLORIDA INTO MIDWEEK (AFTER AN
INITIAL FRONT STALLS ACROSS SOUTHERN FLORIDA EARLY NEXT WEEK) WILL
MAINTAIN SEVERAL DAYS OF THUNDERSTORM CHANCES THERE. FLOODING
ISSUES ARE A POSSIBILITY IF HEAVY RAIN FALLS IN URBAN AREAS. THIS
FRONT MOVES SLOWLY AS IT BENDS BACK TO THE WEST ATOP THE CENTRAL
GULF COAST AND BACK NORTH INTO THE PLAINS. SCATTERED STORMS ARE
POSSIBLE NEAR THE FRONT AS IMPULSES "RIDE THE RIDGE" ALOFT, BUT
WITH GENERALLY LOW CONFIDENCE IN PLACEMENT.
MONSOONAL CONVECTION WILL AFFECT THE FOUR CORNERS STATES NORTHWARD
INTO EARLY NEXT WEEK WITH MOIST SOUTHERLY FLOW WEST OF THE UPPER
RIDGE AXIS. MARGINAL RISKS ARE IN PLACE SUNDAY-MONDAY FOR PORTIONS
OF ARIZONA, UTAH, COLORADO, AND EVEN REACHING FARTHER NORTH INTO
THE NORTHERN ROCKIES WHERE PWS WILL BE ABOVE AVERAGE. STORMS WITH
EMBEDDED HEAVY RAIN COULD CAUSE ISOLATED FLOODING, PARTICULARLY IN
VULNERABLE AREAS LIKE THE SLOT CANYONS OF SOUTHERN UTAH. MONSOONAL
CONVECTION LOOKS TO GRADUALLY REDUCE IN SCOPE INTO MIDWEEK. FARTHER
NORTH, THE PACIFIC NORTHWEST CAN EXPECT RAIN PARTICULARLY ON
SUNDAY WITH THE EASTERN PACIFIC UPPER LOW NEARBY. SOME RAIN IS
FORECAST TO MOVE ACROSS THE NORTHERN TIER AS THE WEEK PROGRESSES.
UPPER RIDGING OVER THE SOUTH-CENTRAL U.S. WILL MAINTAIN ABOVE
NORMAL TEMPERATURES BY 5-15 DEGREES FROM THE HIGH PLAINS EAST
INTO THE CENTRAL GULF COAST. HIGHS WELL INTO THE 100S IN MUCH OF
TEXAS COULD SET DAILY RECORDS. FROM EASTERN TEXAS INTO THE LOWER
MISSISSIPPI VALLEY, HIGH DEWPOINTS WILL COMBINE WITH THE HOT
TEMPERATURES TO PRODUCE HEAT INDICES LIKELY ABOVE 110 DEGREES. LOW
TEMPERATURES IN THE UPPER 70S TO LOW 80S WILL EXACERBATE HEAT
STRESS AND COULD SET RECORDS FOR WARM MINIMUM TEMPERATURES.
EXPERIMENTAL HEATRISK CONTINUES TO SHOW MAJOR TO EXTREME IMPACTS
FROM THIS LENGTHY HEAT WAVE. NORTH OF THIS REGION, MOST AREAS
SHOULD SEE NEAR TO SLIGHTLY BELOW NORMAL HIGHS, WITH SOME BELOW
NORMAL LOWS AS WELL FOR THE OHIO VALLEY, WHILE THE INTERMOUNTAIN
WEST CAN EXPECT ABOVE AVERAGE LOWS. THE PACIFIC NORTHWEST INTO
NORTHERN CALIFORNIA SHOULD SEE TEMPERATURES A BIT BELOW NORMAL IN
RESPONSE TO THE LOW ALOFT.
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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