494
FXUS02 KWBC 151857
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 PM 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 CENTERED OVER 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
LATEST MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE
OVERALL LARGE SCALE PATTERN DURING THE PERIOD, BUT PLENTY OF
DISAGREEMENT ON THE SMALLER SCALE DETAILS. THE MAIN AREA OF
UNCERTAINTY SURROUNDS A COMPACT UPPER LOW OVER SOUTH-CENTRAL CANADA
WHICH MAY ACT TO ERODE THE NORTHERN EXTENT OF THE CENTRAL U.S.
RIDGE A BIT. THERE IS VERY POOR RUN TO RUN CONTINUITY AND NO CLEAR
TREND ONE WAY OR THE OTHER ON THIS WITH SOME GUIDANCE SUGGESTING IT
DROPS SOUTHWARD TOWARDS THE GREAT LAKES, AND SOME KEEPING IT NORTH
AND THUS, STRONGER RIDGING. OTHERWISE, DETAIL DIFFERENCES IN THE
EVOLUTION/SHAPE OF TROUGHING OVER THE EAST AND OFF THE NORTHWEST
COAST MID-LATE PERIOD. THE MAIN OUTLIER WAS THE 00Z CMC WHICH
DROPPED ENERGY DUE SOUTH ACROSS EASTERN CANADA RESULTING IN A DEEP
UPPER LOW JUST NORTH OF THE LOWER LAKES NEXT TUESDAY, BUT THE 12Z
RUN HAS SINCE BACKED OFF ON THIS IDEA AND IS MORE IN LINE WITH THE
REST OF THE GUIDANCE.
THE WPC UPDATED PROGS FOR TODAY LEANED HEAVILY ON THE BETTER
AGREEABLE GFS AND ECMWF FOR THE FIRST HALF OF THE PERIOD. INCREASED
CONTRIBUTIONS FROM THE ENSEMBLE MEANS LATE PERIOD TO MITIGATE
DIFFERENCES DISCUSSED ABOVE. THIS MAINTAINED GOOD AGREEMENT WITH
THE PREVIOUS WPC FORECAST AS WELL.
..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 STILL AT THIS TIME.
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, HOT TEMPERATURES AND HIGH DEWPOINTS SHOULD
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.
SANTORELLI/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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