986
FXUS02 KWBC 310700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT THU AUG 31 2023
VALID 12Z SUN SEP 03 2023 - 12Z THU SEP 07 2023
...HEAT FORECAST TO INCREASE ACROSS THE PLAINS, MIDWEST, AND
MID-ATLANTIC/NORTHEAST NEXT WEEK...
...WET PATTERN ACROSS THE WEST INTO EARLY NEXT WEEK, WITH FLOODING
POSSIBLE...
..OVERVIEW
AN AMPLIFIED UPPER-LEVEL PATTERN WILL BE IN PLACE TO BEGIN NEXT
WEEK, COMPOSED OF AN UPPER TROUGH OVER THE WEST COAST STATES AND
AN UPPER RIDGE OVER THE CENTRAL CONUS. THESE FEATURES SHOULD SHIFT
GRADUALLY EASTWARD NEXT WEEK. THIS PATTERN WILL ALLOW FOR WELL
ABOVE AVERAGE TEMPERATURES TO BUILD ACROSS THE PLAINS AND UPPER
MIDWEST AND STRETCHING INTO THE EAST. MEANWHILE, WIDESPREAD ACROSS
PARTS OF THE WEST THIS HOLIDAY WEEKEND COULD CAUSE FLOODING
CONCERNS. RAIN CHANCES WILL SHIFT INTO THE NORTH-CENTRAL U.S.
AHEAD OF A COLD FRONT AS THE WEEK PROGRESSES. SCATTERED SHOWERS
AND STORMS COULD PERSIST ACROSS THE GULF COAST REGION INTO FLORIDA
ESPECIALLY EARLY IN THE WEEK AS A SHORTWAVE AND FRONTAL BOUNDARY
LINGER THERE.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE STARTS IN REASONABLY GOOD AGREEMENT ACROSS THE
CONUS IN SHOWING THE MAIN UPPER TROUGH WITH ITS AXIS OVER THE WEST
COAST STATES AND BROAD RIDGING FARTHER EAST. HOWEVER, EVEN BY DAY
3/SUNDAY, DIFFERENCES IN TIMING SHOW UP WITH A SHORTWAVE COMING
THROUGH THE NORTHEASTERN PACIFIC TOWARD THE TROUGH. THE
INTERACTION OF THIS UPSTREAM ENERGY WITH THE PREEXISTING LOW
SHOULD COMBINE WITH THE TROUGH AT SOME POINT AND HELP PUSH IT
EASTWARD, WITH VARIATIONS AMONG 12/18Z MODEL GUIDANCE. DID NOT
THINK THERE WERE ANY MODELS THAT COULD BE CONSIDERED OUTLIERS AT
THIS POINT WITH THE SHORTWAVE, JUST UNCERTAINTIES THAT ARISE FROM
THE FEATURE COMING FROM THE TYPICALLY UNCERTAIN NORTHERN PACIFIC.
THE NEWER 00Z GUIDANCE INDICATES THE SHORTWAVE MAY GET ABSORBED
INTO THE TROUGH SOONER RATHER THAN LATER. THE DETAILS OF THIS FLOW
DO HAVE IMPACTS FOR POSITIONS OF SURFACE FRONTS MOVING ACROSS THE
WEST AND BEYOND. IN ANY CASE, MODELS SHOW THAT THE TROUGH SHOULD
DEAMPLIFY A BIT AS IT MOVES DOWNSTREAM THROUGH MIDWEEK, WHILE
RIDGING MOVES INTO THE EAST. ANOTHER TROUGH WILL APPROACH THE
NORTHWEST BY WEDNESDAY-THURSDAY, WHILE THE RIDGING IN THE EAST
ERODES--HELPED POSSIBLY BY ENERGY THAT WAS ONCE IDALIA. IDALIA'S
TRACK IN THE WESTERN ATLANTIC DOES BECOME UNCERTAIN AS MANY MODELS
CURVE THE STORM BACK TO THE WEST EVENTUALLY BY AROUND MIDWEEK,
EVEN APPROACHING (BUT NOT HITTING) THE NORTHEAST OR ATLANTIC
CANADA. SO WILL CONTINUE TO MONITOR.
GIVEN THE SOMEWHAT AGREEABLE LARGE-SCALE FLOW BUT THE DETAIL
DIFFERENCES, THE WPC FORECAST BLEND UTILIZED A MULTI-MODEL
DETERMINISTIC BLEND OF THE 18Z GFS AND THE 12Z ECMWF/CMC/UKMET FOR
THE EARLY PART OF THE FORECAST PERIOD. AS IS TYPICAL, INTRODUCED
THE AGREEABLE EC AND GEFS ENSEMBLE MEANS INTO THE BLEND AND
GRADUALLY INCREASED THEM WITH TIME TO SMOOTH OUT SMALLER-SCALE
DIFFERENCES FROM MODEL TO MODEL. BUT WAS ABLE TO KEEP A MAJORITY
OF DETERMINISTIC GUIDANCE THROUGH DAY 7, WHICH CAN BE GOOD FOR
MAINTAINING STRENGTH OF SYSTEMS.
..WEATHER/HAZARDS HIGHLIGHTS
RAINY CONDITIONS ARE LIKELY ACROSS MUCH OF THE WEST INTO SUNDAY
WITH THE AMPLIFIED UPPER TROUGH SETTLED OVER THE WEST COAST. EAST
OF THE TROUGH, STRONG MEAN SOUTHERLY FLOW WILL CARRY MONSOONAL
MOISTURE WITH PRECIPITABLE WATER VALUES WELL ABOVE NORMAL AND
INSTABILITY INTO THE INTERIOR WEST. ON SUNDAY, THE HEAVIEST
RAINFALL IS FORECAST FOR NORTHERN UTAH INTO EASTERN IDAHO AND
WESTERN WYOMING, AND A SLIGHT RISK REMAINS IN PLACE IN THE ERO FOR
THESE AREAS WITH A WIDER MARGINAL RISK ACROSS PARTS OF THE GREAT
BASIN AND ROCKIES. BURN SCARS, SLOT CANYONS, AND URBAN AREAS WILL
BE PARTICULARLY VULNERABLE TO FLOODING. AS THE TROUGH CONTINUES TO
SHIFT INLAND, PRECIPITATION SHOULD EXPAND/PUSH INTO THE NORTHERN
PLAINS BY MONDAY. A MARGINAL RISK IS PLANNED FROM THE NORTHERN
ROCKIES TO NORTHERN HIGH PLAINS FOR MONDAY AS MOISTURE LEVELS
REMAIN ABOVE NORMAL, BUT NOT QUITE TO THE SAME EXTENT AS ON
SUNDAY. SHOWERS AND STORMS ALSO MAY BE MOVING FASTER THAN ON
SUNDAY. SO A SLIGHT RISK IS NOT PLANNED INTO MONDAY FOR NOW, BUT
AN UPGRADE IS NOT OUT OF THE QUESTION IN FUTURE FORECAST UPDATES
IF BETTER MODEL AGREEMENT IN RAINFALL POSITIONS AND FOR HEAVIER
TOTALS OCCURS. RAIN CHANCES ARE EXPECTED TO SPREAD EAST INTO THE
MIDWEST BY TUESDAY-WEDNESDAY AS THE UPPER TROUGH AND COLD FRONT
MOVE EASTWARD. FARTHER SOUTH, A SHORTWAVE IS FORECAST TO HOVER
OVER THE LOWER MISSISSIPPI VALLEY, WHICH IN COMBINATION WITH A
FRONTAL BOUNDARY WILL PROVIDE SUPPORT FOR SCATTERED SHOWERS AND
STORMS. RAINFALL AMOUNTS DO NOT LOOK TO BE AS HEAVY AS IN THE
SHORT RANGE PERIOD THOUGH. SIMILARLY, SOME SHOWERS ARE POSSIBLE
ACROSS ESPECIALLY THE SOUTHERN PART OF THE FLORIDA PENINSULA.
HEAT IS FORECAST TO BUILD ACROSS PARTS OF THE CENTRAL AND EASTERN
U.S. NEXT WEEK UNDERNEATH UPPER RIDGING. THE NORTHERN PLAINS AND
THE UPPER MIDWEST ARE LIKELY TO SEE TEMPERATURES 15-25 DEGREES
ABOVE AVERAGE ON SUNDAY AND MONDAY, AND RECORD WARM MAXIMUM AND
MINIMUM TEMPERATURES COULD BE WIDESPREAD. HIGHS WILL BE WELL INTO
THE 90S IF NOT POSSIBLY EVEN HITTING THE 100F MARK THERE, WITH
LOWS POTENTIALLY STAYING IN THE MID-70S FOR LIMITED OVERNIGHT
RELIEF. THE NORTH-CENTRAL U.S. SHOULD SEE SOME RELIEF WITH COOLER
TEMPERATURES PUSHING WEST TO EAST BY TUESDAY-THURSDAY AS AN UPPER
TROUGH AND SURFACE COLD FRONT MOVE THROUGH. BUT THIS WILL PUSH
ABOVE NORMAL TEMPERATURES INTO THE OHIO VALLEY AND GREAT LAKES
REGION AND THE MID-ATLANTIC AND NORTHEAST, WITH ANOMALIES AROUND
10-15 DEGREES. FARTHER SOUTH, TEMPERATURES GENERALLY 10-15 DEGREES
ABOVE AVERAGE (AROUND 100F FOR HIGHS) WILL EXTEND SUMMER FOR THE
SOUTHERN HALF OF THE PLAINS THROUGH NEXT WEEK. THE WESTERN U.S. ON
THE OTHER HAND LOOKS TO BE ON THE COOL SIDE ESPECIALLY FOR HIGH
TEMPERATURES PARTICULARLY EARLY IN THE WEEK, WITH SOME MODERATION
BY MIDWEEK.
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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