717
FXUS02 KWBC 130716
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
316 AM EDT FRI SEP 13 2024
VALID 12Z MON SEP 16 2024 - 12Z FRI SEP 20 2024
..OVERVIEW
RELOADING UPPER TROUGHING WILL BE IN PLACE ACROSS THE WESTERN U.S.
THROUGH NEXT WEEK, PROMOTING COOLER WEATHER AND RAIN AND HIGHER
ELEVATION SNOW. SOME RAIN MAY SPREAD INTO THE PLAINS AS WELL, BUT
WITH WARMER THAN AVERAGE TEMPERATURES. MEANWHILE IN THE EAST, EARLY
IN THE WEEK A REX BLOCK TYPE PATTERN WILL SET UP WITH AN UPPER
HIGH SITUATED OVER THE NORTHEAST AND A WEAK UPPER LOW FARTHER
SOUTH, ACCOMPANIED BY A COASTAL SURFACE LOW PRESSURE SYSTEM. THE
EVOLUTION OF THE PATTERN IN THE EAST IS RATHER UNCERTAIN AS THE
WEEK PROGRESSES, INCLUDING THE SURFACE LOW'S POSSIBLE TROPICAL OR
SUBTROPICAL EVOLUTION AND THE COASTAL AND INLAND RAINFALL FORECAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS PERSISTENT IN INDICATING A DEEP UPPER LOW
CENTERED OVER NORTHERN CALIFORNIA AS THE PERIOD BEGINS MONDAY,
WHICH ALONG WITH AN ACCOMPANYING SURFACE LOW SHOULD LIFT NORTHEAST
WITH TIME. THERE IS SOME MINOR INCREASING SPREAD WITH THESE
UPPER/SURFACE LOWS BY WEDNESDAY-THURSDAY. THE 18Z GFS/ECMWF WERE
PRETTY AGREEABLE IN THEIR POSITIONS JUST NORTH OF MONTANA, BUT
THESE WERE SOUTH OF THE ENSEMBLE MEAN POSITIONS AND NOW SOUTH
OF/SLOWER THAN THE 00Z MODEL POSITIONS. MEANWHILE THE 12Z CMC
SEEMED TO GET RID OF THIS COMPACT UPPER LOW TOO QUICKLY, BUT THE
00Z RUN LOOKS IN BETTER ALIGNMENT. ANOTHER ROUND OF VORTICITY IS
FORECAST TO DIG SOUTH ALONG THE WEST SIDE OF THE TROUGH ATOP THE
WEST COAST STATES DURING TUESDAY-THURSDAY BEFORE MOVING THE TROUGH
EAST BY FRIDAY. ONCE AGAIN THE 00Z CMC SEEMS MORE AGREEABLE WITH
CONSENSUS THAN THE 12Z RUN DID. UPSTREAM BY FRIDAY THERE IS
UNCERTAINTY WITH THE FLOW PATTERN COMING INTO BRITISH COLUMBIA AND
POTENTIALLY AFFECTING PACIFIC NORTHWEST FRONTS, WHICH WILL HAVE TO
BE MONITORED AS CURRENTLY MODELS ARE OUT OF PHASE, GFS RUNS WITH
RIDGING AND CMC/EC WITH TROUGHING.
THE EASTERN U.S. PATTERN SHOWS PLENTY OF UNCERTAINTY IN THE
DETAILS OF THE ENERGY EMBEDDED WITHIN THE BROADER UPPER LOW
GRADUALLY DRIFTING NORTH WITH TIME, AFFECTING THE SURFACE PATTERN
WITH A COASTAL STATIONARY FRONT WITH WAVES OF LOW PRESSURE, ONE OF
WHICH HAS SOME POTENTIAL FOR SUBTROPICAL OR TROPICAL DEVELOPMENT.
SOME GFS RUNS REMAIN ON THE AGGRESSIVE SIDE WITH THE SURFACE LOW
STRENGTHENING, BUT ENSEMBLE MEMBERS DO SHOW SOME SUPPORT FOR A LOW
THAT DEEP. THE NEW 00Z ECMWF SHOWS ANY NOTABLE QPF STAYING OFFSHORE
OF THE EAST COAST THROUGH THE FIRST HALF OF THE WEEK, UNLIKE THE
GFS IN PARTICULAR. SO THESE ENERGY/LOW DIFFERENCES DO CREATE
SIGNIFICANT SENSIBLE WEATHER DIFFERENCES, AND THE FORECAST IS
LIKELY TO CONTINUE TO FLUCTUATE.
THE WPC FORECAST WAS BASED ON A BLEND OF DETERMINISTIC MODELS (18Z
GFS AND 00Z ECMWF/CMC/UKMET) EARLY IN THE PERIOD, INTRODUCING AND
INCREASING THE PROPORTION OF THE EC AND GEFS ENSEMBLE MEANS AS THE
PERIOD PROGRESSED GIVEN INCREASING SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER LOW/TROUGH ACROSS THE WEST WILL SUPPORT ROUNDS OF
PRECIPITATION THERE NEXT WEEK, PROVIDING AMPLE LIFT/FORCING. ON
MONDAY, LINGERING MOISTURE FROM THE PACIFIC TROPICAL STORM ILEANA
(THAT WILL BE DISSIPATED BY THEN) COULD ALSO JOIN AND LEAD TO HEAVY
RAIN FOR PORTIONS OF THE FOUR CORNERS STATES, PROMPTING A MARGINAL
RISK THERE. PRECIPITATION MAY ALSO BE SOMEWHAT HEAVY ON MONDAY
AROUND NORTHERN NEVADA, BUT HELD OFF ON ANY EXCESSIVE RAINFALL RISK
DUE TO UNCERTAINTY IF THE RAIN WOULD BE IMPACTFUL THERE. BY
TUESDAY, THE PIVOTING UPPER AND SURFACE LOWS SHOULD SPREAD HEAVY
PRECIPITATION INTO THE NORTHERN ROCKIES/HIGH PLAINS. DID INTRODUCE
A DAY 5/TUESDAY MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE WESTERN
HALF OF MONTANA AS HEAVY RAIN RATES NEAR THE LOW TRACK COULD
PERSIST OVER THE SAME AREAS FOR MULTIPLE HOURS, AND THERE COULD BE
SOME INSTABILITY IN PLACE. THE HIGHER ELEVATIONS CAN EXPECT SNOW
WITH THIS SYSTEM GIVEN THE DEPTH OF THE UPPER LOW. ANOTHER ROUND OF
ENERGY WILL CAUSE THE PACIFIC NORTHWEST TO SEE PRECIPITATION ON
TUESDAY, SHIFTING INTO MUCH OF THE GREAT BASIN MIDWEEK AND THEN
INTO THE NORTHERN ROCKIES. FARTHER EAST, SCATTERED RAIN AND
THUNDERSTORMS ARE LIKELY IN THE PLAINS FOR MULTIPLE DAYS NEXT WEEK
NEAR AND AHEAD OF FRONTAL SYSTEMS.
A STATIONARY FRONT STRETCHING FROM THE FAR WESTERN ATLANTIC AND
BACK ACROSS THE GULF COAST WITH WAVES OF LOW PRESSURE ALONG IT
COULD MAINTAIN RAIN CHANCES ACROSS THE SOUTHEAST TO CAROLINAS FOR
THE EARLY PART OF NEXT WEEK AT LEAST, PERHAPS INTO MID AND LATER
WEEK. THE QPF REMAINS HIGHLY UNCERTAIN GIVEN THE MODEL DIFFERENCES,
INCLUDING HOW FAR WEST AND NORTH RAIN COULD REACH. AT THIS POINT,
WPC SHOWS A MARGINAL RISK FOR DAY 4/MONDAY ACROSS EASTERN PORTIONS
OF THE CAROLINAS NEAR WHERE A SURFACE LOW IS MOST LIKELY TO BE JUST
OFFSHORE. HELD OFF ON ANY RISK AREA FOR DAY 5/TUESDAY BECAUSE OF
THE INCREASING SPREAD IN POSITION OF ANY HEAVY RAINFALL, BUT IN
GENERAL THERE COULD BE SOME RAIN IN THE CAROLINAS INTO THE MID-
ATLANTIC TUESDAY-THURSDAY.
THE WEST CAN EXPECT MUCH COOLER THAN NORMAL TEMPERATURES NEXT WEEK
UNDER THE UPPER TROUGH. HIGH TEMPERATURES WILL BE 15-25F BELOW
NORMAL ACROSS MUCH OF THE WEST, GRADUALLY MODERATING CLOSER TO
NORMAL ACROSS THE WEST COAST STATES BUT PERSISTING IN THE INTERIOR
WEST. SOME AREAS MAY REACH THE FREEZING MARK IN THE HIGHER
ELEVATIONS OF THE GREAT BASIN/ROCKIES. MEANWHILE, THE CENTRAL U.S.
SHOULD SEE WARMER THAN NORMAL HIGHS BY 10-15F AND LOWS BY 15-20F,
INCLUDING HIGHS IN THE 90S IN THE SOUTHERN HALF OF THE PLAINS.
WARMER THAN AVERAGE TEMPERATURES WILL STRETCH INTO THE GREAT LAKES
AND NORTHEAST, WITH MORE TYPICAL TEMPERATURES SOUTH OF THERE.
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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