128
FXUS02 KWBC 141858
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 PM EDT SUN MAY 14 2023
VALID 12Z WED MAY 17 2023 - 12Z SUN MAY 21 2023
...WARMER THAN AVERAGE TEMPERATURES WILL PERSIST ACROSS THE
NORTHWEST...
...THE SOUTH-CENTRAL ROCKIES/PLAINS COULD SEE HEAVY RAINFALL
AROUND THURSDAY-FRIDAY...
..OVERVIEW
THE OVERALL WEATHER PATTERN THROUGH THE WEEK INTO NEXT WEEKEND
SHOULD REMAIN FAIRLY STAGNANT ACROSS THE COUNTRY. AN ANOMALOUSLY
STRONG UPPER RIDGE OVER THE WESTERN HALF OF THE U.S. AND INTO
WESTERN CANADA WILL BE PERSISTENT ASIDE FROM AN UPPER LOW
GRADUALLY SHEARING OUT/WEAKENING ATOP THE NORTHWEST THROUGH AROUND
MIDWEEK. THIS PATTERN WILL MAINTAIN WARM TO HOT TEMPERATURES
ACROSS THE NORTHWEST AND TO A LESSER EXTENT OVER OTHER PARTS OF
THE WEST. DOWNSTREAM, MEAN TROUGHING IS LIKELY ACROSS EASTERN
NORTH AMERICA, WITH INDIVIDUAL SHORTWAVES SUPPORTING COLD FRONTS
THAT SHOULD PUSH SOUTHEASTWARD TO THE EAST OF THE ROCKIES AND
PRODUCE AREAS OF SHOWERS AND THUNDERSTORMS, INCLUDING THE
POSSIBILITY FOR SOME ENHANCED RAINFALL AMOUNTS IN THE
SOUTH-CENTRAL ROCKIES/HIGH PLAINS LATE THIS WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AS HAS BEEN THE CASE IN RECENT DAYS, LATEST GUIDANCE AGREES WELL
ON THE OVERALL MEAN PATTERN THAT HAS ABOVE AVERAGE PREDICTABILITY
GIVEN THE LARGE SCALE OF THE WESTERN RIDGE/EASTERN TROUGH
CONFIGURATION. THERE ARE THREE PRIMARY MEDIUM-SCALE EMBEDDED OR
PERIPHERAL ASPECTS OF THE FORECAST THAT SHOW SPREAD AND
VARIABILITY: THE SYSTEM DROPPING INTO THE NORTHERN TIER U.S./GREAT
LAKES LATE THIS WEEK, SHORTWAVE ENERGY CROSSING THE SOUTHEAST
QUADRANT OF THE LOWER 48 MID-LATE WEEK, AND HOW MUCH NORTHEASTERN
PACIFIC ENERGY MAY BEGIN TO PUSH INTO THE WESTERN CANADA/PACIFIC
NORTHWEST PART OF THE RIDGE BY AROUND NEXT SUNDAY.
FOR THE CANADIAN SYSTEM REACHING OVER OR NEAR THE NORTHERN
TIER/GREAT LAKES LATE IN THE WEEK, THERE IS REASONABLE CLUSTERING
AMONG THE 00Z/06Z RUNS FOR A CLOSED OR NEARLY CLOSED UPPER LOW
INTO DAY 4 THURSDAY BUT THEN SOLUTIONS DIVERGE WITH RESPECT TO
WHEN/HOW MUCH PHASING OCCURS WITH FLOW TO THE NORTH. THE PAST
THREE ECMWF RUNS HAVE KEPT THE TWO STREAMS SEPARATED INTO SATURDAY
WHILE THE PAST 24 HOURS OF GFS RUNS THROUGH 00Z WERE TRENDING IN
THAT MANNER (WITH THE 00Z RUN ACTUALLY TRACKING THE UPPER LOW
SOUTH OF THE 00Z ECMWF). TO VARYING DEGREES THE 00Z CMC/UKMET AND
06Z GFS ALL DEPICTED EARLIER PHASING ALOFT AND INCREASINGLY FASTER
SURFACE LOW PROGRESSION THROUGH EASTERN CANADA INSTEAD OF A SLOWER
PATH ACROSS NEW ENGLAND. THE NEW 12Z CYCLE DOES LITTLE TO IMPROVE
CONFIDENCE. THE CMC HAS REVERTED BACK TO AN EXTREMELY SLOW TIMING
(A TRAIT OF ONE OR MORE EARLIER CMC RUNS) WHILE THE GFS STILL
OPENS THE SYSTEM UP AFTER EARLY FRIDAY BUT THEN PULLS OFF
ADDITIONAL ENERGY TO THE SOUTH BY SUNDAY. THE 12Z ECMWF HAS
TRENDED NOTABLY SLOWER WITH NO PHASING INTO SUNDAY. A BLENDED
APPROACH BASED ON 00Z/06Z GUIDANCE TILTED CLOSER TO THE 00Z ECMWF
IDEA LATER IN THE PERIOD, NUDGING CONTINUITY A BIT FASTER WITH THE
MAIN SURFACE SYSTEM. THE NEW RUNS SUGGEST THE TIMING MAY REVERT
BACK THOUGH.
FARTHER SOUTH, 00Z/06Z MODELS HAVE MADE A PRONOUNCED SWITCH FOR
SHORTWAVE DETAILS FROM THE CENTRAL-SOUTHERN PLAINS EASTWARD. THE
GFS HAD GENERALLY BEEN ON ITS OWN WITH GREATER EMPHASIS ON A
COMPACT CENTRAL PLAINS INTO SOUTHEAST FEATURE RELATIVE TO A
SEPARATE ONE EMERGING FROM TEXAS. HOWEVER THE 00Z RUNS GRAVITATED
TO A SCENARIO MUCH MORE LIKE THE GFS. NEW 12Z RUNS APPEAR SIMILAR
(EXCEPT FOR THE CMC WHOSE SLOWER TIMING ALONG THE NORTHERN TIER
BEGINS TO HAVE SOME EFFECT AFTER WEDNESDAY) WITH THE SHORTWAVE BUT
IN GENERAL GUIDANCE HAS BEEN VERY INCONSISTENT WITH ASSOCIATED QPF
DETAILS OVER THE GULF COAST/SOUTHEAST. THE 00Z/06Z GUIDANCE
SUGGESTS THAT THE ASSOCIATED SURFACE WAVE COULD ULTIMATELY LIFT
NORTHWARD NEAR THE EAST COAST AHEAD OF THE APPROACHING NORTHERN
STREAM SYSTEM. WHILE REFLECTED AS A RESULT OF FOLLOWING THE MOST
COMPATIBLE MODEL/MEAN CLUSTER, CONFIDENCE FOR THIS DETAIL IS
FAIRLY LOW.
BY NEXT WEEKEND THE 00Z CMC MEAN AND LATEST GFS/GEFS MEAN RUNS
BRING MORE PACIFIC SHORTWAVE ENERGY INTO THE WESTERN
CANADA/PACIFIC NORTHWEST PART OF THE WESTERN NORTH AMERICA RIDGE
VERSUS MOST OTHER SOLUTIONS. FOR NOW PREFER AN INTERMEDIATE
SOLUTION THAT SHOWS ONLY A HINT OF SUCH ENERGY (BY WAY OF TILTING
SOMEWHAT MORE IN THE ECMWF/ECMWF MEAN DIRECTION RELATIVE TO THE
GFS/GEFS). IN RECENT DAYS, INDIVIDUAL SOLUTIONS THAT TRIED TO
WEAKEN THE RIDGE ULTIMATELY ADJUSTED BACK TO A STRONGER RIDGE,
THOUGH OF COURSE AT SOME POINT IT WILL EVENTUALLY WEAKEN. THE NEW
12Z ECMWF DOES BEGIN TO ERODE THE RIDGE A LITTLE MORE THAN THE 00Z
RUN.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE OVER PARTS OF THE
SOUTHEAST WEDNESDAY-THURSDAY AS A WAVY WEST-EAST FRONTAL BOUNDARY
LINGERS NEAR THE GULF COAST AND THROUGH THE SOUTHEAST ON WEDNESDAY
AND ANOTHER FRONT DROPS DOWN TO TAKE ITS PLACE BY THURSDAY. RECENT
GUIDANCE HAS BEEN DIVERGENT AND INCONSISTENT WITH RAINFALL
SPECIFICS ACROSS THE SOUTHERN TIER DUE TO ISSUES WITH UPPER LEVEL
SHORTWAVE DETAILS, WITH 00Z AND LATER RUNS TRENDING DRIER ALONG
THE WESTERN-CENTRAL GULF COAST BUT SOME 12Z RUNS ALSO REDUCING
COVERAGE WHERE THERE IS A GENERAL SIGNAL FOR PERHAPS LOCALIZED
HEAVY ACTIVITY OVER THE FAR SOUTHEAST. THESE AREAS WHERE ENHANCED
RAINFALL IS STILL A POSSIBILITY (GIVEN SOME FRONTAL FOCUS ALONG
WITH INSTABILITY/ABOVE-CLIMO MOISTURE THAT COULD YIELD HIGH RAIN
RATES) SHOULD BE LESS SENSITIVE TO RAIN SINCE THE RECENT PATTERN
HAS BEEN DRIER AND FLASH FLOOD GUIDANCE IS HIGH ESPECIALLY CLOSE
TO THE COAST. THE DAYS 4-5 (WEDNESDAY-THURSDAY NIGHT) EXCESSIVE
RAINFALL OUTLOOKS CONTINUES TO HOLD OFF WITH DEPICTING A SPECIFIC
RISK AREA GIVEN THE CONTINUED GUIDANCE INCONSISTENCY.
FARTHER NORTH, SHOWERS AND STORMS ARE LIKELY TO OCCUR AHEAD OF A
COLD FRONT DROPPING SOUTH INTO THE NORTH-CENTRAL CONUS. ON
WEDNESDAY STORMS LOOK PROGRESSIVE ENOUGH THAT THEY MAY NOT CAUSE
FLASH FLOODING ISSUES IN THE NORTHERN PLAINS, BUT BY THURSDAY THE
FRONT COULD HANG UP A BIT IN THE SOUTH-CENTRAL ROCKIES/HIGH
PLAINS. THIS STALLING FRONT AND MOISTURE ANOMALIES THAT COULD BE
ABOVE THE 90TH PERCENTILE COULD LEAD TO SOME FLASH FLOODING
ISSUES, ESPECIALLY WHERE UPSLOPE FLOW COULD ENHANCE RAIN AMOUNTS
IN THE HIGHER ELEVATIONS OF THE CENTRAL ROCKIES. THIS WOULD ALSO
BE ATOP WET GROUND WITH HIGH STREAMFLOWS THAT COULD EXACERBATE THE
RISK. THE PATTERN BY DAY 5/THURSDAY IN SOME WAYS SEEMS MORE
PREDICTABLE THAN THE SHORT RANGE INTO DAY 4 SINCE IT IS DRIVEN BY
THE RELATIVELY LARGER-SCALE FEATURES OF THE UPPER LOW AND SURFACE
FRONTAL SYSTEM, THOUGH SOME TIMING DIFFERENCES REMAIN. THUS THERE
SEEMS TO BE ENOUGH CONFIDENCE FOR A SLIGHT RISK IN THE DAY 5 ERO
FOR THE CENTRAL ROCKIES WITH A MARGINAL RISK EXTENDING INTO MUCH
OF THE SOUTH-CENTRAL PLAINS AND SOUTHERN ROCKIES DOWNSTREAM. THE
AFTERNOON UPDATE SHOULD REFLECT MINIMAL ADJUSTMENT TO THESE AREAS
WHILE AWAITING ANY NEW TRENDS/CLUSTERING IN THE GUIDANCE. SOME
RAIN COULD LINGER IN THOSE AREAS INTO FRIDAY AS WELL. FARTHER
NORTH THE COLD FRONT SHOULD BE MORE PROGRESSIVE BUT MAY STILL
PRODUCE A BRIEF PERIOD OF FAIRLY INTENSE RAINFALL IN LIGHT OF THE
STRONG DYNAMICS ALOFT. EXPECT THIS SYSTEM TO BRING RAINFALL INTO
THE EAST-CENTRAL U.S. ON FRIDAY AND THE EASTERN SEABOARD ON
SATURDAY. PORTIONS OF THE INTERIOR WEST SHOULD SEE SCATTERED
SHOWERS FROM LATE WEEK THROUGH THE WEEKEND IN A MONSOON-LIKE
PATTERN AS ANOMALOUSLY HIGH MOISTURE REMAINS OVER A LARGE PORTION
OF THE WESTERN U.S. UNDERNEATH THE UPPER RIDGE.
TEMPERATURE-WISE, THE PERSISTENT LARGE SCALE WESTERN NORTH AMERICA
RIDGE WILL SUPPORT WARM TO HOT TEMPERATURES ACROSS MOST OF THE
WEST. THE PACIFIC NORTHWEST SHOULD SEE HIGH TEMPERATURES AROUND
15-25 DEGREES ABOVE AVERAGE FOR WEDNESDAY-THURSDAY ALONG WITH LOWS
IN THE 50S AND 60S THAT COULD SET RECORD HIGH MINIMUM
TEMPERATURES. LOCATIONS WEST OF THE CASCADES COULD SEE SOME
GRADUAL MODERATION LATE WEEK INTO THE WEEKEND (PERHAPS GETTING
CLOSE TO NORMAL NEAR THE COAST), BUT INTERIOR AREAS LIKE THE GREAT
BASIN CAN EXPECT MUCH ABOVE AVERAGE TEMPERATURES TO CONTINUE INTO
THE WEEKEND, WITH HIGHS INTO THE 90S POSSIBLY SETTING RECORDS AS
WELL. HIGHS IN THE 90S AND EVEN NEARING 100F ARE POSSIBLE IN THE
CALIFORNIA CENTRAL VALLEY, WITH HIGHS ABOVE 100F IN THE DESERT
SOUTHWEST ON WEDNESDAY (THOUGH THE LATTER IS LESS ANOMALOUS FOR
MAY). THE NORTH-CENTRAL U.S. COULD SEE ABOVE AVERAGE TEMPERATURES
BY UP TO 10-15F ON WEDNESDAY, FOLLOWED BY A COOLER TREND
THURSDAY-FRIDAY AFTER THE FRONTAL PASSAGE, BUT POSSIBLY WARMING UP
AGAIN BY THE WEEKEND. THE SOUTHERN TIER HOWEVER MAY SEE
TEMPERATURES AROUND TO SLIGHTLY BELOW AVERAGE GIVEN THE PERIODIC
RAIN CHANCES. THE NORTHEAST COULD SEE A COOL WEDNESDAY THROUGH
THURSDAY MORNING WITH TEMPERATURES AROUND 10-15F BELOW NORMAL, BUT
SHOULD MODERATE TO NEAR NORMAL THEREAFTER.
RAUSCH/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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES 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
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