607
FXUS02 KWBC 150701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT MON MAY 15 2023
VALID 12Z THU MAY 18 2023 - 12Z MON MAY 22 2023
...WARMER THAN AVERAGE TEMPERATURES WILL PERSIST ACROSS THE
NORTHWEST...
...THE SOUTH-CENTRAL ROCKIES AND PLAINS COULD SEE HEAVY RAINFALL
THURSDAY-FRIDAY...
..OVERVIEW
THE OVERALL WEATHER PATTERN THIS WEEK AND INTO EARLY NEXT WEEK
SHOULD REMAIN FAIRLY STAGNANT ACROSS THE COUNTRY WITH AN
ANOMALOUSLY STRONG UPPER RIDGE OVER THE WEST CAUSING WARM TO HOT
TEMPERATURES THERE, WHILE MEAN TROUGHING IS LIKELY DOWNSTREAM
ACROSS EASTERN NORTH AMERICA. INDIVIDUAL SHORTWAVES WILL SUPPORT
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 POTENTIALLY
CAUSING FLASH FLOODING IN THE SOUTH-CENTRAL ROCKIES AND PLAINS
LATE THIS WORKWEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS QUITE AGREEABLE WITH THE LARGE SCALE RIDGE
IN THE WEST, WITH ABOVE AVERAGE PREDICTABILITY. DETAILS WITHIN THE
MEAN TROUGHING DOWNSTREAM IN THE EAST STILL SHOW SOME VARIABILITY
THOUGH. IN PARTICULAR, A COMPACT UPPER LOW IS FORECAST TO DROP
SOUTHEAST FROM SOUTH-CENTRAL CANADA THURSDAY AND MOVE EASTWARD
THROUGH THE GREAT LAKES LATE WEEK ALONG WITH ITS SURFACE LOW
REFLECTION AND ITS ACCOMPANYING COLD FRONT MOVING THROUGH THE
CENTRAL U.S. AND THEN THE EAST. OVERALL, THE 12/18Z MODEL CYCLE
SLOWED DOWN WITH THESE FEATURES COMPARED TO PREVIOUS CYCLES, WHICH
WAS REFLECTED IN THE WPC 500MB HEIGHTS AND SURFACE FRONTAL CHARTS.
WITHIN THE SLOWER TREND, GFS RUNS AS WELL AS GEFS MEMBERS WERE
GENERALLY ON THE FASTER SIDE OF GUIDANCE COMPARED TO THE ECMWF
SUITE. BUT THE 12Z AND NOW THE NEWER 00Z CMC WERE WELL ON THE
SLOWER SIDE OF THE GUIDANCE ENVELOPE TO THE POINT WHERE THEY COULD
NOT REALLY BE USED IN THE RECENT WPC FORECAST. THUS THE FORECAST
BLEND WAS BASED ON THE 12/18Z GFS AND 12Z ECMWF AND UKMET EARLY IN
THE FORECAST PERIOD THAT SHOWED BETTER AGREEMENT ON TIMING, THOUGH
THESE DO SHOW DIFFERENCES IN THE PHASING OF THE UPPER LOW BACK
INTO THE MEAN FLOW LATE WEEK (ECMWF MAINTAINS THE SEPARATION
LONGER). THE INCOMING 00Z ECMWF CONTINUED THE SLOWER TREND WITH
SLOWER MOVEMENT THAN ITS PREVIOUS RUN, BUT NOT QUITE TO THE SLOW
EXTENT OF THE CMC RUNS.
DIFFERENCES IN THE TIMING OF THE UPPER AND SURFACE LOW AND FRONTAL
SYSTEM CONTINUE INTO EARLY NEXT WEEK AND CREATE A VARIETY OF
SOLUTIONS FOR WHEN THE FRONT AND PRECIPITATION AHEAD OF IT CLEAR
THE EAST COAST. BUT THE EASTERN PACIFIC ALSO BECOMES AN AREA OF
FOCUS FROM A MODEL DIAGNOSTICS STANDPOINT, WITH THE POTENTIAL FOR
AN UPPER LOW OFFSHORE OF CANADA TO FINALLY SPIN SOME
ENERGY/TROUGHING TOWARD THE PACIFIC NORTHWEST FOR SOME COOLER
TEMPERATURES WITH A COLD FRONT OR TWO AS THE PREVAILING RIDGE
WEAKENS A BIT AND PUSHES EASTWARD EARLY NEXT WEEK. SOME
DISCREPANCIES REMAIN IN THE MODEL AND ENSEMBLE GUIDANCE WITH
TIMING AND DEPTH, BUT THE MAIN OUTLIER IN THE 12/18Z CYCLE SEEMED
TO BE THE CMC AGAIN, WHICH SPLIT SOME ENERGY OFF AND TRACKED IT
THROUGH WESTERN CANADA WHILE MAINTAINING A WEST COAST RIDGE NEXT
MONDAY, OUT OF PHASE WITH OTHER GUIDANCE THAT PROGRESSED AT LEAST
SOME SEMBLANCE OF THE UPPER LOW EASTWARD. OVERALL GIVEN INCREASING
MODEL SPREAD, THE FORECAST BLEND TENDED TOWARD MORE GEFS AND EC
ENSEMBLE COMPONENTS THROUGH THE MID TO LATE MEDIUM RANGE PERIOD,
TO ABOUT HALF THE BLEND BY THE END.
..WEATHER/HAZARDS HIGHLIGHTS
A LOW PRESSURE SYSTEM MOVING THROUGH SOUTH-CENTRAL CANADA AND
ACROSS THE GREAT LAKES WILL PUSH A COLD FRONT INTO THE CENTRAL AND
THEN EASTERN U.S. DURING THE LATTER PART OF THE WEEK INTO THE
WEEKEND. THE COLD FRONT LOOKS TO HANG UP A BIT IN THE
SOUTH-CENTRAL ROCKIES ON THURSDAY AND FRIDAY AND PRODUCE UPSLOPE
FLOW IN AN ENVIRONMENT WITH 90+ PERCENTILE MOISTURE ANOMALIES.
THIS WOULD ALSO BE ATOP WET GROUND WITH HIGH STREAMFLOWS THAT
COULD EXACERBATE FLASH FLOODING RISK. THEN FARTHER EAST INTO THE
SOUTH-CENTRAL PLAINS, THE WEST-EAST ORIENTATION OF THE FRONT AND
ITS POSSIBLY SLOW MOVEMENT LATE THURSDAY COULD LEAD TO TRAINING OF
STORMS WITH POSSIBLY HIGH RAIN RATES THAT FORM ON THE CUSP OF THE
INSTABILITY/MOISTURE NEAR AND AHEAD OF THE FRONT. THUS THE SLIGHT
RISK FOR DAY 4/THURSDAY IN THE ERO WILL EXPAND EASTWARD INTO THE
PLAINS FOR THIS FORECAST CYCLE. ONE LIMITING FACTOR THERE COULD BE
DRIER ANTECEDENT CONDITIONS THOUGH. THE FRONT WILL CONTINUE TO
SHIFT SOUTH AND EAST LATE WEEK, WITH A MARGINAL RISK IN THE DAY
5/FRIDAY ERO FROM THE SOUTH-CENTRAL ROCKIES INTO THE SOUTH-CENTRAL
PLAINS/MISSISSIPPI VALLEY. AT THIS POINT IT APPEARS THE FRONT MAY
BEGIN TO MOVE FASTER FOR DAY 5, PRECLUDING AN EMBEDDED SLIGHT
RISK, AS STORMS WITH HIGH RAIN RATES MAY NOT TRAIN AS MUCH. BUT A
SLIGHT RISK MAY NEED TO BE ADDED IN FUTURE CYCLES IF HEAVY RAIN
ENDS UP FALLING OVER SENSITIVE AREAS OR THE FRONT STALLS MORE.
MEANWHILE THE COLD FRONT WILL MOVE MORE PROGRESSIVELY EASTWARD
ACROSS THE MIDWEST/OHIO VALLEY ON FRIDAY WITH ENHANCED RAINFALL
CHANCES SHIFTING INTO THE EAST ON SATURDAY ALONG AND AHEAD OF THE
FRONT, PERHAPS LINGERING ALONG PARTS OF THE EAST COAST SUNDAY.
ELSEWHERE, SHOWERS AND STORMS MAY HAVE SOME FOCUS ALONG A FRONTAL
BOUNDARY IN THE SOUTHEAST ON THURSDAY IN PARTICULAR. WITH AMPLE
MOISTURE AND INSTABILITY IN PLACE, THERE COULD BE SOME LOCALIZED
HEAVY RAIN IN AND AROUND SOUTHERN GEORGIA/NORTHERN FLORIDA, BUT
AMOUNTS ARE NOT CURRENTLY EXPECTED TO SURPASS THE HIGH FLASH FLOOD
GUIDANCE. SCATTERED SHOWERS AND THUNDERSTORMS ARE ALSO POSSIBLE IN
THE WEST AT TIMES LATE THIS WEEK INTO EARLY NEXT WEEK ESPECIALLY
IN THE AFTERNOONS/EVENINGS WITH A COMMON DIURNAL CYCLE, AS A
MONSOONAL PATTERN SETS UP WITH ANOMALOUSLY HIGH MOISTURE REMAINING
OVER A LARGE PORTION OF THE WESTERN U.S. UNDERNEATH THE UPPER
RIDGE. COVERAGE OF THE PRECIPITATION MAY NOT BE TERRIBLY BROAD,
BUT THE DETAILS ARE YET TO BE DETERMINED.
TEMPERATURE-WISE, THE PERSISTENT LARGE SCALE WESTERN NORTH AMERICA
RIDGE WILL SUPPORT WARM TO HOT TEMPERATURES ACROSS MOST OF THE
WEST. THE PACIFIC NORTHWEST IS LIKELY TO BE WARMEST ON THURSDAY IN
THE MEDIUM RANGE PERIOD, WITH HIGHS AROUND 20 DEGREES ABOVE
AVERAGE BEFORE SOME GRADUAL MODERATION LATE THIS WEEK INTO EARLY
NEXT WEEK FOR AREAS WEST OF THE CASCADES. HOWEVER, INTERIOR AREAS
LIKE THE GREAT BASIN CAN EXPECT THE HEAT TO PERSIST INTO THE
WEEKEND. HIGHS IN THE 90S THERE COULD BE RECORD-SETTING, WHILE
LOWS IN THE 50S AND 60S COULD BREAK RECORDS FOR WARM MINIMUM
TEMPERATURES ACROSS MUCH OF THE WEST. HIGHS ABOVE 100F ARE LIKELY
IN THE DESERT SOUTHWEST, BUT THIS IS NOT TERRIBLY ANOMALOUS FOR
MAY. MEANWHILE, THE NORTHEAST SHOULD SEE CHILLY TEMPERATURES
THURSDAY MORNING, POSSIBLY APPROACHING THE FREEZING MARK IN
INTERIOR AREAS AND POTENTIALLY SETTING A FEW RECORDS. PERIODS OF
RAIN AND CLOUDS AND THEN POST-FRONTAL COOLER AIR COULD LEAD TO
TEMPERATURES SLIGHTLY BELOW AVERAGE IN THE PLAINS AT TIMES.
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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