515
FXUS02 KWBC 201906
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SAT MAY 20 2023
VALID 12Z TUE MAY 23 2023 - 12Z SAT MAY 27 2023
...WATCHING COASTAL FRONT/LOW FOR THE POSSIBILITY OF BRINGING
HEAVY RAIN TO THE SOUTHEAST/FLORIDA...
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS AND ENSEMBLES REMAIN IN GOOD AGREEMENT THAT A WARMING UPPER
RIDGE NEXT WEEK WILL SHIFT SLOWLY EASTWARD OVER THE PLAINS BEFORE
NUDGING INTO THE MISSISSIPPI VALLEY/MIDWEST, WITH THE ENTIRE
PATTERN BARELY MOVING THROUGH NEXT WEEK. EARLY-MID NEXT WEEK WILL
ALSO SEE AN UPSTREAM UPPER TROUGH FORM OVER THE NORTHWEST AS A
NORTHEAST U.S. UPPER TROUGH LIFTS INTO THE CANADIAN MARITIMES.
VORTICITIES SEPARATED FROM THE SOUTHERN END OF BOTH TROUGHS ARE
FORECAST TO AMPLIFY THE UPPER-AIR PATTERN TOWARD THE END OF NEXT
WEEK, WITH THE POSSIBILITY OF A CLOSED LOW DEVELOPING NEAR
CALIFORNIA, AND A BROADER TROUGH/CLOSED LOW DEVELOPING OVER THE
SOUTHEAST/FLORIDA.
REGARDING SURFACE FEATURES, MUCH OF THE UNCERTAINTIES WAS NOTED
ACROSS THE NORTHERN PLAINS AS WELL AS WHAT MIGHT HAPPEN ALONG THE
EAST COAST. AS HAS BEEN THE CASE OVER THE LAST FEW DAYS, MODEL
CONSENSUS HAS BEEN TRENDING SLOWER REGARDING THE LOW PRESSURE WAVE
FORECAST TO DEVELOP OVER THE NORTHERN ROCKIES/HIGH PLAINS EARLY
NEXT WEEK. THE GFS HAS BACKED OFF ON BRINGING A FRONTAL WAVE
EASTWARD ACROSS THE NORTHERN PLAINS IN MUCH BETTER AGREEMENT WITH
THE REST OF THE GUIDANCE BY ALLOWING A STRONGER HIGH PRESSURE
SYSTEM FROM CENTRAL CANADA TO REACH INTO THE GREAT LAKES.
MEANWHILE, NEAR AND JUST OFF THE SOUTHEAST AND INTO FLORIDA,
ENSEMBLE MEANS CONTINUE TO SUPPORT AN UPPER TROUGH/LOW TO LINGER
OVER THE VICINITY LATER NEXT WEEK AND INTO NEXT WEEKEND. GIVEN
WARM SEA-SURFACE TEMPERATURES ALONG THE GULF STREAM AND THE
PRESENCE OF A LINGERING FRONT, SOME OF THE DETERMINISTIC GUIDANCE
HAS OCCASIONALLY SPUN UP A TROPICAL SYSTEM OFF THE COAST OF THE
SOUTHEASTERN U.S. AND TRACKED IT TOWARD THE COAST BY THE MEMORIAL
DAY WEEKEND. THERE IS ALSO THE POSSIBILITY FOR ANOTHER PIECE OF
VORTICITY TO BREAK OFF OVER THE NORTHEAST AND THEN INTERACT WITH
WHAT MAY BE PRESENT FROM FLORIDA TO OFF THE SOUTHEAST U.S. COAST.
NEEDLESS TO SAY, UNCERTAINTY IS TOO HIGH TO PROVIDE THE SPECIFICS
AT THIS POINT. NONETHELESS, THERE APPEARS TO BE A HIGHER THAN
NORMAL CHANCE FOR THE HEAVY RAIN ASSOCIATED WITH THE SYSTEM TO
PUSH ONSHORE FROM THE EAST COAST OF FLORIDA TO THE SOUTHEAST AND
POSSIBLY FARTHER UP THE EAST COAST AS WE HEAD CLOSER TO THE
MEMORIAL DAY WEEKEND.
A COMPOSITE BLEND WAS USED TO COMPOSE THE WPC FORECAST PACKAGE
BASED ON 40% FROM THE 00Z ECMWF/EC MEAN, 20% FROM THE 00Z CMC/CMC
MEAN, WITH THE REMAINING PORTIONS BASED MAINLY ON THE 06Z GEFS
MEAN. ONLY MINIMAL PORTIONS FROM THE GFS WERE USED IN THIS
FORECAST CYCLE DUE TO INCOMPATIBILITY WITH THE REST OF THE
GUIDANCE FROM DAY 5 ONWARD. THIS OVERALL SOLUTION MAINTAINS
BROADLY GOOD WPC PRODUCT CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
A SLOW-MOVING AND SOUTHERN STREAM SEPARATED UPPER TROUGH OVER
SOUTHEAST U.S. NEXT WEEK SEEMS TO BROADLY FAVOR PERIODS OF HEAVY
MARITIME RAINS BUT ALSO ONSHORE FROM MAINLY ATLANTIC COASTAL
PORTIONS OF THE SOUTHEAST/FLORIDA AS SERIES OF SURFACE FRONTAL
LOWS ROTATE IN PROXIMITY TO TAP DEEPENED POOLED MOISTURE, BUT
DETAILS REMAIN UNCERTAIN. STRONG ONSHORE FLOW CAN BE EXPECTED TO
SET UP NORTH OF THESE SURFACE LOWS UNDER SUCH SCENARIO.
THEREAFTER, SOME GUIDANCE INDICATED SCENARIOS THAT SHOW LESS
LIKELY COASTAL WAVE DEVELOPMENTS FARTHER UP/OFF THE EAST COAST,
BUT WITH PLENTY OF UNCERTAINTY. MEANWHILE, A FRONTAL WAVE SLIDING
ACROSS THE SOUTHEAST BY AROUND NEXT TUESDAY IS FORECAST TO TRIGGER
SHOWERS AND THUNDERSTORMS ACROSS SOUTHERN GEORGIA TO NORTHERN
FLORIDA WHERE A MARGINAL RISK OF FLASH FLOODING HAS BEEN
INTRODUCED ON DAY 4. OTHERWISE, THE AFOREMENTIONED MAIN UPPER
RIDGE POSITION SLOW TRANSLATION SHOULD SUPPORT SUMMERTIME
TEMPERATURES WITH VALUES UPWARDS TO 15-20 DEGREES ABOVE NORMAL
INTO EARLY-MID NEXT WEEK THAT SHOULD SPREAD SLOWLY EAST WITH TIME
ACROSS THE NORTH-CENTRAL U.S. TO THE MIDWEST INTO LATER NEXT WEEK,
ALL ONLY TEMPERED LOCALLY BY SCATTERED AND POTENTIALLY STRONG
THUNDERSTORM ACTIVITY BOTH WITH EJECTION OF IMPULSES FROM THE WEST
AND INTERACTION WITH NORTHERN STREAM SYSTEM/FRONTAL APPROACHES
FROM SOUTHERN CANADA. LOCALLY HEAVY THUNDERSTORM ACTIVITY MAY ALSO
FOCUS HEAVY LOCAL DOWNPOURS DOWN OVER THE SOUTHERN PLAINS AS WELL
WITH MOISTURE/INSTABILITY POOLING WITH SOUTHERN STREAM IMPULSES.
ABOVE TO MUCH ABOVE NORMAL OVERNIGHT TEMPERATURES AND A PATTERN
FAVORING SCATTERED TO WIDESPREAD SHOWERS AND THUNDERSTORMS ARE
ALSO EXPECTED TO LINGER ALL NEXT WEEK OVER MUCH OF INTERIOR WEST
THROUGH THE ROCKIES IN MOISTENED FLOW CHANNELED BETWEEN WEST COAST
UPPER TROUGHING AND THE DOWNSTREAM CENTRAL U.S. UPPER RIDGE, WITH
DETAILS/LOCAL FOCUS DEPENDENT ON THE ULTIMATE SHAPE OF THE
UNCERTAIN LATER NEXT WEEK UPPER TROUGH POSITION.
KONG/SCHICHTEL
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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