821
FXUS02 KWBC 190717
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
317 AM EDT MON MAY 19 2025
VALID 12Z THU MAY 22 2025 - 12Z MON MAY 26 2025
..OVERVIEW
HINTS OF AN OMEGA BLOCK SET UP ACROSS NORTH AMERICA LATER THIS
WEEK, AS RIDGING IS LIKELY OVER THE SOUTHWESTERN TO CENTRAL U.S.
ANCHORED BY AN UPPER HIGH IN CENTRAL CANADA, WHILE ROUNDS OF
TROUGHING MOVE THROUGH THE NORTHWEST AND AN UPPER LOW SPINS OVER
THE GREAT LAKES. THE EASTERN TROUGH WILL PROMOTE COOLER THAN
AVERAGE TEMPERATURES AND RAIN IN THE NORTHEAST, WHILE THE RIDGE
LEADS TO WARMER THAN AVERAGE TEMPERATURES IN THE SOUTHWESTERN
QUADRANT OF THE LOWER 48. FRONTAL SYSTEMS EMERGING FROM THE WEST
TOWARD THE PLAINS AND MOIST INFLOW AHEAD OF THEM WILL ALLOW FOR
INCREASING RAIN AND THUNDERSTORM CHANCES IN THE CENTRAL U.S. FROM
LATE WEEK INTO THE WEEKEND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE SHOWS REASONABLY GOOD AGREEMENT WITH AN UPPER LOW
OVER THE GREAT LAKES AND ENERGY TRANSFERRING TOWARD A COASTAL
SURFACE LOW IN THE WESTERN ATLANTIC AS THE PERIOD BEGINS THURSDAY.
A MULTI-MODEL BLEND SUFFICED FOR THESE FEATURES, THOUGH MINOR
DISCREPANCIES IN THE LOW TRACK WOULD YIELD QPF DIFFERENCES IN THE
NORTHEAST THAT COULD BE NOTABLE. MODELS AGREE THAT THE
UPPER/SURFACE LOWS SHOULD MOVE SLOWLY NORTHEAST AND GRADUALLY LIFT
THE TROUGHING OUT OF THE SOUTHEAST AT LEAST.
THE UPPER RIDGE BEHIND THIS TROUGHING ALSO SEEMS WELL PREDICTED BY
THE MODELS, MOVING FROM THE SOUTHWESTERN TO SOUTH-CENTRAL U.S.
WITH TIME. BY FAR, THE MOST UNCERTAIN PART OF THE FORECAST IS WITH
THE FLOW PATTERN IN THE NORTHEASTERN PACIFIC UPPER TROUGH/LOW(S)
COMING INTO THE NORTHWEST. AFTER A SHORTWAVE OR TWO MOVING THROUGH,
THE MAIN UNCERTAINTY IS WITH THE STRENGTH AND TIMING OF ENERGY
ENTERING FRIDAY-SATURDAY. THE 12/18Z MODEL CYCLE SHOWED SIGNIFICANT
SPREAD; THE ECMWF WAS PARTICULARLY WEAK WITH THE ENERGY AND
SHUNTED IT EAST MORE QUICKLY THAN CONSENSUS, WHILE THE CMC (AND NEW
00Z CMC) WAS DEEP ENOUGH TO SPIN UP A MEDIUM SIZED UPPER LOW IN
THE NORTHWEST. GFS RUNS, ESPECIALLY THE 18Z, SHOWED AN IN BETWEEN
SOLUTION WITH THE TIMING OF TROUGHING, THOUGH PERHAPS ON THE DEEP
SIDE ON SATURDAY OVER THE GREAT BASIN. THIS TIMING MATCHED
REASONABLY WELL WITH MOST EC- AND GFS-BASED AI MODELS. SO FAVORED
THE GFS WITH THE ENSEMBLE MEANS IN GENERAL. THE NEWER 00Z GUIDANCE
MAY BE COMING TO A BETTER CONSENSUS ON THIS, WITH THE GFS AND ECMWF
WITH BETTER ALIGNMENT ON TIMING AND A WEAKER TROUGH FEATURE. ALL
THIS ENERGY ARISES FROM THE ARCTIC/RUSSIA/ALASKA REGION AND IS THUS
UNCERTAIN, AND FURTHER CHANGES TO THE FORECAST ARE POSSIBLE.
THE WPC FORECAST WAS BASED ON A BLEND OF THE DETERMINISTIC MODELS
EARLY IN THE PERIOD. GIVEN THE SPREAD IN THE WEST, RAMPED UP THE
PROPORTION OF ENSEMBLE MEANS MORE QUICKLY THAN NORMAL, REACHING
OVER HALF BY DAY 5. BUT HOPEFULLY THE SLIGHTLY BETTER CLUSTERING IN
THE NEWER 00Z MODEL GUIDANCE WILL HELP REFINE THE PATTERN.
..WEATHER/HAZARDS HIGHLIGHTS
RAINFALL WRAPPING AROUND THE COASTAL LOW ON THURSDAY COULD LEAD TO
ISOLATED FLASH FLOODING IN THE NORTHEAST THURSDAY, THOUGH
INSTABILITY SHOULD KEEP THE THREAT PRETTY LOW END. A DAY 4 MARGINAL
RISK REMAINS IN PLACE FOR PARTS OF SOUTHERN NEW ENGLAND, BUT WAS
ABLE TO BE REMOVED FROM THE NYC METRO AREA PER MODEL GUIDANCE THAT
WAS A LITTLE FASTER TO TRACK THE LOW NORTHEAST. RAIN SHOULD LINGER
IN THE NORTHEAST FRIDAY AND SATURDAY AS THE LOW PULLS AWAY SLOWLY.
PRECIPITATION, MOSTLY RAIN EXCEPT FOR THE HIGHEST PEAKS, IS
POSSIBLE ACROSS THE NORTHWESTERN U.S. UNDERNEATH THE ROUNDS OF
UPPER TROUGHING AND AS FRONTAL SYSTEMS MOVE THROUGH. THE DETAILS
ARE UNCERTAIN BASED ON THE MODEL SPREAD WITH THE SUPPORTING
ENERGIES. THE SAME IS TRUE FARTHER EAST MOISTURE STREAMS INTO THE
CENTRAL U.S. AHEAD OF FRONTAL SYSTEMS. BUT IN GENERAL, SCATTERED
THUNDERSTORMS ARE POSSIBLE AHEAD OF A DRYLINE IN THE SOUTHERN
PLAINS AND AHEAD OF A WARM FRONT FARTHER NORTH ACROSS THE CENTRAL
AND NORTHERN PLAINS BEGINNING THURSDAY. MORE ORGANIZED CONVECTION
MAY START TO EMERGE BY FRIDAY AND ESPECIALLY OVER THE WEEKEND IN
THE CENTRAL PLAINS/MIDDLE MISSISSIPPI VALLEY. LEFT THE DAY 5/FRIDAY
ERO BLANK FOR NOW DUE TO UNCERTAINTIES IN PLACEMENT OF ANY HEAVIER
RAIN AMOUNTS THAT COULD CAUSE FLASH FLOODING, BUT MARGINALS MAY BE
NEEDED IN FUTURE CYCLES. THE DYNAMICAL SUPPORT MAY GET BETTER FOR
HEAVIER AND MORE WIDESPREAD RAIN IN THE CENTRAL U.S. BY THE
WEEKEND.
UPPER RIDGING WILL RAISE TEMPERATURES ABOVE NORMAL BY AROUND 10-15
DEGREES OVER CALIFORNIA, THE CENTRAL GREAT BASIN, AND THE DESERT
SOUTHWEST THURSDAY. HIGHS WILL BE GENERALLY IN THE MID-100S IN THE
DESERT SOUTHWEST, WHILE BACK IN THE LOS ANGELES METRO THERE MAY BE
RECORD MAX/MIN TEMPERATURES. THE WARMER THAN NORMAL TEMPERATURES
WILL SHIFT INTO THE FOUR CORNERS STATES AND THE SOUTHERN PLAINS
LATE WEEK, WITH GRADUAL MODERATING INTO EARLY NEXT WEEK WHILE THE
GREAT BASIN RISES TO A FEW DEGREES ABOVE AVERAGE. MEANWHILE, COOLER
THAN NORMAL TEMPERATURES WILL TAKE HOLD OVER THE NORTH-CENTRAL TO
EASTERN U.S. LATE THIS WEEK AND INTO THE WEEKEND UNDERNEATH UPPER
TROUGHING. HIGHS OF 10-20 DEGREES BELOW AVERAGE WILL BE COMMON IN
THE APPALACHIANS TO NORTHEAST ON THURSDAY IN PARTICULAR BEFORE
SLOWLY MODERATING.
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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