Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat May 27 2023 Valid 12Z Tue May 30 2023 - 12Z Sat Jun 03 2023 ...East Coast system to weaken and track offshore... ...Broad multi-day thunderstorm and local heavy rain threat continues for the Plains... ...Overview... Guidance continues to show a transition from one blocking pattern to another next week as a Rex block pattern initially over the East gives way to an Omega block, with Great Lakes and southern Rockies upper ridges ultimately merging/amplifying over central Canada and the Mississippi Valley/eastern Plains. Western U.S./eastern Pacific troughing will consist of an initial California upper low, some energy immediately upstream, a shortwave reaching/ejecting from the Pacific Northwest, and then a trough reloading more offshore. Meanwhile the building central Canada upper ridge will likely force troughing into the Northeast toward the end of next week. The forecast pattern will support rainfall of varying intensity from the Sierra Nevada through the Great Basin and northern Rockies, while portions of the Plains will see potential for locally heavy rainfall. One or more fronts should provide added focus over the northern Plains and a shortwave traversing the southern tier could enhance activity near the Gulf Coast and eventually into the Florida Peninsula. Rainfall over the Mid-Atlantic should trend lighter with time as the East Coast system weakens and departs. Expect temperatures to be above normal over a majority of the northern U.S. while near to moderately below normal readings should prevail across the southern tier. ...Guidance Evaluation/Predictability Assessment... The models and ensemble means continue to depict a similar large scale pattern evolution through the period but with some typical embedded detail issues and trends/refinement. Recent GFS runs have been on the faster side of the spread for the upper low initially forecast to be over/near California and eventually open up over the West as it continues inland, with leading height falls also advancing farther east into the Plains than most other solutions. Behind this feature, guidance generally suggests that upstream energy will help to maintain a diffuse trough with a neutral or negative tilt into days 6-7 Friday-Saturday. Over the northeastern Pacific/Pacific Northwest, guidance is refining the forecast toward a defined shortwave arriving into the Pacific Northwest by around day 5 Thursday followed by ejection of this shortwave and farther west trough emphasis in response to digging of energy from the eastern Bering Sea/southwestern Alaska. Farther east, guidance has come into much better agreement for the initial East Coast system versus 24 hours ago with timing on the slower side of the prior spread (which was represented by the GEFS mean). Within developing eastern Canada/northeastern U.S. mean troughing, the 18Z/00Z GFS runs differ from most other guidance for how an initial upper low near Newfoundland may interact with upstream energy but at least the end result is a trough either way. The new 00Z CMC is the extreme solution at the moment as it does not develop this trough. Finally, guidance exhibits typical spread for the upper feature tracking along the Gulf Coast given the forecast time frame and medium scale that can sometimes temper predictability. Guidance preference generally reflected an operational model emphasis early and a model/mean blend later in the period, though with some GEFS input even early in the forecast to mitigate less confident aspects of the GFS in the West. ...Weather/Hazards Highlights... Expect rainfall over the Mid-Atlantic to decrease in coverage and intensity from Tuesday onward as the upper low and surface system initially affecting the region weaken as they track eastward. Farther west, the pattern evolution will favor multiple days of showers and thunderstorms from the Sierra Nevada into the northern Rockies/Montana as well as portions of the Plains and Upper Midwest. By Day 5 (12Z Wednesday-12Z Thursday) the Sierra Nevada through Montana axis may see sufficient localized enhancement after somewhat more modest activity from prior days to merit a Marginal Risk area in the Excessive Rainfall Outlook for that time frame. Also during that period there is a signal for increasing coverage/intensity of rainfall over the southern High Plains with the approach/arrival of height falls from the west, with a Marginal Risk indicated over that region as well. A wavy front may produce some locally heavy rainfall over the eastern Dakotas/Minnesota Tuesday-Wednesday but this area has had below average rainfall over the past couple weeks and guidance has continued to disagree on specifics, so outlooks do not show any risk area thus far. Improved guidance clustering and/or a heavier consensus could eventually favor one though. Rainfall may trend heavier and more expansive over the High Plains and vicinity toward the end of next week. Meanwhile the Day 4 outlook holds onto a smaller Marginal Risk area along the western Gulf Coast with the possibility for lingering activity with the southern tier upper shortwave, with guidance trends reducing the northwestward extent versus continuity. Through the rest of the period as the shortwave drifts eastward, guidance diverges considerably regarding how much rain may fall along the Gulf Coast. There is a little better signal for increasing moisture to lift northward through Florida late in the week, leading to potential for heavier rainfall at that time. Much of the northern part of the country and the Middle Mississippi Valley will see above normal temperatures through next week, with areas from the northern Plains through Great Lakes and Northeast most likely to see multiple days of plus 10-20F anomalies. The Northwest should see moderately above normal readings early in the week, followed by a trend toward normal as an upper shortwave reaches the region mid-late week and then a rebound as it departs. The Northeast should become noticeably cooler by next Saturday behind a passing cold front supported by an amplifying upper trough. Meanwhile much of the southern tier, as well as Mid-Atlantic for highs early in the period, will tend to see near to moderately below normal temperatures. Rausch 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