Extended Forecast Discussion NWS Weather Prediction Center College Park MD 441 PM EDT Fri May 26 2023 Valid 12Z Mon May 29 2023 - 12Z Fri Jun 02 2023 ...Coastal low with heavy rain and coastal/maritime threats for the Southeast/Mid-Atlantic during the weekend to track offshore next week... ...Broad multi-day thunderstorm and local heavy rain threat continues for the Plains... ...Overview... Over the course of next week expect an initial Rex block pattern over the East to transition toward more of an Omega block as Great Lakes and southern Rockies upper ridges ultimately merge/amplify over central Canada and the Mississippi Valley. A California upper low should anchor West Coast mean troughing to start and then a northeastern Pacific/Pacific Northwest trough should become better defined by mid-late week. The lingering system near the Carolinas should weaken as it drifts offshore. Precipitation over the Mid-Atlantic will wane through the coming week as result. The forecast pattern will support periods of rainfall 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 over the southern Plains. 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 latest runs of guidance continue to show fairly good agreement with the large-scale pattern as it transitions from a blocky regime over the eastern U.S. to another blocky regime. Once again, the GFS is favoring a a faster/eastward position with the initial Low ejecting from California into the southern stream. Trend has persisted now with the past four runs, however by the middle of the extended period it flips to then become the slower/west side of the cluster by the end of the period. The GEFS mean remains closer to the cluster. There continues to be variations in the the details with the eastern Canada/New England upper troughing that should begin to take shape in response to central Canada ridging. Differences for the shortwave tracking across the southern tier fall within typical ranges for the time frame involved. The WPC suite of products used a composite of the NBM, continuity, a general blend of the 00Z ECWMF/UKMET/CMC/GFS/GEFS/EC ensemble means and the 06Z/12Z GFS. ...Weather/Hazards Highlights... Holiday weekend hazards of heavy rain and gusty winds associated with the system initially forecast to be near the southern Mid-Atlantic coast is expected to the decrease through the next week as the coastal system weakens and tracks further from the Eastern Seaboard. The potential for periods of heavy rain and isolated flooding will remain elevated along the Mid-Atlantic coast on Day 4 (12Z Monday-12Z Tuesday) as this is where training of thunderstorms along an axis of instability. Therefore, a Marginal Risk for excessive rainfall will remain in effect. The threat will lessen by Day 5 (12Z Tuesday-12Z Wednesday) as the system shifts further offshore, thus no highlighted risk areas for this period at this time. Even as the system itself weakens, northeasterly winds along the coast could still rebound for a time as high pressure drops into the Northeast/Mid-Atlantic by Tuesday-Wednesday. Showers and thunderstorms are expected to break out across the Southern Plains on a daily cadence due to a persistent pattern bringing in low level moisture from the Gulf of Mexico and leading to upslope flow over the High Plains. A southern tier shortwave may also enhance activity during the first half of the week, and height falls emerging from the West may increase coverage/intensity again toward the end of the week. Some of the higher QPF may focus over Texas thus the Days 4-5 Excessive Rainfall Outlooks reflect the early-week potential with Marginal Risk. Further north over the Northern Plains, one or more fronts may focus rain and thunderstorm activity with some signal in the guidance for locally heavy rainfall over parts of the eastern Dakotas into Minnesota. Much of this part of the country has been recently dry, so soil saturation is low as well as the threat for flooding. However, some combination of improved guidance agreement/higher totals could eventually support an area. Areas over and just north of the Sierra Nevada through the Great Basin into the Northern Rockies will see multiple days of showers and thunderstorms ahead of the California upper low and trough evolving near the Pacific Northwest coast. Current guidance signals are do not appear sufficiently organized to depict a large scale risk area in the Excessive Rainfall Outlook but an isolated instance or two of flash flooding in terrain sensitive areas or burn scars will remain possible. 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. Moderately above normal readings over the Northwest early in the week should trend closer to normal as the upper trough reaches the Pacific Northwest coast mid-late week. Meanwhile much of the southern tier will tend to see near to moderately below normal temperatures. Campbell/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 Hazards: - Heavy rain across portions of the Southern Plains, Mon-Tue, May 29-May 30. - Heavy rain across portions of the Southern Plains, Fri, Jun 2. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Central Rockies, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, and the Northern Great Basin. 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