Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun May 07 2023 Valid 12Z Wed May 10 2023 - 12Z Sun May 14 2023 ***Periods of heavy rain expected for portions of the southern Plains and also the northern High Plains and vicinity*** ...Overview... Latest guidance continues to highlight the central U.S. for heaviest rainfall from midweek through the weekend, as a sharp upper trough over the Intermountain West on Wednesday closes off a low that should track through the central Rockies into northern Plains by Friday before likely opening up, while farther south shortwave impulses may interact with a persistent inflow of western Gulf moisture and a stalled surface front to produce heavy rainfall over the southern Plains. Some of the central U.S. moisture may eventually extend into central latitudes of the East by next weekend. The Rockies/Plains system should build a downstream upper ridge from the Gulf Coast to the Midwest, with the northern part eroding as the northern Plains system opens up. Meanwhile eastern Canada into northwest Atlantic mean troughing may start to adjust farther west by next Sunday and beyond in response to the strong ridge building into the Pacific Northwest and western Canada. ...Guidance/Predictability Assessment... Models and ensembles continue to show detail and latitude/track differences for the vigorous upper system expected to track from the Intermountain West into the Plains. The 00Z UKMET had the most diffuse evolution and was not included in the updated forecast blend due to most other solutions still having a better defined/closed system. With other models closer in principle but shuffling around for track, a multi-model/multi-run blend seemed to provide the best approach to represent the system's overall evolution and temper adjustments in successive runs that may or may not turn out to be reliable trends. Meanwhile, the latest cycles of most guidance have trended noticeably stronger with the upper ridge building over the Pacific Northwest and western Canada by late week and next weekend. Currently the ECMWF/CMC are strongest with the ridge while the 00Z/06Z GEFS means were suspiciously weak with the ridge and most aggressive with bringing leading height falls with the upstream Pacific trough toward the Northwest. The 12Z GEFS mean has trended in the right direction though it could still be a little fast with the leading side of the Pacific trough. There is a general theme of a lingering weakness over the southern half or two-thirds of the West by next weekend but minimal confidence in specific upper low(s) that could exist within this area. Finally, a general model/ensemble blend best resolves eastern Canada/northeastern U.S. flow whose shortwave specifics continue to decline in predictability and confidence by mid-late period. Based on the above considerations, the first half of the period started with a composite of the 00Z/06Z GFS, 12Z/06 and 00Z ECMWF, and 00Z CMC. Then the forecast incorporated some 00Z ECens/CMCens means in addition to those model runs, again excluding the GEFS due to its questionable eastern Pacific/western North America solution. ...Weather/Hazards Highlights... Scattered to numerous showers and thunderstorms will likely develop across much of the Plains states mid-late week, as moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the western High Plains and the upper trough/low approaches from the West. The Day 4 (12Z Wednesday-12Z Thursday) Excessive Rainfall Outlook will maintain a modestly adjusted Slight Risk area over parts of the ArkLaTex region where there are continued model signals for locally heavy rainfall totals as multiple MCSs develop in the warm sector of the emerging western low, with the aid of one or more southern stream shortwave impulses. Some of this activity may extend into the Day 5 (12Z Thursday-12Z Friday) period but with a more diffuse signal for specifics anywhere from the Middle Mississippi Valley to the western half of the Gulf Coast--thus the depiction of only a Marginal Risk area. The Middle Mississippi Valley may see a combination of the shortwave energy and moisture from Day 4 lift northeastward and interact with a surface front nearing the region, while persistent moisture and possibly additional upper impulses could produce locally heavy rain farther south. Meanwhile the northern High Plains and vicinity may see a multi-day heavy rain threat with the upper low/surface system tracking over or near the region. The Day 4 ERO maintains the Marginal Risk area over the High Plains while for Day 5 the plan is for a southward expansion of the Slight Risk area over parts of Montana into northeastern Wyoming and far northwest South Dakota. This is based on the best overlap of ensemble guidance for highest expected rainfall along with wet ground conditions likely to exist due to moderate/heavy rainfall the day before. It will take additional time to resolve specifics that will depend on the exact track of the upper low and if/how long it potentially stalls. By late week into next weekend, portions of the southern Plains may get additional rounds of heavier rainfall, with several inches of rain possible in some areas of Texas. The combination of persistent low level flow from the western Gulf, a stalled surface front, and one or more southern stream impulses aloft should contribute to this potentially heavy rainfall. As the upper low crosses the Great Basin and northern Rockies, there should be sufficient cold air aloft to support late season higher elevation snow, especially over the higher mountain ranges of northern Wyoming and western Montana. Some of the central U.S. moisture may extend into central latitudes of the East by the latter half of the period as the Plains system and leading front gradually push eastward. In terms of temperatures, highs will likely be running 5 to 15 degrees above early May averages from the central/northern Plains to the Deep South for the middle of the week, and then reaching into the Midwest states and the Great Lakes/Northeast region going into the end of the week. Northern parts of the East may trend cooler by next Sunday. Warmth will build across the Pacific Northwest by Friday into the weekend with highs potentially up to 20-25 degrees above mid-May averages across western Washington and Oregon. Lows may be a little less extreme but could still challenge daily records for warm lows by the weekend (especially Sunday). Highs will likely be slightly below average across portions of the West Coast states and the Intermountain West in association with the upper trough through Thursday before a moderating trend commences. Overnight lows over those areas early in the period should be closer to average while areas east of the Rockies should see mostly above average lows with increased cloud cover and humidity. Rausch/Hamrick 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