Extended Forecast Discussion NWS Weather Prediction Center College Park MD 317 AM EDT Wed Apr 30 2025 Valid 12Z Sat May 03 2025 - 12Z Wed May 07 2025 ...Overview... Latest guidance continues to show the general theme of an increasingly blocky pattern across the Lower 48, and with better relative agreement for the upper low closing off over California during the weekend and drifting gradually inland versus the pattern over the East. The western upper low will support a broad area of rain and high elevation snow across the region, as well as an increasing threat for areas of heavy rain across parts of the central/southern Plains next week. Meanwhile solutions rapidly diverge over the East, with new 00Z runs adding to recent trends toward closing of an upper low well inland--leaving the more progressive GFS/GEFS that build a ridge into the east-central U.S. in the vast minority. The majority scenario increases the probability of a wetter pattern over portions of the East. ...Guidance/Predictability Assessment... For the developing western upper low, the new 00Z UKMET strays a little southwest of consensus while GFS/GEFS runs have sometimes been leaning a bit fast and north later in the period. Recent machine learning (ML) runs have been favoring the southern half of the envelope, while diverging a little for timing later in the forecast. Multi-day trends have generally been slower but the 00Z CMC may be too slow as it holds the upstream Pacific upper trough (which should bring a cold front into the Northwest Tuesday- Wednesday) west of consensus. Recent ECMWF/ECens runs have been most consistent with the upper low's path and timing with relative support from ML models. Guidance through the 12Z/18Z guidance cycles maintained the tremendous spread for eastern U.S. evolution from Saturday onward. ML models starting with the 12Z/28 cycle have been showing potential for initial Mississippi Valley energy to close off an upper low as far west as the Tennessee/Ohio Valleys, with the ECMWF trending that way over the past day. The 12Z UKMET brought its closed low into the Carolinas while the 12Z CMC was progressive until using Canadian energy to close off a low over the Northeast (but the CMCens mean was a compromise with the ECMWF). On the most progressive extreme were the GFS/GEFS (albeit with a heavy rain producing trough/upper low passing over Florida). ML model consistency along with some trends in non- GFS/GEFS guidance favored some trending toward a more inland upper low, but not yet to the point of matching the ECMWF given the wide guidance spread. By midweek the ML models suggest the upper low would either drift into the northern Mid-Atlantic or lift into the Great Lakes and phase with a Canadian trough. These solutions do not support the new 00Z CMC that drops its upper low to the Southeast Coast. The new 00Z UKMET has joined the latest ECMWF runs. The full array of 00Z runs so far could allow for some added westward trending of the upper low and gradual enhancement of expected rainfall. Combining guidance preferences for the western upper low and the eastern U.S. evolution, the first half of the forecast started with a blend of 12Z and 00Z/29 ECMWF runs, along with input from the 12Z UKMET, ECens, and CMCens. ECens/CMCens weight increased a little after early Monday to make up for the UKMET. Local unavailability of the 12Z ECens valid 12Z Tuesday required greater weight of the CMCens at that exact time. ...Weather/Hazards Highlights... Latest ML models and non-GFS dynamical models are steadily showing more pronounced clustering toward closure of an upper low that may persist for a couple days or so to the west of the Appalachians. This would support some areas of moderate to perhaps locally heavy rainfall depending on the orientation and persistence of any particular bands. There is still a lot of spread for rainfall specifics across areas with varying soil moisture, so for now have opted not to depict any Marginal Risk areas in the Days 4-5 Excessive Rainfall Outlooks. It appears likely that one or more risk areas would become warranted as soon as a little better detail clustering emerges. The drier scenario, aside from some heavy rain potential over Florida/Southeast coast, represented by the GFS seems to be trending lower in probability. Easterly flow and adequate moisture/instability over the far southern Plains should maintain the threat for some locally heavy rainfall over the southwestern half of Texas on Saturday, though with a diminished signal compared to the prior day. The current Marginal Risk over that area in the Day 4 ERO appears reasonable. Meanwhile guidance has been consistent in showing a band of potentially heavy rainfall aligned over west- central Nevada on Saturday as the upper low closes off over California, so that Marginal Risk remains nearly unchanged as well. By Day 5/Monday the rainfall signal over the West looks a little more diffuse but with some guidance clustering toward enhanced rainfall over the sensitive terrain in the eastern half of New Mexico as gradual progress of the Southwest upper low pushes a cold front toward the region. Thus the Day 5 ERO introduces a Marginal Risk area for this threat. Additional gradual drift of the Southwest upper low should raise the potential for areas of heavy rainfall over parts of the central/southern Plains from Monday onward, with details depending on exact path and timing of the upper low. Otherwise, the system moving into the West will bring a broad area of rain and high elevation snow across the region, from the Sierra Nevada and inland Northwest into the northern-central Rockies. Well above normal temperatures should extend from the northern Rockies and eastern Great Basin into the northern Plains on Saturday, with anomalies gradually moderating as the warmth moves eastward toward the Upper Midwest. Plus 15-25F anomalies should be most common over far northern areas during Saturday-Sunday. In contrast, the upper low tracking into the Southwest will bring much cooler temperatures to the region. Highs should be 10-20F below normal on Sunday-Monday, with some record cold highs possible near the upper low's path especially on Sunday. Persistent clouds and precipitation should keep the southern High Plains highs below normal as well. The potentially unsettled pattern over the East may favor temperatures within a few degrees of normal after some modest warmth along the East Coast on Saturday. 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 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