Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Apr 30 2025 Valid 12Z Sat May 03 2025 - 12Z Wed May 07 2025 ...Blocky Closed Lows for the West and the East... ...Overview... Recent 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 diverge over time in the East, albeit with recent guidance runs adding to recent trends toward closing of an upper low well inland, leaving the more progressive GFS/GEFS that alternately builds a ridge into the east-central U.S. as an outlier in the vast minority of solutions. The majority scenario increases the probability of a much wetter pattern for parts of the East this weekend/next week. ...Guidance/Predictability Assessment... Guidance remains clustered better than normal through med-range time scales with the development and slow progression of a significant upper trough/closed low and mid-larger scale features working into/across the West, eventually affecting the central U.S.. A model composite seems to provide a reasonable forecast basis with good continuity and ensemble support. Over the East, recent individual models and ensemble members, but especially the GFS/GEFS, have offered less than stellar agreement from the weekend into early next week with depiction and diggy nature of a potential closed upper trough/low. However, given well clustered guidance showing ample flow amplitude upstream, the pattern continues to strongly favor a solution on the more amplified/closed side of the full envelope of model and ensemble solutions. Accordingly, the WPC product suite was primarily derived from best clustered guidance in this vein with usage of the 00 UTC ECMWF/Canadian/UKMET and ECMWF/Canadian ensemble means along with most machine learning models. This solution maintains reasonably good WPC product continuity. Recent GFS and GEFS runs seemed way too progressive, but the latest 12 UTC GEFS and GFS based Graphcast machine learning model trended toward the aforementioned favored cluster of most other guidance, bolstering forecast guidance. ...Weather/Hazards Highlights... Latest Machine Learning (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 supports some areas of moderate to locally heavy rainfall depending on the orientation and persistence of any particular bands and favored terrain lift. There is still spread for rainfall specifics across areas with varying soil moisture, but given the growing signal and favorable pattern recognition, opted to introduce WPC Day 4/Day 5 Excessive Rainfall Outlook Marginal Risk areas for the southern and central Appalachians region through the northern Mid- Atlantic for at least this Saturday and Sunday. A drier scenario of the GFS is trending lower in probability. Easterly flow and adequate moisture/instability over the far southern Plains still offers some threat for spotty locally heavy rainfall over the southwestern half of Texas into the weekend, though with a diminished signal compared prior days, so no WPC ERO Marginal Risk areas remain in place. 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/Sunday 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 WPC ERO still has 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 to the region, from the Sierra Nevada and inland Northwest to 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/Schichtel 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