Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Tue Apr 29 2025 Valid 12Z Fri May 02 2025 - 12Z Tue May 06 2025 ...Overview... Behind a system tracking across southeastern Canada on Friday with some rain and thunderstorms across the East/South, guidance continues to show a more blocky pattern settling over the Lower 48. With only moderate detail differences/adjustments, most solutions have been fairly well behaved as an upper trough approaches the West Coast by Saturday and closes off an upper low that should drift into the southwestern U.S. early next week, spreading rain and high elevation snow across the West. This system may start to generate heavier southern High Plains rainfall toward the end of the period. A leading ridge should settle over the Plains into the Mississippi Valley. In contrast to the western consensus, individual models and ensembles continue to show remarkably different ideas for the ultimate evolution of energy within an upper trough reaching the East during the weekend into early next week. This spread keeps confidence much lower than average for details of eastern U.S. sensible weather. ...Guidance/Predictability Assessment... Guidance is 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 portion of the nation. A model composite seems to provide a reasonable forecast basis with good continuity and ensemble support. However over the East, individual models and ensemble members and recent machine learning models continue to diverge considerably from the weekend into early next week with depiction and diggy nature of a potential closed upper trough/low. Given well clustered guidance showing ample flow amplitude upstream, favor a solution over the East somewhere on the more amplified/closed side of the full envelope of model and ensemble solutions. Accordingly, the WPC product suite incorporated best clustered guidance in this vein from the 00 UTC ECMWF/Canadian ensemble means that offer better cycle to cycle continuity than individual models. Recent GFS and GEFS runs seem too progressive given upstream amplitude. Latest 12 UTC model guidance remains mixed and uncertain. ...Weather/Hazards Highlights... Expect some showers and thunderstorms ahead of a cold front pushing through the East on Friday. Latest guidance signals have become a little more diffuse regarding how much flash flood potential there may be over the central and northern Appalachians where ground conditions are on the wetter side, but enough instability may exist to promote rain rates that could cause localized runoff issues. Models are starting to develop some clustering for the idea of frontal waviness producing a band of enhanced rainfall over parts of the Northeast on Saturday. This is an emerging trend and there is a question of how much instability may exist in this rain band, so initially opted for no risk area in the Day 5 ERO for this activity. Will continue to monitor this aspect of the forecast in future runs. Precipitation coverage and amounts over the East become increasingly uncertain after Saturday, depending on evolution of upper troughing and possible closed low. Latest guidance preferences suggest more likely closed low development and settling over the East into early next week that would support protracted unsettled conditions including wrapback western Atlantic low moisture to fuel light to moderate rain potential, possibly with focus into the northern Mid-Atlantic. Multiple days of low level flow coming in from the Gulf, along with some weak shortwave energy approaching from the Southwest and eventually height falls ahead of the deeper western upper low, will support multiple episodes of rainfall over portions of the southern and eventually also central Plains. With a fair amount of variance in the details, guidance continues to maintain support in principle for the existing Marginal Risk area from southeastern New Mexico into Texas on Day 4/Friday. Latest solutions are increasing potential for impactful rain to extend farther east along a surface front, now favoring extension of the Marginal Risk area into parts of Louisiana. Moisture/instability and pattern persistence should lead to additional pockets of enhanced rainfall over the southwestern half of Texas into Saturday, meriting a Marginal Risk over that region on Day 5. The Southwest upper low still looks to drift far enough eastward to start generating heavier rainfall over the southern half of the High Plains during the first half of next week. The Pacific system moving into the West will bring rain and high elevation snow into the Northwest around Friday, with precipitation then continuing across the Sierra Nevada/Great Basin/northern parts of the Southwest and into the northern-central Rockies. Highest totals will be dependent on the exact tract of the forecast evolving upper low, and are currently most likely to be over the Great Basin and north-central Rockies. Latest guidance is keying on potential for a band of locally heavy rainfall over Nevada on Day 5/Saturday as the upper low closes off over California, with anomalous moisture and some instability. The Day 5 ERO depicts a Marginal Risk area to reflect this possible threat. Leading weak shortwave/upper low energy may produce lighter precipitation from the Sierra Nevada east into the Rockies on Friday. Well above normal temperatures should migrate from the northern half or so of the West on Friday into the northern Plains by the weekend and early next week. Plus 15-25F anomalies should be most common over far northern areas during Friday-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. Persistent clouds and precipitation should keep the southern High Plains highs below normal as well. Locations near the East Coast will see a warm day on Friday ahead of a cold front, followed by a trend to near or slightly below normal readings. However confidence in specifics from Sunday onward is lower than average due to very uncertain upper level flow details. 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