Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM 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... Over the East, while individual ensemble members diverge considerably after Saturday, the resulting means still provide the most stable solution with an upper trough axis lingering near the East Coast. This alignment also reflects teleconnection tendencies relative to the core of positive height anomalies expected to be west/northwest of Lake Superior late in the period. Guidance spread as of the 12Z/18Z runs favored holding onto a solution closest to the ensemble means while awaiting a better opportunity for more confident adjustments. As for other guidance, CMC/UKMET runs and the 12Z ECMWF/ICON closed off an upper low within the expected mean trough near the East Coast but at very different latitudes. Most 12Z/18Z machine learning (ML) models added even more intrigue, closing off initial Upper Midwest/central Plains shortwave energy to the west of the Appalachians (anywhere between the Great Lakes and Tennessee Valley). The new 00Z ICON has shifted to the ML model cluster while the 00Z ECMWF has shifted somewhat in that direction as well. Latest guidance leaves the 18Z and 00Z GFS runs, which completely clear the trough from the East Coast, as very pronounced extremes. At least the 00Z GEFS mean still holds onto some East Coast troughing. Over recent days the trough nearing the West Coast by Saturday has been exhibiting a slightly slower trend. As the upper low closes off over/near California, the new 00Z GFS favorably adjusted northeastward from the 12Z/18Z runs to the consensus track. Then by Monday-Tuesday the 00Z GFS still looks more favorable in contrast to farther north earlier runs. The 12Z/18Z ML models showed typical spread but generally leaned to the southern half of the dynamical model envelope. Guidance considerations led to starting the updated forecast with a 12Z/18Z operational model composite from Friday into early Sunday, followed by a rapid shift toward 60-70 percent ensemble input (18Z GEFS and 12Z ECens/CMCens) for Monday-Tuesday. ...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. Thus for now maintained the existing Marginal Risk area in the Day 4 Excessive Rainfall Outlook. 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. 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 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