Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Apr 28 2025 Valid 12Z Thu May 01 2025 - 12Z Mon May 05 2025 ...Overview... It overall remains the case that behind a developing storm system tracking from the Midwest through the Great Lakes and southeastern Canada late this week (with associated fronts and rainfall of varying intensity), guidance continues to show a Lower 48 pattern aloft that becomes more blocky heading into the weekend and early next week. An upper trough reaching the West Coast by Saturday will displace a leading weak upper low crossing the Southwest and should ultimately close off a fairly deep low by Sunday-Monday, while a ridge likely sets up over the Plains and then Mississippi Valley. This pattern will favor a broad area of rain and higher elevation snow moving across the West while parts of the southern half of the Plains should see multiple days with episodes of rain/thunderstorms. In contrast to the relative consensus over the western-central U.S. late in the period, models and ensembles are all over the place regarding how much troughing with an embedded upper low over the East. ...Guidance/Predictability Assessment... A main upper trough that reaches into the East shows decent clustering into early Saturday, but solutions with recent guidance runs then rapidly diverge with differences in the character of upstream energy streaming into the mean trough. The 00 UTC ECMWF/GFS show rapid progression of the Saturday trough in favor of much more shallow cyclonic flow over New England thereafter. On the other extreme, the 00/12 UTC CMC/UKMET drop an upper low into the Northeast/Mid-Atlantic. The latest 12 UTC GFS/ECMWF have now trended more into this camp. The full ensemble spread is quite varied as well, but with the resulting means holding onto East Coast troughing into next Monday. Recent ML models differ as well, but with a relative majority supporting a trough with possible upper low. The 00 UTC AIFS shows an upper low just off the New Jersey coast for early Monday, but the 12 UTC AIFS has a closed low way back near the Appalachians. Putting this together the trend theme of ML guidance, plus teleconnection support for some degree of troughing aligned near the East Coast based on a positive height anomaly center likely to be just west of Lake Superior by the end of the period, preference now favors the East Coast trough of at least/more than ensemble means amplitude. The morning WPC medium range product suite maintained WPC continuity given uncertainties, but given amplified 12 UTC guidance trends this now prove quite conservative in that it keeps the upper low only up near Maine. Overall most of the guidance is much better behaved for the upper trough reaching the West Coast by around early Saturday and then closing off an upper low that drifts over the southern/central parts of the West by Sunday-Monday. Some ML models suggest that the upper low could take a slightly farther south track than the average of dynamical models/means. This trends well into WPC continuity that was strongly maintained. ...Weather/Hazards Highlights... Surface low/frontal progression across the eastern half of the country Thursday-Friday will bring a broad area of rainfall with varying intensity, but generally with lighter totals than expected on prior days over the Plains. Adequate instability and moisture may still produce some pockets of rain rates great enough to cause runoff issues, especially where there are wetter ground conditions leading into the event. The Day 4 Excessive Rainfall Outlook maintains a broad Marginal Risk area from parts of the Lower Mississippi Valley and Tennessee Valley into the Great Lakes. What threat of excessive rainfall exists farther east by Day 5 will be more narrow, with best relative potential over the central and northern Appalachians where ground conditions are on the wetter side and a subset of guidance shows locally enhanced rainfall. A Marginal Risk covers this area of interest. Some convection may also extend near the Gulf Coast but with poor agreement for coverage and amounts. Most of the East should trend drier by the weekend but an uncertain lingering upper trough could produce some clouds/showers if it is sufficiently deep. 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. Guidance signals are sufficiently good to support a Marginal Risk area over much of the southwestern two-thirds of Texas into far southeast New Mexico on Day 5/Friday. Some locally heavy totals are possible thereafter, with more organized heavy rainfall becoming possible early next week depending on the eastward drift of the western upper low. 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 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 northern Rockies. Leading weak shortwave/upper low energy may produce lighter precipitation from the Sierra Nevada east into the Rockies Thursday-Friday. Most of the East except for far northern areas should see above normal temperatures late this week with some plus 10-15F anomalies. Cold frontal passage and arrival of upper troughing should bring near to below normal temperatures eastward from the central U.S. into the East by the weekend. Meanwhile above normal temperatures will migrate from the Northwest into the northern/central Plains during the period, corresponding to movement of an upper ridge. Plus 10-20F or so anomalies will be possible over northern locations with warmest readings likely to be during Friday-Sunday. Then the West will trend significantly colder next weekend into Monday with the arrival of the deep upper trough/low, bringing highs down to 5-20F below normal. The most extreme anomalies are for the southern Great Basin and Southwest. 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