Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Apr 28 2025 Valid 12Z Thu May 01 2025 - 12Z Mon May 05 2025 ...Overview... 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 early 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 (and perhaps even an embedded upper low) may linger near the East Coast. ...Guidance/Predictability Assessment... There is still some moderate spread for details of the Midwest through southern Canada system, with dependence on specifics of the shortwave initially ejecting through the Plains/Mississippi Valley and influence from a trailing northern stream trough. CMC runs have been a bit on the slow side relative to other dynamical models as well as machine learning (ML) models. Meanwhile, by early Friday the ML models lean weaker than most dynamical guidance and their average depth is even weaker than an average of the various 12Z/18Z ensemble means. An average of the dynamical models continues to look best at this time. The supporting upper trough that reaches into the East shows decent clustering into early Saturday but then solutions rapidly diverge with differences in the character of upstream energy streaming into the mean trough. Latest ECMWF runs and the 00Z GFS/ICON show rapid progression of the Saturday trough in favor of much more shallow cyclonic flow over New England thereafter. On the other extreme, CMC runs and the 00Z UKMET drop an upper low into the Northeast/Mid-Atlantic. The full ensemble spread is quite varied as well, but with the resulting means holding onto some East Coast troughing into Monday. Recent ML models differ as well but with a relative majority supporting a trough with possible upper low over the Northeast. The new 00Z AIFS has come in with an upper low just off the New Jersey coast for early Monday. Putting together the general 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 leans to the East Coast trough of the ensemble means with modest input of the 18Z/12Z GFS that also had troughing. Within the trough cluster, this is somewhat conservative in that it keeps any potential upper low north of Maine for now. 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. By Monday the 12Z ECMWF strayed a tad faster than most other dynamical/ML model solutions, while some ML models suggest that the upper low could take a slightly farther south track than the average of dynamical models/means. Based on 18Z/12Z guidance comparisons, the Thursday-Friday forecast emphasized an average of the 18Z GFS/12Z ECMWF with modest input from the 12Z UKMET. The UKMET compared less favorably to consensus by Saturday so the 18Z GEFS/12Z ECens means replaced it at that time. Then the blend rapidly shifted toward 60 percent weight of the GEFS/ECens/CMCens means with the rest consisting of the 12Z/18Z GFS. ...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 should be over the southern Great Basin and Southwest. 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