Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Apr 26 2025 Valid 12Z Tue Apr 29 2025 - 12Z Sat May 03 2025 ...Heavy rainfall threat over the southern half of the Plains Tuesday-Wednesday... ...Overview... A cold front pushing across the northeastern quadrant of the lower 48 will hang back over the Plains while awaiting ejection of a Southwest U.S. shortwave. This feature and dynamics coming in behind it should ultimately generate a surface low that brings a front through more of the East by the end of the week as upper troughing amplifies. The Tuesday-Wednesday pattern will favor episodes of heavy rainfall over and near the southern half of the Plains before system progression spreads less pronounced rainfall totals across the East. A trailing upper ridge should move into the Northwest after early Wednesday, reaching the Plains by the start of the weekend, while a Pacific upper trough should reach the West Coast around Saturday with some precipitation over the Northwest. ...Guidance/Predictability Assessment... Guidance is similar in principle for the forecast over the central and eastern U.S. but with some typical differences and minority stray solutions. There are still some timing differences as low pressure crossing southeastern Canada Tuesday-Wednesday pushes a trailing cold front through the Great Lakes/Northeast, with an average providing a reasonable starting point. Then as Southwest U.S. shortwave energy ejects northeastward, solutions still vary for details of developing low pressure that tracks northeastward. At least the surface low's position over the central-eastern Great Lakes by Thursday night-Friday in the majority cluster has held up rather well over the past day. In addition the latest machine learning (ML) models seem to be gaining some definition for this waviness compared to 24 hours ago. Through Thursday or so, the 12Z CMC started to stray on the fast side while the 12Z UKMET lagged other guidance for the ejecting Southwest upper feature. The 00Z solutions have trimmed the extreme sides of the envelope. However toward the end of the week there is a fair amount of spread/variability for how the overall eastern North America upper trough will evolve in both the dynamical and ML guidance, lowering confidence in specifics. This favors a blended/ensemble mean approach late in the period. Over the West, some weak shortwave/upper low energy may briefly reach the Southwest before an amplified upper trough with leading cold front reaches the West Coast by early Saturday. Recent GFS trends have led to improved clustering into Friday, though with consensus as a whole adjusting a bit slower compared to yesterday. By early Saturday the latest dynamical/ML majority suggests the upper trough will remain fairly phased, versus some earlier model runs that had various ideas for how flow could separate. However multiple solutions show separation by later Saturday or Sunday. Guidance comparisons based on 12Z/18Z guidance led to emphasizing the 18Z GFS/12Z ECMWF along with a little 12Z CMC Tuesday- Wednesday, followed by a transition toward an even weight of the GFS/ECMWF and their corresponding ensemble means by next Saturday. ...Weather/Hazards Highlights... During Tuesday-Wednesday, low pressure lifting away from Lake Superior will push a cold front across the northeastern quadrant of the lower 48 while the trailing part over the Plains stalls and develops a wave as a Southwest U.S. shortwave ejects northeastward. This pattern combined with anomalous moisture and sufficient instability will favor a heavy rainfall threat over and near the southern half of the Plains in this time frame. The Day 4/Tuesday Excessive Rainfall Outlook maintains a Marginal Risk area from parts of Texas into the lower Ohio Valley. There still seems to be enough guidance spread within this area to favor waiting for improved clustering before introducing an embedded Slight Risk. Models/ensembles and first-guess fields generally strengthen the heavy rainfall signal on Day 5/Wednesday. Thus the Day 5 ERO introduces a Slight Risk area encompassing parts of Texas/Oklahoma and nudging into far western Arkansas. The surrounding Marginal Risk covers a similar area as the Day 4 ERO. The Storm Prediction Center is also monitoring for severe threats on Tuesday from the eastern Great Lakes and vicinity southwest into northern Texas. Check the SPC outlooks for the latest info on severe threats. After early Wednesday, continued progress of low pressure and its trailing cold front should push rainfall across much of the East but with lesser totals than forecast over the central U.S. Farther west, shortwave energy and a frontal system should spread rain and some high elevation snow from the Northwest through the northern and central Rockies Tuesday-Thursday. By the end of the week some rain may develop over the southern High Plains with the help of easterly low level flow. An upper trough/cold front reaching the West Coast by Friday-Saturday should bring rain and high elevation snow into the Northwest. Expect unseasonably warm temperatures over the eastern U.S. on Tuesday ahead of an advancing cold front. Plus 10-20F anomalies will equate to highs reaching into the 80s over the Ohio Valley. Morning lows will be even a little more anomalous. Northern parts of the East will trend cooler by midweek but locations farther south should remain somewhat above normal through Friday. A more amplified upper trough will likely push the next cold front farther south, bringing temperatures over the East to within a few degrees on either side of normal by Saturday. Meanwhile an upper ridge drifting from the Northwest into the Plains after Wednesday will support corresponding eastward progress of an area of highs 10-20F above normal. 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