Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Wed Apr 24 2024 Valid 12Z Sat Apr 27 2024 - 12Z Wed May 01 2024 ***Severe thunderstorms and heavy rain expected for the Plains and Midwest over the weekend*** ...General Overview... An amplified upper level flow pattern will to be in place across the Continental U.S. this weekend, with a building ridge across the East and a trough across the West/High Plains. Behind a leading Upper Midwest shortwave, another vigorous system will eject into the Plains and continue the threat for heavy rain and strong to severe thunderstorms, especially from central Texas to Iowa/Missouri. A pronounced warming trend is likely for the eastern half of the U.S. going into the beginning of next week, and colder temperatures out West will support late season snowfall for the higher terrain of the Central and Northern Rockies. The unseasonable cold should extend into the Northern Plains for a day or so as well. Expect a second upper trough to settle near the West Coast by the middle of next week as an upper low now over the Bering Sea approaches Vancouver Island or the Pacific Northwest. ...Guidance/Predictability Assessment... Most dynamical models/ensemble means and ECMWF-initialized machine learning models show better than average agreement with most aspects of the forecast through the period. With timing discrepancies for the Sunday Plains storm significantly reduced from 24 hours ago, primary differences that remain are later in the period and involve details of the upper trough nearing the West Coast, leading energy, and some details over the Northeast. Latest CMC runs produce most of these differences--being more closed with upstream energy feeding into the East Pacific/West Coast upper trough, more amplified with a leading shortwave tracking near the Canadian border by midweek, and the 00Z run in particular showing a suppressed surface pattern over the Northeast. The midweek West Coast upper trough has trended a little deeper in consensus, pulling back the western U.S. portion of the front trailing from the system traversing the northern tier by Wednesday. Meanwhile, individual ensemble members develop quite a bit of spread for the character of the modest upper trough nearing the East Coast by the middle of next week but averages of the dynamical/machine learning models and the ensemble means have been similar and steady with the depiction of this trough so far. The current array of guidance supported an operational model composite (with greater GFS/ECMWF emphasis) for the first half of the period and then a transition to a model/mean blend along with eventual elimination of CMC input. ...Weather/Hazards Highlights... The overall synoptic environment will be increasingly conducive for episodes of excessive rainfall across portions of the Central/Southern Plains and into portions of the Midwest states over the weekend. There will likely be multiple mesoscale convective systems that develop from central Texas to Missouri, and there is a good chance of training convection given the slow overall progression of the frontal boundary and nearly unidirectional flow through the lower to middle troposphere. For the Day 4 period Saturday, the Slight Risk area extending from north-central Texas to around the Iowa-Missouri border has been maintained from previous issuance. Potential continues for multiple cases of MCS development within this area and rainfall rates may reach two inches per hour with some storms. The core of the heaviest rainfall should slowly progress eastward for the Day 5 period Sunday across the ArkLaTex region and into southern Missouri, where a Slight Risk area persists given continued multi- model signal for some 2-4 inch totals where storms train over the same areas. There is a good chance that portions of both outlook areas may eventually need a Moderate Risk in later forecast updates if the QPF signal continues to remain robust, so this will continue to be closely monitored. This potential may be enhanced by wet conditions from rainfall forecast over portions of Plains and vicinity prior to the Days 4-5 period. Meanwhile there is some uncertainty over potentially greater northeastward extent of the Day 4 Slight Risk area, as some solutions could argue for, but with too much spread for specifics to make a confident adjustment at this time. Beyond early Monday, deceleration of the front reaching the Lower Mississippi Valley could continue the threat for locally heavy rainfall over that region for another day before the front weakens and eventually dissipates. The more progressive northern part of the front should bring lesser rainfall to the Great Lakes and Northeast during the first half of next week. Elsewhere across the U.S., anticipate widespread moderate to locally heavy rain from Minnesota to northern Michigan on Sunday as the surface low lifts northeastward, and a Marginal Risk area reflects this potential. Another area of enhanced rainfall is probable over northeastern Colorado and portions of adjacent states, where a Marginal Risk remains valid for Saturday. Colder temperatures moving in across the Intermountain West and the Rockies behind this storm system will likely result in several inches of snow for the higher mountain ranges. Flow ahead of the developing East Pacific/West Coast upper trough should support multiple days of mostly light to moderate rain and mountain snow over the Pacific Northwest from this weekend through the middle of next week. Temperatures will also be making weather headlines as most of the country east of the Rockies will likely trend well above normal by the weekend into early next week. Expect highs to rise well into the 80s from the Central/Southern Plains (90s to around 100 on Saturday over the typically hottest parts of far southern Texas) to the Ohio Valley and then reaching the East Coast by Monday, perhaps reaching 90 degrees in some cases. These readings are on the order of 10-20 degrees above normal for late April, and there may be some daily record highs set. Morning lows could reach 20-25 degrees above normal over parts of the Great Lakes with potential for record warm lows. On the other hand, locations from the Intermountain West into Northern Plains should see a day or so with highs 5-15 degrees below average during the weekend. Next week's pattern will favor some moderation over the East (but keeping temperatures somewhat above normal) while areas from the central U.S. through Intermountain West trend 5-15 degrees above normal by Tuesday-Wednesday. At that time, modestly below normal highs should be confined to the Pacific Northwest due to the nearby upper trough. Rausch/Hamrick 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