Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Tue May 28 2024 Valid 12Z Fri May 31 2024 - 12Z Tue Jun 04 2024 ...General Overview... Guidance shows an East Coast upper trough as of Friday moving into the western Atlantic/Canadian Maritimes thereafter (with possible embedded upper low), while cool Canadian surface high pressure provides the East with pleasant weather for the end of this week. Expect a warming trend over the East by early next week with above normal temperatures also gaining coverage over other parts of the lower 48 as well. Meanwhile, a rather unsettled weather pattern should be in place across the Central U.S. with multiple shortwave disturbances aloft and surface waves/fronts fueling rounds of showers and thunderstorms late this week into early next week. Expect the Pacific Northwest to trend wetter over the course of the period as a strong jet underneath a Northeast Pacific mean low brings in moisture and a series of fronts. Most of the Southwest and the Northeast should remain mostly dry, and the upper ridge anchored over Mexico may gradually increase its northward extent and thus maintain hot and humid conditions over southern Texas. ...Guidance/Predictability Assessment... Early in the period there is decent agreement in principle for a Plains/Mississippi Valley shortwave and separate northern tier flow to the south of a central Canada upper low, but scale of the initial Plains shortwave reduces predictability for specifics and associated surface reflection. Meanwhile consensus shows a Pacific shortwave moving into the West during the weekend. By early next week solutions diverge regarding the relative strength of this shortwave and a potential upstream feature, as well as strength of upper ridging that may reach the northeast. Among recent guidance, the GFS/ECMWF and their means provided a relative majority depicting continued eastward progression of this western shortwave into east-central North America by next Tuesday. These solutions as well as 00Z ECMWF-initialized machine learning (ML) models suggest that latest CMC runs may be too strong with their Northeast upper ridging late in the period. As for the potential upstream Pacific shortwave arriving into the Northwest by early next week, dynamical models and MLs are split over the relative strength with some run- to-run variability, favoring an intermediate solution at this time. The updated forecast based on 00Z/06Z guidance started with an operational model composite for about the first half of the period, with greater weight on the 06Z GFS/00Z ECMWF relative to the 00Z UKMET/CMC. The remainder of the blend incorporated some of the 06Z GEFS/00Z ECens means to account for increasing guidance spread for details at the more distant time frames. ...Weather/Hazards Highlights... Numerous showers and thunderstorms are likely to remain ongoing across much of the central and southern Plains going into Friday and the weekend. Multiple weak disturbances aloft, with the best- defined one likely drifting from the Plains into Mississippi Valley Friday into early Saturday, will interact with a warm and humid air mass and one or more surface waves/fronts. A broad Marginal Risk area continues for the Day 4 Excessive Rainfall Outlook period Friday-Friday night from parts of Texas and Lower Mississippi Valley into the Midwest. There is an embedded Slight Risk area centered over the ArkLaTex region, where MCS activity is most likely to develop based on the latest deterministic guidance. In addition, wet ground conditions from recently observed rain or what falls in the short term may contribute to the flash flooding threat. There may be some abatement in the coverage of convection on Saturday across the Central U.S., but still enough to merit a few Marginal Risk areas where the best combination of upper level forcing and moisture convergence will likely exist. Of these, one over Texas and another over and south of the Lower Ohio Valley have relatively better confidence based on latest guidance. Some strong to severe thunderstorms are also likely each day. The potential exists for scattered QPF maxima of 2-4 inches for the Friday to Saturday time period for some of these areas that get repeated rounds of convection. After early Sunday, an advancing northern tier frontal system may increase convective emphasis over the Midwest for a time before extending farther southward. Meanwhile the Pacific Northwest should trend wetter by the weekend and early next week, perhaps with some totals on the heavy side for the time of year. Low- predictability shortwave details will determine the magnitude and duration of rainfall on a day-to-day basis. Cool high pressure will bring below-normal temperatures, especially for morning lows, to areas from the Lower Great Lakes/Ohio Valley into Mid-Atlantic Friday into Saturday. Flow around the high as it settles over the western Atlantic will support near to above average readings thereafter, with some locations in the Northeast possibly reaching at least 10F above normal early next week. Meanwhile above normal temperatures should spread across a majority of the West late this week into the weekend. The warmth will likely expand to include the High Plains into Upper Midwest from late weekend into the next work week. Best potential for one or more days of highs at least 10F above normal will be between California and the High Plains. It will remain very hot and humid for much of southern Texas through the period under the northern periphery of persistent upper ridging anchored over Mexico. The wetter pattern forecast over the Pacific Northwest may bring highs down to near or below normal values during the weekend and early next week. 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