Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Wed May 29 2024 Valid 12Z Sat Jun 01 2024 - 12Z Wed Jun 05 2024 ...General Overview... A leading Mississippi Valley shortwave and trailing weaker impulses, plus an active flow pattern from the Pacific across the northern tier U.S. and southern Canada (with multiple frontal systems) should favor multiple episodes of showers and thunderstorms over the Plains/east-central U.S. from the weekend into next Wednesday, along with a wet pattern over the Pacific Northwest--especially with a fairly strong frontal system at the start of the week. Most of the Southwest and the Northeast should remain mostly dry, while southern Texas should see persistent hot and humid conditions under the northern periphery of an upper ridge anchored over Mexico and a developing ridge over the southern half of the West by next Wednesday may begin to raise the potential for hazardous heat over that part of the country. ...Guidance/Predictability Assessment... The updated forecast started with a blend of operational 00Z/06Z models followed by some GEFS/ECens input by the latter half of the period. Early in the period, the primary differences of note were with the initial Mississippi Valley shortwave. 00Z ECMWF- initialized machine learning (ML) models looked more like the 00Z UKMET/CMC that were somewhat more concentrated in appearance, versus the more northward-sheared GFS/ECMWF, and the new 12Z ECMWF looks more like the ML cluster. Then, over the past 24-36 hours guidance has come into somewhat better agreement on stronger emphasis of a trough reaching the Pacific Northwest around Monday and eventual weakening of leading shortwave energy. There is still typical spread for how the vigorous Monday system evolves farther east into midweek, as well as for details of upper ridging expected to start building over parts of the West. By early next Wednesday the ML models generally favor more ridging over the southern half of the West like the ECMWF/ECens, but with less northward amplitude than the operational run. The latest GFS is somewhat east with its ridge at that time. ...Weather/Hazards Highlights... Numerous showers and thunderstorms are likely to remain ongoing across portions of the Southern Plains and eastward to the central Gulf Coast region going into the weekend, with multiple weak disturbances aloft interacting with a warm and humid air mass. Meanwhile most guidance shows a leading better defined shortwave reaching near the Mississippi Valley as of early Saturday, with potential for an associated surface wave and weakening frontal system. Latest guidance does still does not appear to cluster well enough for an embedded Slight Risk yet, but various solutions are now evenly enough distributed to support one broad Marginal Risk area in the Day 4 Excessive Rainfall Outlook to cover these potential areas of convection. Locations currently with wet soil conditions and/or experiencing heavy rain in the Days 1-3 period will have added sensitivity to additional rainfall by Day 4. Going into the Day 5 period Sunday, a more organized storm system approaches the Dakotas and Upper Midwest, and guidance continues to suggest that convection will be widespread/heavy enough to merit a Marginal Risk here as a starting point. Fairly wet soil/above average streamflows over the Upper Midwest add to the potential for a Slight Risk upgrade as details become better resolved closer to the event. Some strong to severe thunderstorms are also likely with favorable parameters in place. The potential exists for scattered QPF maxima of 1-3 inches during this time period for some of these areas that get repeated rounds of convection. Meanwhile the Day 5 ERO depicts a Marginal Risk area parts of the South with the combination of lingering moisture, instability, and shortwave impulses. There is also an increasing likelihood of a late season heavy rain event across northwestern Oregon and into western Washington on Sunday, potentially an atmospheric river event, with a Marginal Risk area maintained over this region. GFS/ECMWF runs indicate precipitable water values may reach at least 3-4 standard deviations above normal for a time, with best agreement for heaviest totals along the coast. Significant rainfall (and possibly some very high elevation snow) may also extend into the Cascades but with less model consensus. Beyond early Monday, precipitation should linger over the Northwest, including the northern Rockies, but with somewhat lighter totals and a drier trend moving up from the south as upper heights rise somewhat. Parts of the Plains and east-central U.S. should continue to see episodes of showers and thunderstorms early-mid week as the Upper Midwest front continues into the Great Lakes and another front reaches the central U.S. Expect coverage of above normal temperatures to increase after Saturday. Best potential for plus 10-15F anomalies and perhaps locally higher is likely to be over the High Plains Sunday- Tuesday, the Northeast by Monday-Wednesday with persistent flow around the northern periphery of Atlantic high pressure, and over California/Nevada into southern Oregon by next Wednesday as upper ridging builds overhead. The combination of heat and humidity will likely lead to hazardous heat index values over southern Texas through the period, and the heat developing over the California Central Valley may become hazardous by next Wednesday. This looks like the beginning of a longer term heat event over the West during the week 2 period per Climate Prediction Center forecasts. As for exceptions to the warmth, clouds and rainfall will promote below normal highs for multiple days over the Pacific Northwest while locations from the Ohio Valley into Mid-Atlantic will see a cool start to the day on Saturday. 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