Extended Forecast Discussion NWS Weather Prediction Center College Park MD 206 AM EDT Sun May 23 2021 Valid 12Z Wed May 26 2021 - 12Z Sun May 30 2021 ...Record heat likely for portions of the Southeast/Mid-Atlantic Wednesday... ...Overview... Upper pattern will feature quasi-zonal flow with relative lower amplitude waves in a trough/ridge orientation over the West/East, respectively. This will favor well above normal temperatures in parts of the East/Southeast and cooler than normal temperatures for parts of the High Plains to the Upper Midwest. In between, rainfall will be focused around a wavy frontal boundary. ...Guidance Evaluation and Preferences... Aside from the 12Z Canadian, the GFS/ECMWF/UKMET offered reasonable clustering near their ensemble means with the first half of the forecast. In question has been upstream flow south of Alaska into the Pacific Northwest, but this uncertainty has narrowed in recent runs. By next weekend, ensemble spread increased (as expected) but was within typical limits. The GFS/GEFS were generally quicker than the ECMWF/ECMWF ensembles and Canadian ensembles in the West but were not unreasonable given the flow. However, favored the slower ECMWF/ECMWF and Canadian ensembles by about 3:1 to the GEFS with a tendency for some individual systems to lag behind despite the overall progressive pattern. ...Weather Highlights/Hazards... Highest chances of rainfall during the period will likely be over the central Plains to the Midwest and Great Lakes along a wavy frontal boundary. Parts of the northern Rockies and High Plains will see some light rain and high mountain snow with the Wednesday system. As low pressure moves through the Central Plains on Thursday, an area of organized and moderate to heavy rain could shift through the northern/central Plains into the Midwest. The cold front may touch off light to modest showers and perhaps some thunderstorms in the warm air mass or just north of the frontal zone. Some rainfall amounts have the potential to be locally heavy in some convectively-enhanced areas (generally over the southern/central Plains). Areas in the East may see some light showers as the front ebbs and flows. Temperatures will be cool in the High Plains to Upper Midwest Wed-Fri, warm/hot in the East (Wed) and Southeast (Wed-Fri), and within several degrees of normal in between. Coldest anomalies of about 5 to 15 degrees below normal will be centered from eastern Montana to northern Minnesota Wed-Thu. In the East temperatures may climb to 10-20F above normal which will bring 80s/90s into the Northeast/Mid-Atlantic and 90s to near 100F over the Southeast on Wednesday. Record highs are likely in some areas along the I-95 corridor (Georgia to New Jersey). Heat indices may be kept in check by dew points in the 50s/60s through the week. Temperatures will cool back toward more typical late May values for Fri-Sun (Memorial Day weekend) but along with increased rain chances. Fracasso 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml