Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM EDT Sat May 22 2021 Valid 12Z Tue May 25 2021 - 12Z Sat May 29 2021 ...Record heat likely for portions of the Southeast/southern Mid-Atlantic next week... ...Overview... An upper low near the Davis Strait will retrograde toward the Canadian Archipelago next week, which favors a low-amplitude trough/ridge pattern across the lower 48. Heights will generally be near to above normal over all of the CONUS by late next week which will translate to surface temperature as well. Over the East, record highs well into the 90s to around 100F are likely for portions of the Southeast into the Mid-Atlantic. A backdoor front may cool some areas briefly in the otherwise very warm pattern. Rainfall will be focused over the Plains to the Midwest/Great Lakes. ...Guidance Evaluation and Preferences... Through the 12Z/18Z cycle, the deterministic models still had trouble forming a consensus on timing the incoming cold front and upper trough or weak upper low into the Northwest Tuesday. The GFS runs remained a bit quicker but a blended/consensus starting point was favored given a bit quicker trend. Upstream over the Gulf of Alaska, much larger differences emerged in the 12Z ensembles regarding the handling of a couple features from the North Pacific and off the mainland, leading to large differences between the ECMWF ensembles and the GEFS/Canadian. However, there was some overlap between some GEFS members and the ECMWF idea with the deterministic models in principle, leading to favor the ECMWF ensembles over the GEFS into mid week. By later in the week, ECMWF ensembles seemed to suffer from a smearing of 50 members with differences in timing smaller-scale features into the West, leading to a broad trough of low amplitude. In the East, cold front may dip into the southern Mid-Atlantic on Friday with perhaps a wave of low pressure along the front draped back to the Plains. Low confidence in the details as the models have little consistency and the ensembles were rather undefined. Maintained a wavy front astride the Ohio Valley that may lift back into the Northeast as upper ridging attempts to build just offshore. ...Weather Highlights/Hazards... Highest chances of rainfall next week will likely be over the central Plains to the Midwest and Great Lakes along a wavy frontal boundary. Parts of the interior Northwest/northern Rockies will see some light rain and high mountain snow with the Tuesday system. This front may touch off light to modest showers and perhaps some thunderstorms east of the Rockies as it weakens/dissipates. Some rainfall amounts have the potential to be locally heavy in some convectively-enhanced areas (generally over the Plains) along the frontal boundary. Backdoor cold front in the East may bring only light showers to some areas on its way south or back north as a warm front. Temperatures will be cool in the West to start, warm/hot in the East, and within several degrees of normal in between. Coldest anomalies of about 5 to 10 degrees below normal will be centered from the Great Basin/Northwest to the High Plains Tue-Wed. In the East, ahead of the backdoor cold front (or behind the warm front as it advances back to the northeast), temperatures may climb to 10-20F above normal. This will bring 80s/90s into the Northeast/Mid-Atlantic and 90s to near 100F over the Southeast. Records highs are likely for some areas and could be widespread over several states Tue-Wed along the I-75 and I-95 corridors, respectively. Heat indices may be kept in check by dew points in the 50s/60s through the week. 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