Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM 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... The latest deterministic models continue to show some trouble forming a consensus on timing of the incoming cold front and upper trough or weak upper low into the Northwest on Tuesday. The 06z GFS is better on timing but is a bit north of the consensus with the vort center, while the CMC is noticeably faster than the rest of the guidance. Late week into the East, the 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. Favor a ECMWF/UKMET blend for this system, with smaller contributions from the GFS. This also helped maintain some definition to the system later in the period as more of the washed out ensemble means were introduced to the blend. Upstream, the next system shows plenty of unresolved details in the Gulf of Alaska which translates later into the period as a cold front enters the West Thursday into Friday. Again, the GFS and ECMWF show ok agreement on timing, but did favor incorporation of the ensemble means by this point into the blend to help mitigate some of the smaller scale differences. This approach maintains fairly good agreement with the previous WPC forecast as well. ...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. As low pressure tries to form in the Central Plains on Thursday, an area of organized and moderate to heavy rain could shift through the north/central Plains into the Midwest though plenty of uncertainty as it depends on strength of the upper level feature. The cold front may touch off light to modest showers and perhaps some thunderstorms elsewhere too. Some rainfall amounts have the potential to be locally heavy in some convectively-enhanced areas (generally over the southern/central 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/Upper Midwest Tue-Thur. 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. Santorelli/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