Extended Forecast Discussion NWS Weather Prediction Center College Park MD 149 PM EDT Fri May 21 2021 Valid 12Z Mon May 24 2021 - 12Z Fri May 28 2021 ...Record heat likely for portions of the Southeast/southern Mid-Atlantic next week... 1800 UTC Update... Not much has changed with the latest model runs from the previous shift. The medium range period (Monday-Friday) continues to feature a trough holding in the West, while the Eastern U.S. bakes underneath of an amplified ridge. For many from the Southeast into the Mid-Atlantic, this will be the first major heat wave of the season. Models are in good agreement with the overall pattern for roughly the first half of the week, but continue to show uncertainty in the embedded flow details of broad Western U.S. troughing. This cycle of the WPC forecast leaned heavily on the deterministic runs (ECMWF/GFS/CMC/UKMET) through day 4, transitioning to majority ensembles by day 7, with some modest inclusion of the ECMWF/GFS for added system definition. This maintains excellent continuity with the previous forecast so only minor adjustments were needed. See previous discussion attached below for more details on the pattern and the associated weather hazards. Santorelli Previous discussion issued at 0627 UTC... ...Overview... Upper lows in the Bering Sea and near the Davis Strait favor a trough/ridge pattern across the lower 48 next week. This will maintain a cool and showery (even snowy) regime over the interior West behind a frontal system moving ashore early in the week. Over the East, ridging will promote hot and dry conditions over the Southeast with record highs well into the 90s to around 100F. A backdoor front may cool some areas of the Northeast/Mid-Atlantic in the otherwise very warm pattern. Rainfall will be focused over the Plains and northern tier until a cold front pushes southeastward late in the week. ...Guidance Evaluation and Preferences... Through the 12Z/18Z cycle, the deterministic models formed a reasonable blended solution despite their differences in timing into the West early in the week. The GFS runs have been generally quicker but within the ensemble spread, and this was balanced by the slower ECMWF/Canadian. To the east, quicker trend in all the guidance with low pressure through southern Canada (and warm fronts just over the border), but less change near the inflection point between the backdoor cold front into NC Mon-Tue. By later in the week, relied on 50/50 model/ensemble blend, as the West will likely see smaller wavelength features dive through/into the broad trough, but predictability is low. In the East, still expect high pressure out of Canada to help push the cold front southeastward into the Carolinas by late next Thu into next Fri, lingering the boundary across the Plains. ...Weather Highlights/Hazards... Highest chances of rainfall next week will likely be over the central Plains to the Midwest as well as the interior Northwest/northern Rockies. Incoming front into the Pacific Northwest and the slowly-moving cold front out of the northern Plains will touch off light to modest showers and perhaps some thunderstorms east of the Rockies. Some snow is possible over the highest elevations, with snow levels lowering a bit underneath the upper trough. Amounts have the potential to be locally heavy in some convectively-enhanced areas (over the Plains) along the frontal boundary. Backdoor cold front in the East may bring only light showers to some areas. By later in the week, approaching cold front could support more organized showers and perhaps a storm. Temperatures will be cool in the West, warm/hot in the East, and within several degrees of normal in between. Coldest anomalies in the West may be early in the week where readings of 5-15F below normal will moderate a bit to about 5 degrees below normal by the end of the week. 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 Mon-Wed. 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