Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sun May 26 2019 Valid 12Z Wed May 29 2019 - 12Z Sun Jun 02 2019 ...Record/Dangerous Heat expected across the Southeast especially through Thursday... ...Episodes of heavy rain possible for portions of the central U.S... ...Overview and Guidance Evaluation/Preferences... Most guidance continues to show a similar pattern evolution but there are multiple persistent uncertainties regarding medium to smaller scale details. Across the eastern Pacific and western North America, expect mid-latitude Pacific jet energy to encroach upon a split flow regime (mean ridging over western Canada and troughing over the West), ultimately leading to a trough reaching the Pacific Northwest coast by next Sun. The flow over the eastern half of the Lower 48 should become less amplified, as broadly cyclonic flow becomes established over northern areas and the northern extension of the upper ridge initially centered over Florida flattens. This subtropical ridge will likely expand westward across the Gulf of Mexico into northern Mexico by Fri-Sun. The expected evolution should provide partial moderation of the heat over the Southeast after Thu but one or more shortwaves ejecting from the western mean trough could maintain the threat for periods of significant rainfall over the Plains and vicinity. Forecast details remain complex for the system initially over the Midwest on Wed. There is a gradually improving signal that some shortwave energy will remain over the Midwest into day 4 Thu (supporting a frontal wave) but it remains to be seen what proportion will be from the feature initially over the region the prior day (with the rest ejecting northeastward) in addition to trailing energy that rapidly moves into the region from the southwest. Uncertainty over amplitude of southern Canada troughing that may extend into the Great Lakes continues to plague the forecast as well. The 00Z UKMET is on the extreme side of the envelope for the combined fast timing and amplitude of the trough. A blend consisting mostly of the other available models should provide a reasonable starting point given the ongoing potential for detail adjustments in future runs. By Sat-Sun the 00Z ECMWF becomes one of the stronger/faster solutions within the ensemble spread for the upper ridge that crosses southern Canada, with that run actually becoming an extreme to marginal outlier solution over Ontario by Sun. Ensemble means including from the ECMWF favor slower progression. The combination of flow details across southern Canada/northern tier states along with one or more shortwaves ejecting from the western U.S. mean trough will determine the day-to-day position of the front dropping into the northern Plains late this week and possibly continuing southward thereafter. Across the West the past cycle or two of guidance has become more split as to locating the core of the mean trough on any particular day, in contrast to prior emerging consensus that had been over the Great Basin. Toward the end of the period the 00Z GFS once again brings stronger troughing to the West Coast while the 06Z GFS and other models/means are easily within typical error ranges for extended time frames. Based on the current array of guidance the updated forecast started with the 06Z GFS and 00Z ECMWF/CMC with slight input from the 06Z GEFS/00Z ECMWF means early in the period and then trended toward a nearly even model/mean weight for next weekend. ...Weather Highlights and Hazards... High temperatures reaching 10-20 deg F above average across much of the Southeast should continue through Wed-Thu with readings likely nearing or surpassing 100 deg F for many areas along/below the fall line. These temperatures will combine with moderate humidity levels to produce dangerous heat index values that could briefly approach or exceed 105 deg over some areas. Daily records for highs/warm lows will be possible. In less extreme fashion some of this heat should spread northward into parts of the Mid-Atlantic Wed-Thu. A front will provide a cooling trend later in the week but temperatures over and near the Southeast may still be 5-10 deg F above normal through next weekend. The mean trough aloft over the West will keep the Great Basin/central Rockies on the cool side for most of the period but with anomalies somewhat closer to normal than the minus 10-15 F departures for highs (also extending into the High Plains) forecast on Wed. The Northwest will see temperatures about 5-12 deg above normal for most of the period. Heavy rainfall will be a threat across portions of the Southern/Central Plains and perhaps the Mid-Mississippi Valley during the upcoming week. The front trailing from the initial Midwest system Wed could produce widespread showers and thunderstorms with potentially heavy rainfall and severe weather across portions of the Southern Plains, with localized heavier rainfall amounts possibly extending northeast along the front into the Mid-Mississippi Valley. One or more shortwaves ejecting from the western trough aloft along with the northward progression of a weakening warm front could once again enhance the rainfall potential for portions of the Southern/Central Plains and possibly into the Mississippi Valley Fri into the weekend. Across the West, expect primarily terrain focused precipitation from the Great Basin to the central/northern Rockies through much of the week into the weekend with the passage of several upper-level shortwaves. Rausch/Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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