Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Fri May 24 2019 Valid 12Z Mon May 27 2019 - 12Z Fri May 31 2019 ...Record/Dangerous Heat expected across the Southeast... ...Heavy rainfall possible for portions of the Plains and Mid-Upper Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... Expect an amplified pattern to prevail over North America for most of the period. The most persistent features will be a strong subtropical ridge near the Southeast U.S., a mean upper trough over the West, and a Hudson Bay upper low whose surrounding circulation should extend into the northern tier states by the latter half of next week. The depth of the western trough will likely moderate after ejection of a deep upper low Mon-Tue and the Southeast ridge may begin to flatten late next week. The models and ensembles continue to struggle with the exact amplitude and longitude of the Canadian upper low and its circulation to the south--affecting how quickly the system ejecting from the West/Plains will interact with the northern stream flow. Thus far no particular model has really settled into a consistent solution, favoring a blend approach to reduce run-to-run variability and yield a solution close to the ensemble means in principle. As it is even consensus has exhibited some fluctuation over the past couple days with the 00-06Z guidance tending to delay phasing somewhat compared to 12-18Z solutions. Late in the period a model/ensemble blend looks reasonable as the GFS/CMC race ahead of the ECMWF with height falls crossing New England and the Canadian Maritimes but with the 00Z ECMWF trending faster than its prior run. Early in the week ahead of the main Plains system, confidence remains low for specifics of waviness along a front forecast to extend into the Great Lakes/Northeast. During the latter half of the period over the West, guidance clustering has noticeably improved. Multi-day trends have seen GFS/GEFS-based solutions generally gravitate toward the ECMWF/ECMWF mean idea of maintaining a decent mean ridge aloft just off the West Coast with some weak energy passing through a weakness and feeding into the trough that lingers over the West. The 00Z CMC strays faster than consensus while the 00Z GFS is more comparable to the ECMWF and ensemble means than the 06Z run. Based on forecast considerations the updated forecast used an operational model blend during the first half of the period and then trended to a model/mean solution incorporating the 00Z ECMWF/ECMWF mean and GFS (more weight on the 00Z run)/06Z GEFS mean. ...Weather Highlights and Hazards... High temperatures should be 10-20 deg F above average across much of the Southeast Mon-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 at least briefly exceed 105 deg over some areas. There will be multiple days with potential for record highs/warm lows. In less extreme fashion some of this heat should spread northward over the East through midweek. A front will provide a cooling trend later in the week. Meanwhile the upper trough initially ejecting from the West will lead to cool conditions with high temperatures 10-20 deg F below average from California through the Great Basin/north-central High Plains Mon-Tue. Temperatures should gradually trend closer to normal mid-late week. At the same time upper ridging will be close enough to the Northwest to provide above normal temperatures for most of next week. Heavy rainfall will be a threat across portions of the central U.S. during next week. The low pressure system crossing the central U.S. early-mid week (and possibly a leading wave) could produce multiple rounds of potentially heavy rainfall and severe thunderstorms across portions of the Plains and the Midwest, with multi-inch rainfall totals possible. Some enhanced rainfall could extend into the Great Lakes/Northeast with any leading frontal waviness, and then later in the week as the front trailing from the main Plains system reaches the area. Late in the week the trailing end of the front should linger across the Southern Plains and Lower/Mid-Mississippi Valley where it could continue to focus areas of showers and storms with at least locally heavy rainfall. Across the West the most-focused precipitation should be associated with the upper low tracking out of the Southwest Mon into Tue while residual troughing aloft could favor periods of diurnally favored activity through the rest of the week. 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 Hazards: - Heavy rain across portions of the Great Plains, the Mississippi Valley, and the Great Lakes, Mon-Thu, May 27-May 30. - Heavy rain across portions of the Central and Northern Rockies, as well as the Central and Northern Plains, Mon-Tue, May 27-May 28. - Severe weather across portions of the Central and Southern Plains, the Great Lakes, the Middle and Lower Mississippi Valley, Tue, May 28. - Severe weather across portions of the Central and Northern Plains, the Middle and Upper Mississippi Valley, as well as the Great Lakes, Mon, May 27. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding likely across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Excessive heat across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Mon-Thu, May 27-May 30. - Enhanced wildfire risk across portions of the the Southern Rockies, the Southern Plains, and the Southwest, Mon, May 27. 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