Extended Forecast Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Thu May 28 2020 Valid 12Z Sun May 31 2020 - 12Z Thu Jun 04 2020 ...Much above normal temperatures settling into the Rockies/High Plains to trend less extreme after Tuesday... ...Overview and Guidance/Predictability Assessment... Teleconnections relative to the strong upper ridging/positive height anomalies forecast to build over the North Atlantic support the general pattern which the best guidance cluster evolves toward upstream--a trough near the East Coast/western Atlantic, a broad ridge over or near the central U.S., and a trough near the West Coast. There is also consistency in the teleconnection signals for above normal temperatures centered over the Plains and extending into parts of the West along with episodes of showers/thunderstorms within the northwesterly flow aloft over the northeast quadrant of the lower 48. The relative agreement/consistency provides above average confidence in the large scale pattern but there is embedded model/ensemble spread for some details, most significantly near and inland from the West Coast mid-late period as well as shortwave energy rounding the upper ridge downstream. This spread tempers confidence somewhat by midweek. Trend has been toward a slower progression of trough energy that reaches near the southern half of the West Coast early next week though the flow separation in the slower solutions does not show up in the teleconnection-favored pattern. But the generally blocky nature of the pattern to the east would seem to favor the middle to slower half of the spread. Similar to the southern part of the overall mean trough, the ensemble means agree better than individual model runs for Northeast Pacific energy that may reach the Northwest by Wed-Thu. The means all show some degree of trough arriving versus a wide variety of shortwave amplitude/timing in the operational runs. Prefer the means as the starting point until guidance provides a more confident alternative. Guidance continues to have difficulty with the ultimate path/evolution of shortwave energy that ejects from the Northwest as well as surrounding and/or upstream pieces of energy that track around the northern side of the upper ridge. Predictability for these features is low due to their small scale. Over the past day there seems to be an attempt at some convergence in the guidance, with the GFS gradually nudging toward the ECMWF/ECMWF mean idea of a wave/frontal system being incorporated into the northwesterly flow aloft over the Northeast. Convergence of systems into the northeast portion of the area (Northeast to southeastern Canada) muddies the forecasting water a bit as the evolution of an Atlantic system remain unclear. Through the 00Z/06Z guidance, favored a blend of the deterministic guidance to start, with increased weight toward the 00Z ECMWF and especially the ECMWF ensemble mean which has been a bit more steady than the GEFS mean and likely has a better handle on the NW Atlantic evolution. ...Weather/Hazard Highlights... Highest anomalies within the area of much above normal temperatures centered over the west-central U.S. should be over the north-central Rockies/High Plains Sun-Tue with readings up to 15-20F or so above normal and some daily record highs/warm lows possible. Temperatures should become a little less extreme by Wed-Thu but a fairly broad area of plus 5-15F anomalies for highs may persist through the end of the period from parts of the central U.S. into/near the Great Basin, with single-digit warm anomalies possibly returning into California if an upper low remains offshore through Thu. Portions of the East will see a couple days or so of temperatures about 5-12F below normal during Sun-Tue followed by a warming trend to within a few degrees on either side of normal. The evolving pattern should favor one or more episodes of showers/thunderstorms over the northeast quadrant of the lower 48. Best potential for this activity, which could be locally moderate to heavy, extends between the Midwest/Upper Mississippi Valley and central Appalachians/Mid-Atlantic. The uncertainty in surface wave/frontal specifics will keep confidence fairly low for details on magnitude and location of highest rainfall totals though. Farther west a wavy front extending into the Northern Plains/Rockies may provide a focus for rainfall but there is still poor agreement on the specifics. The upper trough expected to approach the Northwest around or after midweek may start to generate some rainfall, while diurnal rainfall should continue over the central/southern Rockies. Other parts of the West may see scattered activity depending on shortwaves within the overall West Coast mean trough aloft. Southern parts of Texas and Florida may see some rain during the period as they will remain south of a front that settles over the northern Gulf. Fracasso/Rausch Hazards: - Flooding possible across portions of the Southeast, the Great Lakes, and the northern Great Basin. - Flooding occurring or imminent across portions of the central and northern Plains, the northern Rockies, from the mid-Mississippi Valley to the Midwest, and South Carolina. - Flooding likely across portions of the central Plains, the mid-Mississippi Valley, and the northern Rockies. - Much above normal temperatures across portions of the northern Rockies into the northern High Plains, Sun, May 31. - Much above normal temperatures across portions of the northern and central Rockies, into the northern and central High Plains, Sun-Tue, May 31-Jun 2. 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