Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM 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 provide 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. Especially starting around 24 hours ago guidance diverged significantly for what ultimately happens with trough energy that reaches near the southern half of the West Coast early next week. The GEFS/ECMWF means have settled into a slow solution just a bit east of the upper low in the past two ECMWF runs. 12Z/18Z GFS runs and the 12Z CMC brought the feature inland. The 12Z CMC mean was a bit east of the other means but slower than the GFS/CMC. 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. New 00Z guidance provides reasonable support for above preferences, with the GFS/CMC trending slower with the southern stream feature (though still east of the ECMWF) and better agreement on a trough approaching the Northwest by Thu). 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. Compared to remaining guidance the GFS/GEFS mean still appear somewhat too open and progressive within the amplifying/sharpening upper trough that starts the period over eastern North America early Sun. The 12Z GEFS mean was actually closer to other guidance than the 18Z run so the earlier run was preferred for the GEFS component of the forecast blend. An operational model blend consisting of 40 percent 12Z ECMWF and smaller weights of the 18Z GFS/12Z CMC-UKMET for about the first half of the period reflected the preferred ECMWF cluster that showed a closed upper low just off the New England coast as of early day 5 Tue and yielded a solution close to the ensemble means for the trough near the West Coast. Days 6-7 Wed-Thu went toward greatest emphasis on the 12Z ECMWF mean with remaining input from about 1/3 of the 12Z GEFS mean and minority weight of the 12Z and 00Z/27 ECMWF runs. ...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. Rausch 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