Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Wed May 27 2020 Valid 12Z Sat May 30 2020 - 12Z Wed Jun 03 2020 ...Much above normal temperatures to progress from the Interior West into the northern/central Plains... ...Overview and Guidance/Predictability Assessment... Guidance is suggesting the large scale pattern may become more blocky over the Atlantic and portions of North America over the course of the period. Consensus shows a very strong upper ridge building over the North Atlantic, preventing much progression of the mean trough expected to settle over New England/Canadian Maritimes into the western Atlantic while an initial Rockies upper ridge drifts into the Plains/Mississippi Valley. Likewise expect rather slow movement of a mean trough aloft that develops near the West Coast. Within this agreeable evolution there are multiple embedded uncertainties that will have a meaningful impact on the forecast for some areas. The compact upper low lifting northward along the West Coast during the weekend will be one focus through the period. While GFS runs have tended to be a bit on the faster side of the spread along the West Coast, by the time the feature reaches western Canada the GFS has been interacting with surrounding energy to slow down the overall area of upper dynamics. Most other guidance maintains the energy from the initial closed low as a distinct shortwave that ultimately progresses across southern Canada faster than the GFS/GEFS mean--bringing a wave/frontal system into the northeastern quadrant of the country late in the period. The axis of the trailing mean trough reaching the West Coast has had good continuity over the past day but there has been some debate over where/if an embedded low may close off. Recent GEFS mean trends and a number of GFS/ECMWF/CMC runs are suggesting a better than even probability that there will be enough flow separation for a low--even if not necessarily defined by the standard 6 dm contour interval. Within the upper trough reaching New England and the western Atlantic, by the latter half of the period the 12Z ECMWF strayed toward the southern extreme for its closed low and then brought it back into New England. Multi-day trends of models/means and most of the latest guidance would argue for a slower and more closed evolution than the GFS/GEFS mean but closer to the specifics of the 12Z ECMWF mean. The old 00Z/26 ECMWF run was close to that idea as well. Above considerations and poor UKMET agreement with consensus for some aspects of the forecast led to an initial blend of half 12Z ECMWF and the 18Z GFS/12Z CMC for about the first half of the period. The latter half of the forecast phased out the GFS, steadily increased 18Z GEFS/12Z ECMWF mean input, and placed more emphasis on the 00Z/26 ECMWF than the 12Z run. This yielded a solution close to the ECMWF mean near New England late in the period and ECMWF/ECMWF mean runs farther west across the northern tier states and southern Canada. The West Coast trough represented a reasonable average of guidance. New 00Z guidance provides a mix of trends. Clustering is better for the initial West Coast upper low early in the period and the new ECMWF is a little less extreme near New England late in the period. On the other hand there is now much greater late-period spread over the West as the CMC becomes more progressive than other solutions while the ECMWF forms its closed low well offshore California, reminiscent of what it was showing a couple days ago. ...Weather/Hazard Highlights... The most extreme anomalies should gradually decrease with time within the broad area of much above normal temperatures drifting from the Interior West into the northern/central Plains. Locations in and near Idaho may see highs up to 20-30F above normal on Sat and some highs should be 15-20F or slightly more above normal over the north-central Rockies and the northern half of the High Plains Sun into early next week. It will still be very warm over the central U.S. into Wed but at that time highs should be no more than 10-15F above normal. Meanwhile near to below normal temperatures will prevail on either side of this area of heat. Expect one or more days with highs 5-12F below normal over the East this weekend into the start of next week followed by a warming trend toward normal by midweek. Locations near the West Coast should be near to slightly below normal most days. Extreme northern California and the Pacific Northwest may see locally heavy rainfall during Sat-Sat night as an upper low tracks northward along the West Coast. The upper trough settling near the West Coast will likely produce areas of mostly light to moderate rainfall, with some sensitivity to exactly how energy within the overall trough evolves. The central and southern Rockies should see diurnally favored rainfall through the period. Much of the East will see dry weather from the weekend into early next week, after some lingering East Coast rain on Sat departs with its associated cold front. Parts of Florida and southern Texas may see additional rainfall since those areas will remain south of the front. A warm front crossing the northern tier may generate some showers/thunderstorms from the Great Lakes into parts of the East by Tue-Wed. Farther west a front settling over the Northern Plains could provide a focus for some rainfall but so far guidance is providing mixed signals for specifics of if/where/when it occurs. 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