Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Tue May 26 2020 Valid 12Z Fri May 29 2020 - 12Z Tue Jun 02 2020 ...Much above normal temperatures to progress from the West into the northern/central Plains... ...Overview and Guidance/Predictability Assessment... Guidance agrees that the strong upper ridge and associated much above normal temperatures initially over the West late this week should reach the central U.S. by next Mon-Tue. Behind this ridge an upper low will lift up the West Coast during the weekend, bringing a period of rainfall that could be locally moderate/heavy, immediately followed by a larger scale trough that should reach the West Coast around Mon-Tue. To the east of the upper ridge expect a trough over the Upper Midwest/western Great Lakes on Fri to amplify as it continues eastward, reaching New England and the western Atlantic early next week. Guidance has gravitated toward recent ECMWF-based solutions that started to advertise enough northern stream interaction to accelerate the short-range upper low over the Plains eastward as an open shortwave after early Fri. The combination of these features will produce rain/thunderstorm chances over a large portion of the East late this week into the weekend but rainfall coverage will steadily decrease as a cold front drops southward. From the large-scale perspective most models and means were close enough to favor an 18Z/12Z operational model blend for about the first half of the period followed by a transition toward a half model/half mean weight by day 7 Tue as details become more uncertain. This blend emphasized timing slower than the GFS for the upper low tracking northward along the West Coast during the weekend, as recent GFS runs have been a little faster/more amplified with a piece of shortwave energy just to the west of the feature. Late in the period the 12Z GFS was discounted due to bringing a deep upper system into the central West Coast instead of tracking it over the North Pacific per other guidance. The 18Z/00Z GFS runs were much more reasonable compared to consensus for the late period trough, which it should be noted has adjusted a bit faster over the past day. The upper trough reaching the eastern U.S. and western Atlantic shows a multi-day trend toward being somewhat sharper and deeper at some valid times, nudging the northern-central part of the leading cold front a bit slower during the late week/weekend time frame. ...Weather/Hazard Highlights... Within the large area of much above normal temperatures progressing from the West into the Plains, the forecast has been consistent so far in highlighting plus 20-25F anomalies for highs from the far northeast corner of California across parts of Oregon/Nevada/Idaho during Fri-Sat. Greatest anomalies may be closer to plus 15-20F or so over the northern-central High Plains by Sun-Tue. The West Coast states will see highs settle to near or slightly below normal. Meanwhile cool high pressure behind the cold front dropping through the East will bring an area of temperatures up to 5-12F below normal from the Upper Midwest eastward/southeastward. Ahead of this front the Northeast should see temperatures of 10-20F above normal on Fri. Morning lows over a larger area in the East could be close to record warm values but rainfall later in the day could result in lower mins for the calendar day. The evolution/progression of the large scale pattern will favor decreasing precipitation coverage over the lower 48 with time. Over the East the majority of rain/thunderstorm activity will be during Fri-Sat ahead of the cold front pushing into the region and the upper shortwave crossing the South. Some activity could be locally moderate/heavy. Parts of Florida and southern Texas will remain south of the front through the period and may see some rainfall linger into next week. Potential remains for areas of moderate to heavy rainfall over some locations of extreme northern California and the Pacific Northwest for part of the weekend due to the upper low tracking northward across the region. Later in the period this feature will likely support Canadian low pressure that brings a frontal system across the Northern Plains, with some rainfall possible ahead of it late in the period over the Upper Midwest and/or Great Lakes. The trailing upper trough reaching the West Coast should support mostly light precipitation early next week, mainly over parts of the West Coast states. Expect scattered diurnally favored rainfall over the central-southern Rockies. 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