Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Mon May 18 2020 Valid 12Z Thu May 21 2020 - 12Z Mon May 25 2020 ...Heavy rains for the Mid-Atlantic/Appalachians linger through at least Thursday... ...Overview and Guidance/Predictability Assessment... The first half of the medium range period continues to feature an amplified/Omega flow pattern over the CONUS. Two deep closed lows, one in the West and another in the East, will sandwich a central U.S. ridge. In the East, guidance has come into better agreement on a lingering upper low over the Tennessee/Ohio Valley region which should finally begin to weaken and lift north and east by next weekend. The CMC is farther south/faster with the main energy, while the latest runs of the ECMWF/GFS are in better agreement with the ensemble means. Meanwhile, out west, a closed low lifting into western Canada on Thursday will be replaced by another closed low/renewed amplified troughing late week and next weekend. Through day 4, models and ensembles show good agreement, but beyond this, the models begin to diverge on the evolution of the second low dropping into the West. The 12z/May 17 run of the ECMWF is the farthest south and maintains the closed upper low through day 7 as it shifts into the High Plains. The latest GFS/CMC solutions present more of an eastward moving amplified trough pattern, which is also in line with the ensemble means. It should be noted though that run to run continuity for any one model is quite poor and so it's hard to say which solution is more probable. This cycle of the WPC progs relied on a majority ECMWF/GFS blend for the first half of the period, transitioning to mostly ensemble mean guidance (ECENS/GEFS means) by days 6 and 7 to account for the uncertainty, especially out west. This also maintains good continuity with the previous WPC shift. ...Weather/Hazards Highlights... Heavy rainfall across the Mid-Atlantic/Appalachians and broad vicinity will continue through at least Thursday, with multi-day rainfall totals of several inches possible, especially along the Central Appalachians. Expect showery conditions, with pockets of locally heavy rains, to continue through the weekend as the upper low lifts slowly north and eventually exits the Mid-Atlantic coast late weekend/early next week. Temperatures across the Eastern U.S. will be cool initially on Thursday, but should trend back towards normal as the upper low departs and ridging begins to move into the region. Out West, enhanced precipitation can be expected along the favored terrain of the Pacific Northwest and northern Rockies through this week. Showers should maintain themselves across the Plains states initially along a quasi-stationary frontal boundary, with more widespread rainfall possible by late this weekend and early next week ahead of the Western U.S. trough. Renewed troughing in the West keeps temperatures well below normal with average departures from normal as much as -10 to -15 degrees. On the contrary, the Central U.S. should stay near or above normal underneath of upper level ridging with these temperatures shifting into the Midwest states by next week. Santorelli 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