Extended Forecast Discussion NWS Weather Prediction Center College Park MD 222 AM EDT Sun May 17 2020 Valid 12Z Wed May 20 2020 - 12Z Sun May 24 2020 ...Heavy precipitation likely for parts of the northern Rockies and Montana on Wednesday... ...Areas of heavy rainfall for parts of the Mid-Atlantic and Appalachians next week... ...Overview and Guidance/Predictability Assessment... Forecast guidance continues to advertise a very amplified upper level flow pattern across the CONUS through much of the medium range period featuring two deep closed lows on each coast, with an Omega High sandwiched in between. Out West, one closed low over the Great Basin on Wednesday will lift into western Canada by Friday, only to be replaced by another closed low/renewed amplified troughing later in the week and next weekend. Models and ensembles show better than average clustering through day 5, with some differences arising by day 6-7 regarding amplitude/timing of the second system into the Pacific Northwest. A general model blend with increasing contributions from the ensemble means seems like a good starting point for the West. Meanwhile in the East, another closed low dropping southward from the Midwest early in the week will likely stall/wobble over the Mid-Atlantic/Southeast through about Thursday, before beginning to slowly weaken and lift north then east into next weekend. The guidance continues to show plenty of run to run variability, especially the second half of the period on amplitude and timing as the system finally lifts into the Northeast or off the Northeast coast. The ECMWF/ECENS remain a bit west of the GFS/GEFS through the period, with the CMC largely in the middle. Neither solution seems unreasonable enough to not include in the blend, so WPCs preferred blend for this cycle of progs was based on the GFS/ECMWF/CMC for days 3-4, with more ensemble means beyond given the large discontinuities between runs. ...Weather/Hazards Highlights... In the West, locally heavy rainfall and high elevation snow will accompany the first low as it lifts into western Canada late next week. Low level upslope flow associated with a developing low pressure system over the northern High Plains will likely serve to enhance precipitation across northern Idaho and western Montana next Wednesday. Lighter and more scattered precipitation can be expected to continue across parts of the interior Northwest into next weekend as the troughing is renewed across the West. Out east, the slow moving/stalled upper low will likely bring periods of heavy rainfall to parts of the Mid-Atlantic and Appalachians through Friday. Models show multi-day rainfall totals of a couple of inches possible, especially along the Central Appalachians. For temperatures across the CONUS, the Western and Eastern U.S. systems will each support a period of below normal daytime highs underneath the lows with some places as much as 10 to 15 degrees below normal. In between, expect an area of much above normal temperatures to drift slowly eastward from the High Plains into the Central U.S. Wednesday-Friday, and eventually into the Midwest by the weekend. Temperatures along the East Coast should also return back to near normal by next Friday. 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