Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun May 17 2020 Valid 12Z Wed May 20 2020 - 12Z Sun May 24 2020 ...Heavy precipitation pattern for the northern Rockies mid-late week... ...Heavy rains for the Mid-Atlantic/Appalachians linger this week as Arthur churns up the western Atlantic... ...Overview and Guidance/Predictability Assessment... Latest forecast guidance continues to advertise an amplified upper level/Omega flow pattern over the CONUS through much of the medium range period featuring deep closed lows in the West and the East that sandwich a central U.S. ridge. Out West, a closed low over the Great Basin Wednesday will lift into western Canada Thursday/Friday, only to be replaced by another closed low/renewed amplified troughing late week and next weekend. Models and ensembles show slightly better than average clustering through day 5 outside of Arthur, with more differences arising by day 6-7 regarding amplitude/timing of the second system into the West. A guidance composite seems to work as a good forecast starting point for days 3-5 and GEFS/ECMWF ensemble mean usage days 6/7 maintains high amplitude flow over the West that seems most reasonable considering the blocky nature of the flow. 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 northwest of the GFS/GEFS and are less amplified this period. It is accordingly a complex and uncertain forecast for Arthur. A blend of the 00 UTC ECMWF/ECMWF ensemble mean and the 06 UTC GEFS mean seems to best align with the latest NHC track for Arthur days 3-5, then relied on the ensemble means to maintain good flow amplitude consistent with a blocky pattern days 5-7. ...Weather/Hazards Highlights... Tropical Storm Arthur is churning up the far western Atlantic and the U.S. coastal eastern seaboard. Please refer to ongoing advisories from the National Hurricane Center and your local forecast office for updated info on Arthur. Upstream, a slow moving/stalled upper low will bring multiple periods of heavy rains to the Mid-Atlantic/ Appalachians and broad vicinity through Friday. Models show multi-day rainfall totals of several inches possible, especially along the Central Appalachians. 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. Enhanced precipitation can be expected across parts of the interior Northwest and vicinity into next weekend as the troughing is renewed across the West. 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. Schichtel/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