Extended Forecast Discussion NWS Weather Prediction Center College Park MD 224 AM EDT Sat May 16 2020 Valid 12Z Tue May 19 2020 - 12Z Sat May 23 2020 ...Heavy precipitation likely from the northern Great Basin to the northern Rockies and Montana Tuesday and Wednesday... ...Areas of heavy rainfall for parts of the Mid-Atlantic next week... ...Overview and Weather/Hazard Highlights... Guidance continues to advertise a very amplified upper level flow pattern across the CONUS through much of the medium range period. Two deep closed lows on each coasts, with an Omega High sandwiched in between will be the main highlights. Out West, a closed upper low swings into northern California early Tuesday and should lift steadily north-northeastward into the Great Basin by Wednesday and western Canada by late next week. Locally heavy rainfall and high elevation snow will accompany this system from northern California into the northern Rockies from the early to middle part of next week. In addition, 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 on Wednesday. Meanwhile in the East, another closed upper low dropping southward from the Midwest early next week will likely stall/wobble over the Mid-Atlantic/Southeast through at least the middle of next week before weakening as it lifts northward into the Northeast. This slow to move upper low, combined with associated surface low pressure development, will likely lead to a prolonged period of wet weather for much of the Mid-Atlantic, with models continuing to show a signal for heavy rainfall, particularly across parts of the central Mid-Atlantic and Appalachians. The National Hurricane Center continues to monitor a potential sub-tropical low coming from the Bahamas, and although almost all models show this low staying offshore, tropically sourced moisture may wrap into the system over the East, enhancing rainfall across parts of the Mid-Atlantic on Tuesday. This will need to continue to be monitored. 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 eastward from the High Plains into the Central U.S. with the greatest anomalies of +10 to +20 degrees over the northern High Plains on Tuesday and the northern Plains on Wednesday. ...Guidance/Predictability Assessment... Continuity and clustering over the West continues to be better than average through much of the period, though the latest runs of the GFS/UKMET/GEFS mean are a little bit quicker to lift the closed low through the northern Rockies. Prefer a blend weighted more towards the ECMWF/ECENS for this system. Behind this, another possible closed low may skirt the Pacific Northwest mid to late next week, with the GFS/ECMWF and ensemble means showing the best consensus for this weak system. On the contrary, models and ensembles continue to show considerable run to run variability for both track and depth of the closed low, although the majority of the guidance does support deep troughing and a slow moving system. The latest runs of the GFS/ECMWF continue to show the best clustering with the ensemble means, with the GFS a bit north of the ECWMF/ensemble means after day 4. The UKMET stalls the low over the Southeast, much farther south than the better ensemble clustering, while the latest run of the CMC now presents a much weaker system, ejecting it eastward (rather than slowly northward) into the Atlantic by the middle of next week. Again, confidence in the current forecast for this system is low to average at best given the continued uncertainty with exactly how this evolves. This cycle of the WPC fronts/pressure progs relied on a majority ECMWF/GFS blend for days 3-4 (more weighting on the ECMWF), along with smaller contributions from the ensemble means (GEFS/ECENS). Ensemble mean weighting was increased in place of the GFS after day 5 due to increased model spread for both major systems. This maintains good continuity with the previous WPC forecast. 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