Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat May 16 2020 Valid 12Z Tue May 19 2020 - 12Z Sat May 23 2020 ...NHC monitoring a disturbance off Florida and the Bahamas... ...Areas of heavy rainfall for parts of the Mid-Atlantic next week... ...Heavy precipitation likely from the northern Great Basin to the northern Rockies and Montana Tuesday and Wednesday... ...Guidance/Predictability Assessment... Continuity and forecast clustering over the West continues to be better than average through much of next week, but less amplified guidance runs into days 6/7 such as the 00/06 UTC GFS and 00 UTC GEFS may be less likely considering the overall upper flow amplitude. The 12 UTC GFS and 06/12 UTC GEFS are at least trending toward more amplitude. On the contrary, models and ensembles still continue to show considerable run to run variability for both track and depth of the closed low into the East, although the majority of the guidance does support deep troughing and a slow moving system. A solution slightly more on the amplified side of the full envelope of model and ensemble solution makes sense given the closed nature of the Spring low/trough. This and latest guidance from NHC seems best represented by a composite of the 00 UTC ECMWF and GEFS/ECMWF ensemble means into Tue/Wed. Prefer to transition to the latest GEFS/ECMWF ensemble means into days 6/7 as was hesitant to embrace the 00 UTC ECMWF by days 6/7 that lifts a less separated feature more progressively northward than prior runs. The 12 UTC ECMWF trended toward more amplitude, as well as in general the overall suite of 12 UTC models and ensembles. ...Weather/Hazards Highlights... It remains the case that 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/Sierra 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. 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