Extended Forecast Discussion NWS Weather Prediction Center College Park MD 100 AM EDT Sun May 10 2020 Valid 12Z Wed May 13 2020 - 12Z Sun May 17 2020 ...Record low temperatures possible across the East through Wednesday morning, before a gradual warming trend begins... ...Heavy rains possible for South Florida and portions of the central U.S... ...Overview and Weather/Hazard Highlights... Downstream of a persistent Rex Block extending from the North Pacific across Alaska to the Arctic, relatively zonal flow is forecast across the CONUS through much of the extended forecast period, as an upper-level trough pulls away from the Northeast by midweek. Record low temperatures will still be possible across portions of the Mid-Atlantic and Northeast Wed morning before a warming trend begins after that. Heights are forecast to slowly rise across the Southeast/Mid-Atlantic through late in the week as subtropical ridging becomes a bit stronger, while the trailing end of a stationary frontal boundary washes out in the vicinity of South Florida. Moisture/instability focused along this front will keep scattered to numerous showers and thunderstorms a possibility through late in the week for South Florida, with at least locally heavy rain possible through Fri/Sat. An active upper-level jet is forecast across the CONUS northern tier, south of a persistent upper low across central Canada, with a number of relatively small-scale shortwaves traversing the jet. A Pacific frontal boundary expected to persist across the central U.S. through the week will focus multiple waves of low pressure in response to passing shortwave energy, each of which could produce areas of showers and thunderstorms with locally heavy rains and the potential for severe weather for some areas. A Pacific upper low expected to move onshore in the Northwest on Wed (bringing rain and high elevation snows) is expected to weaken somewhat as it crosses the northern Rockies and emerges into the Northern Plains Fri-Sat. The resultant developing wave along the central U.S. front should bring an increase in coverage and perhaps intensity of convection, with the threat for heavy rain potentially increasing by Fri-Sat across the Central/Southern Plains. Models/ensemble show a fair degree of consensus that a stronger Pacific trough/upper low should approach the West Coast next weekend, bringing moving a robust frontal boundary onshore by Sun, along with an increase in precipitation coverage/intensity from northern California to the Pacific Northwest. ...Guidance/Predictability Assessment... Model consensus was sufficient during days 3-4 (Wed-Thu) to base the forecast on a multi-model deterministic blend, including the 12Z ECMWF/CMC and the 18Z GFS. Differences continued to exist with timing/intensity of smaller-scale shortwave energy crossing the CONUS northern tier during that time frame, but the blend should resolve these differences favorably. Starting on day 5 (Fri) and continuing through day 7 (Sun), weighting of ensemble means (ECENS/GEFS) was gradually increased in the forecast through time. Good ensemble consensus at the large scales through next weekend should lend to an ensemble-based forecast that maintains a reasonable amount of detail while resolving growing differences among members/deterministic solutions that exist at smaller scales. Ryan 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