Extended Forecast Discussion NWS Weather Prediction Center College Park MD 128 AM EDT Fri May 08 2020 Valid 12Z Mon May 11 2020 - 12Z Fri May 15 2020 ...Record cold over the central/eastern U.S. through early next week... ...Wet pattern for southern Florida Sunday onward... ...Overview and Guidance/Predictability Assessment... The large scale pattern from the North Pacific across North America will remain relatively stable through the medium range. A Rex Block, comprised of anomalous upper-level ridge extending from Alaska into the Arctic Ocean and a trough between Alaska and Hawaii, is forecast by ensemble guidance to persist through and beyond the medium range forecast period. The implications of this for North America are modestly anticyclonic flow along the CONUS West Coast (an extension of the ridge farther north), and broad cyclonic flow across central/eastern portions of Canada and the U.S. Embedded within the broad cyclonic flow are forecast to be a couple relatively deep/anomalous upper-level lows, one across eastern Canada through much of next week, and another diving south from the arctic into central Canada by late next week. Numerous smaller scale shortwaves are expected to rotate around these larger upper lows, undergoing some degree of amplification from the Midwest/Great Lakes to the Northeast next week. Along the West Coast, an upper low forecast to approach the coast early next week should gradually weaken as it encounters the anticyclonic flow - but a weakened shortwave is still expected to cross the Rockies in a region of more zonal flow and emerge into the central U.S. by mid to late next week, with cyclogenesis along a lingering frontal boundary expected. Models struggled even early in the medium range with the timing and amplitude of multiple shortwaves crossing the Midwest/Great Lakes/Northeast as described above. Run-to-run variability was overall quite high among most guidance. Overall, the ECMWF has shown perhaps somewhat less variability although it has not been immune either. Further, the ECMWF had some degree of ensemble support, and was relatively centered within the ensemble spread through much of the forecast period. The GFS, in addition to high run-to-run variability, strayed quite a bit from ensemble consensus both in the eastern U.S. with the passing shortwaves, and in the West by later next week, bringing an upper trough inland while ensembles all maintain some degree of ridging. At the larger scales, with the Pacific Rex Block persisting through the forecast period, predictability is relatively good based on ensemble means, but this does not extend to the smaller scales where chaotic interactions between arctic and Pacific energy across Canada and the CONUS northern tier make for below average predictability with it comes to the finer-scale forecast details. Thus, opted to lean heavily on the 12Z ECMWF initially on Days 3-4 (Mon-Tue), with a gradual trend toward heavier/majority ensemble means (ECENS and NAEFS) through time during days 5-7 (Wed-Fri). ...Weather/Hazard Highlights... The Alaskan/Arctic ridge and broad cyclonic flow farther east will support well below average temperatures across much of the eastern half of the CONUS through early next week, with record cold temperatures possible for a number of locations. Both high and low temperatures are forecast to be 10 to 20 deg F (in some cases as much as 25 deg) below normal Mon-Tue across much of the central and eastern U.S. Scattered records are possible, including record low minimum and maximum temperatures. Temperatures are forecast to gradually moderate Wed-Thu as upper-level flow becomes a bit more zonal, but the Northeast should remain below average into late next week. In terms of precipitation, South Florida will see persistent showers and thunderstorms through much of next week as a trailing frontal boundary stalls in the vicinity. Locally heavy rains will be possible through much of next week, with some locations perhaps seeing several inches of rain. Areas from central/northern California and the Pacific Northwest to the northern Rockies will see precipitation Mon-Wed as the upper low/shortwave move inland. Showers and thunderstorms with at least locally heavy rain will also be possible across portions of the central/southern Plains Mon-Tue along a quasi-stationary frontal boundary and dry line, with convective activity likely lifting northeastward across the central U.S. by the middle to latter portion of next week as shortwave energy crosses the Rockies and reaches the central U.S., resulting in development of a frontal wave. 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