Extended Forecast Discussion NWS Weather Prediction Center College Park MD 102 AM EDT Sat May 09 2020 Valid 12Z Tue May 12 2020 - 12Z Sat May 16 2020 ...Below average temperatures expected to persist across portions of the central/eastern U.S. through early next week... ...Stalled frontal boundary to bring potentially heavy rains to South Florida next week... ...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 an 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 forecast period, with the ridge component of the block gradually sliding farther north into the Arctic Ocean by late next week. The implications of this downstream are modestly anticyclonic flow along the CONUS West Coast (an extension of the the ridge farther north), and broad cyclonic flow across central/eastern portions of Canada and the U.S. A deep upper-level low initially in place early next week east of Hudson Bay is expected to lift northeastward, eventually into the North Atlantic, leaving broad cyclonic but somewhat zonal flow in its wake. Numerous smaller scale shortwave disturbances are expected to traverse the region of cyclonic flow, from the Midwest/Great Lakes to the Northeast next week. Along the West Coast, an upper low forecast to approach the coast Tue-Wed should gradually weaken as it encounters the modest ridging along the coast - but a weakened shortwave is expected to cross the Rockies and emerge into the central U.S. by the middle of next week, with cyclogenesis along a lingering frontal boundary expected. The resultant frontal wave is forecast to move northeast toward the Great Lakes and Northeast Thu night through Fri/Sat. Despite good large scale agreement, models have continued to struggle with the details of the smaller-scale shortwaves expected to cross the north central/northeastern U.S. through next week. Some improvement/reduction in spread was noted over the last couple model/ensemble cycles, but the timing and amplitude of these features remains somewhat tough to pin down. A multi-model blend approach was preferred for days 3-4 (Tue-Wed), including the 12Z ECMWF/UKMET and 18Z GFS (with a bit more weight toward the ECMWF/UKMET). This blend seemed to reasonably represent the guidance consensus at this time, while smoothing out some of the differences on timing/amplitude of shortwaves. By later in the forecast period, as spread increased, ECENS and GEFS ensemble means were weighted more heavily, with majority ensemble means during days 6-7 (Fri-Sat). Given the relatively good large scale consensus, the ensemble-based blend should favorably resolve differences with continued shortwaves traversing the northern tier, and additional Pacific energy moving into the western U.S., weakening as it encounters the region of anticyclonic flow. ...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 below average on Tue across much of the central and eastern U.S., with temperatures moderating slightly on Wed, and returning closer to seasonal norms by Thu-Fri. Scattered records are possible, including record low minimum and maximum temperatures. In terms of precipitation, South Florida will see persistent showers and thunderstorms through much of next week as a frontal boundary stalls in the vicinity. Locally heavy rains will be possible through much of next week, with a few locations perhaps seeing several inches of rain. Areas from central/northern California and the Pacific Northwest to the northern Rockies will see fairly widespread precipitation (snows are likely at the higher elevations of the northern Rockies) Tue-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 southern Plains/Mississippi Valley on Tue along a quasi-stationary frontal boundary, with convective activity likely expanding across the central U.S. (and eventually the Northeast) by the middle to latter portion of next week as shortwave energy crosses the Rockies the frontal wave develops. Given the potential for multiple rounds of showers and thunderstorms to affect the same areas over the course of several days - areas of flooding are a possibility. 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