Extended Forecast Discussion NWS Weather Prediction Center College Park MD 101 AM EDT Tue May 12 2020 Valid 12Z Fri May 15 2020 - 12Z Tue May 19 2020 ...Active weather pattern expected to develop for portions of the central and east central U.S. later this week... ...Overview and Weather/Hazard Highlights... A Rex Block extending from the North Pacific to the Arctic which has been persistent in recent days appears likely to continue, but become somewhat reoriented during the medium range. The block should weaken somewhat as the area of negative height anomalies in the North Pacific shifts east into the western U.S. Another area of persistent negative height anomalies looks to become established across the northwest Pacific, changing the orientation of the block. The implications of this will be a trend toward at least a somewhat more amplified pattern across the CONUS, after beginning the forecast period a bit more zonal, as troughing becomes more persistent in the West and the subtropical ridge builds northward initially into the Southeast, and shifting to the south central U.S. Sun through early next week. Shortwave energy ejecting from the broad western trough will interact with a wavy frontal boundary and then traverse the northern periphery of the ridge. This setup will keep areas from the Southern/Central Plains to the Ohio Valley/Great Lakes in a relatively active weather pattern with several rounds of showers and thunderstorms possible through the medium range. Rising heights over the southern tier will bring warming temperatures. Highs are forecast to be 5 to 10 deg F above average Fri through the weekend from parts of the Lower Mississippi Valley/Tennessee Valley to the Mid-Atlantic. As the strongest upper-level ridging shifts to the south central U.S. early next week, temperatures should warm across much of the Southern/Central Plains, with highs forecast to reach 10 to 20 deg above average. A cold front is expected to bring showers and thunderstorms from the Mid-Mississippi Valley to the Great Lakes on Fri (before the boundary becomes stationary), with areas of heavy rainfall possible. As more pronounced shortwave energy moves from the northern Rockies into the central U.S. late Fri into Sat, the Central/Southern Plains are expected to become increasingly active, with the potential for multiple waves of low pressure to move along a quasi-stationary front, eventually consolidating into a more organized frontal wave reaching the Great Lakes Sun morning. Areas of heavy rainfall as well as strong to severe thunderstorms will be possible, although the specific areas affected will likely depend on convection-scale processes with low predictability in the medium range. Farther west, heights should fall across the Northwest late Sat through Tue as the aforementioned southern component of the Rex Block shifts eastward. A vigorous Pacific cold front is expected to push inland along with the height falls, bringing dropping temperatures and widespread precipitation (rain and high elevation snows) to areas from central California and the Pacific Northwest eastward into the northern Rockies. High temperatures by Sun-Tue are forecast to be 5 to 10 deg below average across portions of the West Coast and interior Northwest. Elsewhere, the trailing end of a stationary frontal boundary is forecast to linger in the vicinity of South Florida and the Bahamas through late in the week, before an area of low pressure develops along the boundary and moves northeastward away from the state. This boundary is expected to focus showers and thunderstorms, with the potential for locally heavy rains across South Florida through Fri before activity shifts east of the state over the weekend. The National Hurricane Center and Tropical Analysis and Forecast Branch (TAFB) are monitoring this area for potential development as a sub-tropical or tropical system by this weekend. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/GFS was used as a basis for the forecast through much of the extended forecast period, with increasing weight through time placed on ECENS/GEFS ensemble means starting on day 5 (Sun) and continuing through day 7 (Tue). The 12Z run of the GFS was preferred over the 18Z run primarily for handling of the deep West Coast trough late in the weekend through early next week. With this feature, the 18Z GFS was on the faster side of the spread with moving the feature inland, while the 12Z run was much closer to the ECMWF and the ensemble means. Off the Southeast coast, a middle ground solution between the more aggressive GFS and less aggressive ECMWF solutions was preferred given the timing uncertainty in development. 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