Extended Forecast Discussion NWS Weather Prediction Center College Park MD 349 PM EDT Tue May 04 2021 Valid 12Z Fri May 07 2021 - 12Z Tue May 11 2021 19Z Update... 00Z/06Z guidance maintains the overall pattern evolution, with good agreement among the GFS/ECWMF and Canadian (less so for the UKMET). Trend has been for a bit quicker pace with the reinforcing trough into/through the Great Basin next Mon/Tue, but at that lead time it was still within the expected error bounds. Utilized a consensus blend approach with increased ensemble weighting (evenly between the GEFS/CMCE and ECENS mean) by next week, but retaining the 06Z GFS and 00Z ECMWF for some details. The previous discussion from 07Z is below. Fracasso ...Weather Pattern and Model Guidance... An amplified upper level pattern will initially be in place across the nation on Friday with well developed troughs near the West Coast and East Coast, with an upper ridge over the Rockies that weakens with time as it emerges over the Plains. As the trough over the Northeast lifts out, a more quasi-zonal pattern is expected by the weekend across the central and eastern U.S., with a second trough building across the West by early next week. At the surface, a quasi-stationary frontal boundary with multiple waves of low pressure along it will be the main weather maker during the medium range period. Models and ensembles remain in above average agreement for the end of the week, although there is some variance with the timing and strength of smaller scale systems, but guidance composites tend to mitigate these differences that is consistent with predictability. The GFS is now solidly in line with keeping the developing low pressure system well offshore the East Coast and taking the storm out to sea for Friday/Saturday, compared to the nor'easter type solution portrayed in earlier model runs. However, the GFS becomes much faster with ejecting the western U.S. trough over the Plains during the Monday/Tuesday time period, whereas the CMC/ECMWF and ensembles are favoring a considerably slower solution. The WPC fronts/pressures forecast was based on a CMC/ECMWF/GFS blend along with some of the ensemble means through Day 5, and then primarily ECMWF/EC mean/GEFS means by days 6 and 7. ...Weather Highlights/Hazards... The best rainfall prospects will likely be from the Central Plains to the Ohio Valley where multiple convective complexes are likely in the vicinity of a slow moving front. The potential exists for 2 to 4 inches of rain, with locally higher amounts, from eastern Kansas to western Kentucky. Some training of thunderstorms may lead to instances of flooding, and this is something that will continue to be monitored in the days ahead. Widespread moderate to locally heavy rain can also be expected from eastern Colorado to central Montana as the upper trough/closed low builds in overhead combined with moist upslope flow. Dry conditions should persist across most of the Intermountain West and the Desert Southwest, as well as the northern Great Lakes. Temperatures will likely be more reminiscent of March across the western High Plains and adjacent portions of the northern Rockies for the Sunday to Tuesday time period, with highs up to 20 degrees below early May averages. This equates to readings maxing out in the mid 40s to mid 50s for many of those areas, and the higher elevations of Wyoming, Montana, and northern Colorado can expect some late season accumulating snow, perhaps exceeding six inches for some of the higher ranges. For the Desert Southwest and Great Basin, the above average readings this week will likely persist until Friday, after which a return to more seasonable conditions can be expected. However, it should remain warm and humid south of the frontal boundary across the Deep South and Gulf Coast region with highs up to 10 degrees above normal. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Central Plains, the Middle/Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Sat-Tue, May 8-May 11. - Flooding occurring or imminent across portions of the Southern Plains. - High winds across portions of California and the Pacific Northwest, Fri-Tue, May 7-May 11. 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