Extended Forecast Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Thu May 05 2022 Valid 12Z Sun May 08 2022 - 12Z Thu May 12 2022 ...Heavy snow threat over a cold north-central Great Basin/Rockies to contrast north-central U.S. convection and south-central U.S to Midwest record heat as part of Omega-style block development over the lower 48 and adjoining oceans this weekend into next week... 18Z Update: The 12Z model guidance suite is in very good synoptic scale agreement for Sunday and into early Tuesday as an omega block pattern develops over the central U.S., so a multi-deterministic blend sufficed as a starting point in the forecast process for that time period. Noticeable differences emerge with the 12Z CMC across the eastern Pacific and southwestern U.S. by Wednesday, and it is weaker with the shortwave trough ejecting eastward across the Rockies compared to the consensus. The past two runs of the CMC have trended with the GFS and ECMWF regarding a retrograding low over the western Atlantic and towards the Southeast Coast by the end of the week, with the ECMWF taking this low farthest west. Model preferences for the latter half of the period maintained some of the GFS/ECMWF whilst gradually increasing use of the ensemble means and some previous WPC continuity. In terms of sensible weather, the main story will likely be the early season heat wave that is forecast to encompass much of the south-central U.S. and into the Midwest, with afternoon high temperatures running up to 20 degrees above normal, and the potential for multiple daily record highs to be set. The previous forecast discussion follows below. /Hamrick ------------------------- ...Pattern Overview and Guidance Evaluation... A composite of the 18 UTC GFS/GEFS along with the 12 UTC ECMWF/UKMET/Canadian, ECMWF ensemble mean and 01 UTC National Blend of Models seems to present a reasonable forecast basis while tending to mitigate embedded system variance consistent with a pattern with near normal predictability Sunday into Monday. The GFS/ECMWF and GEFS/ECMWF ensemble means remain best clustered and in line with WPC continuity and the NBM Tuesday-next Thursday. Despite better similarities, recent run-run guidance continuity issues still suggests some caution with embedded feature details, especially days 5-7. Most of the newer 00 UTC guidance maintain similar trends from earlier guidance, with the Canadian remaining the most out of phase with other guidance with the timing/emphasis on systems digging into the western U.S.upper trough. ...Weather/Threat Highlights... A closed upper trough and well organized surface low/frontal system will track off the Mid-Atlantic Sunday. The low stalls and retrogrades over the western Atlantic back toward the Southeast coast mid-later next week with wrapping rains within a developing Omega-style block. Robust digging of a series of dynamic upper systems will lead to a dramatic change over much of the West later this weekend into next week as an amplified mean upper trough and wavy frontal systems become established over the region. This will support significant cooling to include spotty record values along with a widespread precipitation focus from the Northwest/Sierra through the north-central Great Basin/Rockies. This will include a substantial and lingering threat for widespread heavy snows as further enhanced in favored terrain. Expect temperatures to range as much as 10-20 degrees below normal. Upper system ejections channeled between amplified eastern Pacific upper ridging/upper troughing over the West and amplified east-central U.S. upper ridging as part of the aforementioned Omega-style block over the lower 48 and adjoining oceans will trap a slow moving and wavy frontal zone over the north-central U.S. to support an emerging but uncertain convection/rainfall pattern. This offers a runoff threat given moist ambient Northern Plains soil conditions. The Storm Prediction Center also highlights a concern for severe weather into the eastern Central Plains for at least Sunday and Monday. The pattern meanwhile favors record high temperatures over the south-central U.S., with pre-frontal warmth gradually building northeastward next week through the MS/TN Valleys/Midwest under the upper ridge, in stark contrast to the wintry conditions over the West and moderated/cooled high pressure dammed airmass to linger over the East on the backside of a difficult to dislodge western Atlantic circulation. Schichtel/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 precipitation across portions of California, the Central Great Basin, and the Pacific Northwest, Sun-Mon, May 8-May 9. - Heavy snow across portions of the Central/Northern Rockies, the Northern Plains, and the Northern Great Basin, Sun-Mon, May 8-May 9. - Severe weather across portions of the Central/Northern Plains, the Middle/Upper Mississippi Valley, Mon, May 9. - Flooding possible across portions of the Great Plains, the Mississippi Valley, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Great Plains, and the Mississippi Valley. - Flooding likely across portions of the Great Plains, and the Middle/Lower Mississippi Valley. - Excessive heat across portions of the Central/Southern Plains, and the Lower Mississippi Valley, Sun-Tue, May 8-May 10. - High winds across portions of the Great Plains, the Rockies, the Central Great Basin, the Mid-Atlantic, California, and the Southwest, Sun-Mon, May 8-May 9. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Sun, May 8. - Flooding possible across portions of mainland Alaska, Sun-Mon, May 8-May 9. - Flooding possible across portions of mainland Alaska, Sun-Thu, May 8-May 12. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml