Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Sun May 01 2022 Valid 12Z Wed May 04 2022 - 12Z Sun May 08 2022 ...Model Guidance Evaluation... Guidance remains varied with the progression of a closed upper low tracking out of the West into the Plains and eastward, with the GFS/GEFS mean (including the new 12z runs today) still on the fast side compared to the better clustered and slower 00/12 UTC solutions of the ECMWF/UKMET/Canadian and ECMWF ensemble mean. The WPC forecast continues to lean towards the slower side of the forecast envelope, given continuity, but also the general nature of a separated southern stream flow with these embedded closed systems. By late period, some larger scale differences begin to emerge between the CMC and ECMWF with the EC wanting to maintain a stronger/closed low while the CMC opens up into general amplified troughing along the East Coast. Felt leaning towards the ensemble means by next weekend helped mitigate these variances. This general combination also worked well for the rest of the country and maintains good WPC continuity. Out West, there is a general theme of a leading shortwave reaching the coast Thursday and then another arriving around Saturday. Models/ensembles continue to vary on amplitude and timing for the later troughing, though the most notable outlier continues to be the CMC which features a faster and much weaker/less amplified progression of the Saturday wave into the West, with a faster system upstream dropping out of the Gulf of Alaska by Sunday compared to the ECMWF and GFS and the ensembles. ...Pattern Overview and Weather/Threat Highlights... An active spring pattern continues across much of the lower 48 states this week. A leading system into the Northeast should focus some moderate rains on Wednesday, with cooling post-frontal Canadian high pressure settling in the wake of the main low. Meanwhile, a closed upper low forecast to track through the central Rockies midweek and associated surface system development will spread rain and higher elevation snow across the northern-central Rockies and High Plains, with some locally terrain enhanced totals. This system will then emerge over the Plains and present a potentially heavy rainfall threat, with the WPC experimental Excessive Rainfall Outlook still showing a Slight Risk over/near parts of the south-central Plains Wednesday into Thursday. The threat signal is stronger than average as the southern stream closed upper system and upper diffluence/instability combine with increasing inflow of Gulf moisture into a wavy front. The Storm Prediction Center is also monitoring severe threats for this area Wednesday-Thursday. Organized rain should translate generally eastward through the rest of the week with low/frontal system progression that will focus some widespread moderate to heavy amounts. The most likely axis of highest rainfall totals extends from the mid-lower MS and OH Valleys through the central Appalachians and Mid-Atlantic given expected upper low/trough and surface system track and trailing cold front. Upstream, a moderately amplified upper trough and surface frontal system forecast to reach the West Coast and progress inland Thursday into next weekend should bring a renewed precipitation focus into the Pacific Northwest and then farther eastward. The southern extent of precipitation will depend on the upper trough's precise amplitude which has been quite uncertain recently in guidance, but latest trends suggest that most moisture should be confined to the northern half of the West. The upper dynamics/surface front could reach far enough east to start producing at least scattered rainfall over the northern High Plains by next weekend. Expect renewed potential again by next weekend to progress into and across the West with another frontal surge as unsettling upper trough energies earnestly dig into the region. Santorelli/Schichtel 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 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