Extended Forecast Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Wed May 03 2023 Valid 12Z Sat May 06 2023 - 12Z Wed May 10 2023 ...Overview... The flow pattern over North America from the weekend into early next week is forecast to slowly evolve away from stubborn blocking towards mean ridging building over the central/eastern CONUS and mean troughing over the central/western CONUS. Conditions will be cool and wet through the weekend in the West under the influence of the upper-level troughing. Ejecting shortwave energy over the central/eastern U.S. will trigger multiple areas of showers and thunderstorms very broadly from the Plains to the east-central U.S.. ...Guidance/Predictability Assessment... Guidance shows a generally similar and slowly evolving larger scale pattern through the weekend into Monday despite transitional blocky flow over North America and adjoining waters. Even so, ample differences and run to run consistency issues remain with local weather focusing smaller scale systems embedded within the flow. A composite blend of reasonably compatible model guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian seems to provide a good forecast basis that tends to mitigate smaller scale variance consistent with system predictability, but WPC system continuity is not stellar. This solution though does have good ensemble and National Blend of Models (NBM) support. Ensemble spaghetti and cluster plots show enough of an increase in forecast spread/uncertainly into next week to then switch preference to the still compatible 06 UTC GEFS/00 UTC ECMWF ensemble means and the NBM. Run to run continuity with newer 12 UTC guidance is not encouraging as early as this weekend however, lowering forecast confidence. ...Weather/Hazards Highlights... The overall pattern looks wet for the central to southeastern CONUS through the period as mean southwesterly upper flow sets in, with two areas of potentially greater focus of showers/storms. Moist, upslope flow along portions of the Northern/Central High Plains ahead of uncertain surface frontal systems passing through the Plains/Upper Midwest look to bring multiple days of rainfall this weekend into early next week. Further South, Gulf moisture will spread northward across the Central/Southern Plains and Middle/Lower Mississippi Valley into the Southeast/East leading to a broad region of showers and thunderstorms. In both cases, the evolution of the pattern has remained rather nebulous the last few days as the guidance has varied on the presence and timing of embedded shortwave energy aloft, lessening confidence on the specifics of regions of heavier rainfall and potential flash flooding concerns. Currently, the greatest chance of locally heavy rainfall is on Sunday over portions of the Upper Missouri Valley and for early next week down to the Lower MS/TN Valleys. High temperatures will be seasonably warm for early May from the Plains to the Southeast/Mid-Atlantic under the influence of mean ridging aloft, with some highs running 15+ degrees above average in the Central Plains this weekend. The persistent cool/wet pattern over the Great Lakes and Northeast due to a stubborn upper-level low this past week/weekend will finally moderate this upcoming weekend, with high temperatures returning to near to slightly above average levels into early next week. Temperatures may eventually trend cooler depending on the evolution of a potential upper-level closed low to the northeast. Precipitation chances will remain low through the weekend under the influence of a broad area of high pressure in place. Shower and thunderstorm chances will increase for the Carolinas south into Florida this weekend due to a lingering frontal boundary and potential downstream MCS activity. Periodic moderate rainfall chances are now slated to linger through the weekend for the Pacific Northwest/Great Basin/Northern Rockies as upper-level troughs/surface systems lifts over the region. Additional shortwave impulses/surface frontal systems pushing through the West will keep some precipitation chances into early next week, particularly across the northern tier. Some snow showers will be possible at higher mountain elevations. Temperatures will remain a cool, especially across the West Coast states through the weekend into next week. Putnam/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 Hazards: - Flooding possible across portions of the Middle Mississippi Valley, the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding occurring or imminent across portions of the Northeast, the Northern Plains, the Northern Rockies, the Middle Mississippi Valley, the Mid-Atlantic, the Northern Great Basin, and the Upper Mississippi Valley. - Flooding likely across portions of the Middle Mississippi Valley, the Pacific Northwest, the Northern Rockies, the Northern Great Basin, and the Northern Plains. 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