Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Tue May 02 2023 Valid 12Z Fri May 05 2023 - 12Z Tue May 09 2023 ...Cool/wet pattern for the Pacific Northwest/Great Basin/Rockies into the weekend with localized flooding concerns... ...Overview... The upper level pattern is forecast to evolve from the stubborn omega block centered over the CONUS this past week/weekend and towards mean ridging building over the central/eastern CONUS and mean troughing over the central/western CONUS. Conditions will be cool and wet in the West under the influence of the upper-level troughing, with heavy rainfall on top of mountain snow melt leading to localized flooding concerns in the Great Basin/Northern Rockies. Moisture sourced from the Gulf and Pacific will bring the potential for multiple regions of locally heavy rain to the Plains/Mississippi Valley through the weekend and into next week depending on the evolution of shortwave energy aloft. ...Guidance/Predictability Assessment... The 00z/06z guidance package did a good job of capturing the overall synoptic evolution across the CONUS early in the period before model spread increased toward the end. A general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS were utilized through day 4 before introducing the ensemble means incrementally through day 7 and phasing out the 06z GFS. The 06z GFS/GEFS were weighted less on days 6 and 7 due to run to run and ensemble member inconsistencies with respect to the timing of upper-level energy and moving across the West and Central U.S.. The 00z EC/ECE appeared to have better run-run consistency, ensemble member agreement and closer clustering with the 00z CMC/CMCE. ...Weather/Hazards Highlights... Precipitation chances will remain high and widespread for the Pacific Northwest/Great Basin/Northern Rockies late this week/weekend as an upper-level wave and multiple frontal systems impact the region. Rainfall may be locally heavy at times, with some scattered thunderstorm chances. The rainfall on top of ongoing snow melt may lead to some flooding concerns in terrain sensitive areas around the foothills of mountain ranges as well as along local streams/rivers Friday, with a broad "Marginal Risk" Excessive Rainfall Outlook in place. Some snow showers will be possible at higher mountain elevations. Temperatures will remain a bit cool more broadly across the West, with highs running 5-10 degrees below average through the period. Moist, upslope flow settling in along portions of the Northern/Central High Plains ahead of a potential better organized surface frontal system passing through the Plains/Upper Midwest look to bring multiple days of rainfall Friday-Sunday. A "Marginal Risk" has been maintained for portions of the Northern/Central Plains Friday where more organized convection may produce some locally heavy rainfall totals. Further South, Gulf moisture flowing northward as well as additional moisture sourced from the Pacific will spread northeastward across the Central/Southern Plains and Middle/Lower Mississippi Valley into the Southeast late week leading to a broad region of showers and thunderstorms. The evolution of the pattern has remained rather nebulous the last few days as the guidance has varied on the timing and evolution of embedded shortwave energy aloft, lessening confidence on the specifics of regions of heavier rainfall and potential flash flooding concerns. However, the guidance has shown the highest confidence in widespread convection and high QPF amounts from the Middle Mississippi to Tennessee Valley Friday, with the potential for training appearing sufficient to support an upgrade to a "Slight Risk" area. Additional heavy rainfall chances look possible over the weekend more broadly across the central/southeastern U.S., particularly the Lower Mississippi Valley, but confidence remains low given the guidance spread in how the surface pattern and supporting shortwave dynamics will evolve. High temperatures will be seasonably warm for early May from the Plains to the Southeast under the influence of mean ridging aloft, with some highs running 10-15 degrees above average in the Central Plains. 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. Precipitation chances will remain low 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, with amounts and coverage likely dictated by a weak wave aloft and surface frontal boundary influence that remain hard to pin down at this timeframe. Kebede/Putnam 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