Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Fri May 5 2023 Valid 12Z Mon May 8 2023 - 12Z Fri May 12 2023 ...Synoptic Overview... A quasi-zonal upper level flow pattern with a broad upper ridge over the Ohio Valley and an arriving upper trough near the West Coast is expected for Monday. This trough is forecast to amplify further going into the middle of the week across the Intermountain West, and this will tend to build the ridge that will be downstream across the Plains, and a trough axis across the Northeast states. The western U.S. trough should then reach the western High Plains to close out the work week with an increase in shower and thunderstorm chances across the central U.S., and generally above normal temperatures east of the Rockies in response to the upper ridge aloft. ...Guidance/Predictability Assessment... The 00Z model guidance suite agrees well on the overall synoptic scale pattern across the continental U.S. for the beginning of next week, and a general model blend suffices as a starting point in the forecast process for Monday. The UKMET strays from the consensus by Tuesday across the Northern Rockies with a stronger shortwave trough and faster with the low off the West Coast. The guidance has continued to trend more amplified with the trough reaching the Desert Southwest by Wednesday, with the CMC and ECMWF trending towards the GFS compared to earlier model runs, even though the 00Z ECMWF is still slightly north of the CMC/GFS by Wednesday morning. The models are also trending a little faster bringing the trough across the Rockies, with the 12Z ECENS now slower than the 00Z consensus. Model uncertainty increases going into Friday, but still reasonable on depiction of the main features. The ensemble means accounted for about half of the forecast blend by the second half of the week. ...Weather/Hazards Highlights... Gradually increasing moisture across the eastern half of the nation, along with some weak shortwave disturbances aloft, will tend to result in scattered to numerous showers and thunderstorms developing from the Deep South to the southern Great Lakes. A Marginal Risk for the Day 4 ERO is planned from central Missouri to southern Michigan where the model signal is best for highest QPF, and the potential exists for eventual Slight Risk in future updates once the main QPF axis becomes more apparent. The heaviest convection for Day 5 is currently favored across portions of the Central Plains, and a Marginal Risk area is planned here from central NE/KS to far western KY/TN. Looking ahead to Wednesday and Thursday, a broader expanse of moderate to locally heavy rainfall is becoming more likely from Texas to the Dakotas in association with the ejecting western U.S. storm system. There will likely be enough cold air aloft to support lowering snow levels across portions of the Northern Rockies, and late season snow is probable for the higher elevations of Wyoming and Montana during this time. In terms of temperatures, highs will likely be running 5 to 15 degrees above early May averages from the central/southern Plains to the Ohio Valley to start the week, and then reaching into the Midwest states going into the middle of the week. It should initially be above average across the eastern U.S. before a cold front ushers in a cooler and more refreshing airmass for the Wednesday-Friday time period. It will likely be slightly below average across portions of the West Coast states and the Intermountain West in association with the upper trough, but overnight lows should be closer to average here and generally above average east of the Rockies. 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 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