Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Fri May 05 2023 Valid 12Z Mon May 08 2023 - 12Z Fri May 12 2023 ...Overview... A quasi-zonal upper level flow pattern with a broad, low-amplitude 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 and close off a low through midweek in the Intermountain West, which should build ridging downstream across the Plains, while a trough axis remains atop the Northeast. This pattern should slowly translate east by late week, with the western trough/low shifting into the High Plains and ridging across the east-central U.S. pushing the trough axis out of the Northeast eventually. Warm to hot temperatures are likely east of the Rockies in response to the upper ridge aloft, while shower and thunderstorm chances are likely to increase across the central U.S. for the latter half of next week. ...Guidance/Predictability Assessment... Model guidance agrees well on the overall synoptic pattern described above through the early part of the period. In terms of smaller-scale features, the 00Z/06Z model guidance varied somewhat with the position of a shortwave in the north-central U.S./south-central Canada Monday-Wednesday, with the 00Z ECMWF perhaps holding onto a small closed low feature too long. Differences are also evident with surface frontal positions in the eastern half of the U.S. But with the overarching features in place, the updated WPC forecast utilized a blend of the 00Z/06Z GFS and 00Z ECMWF and CMC early on. By mid- to late week, models have waffled a bit with how early to close off a low in the West within the trough axis (most end up having a closed low by Thursday) and with its track eastward. The 00Z/06Z model guidance was reasonably agreeable with the trough's timing trending a little faster, as well as showing good consensus on the downstream ridging, so a majority deterministic model blend was able to be used through the period but including some ensemble mean guidance. However, the incoming 12Z model cycle shows some divergence in the trough's timing/track east along with its associated surface low, with the GFS speeding up and the ECMWF slowing down. So there could certainly be more shifts in the forecast going forward. ...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 (Monday-Monday night) has been maintained for portions of the Mid-Mississippi Valley into the Ohio Valley and lower Michigan, with a minor extension eastward per latest model guidance. There may be more than one axis of heavy rainfall within this region but the placement remains quite uncertain for now. Some rain is forecast to shift into the Eastern Seaboard on Tuesday, but beginning late Tuesday and beyond, the central U.S. should be the main focus for higher rainfall amounts as moisture levels increase ahead of the amplifying western trough. For Day 5/Tuesday, a Marginal Risk of excessive rainfall will be introduced across much of Texas, the first of likely multiple days of increased convection there with ample moisture and instability in place. Farther north, a Marginal Risk is in place for portions of the central Plains where there is some agreement for heavy rain that could cause isolated flash flooding. Rain and storms are likely to extend southeast of there, but models have not narrowed in on a location/axis for impactful storms yet, as it will depend on small-scale frontal positions and other model differences. Looking ahead to Wednesday and Thursday, a broader expanse of moderate to locally heavy rainfall should continue 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 workweek, and then reaching into the Midwest states going into the middle of the week. Portions of Texas into the Deep South could reach or exceed 90 degrees. It should initially be above average across the eastern U.S. before a cold front ushers in a cooler and drier 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. Tate/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