Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sun May 7 2023 Valid 12Z Wed May 10 2023 - 12Z Sun May 14 2023 ***Periods of heavy rain expected for portions of the southern Plains and also Montana and North Dakota*** ...Overview... A well defined trough expected over the Intermountain West mid-week is expected to slowly lift northeastward across the Rockies and then the northern Plains by Friday. This will tend to build the ridge that will be downstream from the Gulf Coast to the Midwest, and a trough axis across the Northeast states that exits on Thursday with a second trough building in over southeast Canada next weekend. There will be an increase in shower and thunderstorm activity across much of 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 has a good overall synoptic scale depiction of the trough crossing the Intermountain West early in the period, but some mesoscale differences are apparent with shortwave energy and convective evolution across the Gulf Coast region. By Thursday the GFS and the ensemble means are farther north with the closed upper low over the western High Plains compared to the ECMWF/CMC solutions, and this remains the case going into Friday. Looking ahead to next weekend, the models agree on a strong upper ridge building into southwestern Canada, and the latest CMC has trended closer to the model consensus compared to its 12Z run. The GFS is a little slower with bringing the northern Plains low eastward by Sunday, but for a day 7 forecast it is close enough to the consensus to include it in the forecast blend. A multi-deterministic model blend was used through Thursday, followed by gradually increasing contributions from the ensemble means by the weekend and dropping use of the 12Z CMC after Friday owing to the greater western U.S. differences in that model run. ...Weather/Hazards Highlights... Scattered to numerous showers and thunderstorms are expected to develop across much of the Plains states going into the second half of the work week, as moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the western High Plains and the upper trough approaching from the West. The new Day 4 (12Z Wednesday-12Z Thursday) Excessive Rainfall Outlook plans to introduce a Slight Risk area over parts of the ArkLaTex region as multiple models are now indicating a broken axis of 2 to locally 4 inch rainfall totals as multiple MCSs develop in the warm sector of the low. The heaviest rainfall during the Day 5 (12Z Thursday-12Z Friday) period is currently favored across much of central/eastern Montana and extending into western North Dakota, and a Slight Risk is planned for portions of those areas owing to widespread 1-2 inch rainfall on top of expected moderate to heavy rainfall the day before. Specifics over the northern High Plains will depend on the exact track of the upper low expected to cross the region and how long it potentially stalls. Going into next weekend, portions of the southern Plains may get additional rounds of heavier rainfall, with several inches of rain possible in some areas of Texas. As the upper low crosses the Great Basin and northern Rockies, there should be sufficient cold air aloft to support late season higher elevation snow, especially over the higher mountain ranges of northern Wyoming and western Montana. In terms of temperatures, highs will likely be running 5 to 15 degrees above early May averages from the central/northern Plains to the Deep South for the middle of the week, and then reaching into the Midwest states and the Great Lakes/Northeast region going into the end of the week. Warmth builds across the Pacific Northwest by Friday into the weekend with highs potentially up to 20 degrees above mid-May averages across western Washington and Oregon. It will likely be slightly below average across portions of the West Coast states and the Intermountain West in association with the upper trough through Thursday before a moderating trend commences, but overnight lows should be closer to average here and generally above average east of the Rockies with increased cloud cover and humidity. 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