Extended Forecast Discussion NWS Weather Prediction Center College Park MD 241 PM EDT Mon May 15 2023 Valid 12Z Thu May 18 2023 - 12Z Mon May 22 2023 ...Warmer than average temperatures will persist across the Northwest... ...Portions of the south-central Rockies and Plains could see flash flooding Thursday-Friday... ...Overview... The overall weather pattern this week and into early next week should remain fairly similar with time across the country with an anomalously strong upper ridge over the West causing warm to hot temperatures there, while mean troughing is likely downstream across eastern North America where individual systems will be moving through the progressive flow. Individual shortwaves will support cold fronts that should push southeastward to the east of the Rockies and produce areas of showers and thunderstorms, including the possibility for some enhanced rainfall amounts potentially causing flash flooding in the south-central Rockies and Plains late this workweek. ...Guidance/Predictability Assessment... Details within the mean troughing downstream and related frontal system in the East show variability, while the eastern Pacific also becomes an area of focus from a model diagnostics standpoint, with the potential for an upper low offshore of Canada to finally spin some energy/troughing toward the Pacific Northwest for some cooler temperatures with a cold front or two as the prevailing ridge weakens a bit and pushes eastward early next week. Overall, the 06z GFS is on the fast side of the guidance. The 00z Canadian was uniquely slow in the Eastern US and could not be used. The breadth of the ridge upstream suggests a broader trough with decent flow through it, arguing against the Canadian. Overall, a compromise of the 00z ECMWF and 06z GFS works well across the country for 500 hPa heights and pressures, with some input from the 00z ECMWF and NAEFS means late in the period to deal with uncertainty. QPF in the Southeast looks overdone in the 00z ECMWF. While the approach of a shortwave trough should spur a surface low which should be convective, it should either remain stationary and/or drift southeast as the upper trough moves overhead and passes by. Used minimal 00z ECMWF weight in the QPF. ...Weather/Hazards Highlights... A front looks hangs up in the south-central Rockies on Thursday and Friday, with upslope flow on its backside in an environment with 90+ percentile moisture anomalies. This would also be atop wet ground with high streamflows that could exacerbate flash flooding risk. Farther east into the south-central Plains, the west-east orientation of the front and its possibly slow movement late Thursday could lead to training of storms with possibly high rain rates that form on the cusp of the instability/moisture near the boundary. The Slight Risk for Day 4/Thursday in the ERO is a little smaller but similar in placement, which is a better match to the best heavy rain signal in the 15/12z guidance. The front will shift south and east late week, with a Marginal Risk in the Day 5/Friday ERO across portions of the the south-central Rockies and the south-central Plains/Mississippi Valley. It appears the front may begin to move faster for Day 5, precluding an embedded Slight Risk, as storms with high rain rates may have less opportunity to train. The cold front will move more progressively eastward across the Midwest/Ohio Valley on Friday with enhanced rainfall chances shifting into the East on Saturday along and ahead of the front, perhaps lingering along parts of the East Coast Sunday. Elsewhere, showers and storms may have some focus along a frontal boundary in the Southeast on Thursday in particular. With ample moisture and instability in place, there could be some localized heavy rain in and around southern Georgia/northern Florida. While amounts are not currently expected to surpass the high Flash Flood Guidance, it may fall fast enough to overwhelm urban environments, so a Marginal Risk was added for Day 4/Thursday into early Friday. Scattered showers and thunderstorms are also possible in the West at times late this week into early next week especially in the afternoons/evenings with a common diurnal cycle, as a monsoon-like pattern sets up with anomalously high moisture remaining over a large portion of the western U.S. under the base of the upper ridge. Temperature-wise, the persistent large scale western North America ridge will support warm to hot temperatures across most of the West. The Pacific Northwest is likely to be hottest around Friday (during the medium range period), with highs around 20 degrees above average before some gradual moderation late this weekend. However, interior areas like the Great Basin can expect the heat to persist through the weekend. Highs in the 90s there could be record-setting, especially Friday and Saturday, while lows in the 50s and 60s could break records for warm low temperatures across an increasing area of the West. Highs above 100F are likely in the Desert Southwest, par for the course for May. The Northeast should see chilly temperatures Thursday morning, possibly approaching the freezing mark in interior areas and setting a few records. Periods of rain and clouds and then post-frontal cooler air could lead to temperatures slightly below average in the Plains at times. Roth/Tate 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