Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun May 14 2023 Valid 12Z Wed May 17 2023 - 12Z Sun May 21 2023 ...Warmer than average temperatures will persist across the Northwest... ...Overview... The overall weather pattern through the week into next weekend should remain fairly stagnant across the country. An anomalously strong upper ridge over the western half of the U.S. and into western Canada will be persistent aside from an upper low gradually shearing out/weakening atop the Northwest through around midweek. This pattern will maintain warm to hot temperatures across the Northwest and to a lesser extent over other parts of the West. Downstream, mean troughing is likely across eastern North America, with individual shortwaves supporting 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 in the south-central Rockies/High Plains late this week. ...Guidance/Predictability Assessment... Model guidance continues to show general agreement on the large scale but lingering differences in the middle and smaller scales of the upper-level and surface features. As one spoke of upper troughing lifts out of the Northeast on Wednesday, another compact upper low is likely to drop through south-central Canada and atop the Great Lakes late Thursday into Friday. There remain some minor timing differences in the 12/18Z model cycle with this feature and its surface low reflection--the GFS runs and the 12Z CMC were a bit slower than the UKMET and ECMWF. As all were within the spread of the ensemble members envelope, a middle ground for timing seemed best. The 12Z CMC and the newer 00Z CMC run open up the upper low more quickly and the 00Z shows more phasing than the other guidance by Friday-Saturday, which affects the timing of the shortwave eastward over the weekend. Meanwhile low predictability smaller-scale features continue to plague the forecast from the southern half of the Plains eastward. Overall, recent models have trended toward a shortwave moving more quickly eastward midweek compared to previous cycles, lessening QPF over the the south-central U.S. and raising it into the Southeast for Wednesday. Additional refinements will likely be needed even into the short term with the small-scale elements of the forecast causing QPF and other differences through the period. Then models handle the potential for upper troughing influence to approach the Northwest this weekend differently, with low confidence on any particular solution. The WPC forecast was based on a multi-model deterministic blend of the 12Z and 18Z model guidance early in the period. Gradually increased the GEFS and EC ensemble means through the latter part of the period to just over half by day 7 given increasing uncertainty. ...Weather/Hazards Highlights... As a west-east frontal boundary lingers near the Gulf Coast on Wednesday, scattered thunderstorms are possible from the south-central U.S. into the Southeast. This forecast favored more rainfall farther east across the Southeast rather than back across Texas per the newer model guidance. With amounts trending down considerably in Texas, the Marginal Risk was eliminated there, and areas farther east should be less sensitive to rain. Plus, this trend for heavier QPF there in the Southeast is somewhat new, so did not want to add any ERO risk at this lead time. However, instability and moisture in place means rain rates could be high, and isolated flash flooding cannot be completely ruled out, so a Marginal Risk may have to be added there in the future. Farther north, showers and storms are likely to occur ahead of a cold front dropping south into the north-central CONUS. On Wednesday storms look progressive enough that they may not cause flash flooding issues in the northern Plains, but by Thursday the front could hang up a bit in the south-central Rockies/High Plains. This stalling front and moisture anomalies that could be above the 90th percentile could lead to some flash flooding issues, especially where upslope flow could enhance rain amounts in the higher elevations of the central Rockies. This would also be atop wet ground with high streamflows that could exacerbate the risk. The pattern by Day 5/Thursday in some ways seems more predictable than the short range into Day 4 since it is driven by the relatively larger-scale features of the upper low and surface frontal system. Thus there seems to be enough confidence for a Slight Risk in the Day 5 ERO for the central Rockies with a Marginal Risk extending into much of the south-central Plains and southern Rockies downstream. Some rain could linger in those areas into Friday as well, while farther north the cold front should be more progressive toward the east, pushing rain chances into the east-central U.S. on Friday and into the Eastern Seaboard on Saturday. Scattered showers may also be possible for parts of the interior West by late week. Temperature-wise, expect warm to hot temperatures to persist across the West as large scale upper ridging prevails on the whole. The Pacific Northwest should see high temperatures around 15-25 degrees above average for Wednesday-Thursday along with lows in the 50s and 60s that could set record high minimum temperatures. These coastal areas could see some gradual moderation closer to normal for late week into the weekend, but interior areas like the Great Basin can expect much above average temperatures to persist into the weekend, with highs into the 90s possibly setting records as well. Highs in the 90s and even nearing 100F are possible in the California Central Valley, with highs above 100F in the Desert Southwest on Wednesday (though the latter is less anomalous for May). The north-central U.S. could see above average temperatures by about 10F on Wednesday, followed by a cooler trend Thursday-Friday after the frontal passage, but possibly warming up again by the weekend. The southern tier however may see temperatures around to slightly below average given the periodic rain chances. The Northeast could see a cool Wednesday and Thursday morning with temperatures around 10-15F below normal, but should moderate to near normal after that. 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