Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM 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... ...The south-central Rockies/Plains could see heavy rainfall around Thursday-Friday... ...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... As has been the case in recent days, latest guidance agrees well on the overall mean pattern that has above average predictability given the large scale of the western ridge/eastern trough configuration. There are three primary medium-scale embedded or peripheral aspects of the forecast that show spread and variability: the system dropping into the northern tier U.S./Great Lakes late this week, shortwave energy crossing the southeast quadrant of the lower 48 mid-late week, and how much northeastern Pacific energy may begin to push into the western Canada/Pacific Northwest part of the ridge by around next Sunday. For the Canadian system reaching over or near the northern tier/Great Lakes late in the week, there is reasonable clustering among the 00Z/06Z runs for a closed or nearly closed upper low into day 4 Thursday but then solutions diverge with respect to when/how much phasing occurs with flow to the north. The past three ECMWF runs have kept the two streams separated into Saturday while the past 24 hours of GFS runs through 00Z were trending in that manner (with the 00Z run actually tracking the upper low south of the 00Z ECMWF). To varying degrees the 00Z CMC/UKMET and 06Z GFS all depicted earlier phasing aloft and increasingly faster surface low progression through eastern Canada instead of a slower path across New England. The new 12Z cycle does little to improve confidence. The CMC has reverted back to an extremely slow timing (a trait of one or more earlier CMC runs) while the GFS still opens the system up after early Friday but then pulls off additional energy to the south by Sunday. The 12Z ECMWF has trended notably slower with no phasing into Sunday. A blended approach based on 00Z/06Z guidance tilted closer to the 00Z ECMWF idea later in the period, nudging continuity a bit faster with the main surface system. The new runs suggest the timing may revert back though. Farther south, 00Z/06Z models have made a pronounced switch for shortwave details from the central-southern Plains eastward. The GFS had generally been on its own with greater emphasis on a compact central Plains into Southeast feature relative to a separate one emerging from Texas. However the 00Z runs gravitated to a scenario much more like the GFS. New 12Z runs appear similar (except for the CMC whose slower timing along the northern tier begins to have some effect after Wednesday) with the shortwave but in general guidance has been very inconsistent with associated QPF details over the Gulf Coast/Southeast. The 00Z/06Z guidance suggests that the associated surface wave could ultimately lift northward near the East Coast ahead of the approaching northern stream system. While reflected as a result of following the most compatible model/mean cluster, confidence for this detail is fairly low. By next weekend the 00Z CMC mean and latest GFS/GEFS mean runs bring more Pacific shortwave energy into the western Canada/Pacific Northwest part of the western North America ridge versus most other solutions. For now prefer an intermediate solution that shows only a hint of such energy (by way of tilting somewhat more in the ECMWF/ECMWF mean direction relative to the GFS/GEFS). In recent days, individual solutions that tried to weaken the ridge ultimately adjusted back to a stronger ridge, though of course at some point it will eventually weaken. The new 12Z ECMWF does begin to erode the ridge a little more than the 00Z run. ...Weather/Hazards Highlights... Showers and thunderstorms will be possible over parts of the Southeast Wednesday-Thursday as a wavy west-east frontal boundary lingers near the Gulf Coast and through the Southeast on Wednesday and another front drops down to take its place by Thursday. Recent guidance has been divergent and inconsistent with rainfall specifics across the southern tier due to issues with upper level shortwave details, with 00Z and later runs trending drier along the western-central Gulf Coast but some 12Z runs also reducing coverage where there is a general signal for perhaps localized heavy activity over the far Southeast. These areas where enhanced rainfall is still a possibility (given some frontal focus along with instability/above-climo moisture that could yield high rain rates) should be less sensitive to rain since the recent pattern has been drier and Flash Flood Guidance is high especially close to the coast. The Days 4-5 (Wednesday-Thursday night) Excessive Rainfall Outlooks continues to hold off with depicting a specific risk area given the continued guidance inconsistency. 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, though some timing differences remain. 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. The afternoon update should reflect minimal adjustment to these areas while awaiting any new trends/clustering in the guidance. Some rain could linger in those areas into Friday as well. Farther north the cold front should be more progressive but may still produce a brief period of fairly intense rainfall in light of the strong dynamics aloft. Expect this system to bring rainfall into the east-central U.S. on Friday and the Eastern Seaboard on Saturday. Portions of the interior West should see scattered showers from late week through the weekend in a monsoon-like pattern as anomalously high moisture remains over a large portion of the western U.S. underneath 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 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. Locations west of the Cascades could see some gradual moderation late week into the weekend (perhaps getting close to normal near the coast), but interior areas like the Great Basin can expect much above average temperatures to continue 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 up to 10-15F 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 through Thursday morning with temperatures around 10-15F below normal, but should moderate to near normal thereafter. Rausch/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