Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Sun Sep 22 2019 Valid 12Z Wed Sep 25 2019 - 12Z Sun Sep 29 2019 ...Areas of heavy rain possible over parts of the central U.S.... ...Rain and mountain snow expanding over the West late week/weekend... ...Expanding area of record highs in the Southeast later in the week through the weekend... ...Overview... The overall forecast is consistent, with guidance unanimous in showing the development of a very amplified West Coast trough/eastern U.S. ridge upper pattern by next weekend. The western trough will support much below normal high temperatures along with a broad area of precipitation including some elevation snow, while the ridge will bring much above normal temperatures to most of the eastern half of the lower 48 including potential for numerous record highs over the Southeast. Between the two extremes there will likely be one or more areas of heavy rainfall over the Plains/Upper Mississippi Valley/Midwest. A compact upper low expected to be near the western U.S.-Mexico border as of early Wed still provides some uncertainty in terms of when it ejects northeastward ahead of the developing western trough. ...Guidance Evaluation/Preferences... Longwave pattern remains in better than average agreement across all the ensemble systems through next weekend as the very amplified pattern takes shape. 500mb standardized height anomalies of -4/+2 for the western trough/eastern ridge, respectively, in the ensemble means (!!) by the end of the period were in the extremes for this lead time and this time of year relative to what the ensembles can forecast. However, as is typical, the embedded shorter wavelength features show less predictability. In the Southwest midweek, the deterministic models still showed timing differences for the closed low ejecting through the Southern Plains Friday--00Z UKMET and 06Z GFS were quickest as they have been off an on. The most consistent has been the ECMWF over the past several days and this, along with the 00Z GFS and Canadian, formed a good starting point for the CONUS. By Friday into the weekend, additional uncertainty in the speed of height falls out of British Columbia continue to persist and the 06Z GFS again was quicker than most. Trended toward the ECMWF and GEFS means with retained detail from the 00Z ECMWF as it was closest to the ensemble consensus. This maintained good continuity from the previous forecast. Building heights (eventually a closed 594dm upper high) in the Southeast may start to influence Tropical Storm Karen as it moves north of 20N. That forecast is highly uncertain given the lack of steering between the strong upper high over the lower MS Valley and a deeper trough well east in the central Atlantic. Please consult the NHC forecast for the latest on Karen. ...Weather Highlights/Threats... The deepening West Coast trough aloft will expand precipitation coverage over the western states. The most agreeable area of highest totals currently includes the northern Rockies and northern Great Basin. Higher elevations of western Montana have the best potential for meaningful snow but lowering snow levels could reach some valley locations. In addition the eastern side of the northern Rockies may see a period of upslope flow that would enhance rain/snow activity. Precip amounts closer to the coast will be more sensitive to the exact position of the upper trough/embedded low. Some moisture may reach as far south as southern California as the cold front sinks south of 30N late in the week. High elevation snow will be possible over the Northwest and Sierra Nevada but with lower probabilities for significant totals relative to the northern Rockies. Wind will increase of much of the interior West as well, centered over the Great Basin eastward into the central Rockies. Well ahead of the upper trough, the compact upper low initially near the Southwest should generate some rainfall into midweek. Latest guidance is generally suggesting two episodes of enhanced rainfall over the central U.S.--one over the south-central Plains early in the period as a front reaches the region and then a second centered over the Upper Mississippi Valley/Upper Great Lakes late week into the weekend. This latter area of rainfall may be enhanced by the upper low energy ejecting from the Southwest. It also remains consistent with where teleconnections support precip based on the amplified trough/ridge pattern aloft. The dramatic cooling trend over the West will start with some highs 10F or so below normal over the Northwest on Thu. Then expect a broad area with minus 10-25F max temp anomalies from the West Coast states into Montana during Fri-Sun. A few record cool highs may be possible. Clouds and precip will lead to less extreme min temps but some morning lows may still reach 5-10F below normal by the weekend. Meanwhile the East and to some degree the central/southern Plains should see an expanding area of highs and/or lows 10-20F above normal. Highs over the Southeast could approach daily records at a few places as early as Wed, with records likely becoming more numerous Thu into the weekend. Hottest temperatures (mid/upper 90s) may push a little westward over the course of the weekend as the upper high center retrogrades toward the Lower Mississippi Valley. This pattern of extreme contrasts should persist beyond the weekend based on the CPC day 6-10 and 8-14 day outlooks. Fracasso/Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml