Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 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 late next week through next 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 higher 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. 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... Most models and means through the 12Z/18Z cycles were generally within typical error range for the large scale evolution over the course of the period but there were some details that led to downplaying a particular model solution for parts of the forecast. For the upper low initially near the Southwest, the GFS/ECMWF were reasonable (and close to previous preference) while the questionably fast 12Z CMC timing required that run to be excluded from the blend Wed-Fri. Among new 00Z guidance thus far the CMC has adjusted back to the GFS/ECMWF while the UKMET appears fast. Individual ensemble members are still very diverse so the majority consensus could change at some point. Over the Northeast, the 12Z GFS strayed from consensus Fri-Sun with excessive troughing around a slow moving Canadian Maritimes upper low so that run had to be dropped after Thu. Within the amplifying western trough both the 12Z and 18Z GFS runs were somewhat faster than consensus to bring the embedded upper low into the Northwest late in the week. The new 00Z GFS is slower but still a tad ahead of the ECMWF/UKMET/CMC. On the other hand it remains to be seen if the upper low tracks quite as far southwest as in the past couple ECMWF runs. The ECMWF path would lead to some 500mb heights reaching at least 5 standard deviations below normal over/near California. As it is other models still suggest some minus 3-4.5 standardized anomalies. Days 3-4 Wed-Thu started with primary emphasis on the 12Z ECMWF, 12-18Z GFS, and 12Z UKMET in order from more to less weight with minor editing as appropriate. Later in the period the blend transitioned to a combination of 12Z ECMWF/18Z GFS/12Z CMC and the 12Z ECMWF/18Z GEFS means. Operational guidance still held a slight majority weight over the means through day 7 Sun given their definition with the western trough and eastern ridge (whose core should exceed 594dm Sat-Sun as it builds from the Southeast into the Lower Mississippi Valley). ...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. 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. High elevation snow will be possible over the Northwest and Sierra Nevada but with lower probabilities for significant totals relative to the northern 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 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. 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