Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1111 AM EDT Wed Sep 25 2019 Valid 12Z Sat Sep 28 2019 - 12Z Wed Oct 02 2019 ...Much below normal temperatures expected in the West with winter weather possible for the northern Rockies... ...Much above normal temperatures expected in the eastern third of the nation... ...A couple potential rounds of heavy rain expected across the central U.S... ...Pattern Overview and Guidance/Uncertainty Assessment... Guidance continues to show a highly amplified western trough/eastern ridge pattern whose large scale typically yields fairly high predictability. The relative agreement of latest/recent GFS-ECMWF runs and their respective means supports this idea. Through much of the period the CMC agreed as well, so the updated forecast reflected a blend of the 06Z GFS and 00Z ECMWF/CMC. What differences existed were well within typical error ranges for the forecast time frame. Even to day 7 Wed the GFS/ECMWF were sufficiently close to their respective means that a model/mean blend represented consensus well. Late in the period the greatest uncertainty involved exactly how approaching Pacific flow (south of a trough/upper low reaching the northeastern Pacific) will begin to eject the western trough. As for some forecast details, the latest guidance cluster is consistent with the idea of the western trough containing a Northwest upper low that may not move much between Sat and Mon or early Tue. At the same time eastern ridge aloft amplifies with the anchoring upper high retrograding to over or just east of the Lower Mississippi Valley by early Mon. Shortwave energy ejecting from the western trough should support a surface wave that emerges from the West and tracks across the northern Plains/Upper Great Lakes around Sun-Mon before continuing into Canada. By next Tue-Wed the front trailing from this wave and extending to and beyond the central High Plains may decelerate for a time as it becomes nearly parallel to flow aloft. ...Weather Highlights/Threats... Guidance continues to highlight heavy precipitation potential over locations from the northern Rockies/High Plains into parts of the Great Basin. Expected slow movement of the upper low center over the interior Northwest should lead to a period of enhanced precip over terrain favored by southwesterly flow aloft. Meanwhile eastern slopes of the northern Rockies will see a period of low level upslope flow. Heaviest snow should fall over western Montana while some lower elevations to the east may see meaningful snow as well. Northern California and Pacific northwest rain/high elevation snow still looks to be lighter and more scattered. Areas from the Great Basin into the central Rockies may see a period of relatively stronger winds. Moisture will spread across the northern Plains as well as the Midwest/Upper Mississippi Valley/western Great Lakes as low pressure tracks across the northern tier, with areas of locally heavy rainfall possible. Then another episode of heavy rainfall will be possible from the central Plains northeast into the Great Lakes Tue-Wed as the front trailing from the aforementioned wave potentially slows down and becomes aligned with flow aloft. This latter event has the potential to produce heavier totals due to repeat activity/training, plus possible input of moisture originating from the tropical eastern Pacific--which on the way toward the region may also play a part in promoting pockets of enhanced rainfall over or near the southern High Plains. Easterly flow around the southern side of the strong eastern ridge aloft should still lead to a slow increase of moisture over Florida. The most likely scenario would have Tropical Storm Karen eventually being steered by this flow, though multi-day trends have been considerably slower (reaching near the Bahamas by around day 7 for the past 2-3 days) and the full range of guidance spans a wide envelope. Monitor NHC forecasts for the latest information regarding Karen. Forecasts continue to show a broad area of the western U.S. into northern Plains with highs 10-25F below normal during most of the period--and locally 25-35F below normal over some locations in Montana. Morning lows will tend to be 5-15F below normal from Sun onward, again locally colder in parts of Montana. Eastern areas into the Midwest and central/southern Plains will see much above normal temperatures with anomalies for highs generally plus 10-20F, and some lows possibly 20-25F above normal during the first half of next week from the central Plains into Great Lakes. Expect numerous daily record highs/warm lows during the period. Rausch/Ryan 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