Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1143 AM EDT Thu Sep 26 2019 Valid 12Z Sun Sep 29 2019 - 12Z Thu Oct 03 2019 ...Much below normal temperatures over the West/northern Plains with heavy snow possible in the northern Rockies... ...Much above normal temperatures over the East, extending for a time as far west as parts of the Plains... ...Heavy rainfall potential from the southern High Plains into Great Lakes... ...Pattern Overview and Guidance/Uncertainty Assessment... The large scale pattern appears likely to remain persistent through the medium range with an amplified upper trough in the West, an unseasonably strong ridge (+2 standard deviations in 500 hPa heights) anchored from the lower Mississippi Valley to the Southeast, and a persistent and wavy frontal boundary in between. Ensemble consensus remains quite good, although spread among deterministic guidance has increased a bit compared to yesterday, particularly with respect to energy ejecting eastward from the western trough and crossing the north central U.S. and Canada. A blend of the 00Z ECMWF/CMC and 06Z GFS served as a good forecast starting point during days 3-4 (Sun-Mon) as these solutions showed relatively good consensus around significant features. By late day 4 into day 5 (Tue) the GFS become much faster than consensus with the upper shortwave ejecting from the western trough and crossing the north central U.S., and with the track of the associated surface low moving from the Great Lakes into Ontario and Quebec. Thus, the GFS was removed from the forecast starting on day 5, with a continued blend of the ECMWF/CMC and gradually increasing weight on ensemble means through day 7 (Thu). Confidence is good based on ensembles that another vigorous shortwave should drop into the western U.S. by next Wed-Thu, reinforcing the continued amplified upper trough/ridge pattern across the CONUS. Ryan ...Weather Highlights/Threats... The upper low over the Northwest and strong low level easterly flow north of surface low pressure over the north-central Rockies will support continued heavy precipitation over the northern Rockies through the weekend. The greatest snow totals should be over higher elevations of western Montana with some snow possibly extending to somewhat lower elevations to the east or northeast. The surface low will likely bring areas of locally heavy rainfall across the extreme northern Plains into the Upper Mississippi Valley/Upper Great Lakes Sun into Mon. At the same time flow around the western side of the eastern ridge aloft should begin to bring rainfall-enhancing moisture from the tropical eastern Pacific and Mexico into the southern High Plains. This flow may persist through midweek, leading to multiple episodes of locally heavy rainfall over the region. The interaction of this moisture and wavy front trailing from the northern Plains/Canada low will raise the potential for a heavy rainfall event from the central Plains northeast into the Great Lakes (centered over Iowa). The signal for this event has persisted over the past couple days and the possibility that the front moves slowly for a day or two would allow for significant rainfall totals due to training/repeat activity. To a lesser degree some moisture should extend into New England. Deep-layer easterly flow will promote an increase in rain/showers over Florida. The flow may ultimately contain some moisture associated with Tropical Storm Karen, but Karen itself is currently forecast to weaken slowly to a tropical depression and then a post-tropical remnant low in the coming days. Monitor NHC forecasts for the latest information regarding Karen. The most extreme temperature anomalies over the lower 48 will exist from Sun into early next week. The western U.S. into northern Plains will see highs 10-25F below normal with localized areas in Montana seeing highs at least 30F below normal. Daily records for cold highs are most likely on Sun but some are possible beyond then. The east will see a broad area of plus 10-20F anomalies for highs with numerous daily records. The northern Plains low pressure early in the week will also bring a surge of warmth into parts of the central U.S. with morning lows particularly warm (plus 20F or greater anomalies) from the central Plains into Great Lakes. As the pattern aloft begins to trend less amplified expect a gradual moderation of chilly western U.S./northern Plains temperatures while warm to hot readings become more confined to the East--with some record highs/warm lows continuing to be possible into Wed-Thu. 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