Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Sep 27 2019 Valid 12Z Mon Sep 30 2019 - 12Z Fri Oct 04 2019 ...Heavy rainfall potential from the southern High Plains into Great Lakes early to middle of next week... ...Anomalous warmth continues over the central to eastern U.S. while very chilly conditions remain across the northern and northwestern U.S.... ...Pattern Overview and Guidance/Uncertainty Assessment... A highly amplified large-scale pattern across the U.S. will appear to become more zonal by late next week. However, a cold trough over the Northwest while an anomalously strong upper ridge over the Southeast will likely to remain persistent through the medium range. Meanwhile, a wavy front is forecast to gradually move across the central and eastern U.S. Ensemble consensus remains quite good, while deterministic guidance has increased its speed of ejecting energy from the western trough eastward across the north central U.S. toward the Northeast. The Canadian model appears to eject too much energy farther north into Ontario, Canada by midweek. Therefore, the WPC grids were derived using a blend of the 12Z ECMWF and 18Z GFS with a small percentage of their ensemble means for days 3-4 (Mon-Tue). The 12Z NAEFS was introduced into the blend by day 5, with increasing weights through day 7 together with the 12Z EC mean and 18Z GEFS. Confidence is good based on ensembles that another vigorous shortwave should drop into the Pacific Northwest for the latter part of next week, reinforcing the amplitude of the upper trough over the area, while a less anomalous ridge remains over the Southeast. ...Weather Highlights/Threats... With the guidance trending faster eastward ejection of the energy from the western trough, an early end to the heavy precipitation is indicated over the northern Rockies by Monday as the main surface cyclone moves into the northern Plains. The trailing cold front is expected to move across the Plains relatively slowly as it assumes a more zonal orientation through the end of next week. Meanwhile, 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. The interaction of this moisture with the wavy front 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, associated will the decaying Tropical Storm Karen, will promote an increase in rain/showers over Florida. Please monitor NHC forecasts for the latest information regarding Karen. The most extreme temperature anomalies over the U.S. mainland will exist through 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. However, record highs/warm lows remain a distinct possibility over the southeastern U.S. through late next week. Kong/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