Extended Forecast Discussion NWS Weather Prediction Center College Park MD 500 AM EDT Tue Sep 24 2019 Valid 12Z Fri Sep 27 2019 - 12Z Tue Oct 01 2019 ...Tropical Storm Karen over the Caribbean... ...Much below normal temperatures over the West with rain and areas of heavy mountain snow... ...Heatwave for the South/Southeast U.S... ...Heavy thunderstorms/rain over parts of the Midwest/Great lakes... ...Pattern Overview and Guidance/Uncertainty Assessment... After about a day of some guidance straying from each other and/or continuity for some important details, latest solutions (minus the new 00Z CMC) have returned to a more agreeable consensus representing the evolution of an increasingly amplified western trough/eastern ridge upper pattern. This flow regime will support contrasting extremes of much below normal temperatures over the West into the northern Plains and unusually warm to hot conditions for this late in the year farther to the east. Over the West expect highest precipitation totals to be centered over/near the northern Rockies with best potential for heavy higher elevation snow over western Montana. At the same time a heavy rainfall threat will exist over parts of the Midwest/Upper Mississippi Valley/Great Lakes. The eventual path of Tropical Storm Karen remains very uncertain, with dependence on sensitive details of steering flow and the ultimate strength of the system. Monitor NHC forecasts for the latest information regarding Karen. Overall the 18Z GFS, 12Z ECMWF/CMC, and 18Z GEFS/12Z ECMWF means provided a good starting point for the updated forecast and yielded only modest adjustments in continuity. Consensus shows a leading upper low reaching the Northwest by day 4 Sat with upstream energy dropping down from western Canada then serving to maintain the upper low and perhaps further amplify the overall trough which may finally begin to drift east a bit on day 7 Tue. The majority cluster is also similar in having the eastern ridge's upper high center retrograde close to or over the Lower Mississippi Valley by day 6 Mon. Guidance agreement for the overall pattern leads to fairly high confidence for that part of the forecast but some embedded uncertainties remain, and it is certainly possible that guidance might diverge again for some details as was seen 24-36 hours ago. As for current uncertainties, by the latter half of the period the GFS/ECMWF trended faster than the means with the wave emerging over the northern Plains and then tracking northeast into Canada. An intermediate solution looked best given that flow around the eastern ridge could at least support somewhat faster wave progression than seen in the means. Earlier in the period there was still some lingering spread for timing the ejection of the short range Southwest upper low whose forecast has proven to be challenging over recent days--with solutions tending toward the slower half of prior spread in response to the western trough taking a bit longer to amplify. ...Weather Highlights/Threats... Expect significant precipitation over the western states to be focused over/near the northern Rockies and into favored terrain of the Great Basin. Northern Rockies precip should be enhanced by a period of upslope flow. Recent forecasts have been consistent in showing the greatest snowfall potential over western Montana. While the heaviest snow will be confined to the mountains, temperatures may be cold enough to bring some snow even to lower elevations of the northern High Plains. Rain/high elevation snow should be lighter and more scattered over northern California and the Pacific Northwest. Some locations may see a period of relatively stronger winds, most likely from the Great Basin into the central Rockies. Farther east anticipate some moisture extending across the northern Plains while heavier rainfall is likely within an area centered over the Midwest/Upper Mississippi Valley and western half of the Great Lakes. A leading frontal system will likely stall near the southern edge of the threat area and then lift northward in response to the wave forecast to track across the extreme northern tier. Shortwave energy ejecting from the Southwest may enhance activity for a time as well. Evolution of the eastern ridge aloft continues to favor a gradual increase of moisture over Florida as easterly flow becomes established. It is still very uncertain whether the moisture/rainfall will be enhanced by Tropical Storm Karen. Monitor NHC forecasts for the latest information regarding Karen. Much below normal temperatures will settle into northern parts of the West and northern Plains by Sat and then cover more of the West by early next week. There will be a broad area of minus 10-25F anomalies for high temperatures and parts of Montana could see a day or so with highs at least 30F below normal. Morning lows will be less extreme but may still be at least 10F below normal over some areas by Sun-Tue. Locations over the East and even into the central/southern Plains will see plus 10-20F anomalies for min and/or max temps. Best potential for daily record highs will be over the Southeast, possibly extending a little farther westward with time corresponding to motion of the upper high. A few locations farther north could see records as well. Morning lows may reach 20-25F above normal over parts of the central Plains/Midwest by next Mon-Tue. 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