Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sat Oct 12 2019 Valid 12Z Tue Oct 15 2019 - 12Z Sat Oct 19 2019 ...Pattern Overview and Guidance/Uncertainty Assessment... Guidance shows a fairly amplified but progressive pattern across the CONUS during the extended period Tue-Sat. An amplifying shortwave should cross the northern Plains/Midwest on Tue and the Great Lakes/Northeast Wed-Thu. There has been decent agreement that an embedded upper low will reach the vicinity of New England by Thu and in fact a compact closed low could exist a day or two before then. This feature will push a surface low and frontal system across the eastern half of the nation. Most guidance has been signaling formation of a new Mid-Atlantic wave on Wed and intensification to a relatively deep cyclone near the New England coast Wed night-Thu as the upper low intensifies. Farther west an intense Pacific upper-level jet that will reach the Northwest should carry a train of shortwaves, lowering heights across the Northwest and eventually the northern Rockies as the series of systems moves inland. By the end of the forecast period next Sat, the models and individual ensemble members show a fair amount of spread for details but over the past couple days there has been decent consensus (especially among the ensemble means) toward an amplifying trough over the West. Leading shortwave energy should eject across the central U.S. A compromise among latest guidance represents consensus well for the system crossing the East and intensifying near New England, with deepening ensemble mean trends over recent days providing support for the fairly strong model solutions by early Thu. Meanwhile over the past couple days guidance has essentially fallen into two clusters for the western through central U.S. shortwave during Thu-Sat, the faster and flatter GFS/UKMET versus slower and sharper ECMWF/CMC. Respective ensemble means generally lean toward their operational counterparts. Historical biases favor maintaining a forecast that leans slower than the GFS cluster. However there is some potential that the very fast upstream Pacific flow could lead to somewhat faster timing than the ECMWF. In addition, even with the different specifics the past two days' worth of multi-day means from the various guidance end up with a similar pattern. This suggests lower predictability for the exact forecast of this feature. By next Sat the recent agreement and consistency of the ensemble means for the larger-scale amplifying western U.S. trough favor primary emphasis on the means as a starting point. Recent GFS runs have been on the deeper side of the spread with the trough while the 00Z CMC is a bit on the slower side. ...Weather Highlights/Threats... A stalled and potentially wavy front lingering over the Lower Mississippi Valley to Southeast/far southern Mid-Atlantic on Tue will be the focus for widespread showers and thunderstorms. Some eastern Pacific tropical moisture may help to increase the heavy rainfall potential over this region. A separate area of lighter precipitation (mostly rain but perhaps a few pockets of snow) will accompany the low pressure/front crossing the Upper Midwest and Great Lakes. The northern system will pull the deeper moisture over the South northward, resulting in widespread and locally heavy rainfall over the Northeast as low pressure consolidates/deepens. Highest elevations of northern New England may see a little snow as the upper low passes through. The series of systems affecting the Pacific Northwest will bring an extended period of rain and some high elevation snow, with snow levels expected to fall gradually through the week as lower heights begin to press inland. The coastal ranges of the Northwest and Cascades should see the heaviest precipitation with multi-inch liquid equivalents possible for the five-day period. By late next week or next weekend moisture should increase across the central U.S. It remains a significant question mark how much precipitation may occur with a front reaching the Plains on Fri, with differences in shortwave amplitude/timing leading to a wide spread in amounts. Currently the most probable scenario has fairly light totals. The amplifying western trough aloft and strengthening low level inflow from the western Gulf of Mexico by next weekend should promote increased rainfall along and perhaps north of the western-central Gulf Coast. A lingering surface front over the Gulf of Mexico may also help to focus this activity. Through Tue-Wed temperatures will be well below average across the north central U.S. in the wake of the current winter storm. Highs should be 15-25 deg F below average across portions of the northern Plains and Upper Midwest before temperatures trend warmer after midweek. Expect cooler temperatures to spread across the eastern half of the nation by the mid-late week as Canadian high pressure trails the system consolidating over New England by early Thu. High temperatures 5-15 deg F below average are likely across a large area from the Mississippi River to the Eastern Seaboard Wed-Thu, before temperatures return to near seasonal norms by next Fri-Sat. Below average temperatures (generally minus 5-10F anomalies) will spread into the Pacific Northwest and northern Great Basin by late next week as upper-level heights fall and the series of frontal systems affect the region. The upper trough amplifying over the West by next Sat should then increase coverage of highs 10 deg F or more below normal. 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