Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Thu Nov 01 2018 Valid 12Z Sun Nov 04 2018 - 12Z Thu Nov 08 2018 ...Pattern Overview... Broad cyclonic flow and upper-level troughing is forecast for the central US next week as upper ridging in the Atlantic holds in place. The trough will be reinforced by a series of shortwaves originating in the north Pacific (north of another persistent upper-level ridge), potentially interacting with additional shortwave energy of polar/arctic origin as they cross the Canadian Rockies before amplifying in the Plains. A deepening area of low pressure is expected to lift into the Great Lakes next Tuesday which will bring in another shot of cooler Canadian air. ...Guidance/Predictability Assessment... The recent GFS runs diverged from the consensus over the southern Plains late this weekend as a lead system weakens and lifts northward into the Lakes. The consensus takes one shortwave to the northeast while the GFS had separated the trough into a northern/southern portion which was not shown in the ensembles (except for several GEFS members) nor has been shown in previous runs. Opted to rely on the ECMWF-led consensus with the Canadian/UKMET and the ECMWF ensemble mean (along with the previous forecast). By Monday the next shortwave should be exiting the Rockies into the Plains, with development of yet another surface low pressure system expected. Spread among the deterministic guidance seems to have increased rather substantially with this second system by day 5 (Tue) and onward, partially stemming from the GFS but also from how the models/ensembles handle incoming Pacific shortwaves into the quick NW flow. Preferred the more stable solutions near the ECMWF ensemble mean and even GEFS mean which align with the 00Z ECMWF. This also keeps reasonably good continuity with the previous forecast. This scenario shows a strengthening surface low moving north-northeastward across the Great Lakes on Tue and continuing to deepen (perhaps quite substantially) as the low moves into eastern Ontario toward James Bay on day 6 (Wed). Later in the forecast period (next Wed-Thu), additional mostly northern stream shortwaves will continue to reinforce the mean trough (although the details of these systems are quite murky). Cold air across the Lakes could support lake-enhanced/effect snow (or rain/snow mix). The next Pacific system may stay just offshore British Columbia but spread some lighter rain/snow into especially coastal areas. ...Weather Highlights/Threats... The Mississippi and Ohio Valleys will see the potential for rain to become more widespread late in the weekend into next week as a couple frontal systems traverse the region and tap into Gulf of Mexico moisture. An expanding areas of rain will spread northeastward ahead of the midweek system (Tue-Wed) with an inch or more of rain possible over a large area, particularly over the Ohio/Tennessee Valleys northward to the Lower Great Lakes. Off the coast, the lingering stationary front may push back toward the coast and bring the potential for precipitation back to the I-95 corridor from the Southeast all the way northward to New England. Heavy rain and higher elevation snow will be possible across the Pacific Northwest into early next week as upper shortwaves/surface fronts traverse the northern periphery of the upper ridge. These features will also bring rounds of snow to the higher elevations of the northern and then central Rockies early next week. Chilly conditions will be prevalent across portions of the central U.S. beneath the persistent upper trough, where highs will generally be 5 to 10 deg F below average. Chilly air may make its way into the eastern U.S. by later next week behind a strong cold front but will mostly be focused west of the Appalachians until then. Ahead of the front, temperatures will rise to above average by 5-10 deg F on Tuesday. Meanwhile, above average temperatures will persist across portions of the West, from California into the Great Basin (especially Sun-Tue), where high temperatures may reach 5 to 15 deg F above average downstream of persistent upper-level ridging. Fracasso/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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