Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue Oct 30 2018 Valid 12Z Fri Nov 02 2018 - 12Z Tue Nov 06 2018 ...Pattern Overview... A mean upper-level trough is expected to persist and reload across the central U.S. through the extended period. The trough will be reinforced by a series of shortwaves originating in the north Pacific, potentially interacting with additional shortwave energy of polar/arctic origin as they cross the Canadian Rockies before amplifying as they encounter the area of broad and persistent cyclonic flow across the central U.S. A series of frontal/low pressure systems will accompany these shortwaves across the CONUS, generally 1-2 days apart, resulting in a relatively active period of weather. ...Guidance/Predictability Assessment... Models are still struggling with the wave of low pressure crossing the eastern U.S. Friday with the GFS continuing to show a somewhat weaker and faster low pressure system and the ECMWF/UKMET the slower/stronger deterministic solutions (but with a fairly significant degree of ensemble support). Though a fairly large degree of variability exists over several runs, the ensemble trend has been decidedly slower and preferred to side with the ECMWF-led cluster (including the 00Z Canadian). From Sunday onward, ensemble guidance suggests that the upper ridging just off California will trend eastward and flatten while upper ridging in the NW Atlantic near Bermuda will trend northwestward, favoring a continuation of the central CONUS troughing. This may also have the effect of preventing the lead eastern system from staying offshore after this weekend as it may be shunted back westward toward the coast before weakening. The north Pacific flow may begin to trend toward a more zonal pattern which introduces more chaos into the forecast, particularly by the end of the extended period in the Pac NW into the Gulf of Alaska. Better agreement (though still with spatial/strength details) exists in the mid-Mississippi valley as a strengthening area of low pressure may lift toward the Great Lakes next Tuesday. ECMWF ensemble mean has lowered its pressure ~4mb each cycle since yesterday while the GEFS mean has wavered due to larger spread in its members. ...Weather Highlights/Threats... Rain will be on its way out at the beginning of the extended period on Fri across much of the eastern U.S. in association with a low pressure system/cold front and warm front through New England. The Mississippi Valley will see the potential for rain to become more widespread late in the weekend into early next week as a couple frontal systems traverse the region and tap into Gulf of Mexico moisture. Heavy rain and higher elevation snow will be possible across the Pacific Northwest through the weekend and 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 perhaps the central Rockies by next week as snow levels lower. Chilly conditions will be prevalent through the weekend across much of the central U.S. and portions of the eastern U.S. beneath the persistent upper trough, where highs will generally be 5 to 10 deg F below average. Meanwhile, above average temperatures will persist across portions of the West, from California into the Great Basin, where high temperatures may reach 5 to 15 deg F above average through the weekend and into early next week. 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