Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Thu Oct 17 2019 Valid 12Z Sun Oct 20 2019 - 12Z Thu Oct 24 2019 ...Heavy rain and mountain snow expected for portions of the Pacific Northwest... ...Potential Tropical Cyclone Sixteen to exit away from the North Carolina coast on Sunday... ...Pattern Overview and Guidance/Uncertainty Assessment... Amplified and relatively progressive pattern is expected to continue across the CONUS during the medium range. The National Hurricane Center has recently started advisories on Potential Tropical Cyclone Sixteen in the western Gulf of Mexico that is forecast to exit North Carolina Sunday after merging with a frontal boundary. Meanwhile, a series of rapidly moving shortwaves and frontal systems are forecast to affect the northwestern U.S., spaced approximately 24-48 hours apart through the middle of next week. One such Pacific shortwave will cross the Great Lakes on Sunday, with another behind it late Monday into Tuesday. Another significant trough is forecast to quickly amplify across the Rockies Sunday before reaching the High Plains on Monday. Most deterministic model solutions suggest development of a closed upper low and deep surface cyclone across the Upper Midwest by Monday, which should then lift northward into western Lake Superior and Ontario on Tuesday. A strengthening subtropical ridge off the Southeast U.S. coast early next week will contribute to a highly amplified flow setup, with a broad region of moist southerly flow across the Mississippi, Tennessee, and Ohio Valleys. This will aide in producing widespread modest rains with locally heavier amounts that will then shift eastward into the Mid-Atlantic and Northeast by Tue and Wed. Uncertainty increases significantly by Tue-Wed across the Pacific Northwest and the northeastern Pacific, particularly with respect to a building upper ridge off the West Coast and how it impacts the active train of shortwave energy from the north Pacific to the northwestern U.S. Models have shown quite large timing and amplitude variations from run-to-run, which significantly reduces forecast confidence by that time. This uncertainty extends east into the Rockies and central U.S. by Wed-Thu. Models continued to shift over the Southeast with respect to Potential Tropical Cyclone Sixteen. Latest 00Z/06Z models offered a reasonable clustered starting point that was close to the NHC official forecast. The 00Z/06Z GFS runs became much quicker with the upstream central U.S. trough than all other ensemble members, which allowed it to wrap PTC16 back to the north into New England--but this was considered a low probability solution. Overwhelming consensus would take PTC16 more eastward than northward and get somewhat stuck beneath building heights before dissipating, per the ECMWF and somewhat like the UKMET. Trend both upstream (over the Pacific) and downstream (over the north Atlantic) was toward more amplification and a slower eastward progression of the cold front Mon-Wed to the East Coast. This was captured well in the ECMWF ensemble mean and also the ECMWF and GEFS mean, which formed the basis for the forecast next Wed-Thu. ...Weather Highlights/Threats... Potentially heavy precipitation is expected across the Pacific Northwest into next week as the series of shortwaves and surface fronts move inland. Snow levels should rise some as the upper trough axis shifts to the central/eastern U.S., but some significant snows will still be possible at the higher elevations of the Cascades and portions of the northern Rockies. As the central U.S. trough amplifies this weekend into early next week, moisture return from the Gulf of Mexico will increase, along with a chance of severe weather--SPC has noted both Sunday and Monday for a chance of severe weather over eastern OK and the northern Gulf Coast, respectively. Advancing frontal boundary and PTC16 are expected to focus isolated potentially heavy rainfall into portions of the Southeast this weekend, before deeper moisture and widespread showers/thunderstorms spread northward into the Mississippi, Ohio, and Tennessee Valleys ahead of the strengthening central U.S. low pressure system early next week. By Mon-Tue as the system moves eastward, widespread and locally heavy rains will also be possible across portions of the Northeast. The heavy rain threat may be enhanced/exacerbated by a slower-moving frontal system. GEFS reforecast (M-climate percentile forecasts) highlight this well Tue into Wed (above the 99th percentile). Below average temperatures are forecast across the western U.S. as the upper trough moves inland. High temperatures 5 to 15 deg below average are forecast on Sun from the Northwest and Great Basin eastward to the central/northern Rockies as the main upper trough axis moves inland and amplifies. As the trough reaches the central U.S. next Mon-Tue, highs may be 5 to 10 deg below average across the central/northern Plains. Pre-frontal warmth will be limited to about 5-10 degrees above average for maxes but 5-15 degrees above average for overnight mins. Some records may be possible across Texas on Sunday and over Florida next Mon/Tue. Another surge of cooler than average temperatures may spread into the West with additional troughing by next Thu. Fracasso/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