Extended Forecast Discussion NWS Weather Prediction Center College Park MD 351 PM EST Tue Nov 14 2023 Valid 12Z Fri Nov 17 2023 - 12Z Tue Nov 21 2023 ...Pattern Overview and Guidance Evaluation/Predictability Assessment... A split flow pattern is forecast over much of North America through this weekend and into next week. A couple upper level troughs will swing across the continental U.S. through the period with embedded systems in the northern and southern stream flows, and timing and strength of these features will be dependent on the degree of upstream amplification. Model guidance has been consistently showing a highly amplified wave, which would result in slower system progression. Model solutions are fairly similar for Days 3-5 (Friday through Sunday), but solutions diverge significantly on Days 6 and 7 (Monday and Tuesday). For this forecast package, WPCs model blend favors the 06Z GFS and 00Z ECMWF, which have shown consistent amplification and relatively slower progression of features through the period. The 00Z CMC was the fastest out of the available guidance, though not unreasonable, and was given less weight in the blend for this reason. The GEFS and EC means were factored in heavily for days 6 and 7 to produce a middle of the road solution. The 12Z GFS and CMC runs became available after the forecast was made, but both are trending towards WPCs middle of the road solution with the GFS speeding up slightly and the CMC slowing down. ...Weather/Hazards Highlights... The slow and steady march of an amplified and dynamic upper trough and associated wavy frontal system across the Great Lakes/Midwest and Northeast should favor increasing precipitation chances across the region late this week into this weekend. The current forecast has moisture increasing substantially over the far Northeast as a coastal low lifts northward off the East Coast. This may focus several inches of rain in eastern New England with best likelihood in Downeast Maine, but modest instability and rain rates in combination with the existing antecedent conditions does not seem to support issuance of any Excessive Rainfall Outlook threat areas at this time. For the West/Southwest, the slow approach of the closed southern stream upper low will only gradually bring in bouts of light to moderate terrain enhanced rain and mountain snow that expand eastward in time. The delay in the height falls likely lessens the stronger moisture flux into the area as the system fills before moving ashore, but most areas of California into the Southwest will see at least measurable rainfall (especially coastal areas) outside some of the shadowed deserts. Given the consistent slow trend in the guidance, no excessive rainfall risk areas are in place. As the amplified upper system finally crosses the Divide, return flow out of the Gulf of Mexico will help increase moisture over the Plains and Mississippi Valley, with rain chances expanding in coverage late weekend into early next week. Lead cyclogenesis and frontogenesis may increasingly support a renewed heavy rainfall pattern with focus over the lower Mississippi Valley in about a week to monitor. In the wake of this system, renewed Pacific system energy within northern stream flow is expected to dig more sharply southeastward from the Northwest through the Rockies this weekend into early next week and focus a swath of rain and terrain enhanced mountain snow given upper dynamic support. Dolan/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Northeast, Sat, Nov 18. - Heavy snow across portions of the Northeast, Sat, Nov 18. - Heavy precipitation across portions of the Alaska Panhandle, Fri, Nov 17 and Mon, Nov20. - High winds across portions of the Aleutians, Sun, Nov 19. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw