Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Nov 15 2023 Valid 12Z Sat Nov 18 2023 - 12Z Wed Nov 22 2023 ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite valid Saturday-Monday was primarily derived from a composite blend of reasonably well clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/Canadian along with the 01 UTC National Blend of Models (NBM) and WPC continuity. This solution has good multi-model ensemble support in a pattern with seemingly above normal predictability. Opted to shift guidance preference toward the GEFS/ECMWF/Canadian ensemble means by next Tuesday/Wednesday to mitigate growing model run to run continuty issues and forecast spread. Emphasis on the ECMWF/Canadian means offers slightly more amplified flow than the GEFS. This seems more conistent with recent pattern trends. 00 UTC guidance overall seems in line. ...Weather/Hazards Highlights... The steady march of an amplified upper trough and wavy frontal system across the Northeast will favor a swath of moderately heavy precipitation across the region into Saturday. The interaction of the upper trough and a leading coastal low set to lift northward over the western Atlantic should combine to create a window with substantially increasing moisture inflow into eastern New England. This may focus several inches of rain, with best likelihood in Downeast Maine. However, modest instability and rain rates in combination with the existing antecedent conditions does not seem to support issuance of any WPC Excessive Rainfall Outlook (ERO) threat areas at this time. Meanwhile, cooling post-frontal vertical temperature fields also offer potential in this pattern for wrapback snows over northern New England and also to the lee of the lower Great Lakes in the wake of system passage. For the West, the slow approach of the closed southern stream upper low will gradually bring light to moderate terrain enhanced rain and mountain snow from California to the Southwest/Great Basin into this weekend. Many areas will see at least measurable rainfall (especially coastal areas) outside some of the shadowed deserts. No Excessive Rainfall Risk (EROs) areas are expected, but there may be some gusty winds with system passage. As the amplified upper system finally crosses the Divide into The South, return flow out of the Gulf of Mexico will increase moisture over the Plains and Mississippi Valley, with rain chances expanding in coverage late weekend into early next week. Lead cyclogenesis may increasingly offer a heavy rainfall threat to monitor for Monday as a deepening moisture inflow focus feeds especially into the Lower Mississippi Valley and vicinity. Widespread moderate to locally enhanced rains are likely to then spread downstream through the east-central U.S. and the East Tuesday into next Wednesday with southern stream upper trough and wavy frontal system progression. The extent of northward expansion of the precipitation shield over time will be dependent on influx of digging northern stream upper trough energy that in this pattern may also be sufficient to support cooled temps and modest swath of far northern tier states snow. Meanwhile, ample Pacific system energy within northern stream flow will dig sharply southeastward from the Northwest through the Intermountain West and Rockies this weekend into early next week to focus a swath of rain (enhanced over coastal areas of the Pacific Northwest) and inland/terrain enhanced mountain snows given upper dynamic support. An amplified upper ridge will then build over the West Coast early next week to stablize and warm. However, upstream Pacific system energies may work into enough into this ridge to bring another opportunity for organized precipitation into the Pacific Northwest next Tuesday/Wednesday. 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 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