Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM 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 should remain established over much of Noth America for the next week and guidance overall continues to trend toward amplification with greater system digging and slower progressions within well defined northern and southern stream flows. This seems reasonable considering increasing trends toward amplitude upstream over the Pacific as well as at higher latitudes with deep forcing systems from northeast Asia toward Alaska. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of best clustered and amplified/stream separated guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET valid for Friday into Sunday. GEFS/ECMWF ensemble and National Blend of Model support has also increased for this scenario in this time frame and the latest 00 UTC cycle guidance overall remains in line, bolstering forecast confidence. At longer time frames into early next week, the guidance camp that continues to best extend preference for slowed/amplified flow/trends seems to best fit reasonably clustered guidance from the 18/00 UTC GFS and 12/00 UTC ECMWF/ECMWF ensemble mean versus the somewhat more progressive recent runs of the GEFS/Canadian ensembles and the Canadian model. ...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/amounts across the region later week into the weekend, especially as moisture feed potenitally increases more substantailly over the Northeast with interaction with a lead coastal low set to meanwhile lift northward up off the East Coast toward the Northeast. This may focus/fuel several inches of rain into eastern New England with best likelihood into Downeast Maine, but relatively modest instibitities/intensities and 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 slower and drier 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 then Mississippi Valley, with rain expanding in coverage late weekend into early next week. Lead cyclogensis and frontgenesis may increasingly support a renewed heavy rainfall pattern with focus over the lower Mississippi Valley in about a week to monitor. Meanwhile in the wake of this system, renewed Pacific system energy within northern stream flow is expected to dig more sharply inland/southeastward from the Northwest through the Rockies this weekend into early next week to focus a swath of rain and terrain enhanced mountain snows given upper dynamic support. 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