Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 AM EDT Wed Sep 21 2022 Valid 12Z Sat Sep 24 2022 - 12Z Wed Sep 28 2022 ...Dangerous Fiona forecast to slam toward the Canadian maritimes Saturday with transition from Hurricane to a deep extratropical storm... ...Pattern Overview... Guidance agrees to build an amplified/warming western North American upper ridge this weekend into next week with demise of a hot ridge over the U.S. southern tier. This transition would allow energetic mid-latitude and northern stream shortwave troughs to more robustly dig downstream to carve out an amplified east-central North American upper trough to focus cooler/unsettled weather and possibly offer some connection to tropical lattudes to monitor by mid-later next week. ...Guidance Evaluation/Predictability Assessment... Model guidance remains agreeable with the track of Hurricane Fiona toward the Canadian Maritimes even as it transitions to a very deep extratropical low during the medium range period. WPC and the National Hurricane Center will also continue to monitor the tropics regarding Invest 98L, which is likely to form into a tropical cyclone in the Caribbean per the NHC Tropical Weather Outlook. Recent GFS/ECMWF runs agressively develop the tropical system into next midweek with a more northward track into the Gulf of Mexico compared both CMC runs and the centroid of GEFS/ECMWF ensemble members. As per collaboration with NHC, leaned away from the plausible GFS/ECMWF at this point, but latest 00 UTC GFS run have not backed off. Even so, there is ample uncertainty as the track will depend on multiple factors including the exact upper trough position and amplitude to the north that may provide a conduit to lift any system toward the U.S.. Stay tuned for forecast updates over the next few days as this system could eventually be impactful. Model and ensemble solutions are well clustered this weekend into Monday and a composite of the best clustered GFS/ECMWF/Canadian along with the National Blend of Models (NBM) provides a good basis for the forecast during that period, with enhanced detail consistent with a pattern with above normal forecast predictability. Forecast spread and uncertainty only slowly grows into longer time frames, but aforementioned tropical system differences prompted a switch to forecast usage of the more compatible GEFS/ECMWF ensemble means and the NBM to maintain WPC max product continuity into early-mid next week. ...Weather/Hazards Highlights... A shortwave trough tracking across the north-central U.S. will combine with energy from northern Canada to deepen a trough with its axis in the east-central U.S. for the early part of next week. This trough and a couple of frontal systems at the surface should help produce some increasing rainfall chances from the Great Lakes to the Ohio Valley into the weekend, shifting into the Eastern Seaboard by later Sunday into Monday near the frontal system, with some lake enhanced rainfall behind it. The western U.S. should see generally dry conditions during the medium range period. Showers and storms are possible across the Florida Peninsula and may increase in coverage by around Tuesday and beyond as there is potential for tropical moisture streaming in. However, opted to target a decent decrease in Florida rainfall amounts compared to the NBM in this time window for now given the uncertainty with fueling tropical influences. A lingering upper ridge will favor continued warmth for the Southern Plains and Lower Mississippi Valley through the weekend prior to moderating frontal passages down across much of the central and eastern U.S., with highs in the 90s and perhaps isolated 100s, which are 10-15F above average. As upper ridging develops in the West, warmer than average temperatures by around 5-10F are forecast for the first part of next week, bringing the Desert Southwest temperatures well into the 100s. 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, experimental excessive rainfall outlook, 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/#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