Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Sep 24 2022 Valid 12Z Tue Sep 27 2022 - 12Z Sat Oct 01 2022 ...Tropical Storm Ian expected to intensify as it tracks from the Caribbean toward Florida, with possible effects over other parts of the East as well... ...Overview... Guidance is consistent in showing a fairly rapid flatting of the initially amplified upper pattern consisting of a western ridge/eastern trough, as a shortwave trough approaches from the eastern Pacific and moves inland. However, flow details become more uncertain from the Pacific into North America toward the end of next week. The 03Z National Hurricane Center advisory shows Tropical Storm Ian currently in the Caribbean and intensifying to hurricane status as it tracks into the far southeastern Gulf of Mexico, reaching the Florida Peninsula by Wednesday. There is still some timing/longitude spread as Ian reaches near Florida, and by the latter half of the period the details of the weakening eastern U.S. upper trough will also play a role in how this system affects areas to the north. Expect the Great Lakes region to see some rainfall with the chilly cyclonic flow in place during the first half of the week, while the Northwest may see mostly light rainfall around midweek. Parts of the Southwest and central/southern Rockies may also see some potential for scattered rainfall. ...Guidance Evaluation/Predictability Assessment... For Tropical Storm Ian, guidance is continuing to show meaningful divergence by 4-5 days out in time (through Wednesday-Thursday), with the ECMWF runs through 12Z/23 on the fast/eastern side of the spread and the GFS on the western side. In the new 00Z runs, the GEFS mean is even west of the operational run while the CMC is close to the GFS. The 00Z UKMET is a compromise. After Thursday ECMWF runs have been more amplified/persistent with lingering eastern upper trough energy that interacts with Ian, versus other solutions that lift the trough away more quickly (leading to slower motion for Ian). The official NHC track is an intermediate scenario within this envelope. The new 00Z ECMWF has made notable trends with both aspects of the forecast--trending slower/westward over Florida and then less amplified with what upper trough energy lingers over the East. There is decent clustering for the upper trough and associated surface system/front affecting the Northwest during Wednesday-Thursday. Then models and ensemble members display rapidly increasing spread for upstream flow over the Pacific late week into the weekend. By next Saturday individual solutions range between pronounced flow separation (depositing a closed low west of 140W longitude and allowing a ridge to build near the West Coast by Saturday) and a phased trough approaching the West Coast. Highlighting the uncertainty, the new 00Z CMC/ECMWF have both flipped significantly relative to their previous runs. Prefer a solution closer to the ensemble means with a trough offshore the West Coast until there is better model agreement. Latest GFS runs are a deeper version of this idea. These issues ultimately have some effect on the Northwest shortwave as it continues into central North America. The combination of increasing model divergence for Ian later in the period as well as the developing uncertainty with the Pacific into North America pattern led to transitioning the manual forecast from an early-period model blend (12Z/18Z runs) toward 70 percent emphasis of the GEFS/ECWMF/CMC means by day 7 Saturday. The starting blend required adjustment to accommodate the 03Z NHC forecast for Ian. ...Weather/Hazards Highlights... The dominant rainfall and wind threat during the period will be with Tropical Cyclone Ian that should affect Florida and potentially other parts of the East. A front expected to stall over Florida before arrival of the system could provide an added focus for rainfall. While there is still meaningful spread regarding the ultimate track of this system, there is a strong enough signal for a heavy rainfall threat over the Florida Peninsula Tuesday-Wednesday to support a Slight Risk area over parts of the region in the experimental Days 4-5 Excessive Rainfall Outlooks. Continue to monitor National Hurricane Center products for the latest information regarding this system. It appears likely that Ian will spread rain and wind farther north over the East later in the week into the weekend but with fairly low confidence in the coverage and timing. High pressure tracking across the Great Lakes and Northeast may serve to strengthen the surface gradient to the north of Ian and thus enhance the winds over some areas. The Great Lakes and vicinity will see periods of rain through the first half of the week with chilly and brisk cyclonic flow in place at low levels and the upper trough overhead. A few focused bands to the lee of the Great Lakes could produce some enhanced totals. The Pacific Northwest should see a period of light to perhaps isolated moderate rainfall around Wednesday-early Thursday with the frontal system forecast to push into the region. Increasing moisture may promote scattered rain over portions of the Southwest and into the central/southern Rockies, with best coverage perhaps around Thursday. The evolution of the upper pattern will favor chilly temperatures over a majority of the eastern U.S. with best potential for highs up to 10-15F below normal over and near the Great Lakes/Ohio Valley, especially Tuesday-Thursday. Temperatures will likely start to moderate by the end of the week with the track of Ian determining some of the specifics. Warmth over the West on Tuesday will progress into the Plains with time, with anomalies moderating slightly by Friday-Saturday. Areas from the Northwest/northern Great Basin into the northern High Plains may see one or more days with highs 10-15F above normal. The front pushing into the West after early Wednesday will bring temperatures over that region closer to normal, with lower confidence in temperature specifics by next weekend. Rausch 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