Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Sep 23 2022 Valid 12Z Mon Sep 26 2022 - 12Z Fri Sep 30 2022 ...Possible tropical activity from the Caribbean may affect at least Florida by next week... ...Overview... An amplified upper pattern featuring a western ridge and eastern trough early next week should flatten considerably after Wednesday as an eastern Pacific shortwave comes ashore and progresses inland. Meanwhile most guidance shows an area of disturbed weather (Invest 98L) currently over the Caribbean organizing into a tropical system that may track into the far eastern Gulf of Mexico/Florida, spreading heavy rainfall and strong winds over parts of Florida and possibly other portions of the Southeast. Some specifics of this system will be sensitive to exactly when it reaches near Florida, and its longitude, relative to the expected weakening of the eastern U.S. upper trough. 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 with the Four Corners states also seeing some potential for scattered rainfall. ...Guidance Evaluation/Predictability Assessment... The updated forecast started with a blend of the 18Z GFS and 00Z/22 ECMWF with lesser input from the 12Z UKMET/CMC early in the period and then transitioned to 60 percent of those GFS/ECMWF runs and 40 percent 18Z GEFS/12Z ECMWF means by days 6-7 Thursday-Friday. The more recent 12Z ECMWF was fairly extreme relative to other guidance over the East, both for the initial speed of its tropical system as well as the amplitude/persistence of the eastern trough to the north, which ultimately led to an even faster tropical system progression later in the period. Emphasis on the 18Z GFS and 00Z/22 ECMWF (with a bit of a nudge slower late) yielded a solution fairly close to yesterday's NHC/WPC coordinated track for the system. The new 00Z models through midweek are becoming better clustered, somewhat east of previous GFS runs and west of the 12Z CMC. The new 00Z ECMWF is slower than its prior run into Wednesday but still on the east side of the spread. Otherwise, there is still moderate spread with the details of the eastern trough beyond those noted from the 12Z ECMWF. Differences for the system nearing the Northwest as of day 5 Wednesday are fairly typical for a forecast that far out in time, with some convergence of GFS/ECMWF over recent days but still some run-to-run variability. The new 00Z UKMET is the southwestern extreme at the moment. Upstream flow over the eastern Pacific becomes more uncertain by day 7 Friday with models/ensemble members ultimately ranging between varying degrees of troughing or ridging. The blend noted above reflects the preferred model/mean approach to resolve current differences. ...Weather/Hazards Highlights... The dominant rainfall threat during the period will be with the possible tropical system that could affect Florida and potentially other parts of the Southeast/East Coast. A front expected to stall over Florida before arrival of the system could provide an added focus for rainfall. Note that there is still considerable uncertainty regarding the ultimate track of this system by the latter half of the week. However into early Wednesday there is sufficient potential for heavy rainfall to introduce a Slight Risk area on the experimental Day 5 Excessive Rainfall Outlook over parts of the Florida Peninsula. Continue to monitor National Hurricane Center products for the latest information regarding Invest 98L, the area of disturbed weather currently over the Caribbean. Expect the Great Lakes and vicinity to see periods of rainfall through the first half of the week with chilly and brisk cyclonic flow in place at low levels and the upper trough overhead. The Pacific Northwest should see a period of light to perhaps isolated moderate rainfall around midweek with the frontal system forecast to push into the region. Scattered rain over portions of the Four Corners states may gradually increase in coverage over the course of the week. 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. Meanwhile the West will be on the warm side Monday-Wednesday with some areas from the Northwest into northern Rockies likely seeing highs up to 10-15F above normal, followed by a cooler trend closer to normal later in the week as the warm air reaches the High Plains. 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