Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Sep 27 2022 Valid 12Z Fri Sep 30 2022 - 12Z Tue Oct 04 2022 ...Hurricane Ian forecast to bring heavy/excessive rain and strong winds to Florida and the Southeast, with significant rainfall then pushing northward into the Appalachians and Mid-Atlantic... ...Overview... The 18Z National Hurricane Center advisory has Hurricane Ian reaching northeastern Florida by the start of the medium range forecast period early Friday and tracking generally northward into South Carolina by Saturday. Although Ian will weaken with time, the system will still spread significant rainfall and strong winds northward over the portions of the eastern U.S. into the weekend, with the moisture likely to reach as far north as the central Appalachians and Mid-Atlantic. The pattern farther west becomes somewhat blocky for a time, with a slow moving northern Rockies into High Plains upper low (and accompanying locally enhanced precipitation) between ridges near the West Coast and over the Midwest. ...Guidance Evaluation/Predictability Assessment... Overall there is reasonable consensus that Ian should lift northward over the eastern U.S. with the upper level reflection potentially opening up around Sunday-Monday and the extratropical surface system reaching near the Mid-Atlantic coast. A non-UKMET model blend was reasonable as a starting point to represent the overall pattern followed by some adjustment to conform to the official NHC forecast. Recent cycles have been coming together for a strong eastern Canada/New England surface high around Sunday-Monday, limiting the northward progression of the system to some degree. Meanwhile, the models through 18Z generally appeared to be starting to settle into a consensus evolution for the pattern over the Pacific and western three-fourths of the lower 48 after a number of days with some significant spread and run-to-run variability. One uncertainty by the first part of next week will be if/when the Rockies-High Plains upper low opens up. New 00Z model runs hint that the low could hang on a little longer than depicted by the preferred 18Z/12Z model and 18Z GEFS/12Z ECens blend by day 7 Tuesday. Shortwave energy originating from near the Gulf of Alaska may amplify as it crosses Canada and push a cold front into the northern tier states early next week. Adding in the new 00Z GFS/ECMWF runs, there is notable inconsistency as those solutions have adjusted notably faster/flatter with the upper trough than prior clustering. Also spread persists for how northern stream flow may eventually interact with the upper low well offshore the West Coast, effecting how quickly height falls begin to reach the West Coast during the first part of next week. ...Weather/Hazards Highlights... Expect Hurricane Ian to produce notable rainfall and wind threats to Florida and the Southeast extending into late this week. A front stalling over Florida in the short range will lift north with the system and provide an added rainfall focus. High pressure over the Northeast late this week should be reinforced by another high reaching southeastern Canada and New England by Sunday-Monday. These highs will help to enhance the winds/onshore flow to the north of Ian. The heavy rain threat should push north into parts of the Appalachians and Mid-Atlantic by the weekend, with some rainfall possibly continuing into early next week. The Day 4 experimental Excessive Rainfall Outlook maintains the Moderate Risk from the Charleston to Savannah metro areas with the Slight Risk encompassing far northern Florida, much of Georgia, all of South Carolina, and southern North Carolina. The new Day 5 outlook depicts a Slight Risk area approximately from northern Georgia/far northeastern Alabama into northern South Carolina and northward through much of Virginia. The combination of anomalous moisture and the upper low tracking over the northern Rockies/High Plains should produce some areas areas of locally heavy rainfall within an area from Montana through Wyoming and parts of Utah/Colorado, especially during Friday-Saturday. Rainfall should trend lighter and more scattered thereafter as the moisture shield drifts eastward and the upper low trends a little weaker. Highest elevations under the upper low could see some snow. Cloud cover/rain from Ian will support below normal highs over the southern two-thirds of the East for most of the period, with the coolest anomalies of 10-15F below normal most likely Friday-Saturday. The Plains will be on the warm side Friday with some areas up to 10-15F above normal. Thereafter, northern areas should trend toward normal by Tuesday while plus 5-10F anomalies for highs persist over the southern Plains. The upper low drifting over the Rockies/High Plains will bring cool temperatures along its path with a few locations at least 10F below normal for highs Friday-Saturday. The upper ridge near the West Coast will promote a gradually expanding area of moderately above normal temperatures, with some localized plus 10F or so anomalies over the Northwest from Saturday onward. Rausch/Kebede 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