Extended Forecast Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Wed Sep 28 2022 Valid 12Z Sat Oct 01 2022 - 12Z Wed Oct 05 2022 ...Ian forecast to bring heavy/excessive rainfall and brisk to strong winds to the Appalachians and Mid-Atlantic this weekend and possibly into early next week... ...Overview... The 18Z National Hurricane Center advisory has Hurricane Ian weakening to a tropical depression near northwestern South Carolina by the start of the forecast early Saturday and becoming extratropical shortly thereafter. Due to a fairly blocky pattern taking shape over the lower 48, this system may take until early next week to track off the East Coast and in the meantime should produce areas of significant rainfall and brisk to strong winds over the central-southern portions of the Appalachians into the Mid-Atlantic. At the same time a northern Rockies into High Plains upper low (and accompanying locally enhanced precipitation) will drift along slowly due to its position between ridges near the West Coast and over the Plains/Midwest. Recent guidance has varied with the expected flow near the West Coast by late in the period. ...Guidance Evaluation/Predictability Assessment... Ian's evolution from Tropical depression to extratropical cyclone appears to be captured well by a non-UKMET guidance package through the medium range period. A mainly 06z GFS and 00z EC blend was utilized through day 4 due to the consistency between the two models with respect to the closed upper level low associated with the tropical prior to transition. Some 06z GEFS and 00z ECE were introduced into the blend on Day 5 to mitigate any troughing uncertainty displayed by the deterministic EC and GFS when Ian undergoes Extratropical transition. The blocking pattern over the central CONUS was also captured well by the EC/GFS blend through day 5. The ECE and GEFS were primary inputs on days 6 & 7 due to uncertainty with respect to the timing/location of the eastern trough. The western ridge is supported well by the ensemble guidance at the end of the period as well. Some uncertainty arose with respect to prospective excessive rain development over portions of the Central Appalachians on Monday due to a more southeasterly trend in qpf, caused by building high pressure over the Ohio Valley, during this morning's cycle. ...Weather/Hazards Highlights... While Hurricane Ian will likely weaken to tropical depression status by early Saturday and become extratropical soon thereafter, the lingering system should continue to produce heavy rainfall and wind threats over parts of the Appalachians and Mid-Atlantic through the weekend and possibly into early next week. High pressure weakening in place over the Northeast on Saturday should be reinforced by another high reaching southeastern Canada and New England by Sunday-Monday, helping to maintain a fairly tight gradient to the north of extratropical Ian. Expect the heaviest rainfall to be over parts of the central-southern Appalachians east into the Mid-Atlantic, with much of this area covered in Slight Risk areas in the experimental Days 4-5 Excessive Rainfall Outlooks. Parts of western North Carolina into southwest Virginia are in an embedded Moderate Risk area, given the forecast surface low track and its slow movement. The combination of anomalous moisture and the upper low tracking over the northern Rockies/High Plains should produce some areas of locally heavy rainfall within an area from Montana through Wyoming and parts of Colorado/eastern Utah. Best potential for heavier activity should be on Saturday with a gradual lighter and more scattered trend after then. Highest elevations under the upper low could see some snow. Clouds/rain from Ian will support cool highs from parts of the Southeast into the Mid-Atlantic, with some readings 10-15F below normal from Saturday into Monday. Temperatures should finally rebound closer to normal by the middle of next week. The Plains will see moderately warm anomalies from the weekend into Monday, with the warmth getting suppressed southward thereafter. 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 on Saturday. The upper ridge near the West Coast will promote a gradually expanding area of above normal temperatures, with some locations in the Northwest seeing highs up to 10-15F above normal. Kebede/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 Hazards: - Heavy rain across portions of the Northeast, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Sat-Mon, Oct 1-Oct 3. - Flooding likely across portions of the Southeast. - High winds across portions of the Southeast and the Mid-Atlantic, Sat, Oct 1. - High winds across portions of the Mid-Atlantic, Sun-Mon, Oct 2-Oct 3. 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