Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 03Z 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... For Ian and its extratropical evolution, guidance clusters well for the upper low into day 4 Sunday. After that time there is increased spread related in part to varied influence of northern stream energy which could either push the initial upper low away or reinforce it in place. The new 00Z CMC is the first to stray, becoming weaker/more open than consensus. Then the 00Z GFS becomes somewhat progressive by day 6 Tuesday with the GEFS mean runs supporting a slower motion. Meanwhile for the Rockies/High Plains upper low, GFS runs have tended to be on the faster side of the spectrum. The new 00Z GFS has made some progress in backing up closer to other guidance. Guidance has shown significant spread and variability for the shortwave emerging from the Gulf of Alaska and tracking across Canada, as well as for possible upstream energy that could eventually amplify in response to western Canada ridging. This will affect a cold front that may drop into the northern tier states. 12Z/18Z GFS runs were on the slow/amplified side of the spread but the new 00Z run looks much more like the average of other guidance. Finally, models and ensemble members diverge considerably over the eastern Pacific/West Coast by the latter half of the period. The 12Z ECMWF run was fastest to bring a shortwave to the West Coast (but within the broad ensemble spread, and the new 00Z ECMWF is similar) while the 12Z/18Z GFS were farthest west with the West Coast ridge and upstream trough. The ensemble means overall suggest an intermediate solution. Based on the 12Z/18Z guidance, the updated forecast started the first part of the period with a blend of the 18Z GFS and 12Z ECMWF/CMC since the UKMET did not compare well to consensus for Ian. The mid-late part of the forecast incorporated a model/mean blend to reflect an intermediate scenario and maintain decent continuity where possible, in light of the diverging guidance at that time frame. ...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. 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