Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Sep 30 2022 Valid 12Z Mon Oct 03 2022 - 12Z Fri Oct 07 2022 ...Ian forecast to bring possibly heavy to excessive rainfall and brisk to strong winds to mainly coastal Mid-Atlantic early next week... ...Overview... A somewhat blocky pattern should be in place by the start of the medium range period on Monday, with a trough/upper low over the East (which includes the extratropical reflection of Ian) and a slow moving northern Rockies into High Plains upper low. The latter is sandwiched between ridges over the West Coast and central-southern Plains into the Midwest. Troughing looks to be maintained over the East even as the remnants of Ian move offshore Tues-Wed, but upper level ridging building over the West which should act to accelerate the second shortwave towards the Upper Midwest and further amplify troughing over the East late next week. Sensible weather threats however during the medium range period look to mainly surround the remnants of Ian as they move through the Mid-Atlantic next Monday-Tuesday with some locally heavy rainfall and gusty winds lingering along mainly coastal regions from the Outer Banks, NC to southern New England. ...Guidance Evaluation/Predictability Assessment... Differences in the details continue to surround the remnants of Ian and possible interaction with additional shortwave energy into the East Monday to Tuesday, and then how quickly the shortwave exits the coast. The 18Z/Sept 29 GFS run was slowest with this energy, while the UKMET was much too quick. The ECMWF and CMC have more support from the ensemble means on a middle ground solution and thus WPC relied more closely on these solutions (along with the National Blend of Models) for QPF concerns early in the period. Elsewhere, some timing differences remain with the next shortwave eventually pushing through the northern Plains/Upper Midwest mid next week and eventual amplification of larger scale troughing over the East, with more significant differences over the northeast Pacific/western North America which spill into parts of the Northwest U.S. later next week. For the latter half of next week, WPC preferred a blend more towards the ensemble means which help to mitigate these differences more. ...Weather/Hazards Highlights... The potentially slow moving eastern U.S. upper low and associated low pressure (whether initially from Ian or eventual redevelopment just off the East Coast) may continue to produce locally heavy rainfall over parts of the Mid-Atlantic into early next week. Brisk to strong winds, aided by high pressure to the north, should be most noticeable over coastal areas. Lingering guidance differences for specifics at the surface and aloft directly related to the extratropical transition of Ian this weekend continue to offer a lower confidence forecast in rainfall coverage and amounts, so adjustments to the current forecast is likely in the days to come. The remainder of the CONUS looks to be fairly quiet in terms of any larger scale precipitation focus areas next week. The upper low emerging from the northern Rockies should produce areas of precipitation from the Southwest/Four Corner region into the High Plains and sufficient moisture could support some areas of locally moderate to heavy rainfall early next week. Highest elevations under the upper low may see some snow as well. This precipitation should become lighter/more scattered by the middle of next week and onward as the upper feature opens up and moves eastward. Much of the East will see below normal highs during the first half of next week. Parts of the Mid-Atlantic into Southeast may be 10-15F below normal into early next week with the clouds/rain originally associated with Ian, while cool high pressure prevails over New England. Out West, persistent upper ridging will keep much of the West warmer than average with some locations in the Northwest potentially seeing highs 10-15F above normal for most of the period. Modestly above normal temperatures over the Plains will eventually shift more over southern areas while locations over the northern tier and into the Midwest/Great Lakes should trend colder as Canadian High pressure settles into the region. Santorelli 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