Extended Forecast Discussion NWS Weather Prediction Center College Park MD 311 AM EDT Thu Sep 29 2022 Valid 12Z Sun Oct 02 2022 - 12Z Thu Oct 06 2022 ...Ian forecast to bring heavy/excessive rainfall and brisk to strong winds to parts of the Appalachians and Mid-Atlantic through this weekend and possibly into early next week... ...Overview... The pattern over the lower 48 will be fairly blocky to start the forecast on Sunday, with a trough/upper low over the East (supporting the extratropical reflection of Ian) and a slow moving northern Rockies into Plains upper low between ridges over the West Coast and central-southern Plains/Midwest. After Tuesday the guidance suggests that flow will begin to transition toward larger scale mean troughing over eastern North America in response to more robust upper ridging that extends into far western Canada and the Alaska Panhandle. Meanwhile the models and ensemble means are starting to show a more common evolution over the eastern Pacific/West Coast by Wednesday-Thursday, with some weak southern stream trough energy approaching California while the northern part of the initial trough weakens as it approaches the core of positive height anomalies over the Alaska Panhandle/British Columbia. The two main areas of precipitation focus will be over the Mid-Atlantic where moisture from Ian may continue to produce locally heavy rainfall into early next week, and parts of the Rockies/High Plains where the upper low and associated trough will produce areas of precipitation. ...Guidance Evaluation/Predictability Assessment... Guidance still shows considerable disagreement for the specifics of the evolution over the East from Sunday onward, involving the extratropical reflection of Ian and potential interaction of one or more northern stream impulses. UKMET runs remain on the open and progressive extreme but even among other solutions there is a range between incoming energy reinforcing the initial upper low or ejecting it (then possibly forming a new low). Given the typical sensitivity of surface evolutions to small shortwave details, confidence is even lower for the details of low pressure near the East Coast after Sunday. Meanwhile the GFS continues to gravitate back toward prior remaining guidance for the Rockies/Plains upper low that should open up by midweek or so. A blend/ensemble approach looks best for resolving the varying shortwave details within southern Canada flow that begins to amplify into the northern tier states, gradually pushing a cold front into the region Tuesday-Thursday. For the moment guidance is converging over the eastern Pacific after at least a couple days of significant spread, with consensus now showing a weak southern stream trough nearing California by next Thursday after the rest of the initial trough erodes while downstream mean ridge. The updated forecast started with the 18Z GFS/12Z ECMWF and a little 12Z CMC to represent the overall preferred solution for features of interest early in the period, followed by some inclusion of the 18Z GEFS/12Z ECMWF means as detail spread increases over some areas. ...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/Appalachians through the weekend and possibly into early next week. Brisk to strong winds, aided by high pressure to the north, should be most noticeable over coastal areas. The experimental Day 4 Excessive Rainfall Outlook valid Sunday-Sunday night shows a Slight Risk over parts of the central/south-central Appalachians east into the Mid-Atlantic. The Day 5 outlook focuses a lingering Slight Risk closer to the coast. Guidance differences for specifics at the surface and aloft by this time frame lead to lower confidence in rainfall coverage and amounts, so monitor upcoming forecasts for possible adjustments. The upper low emerging from the northern Rockies and the trough to its south should produce areas of precipitation from the Rockies into the High Plains. Slow movement of the upper low and sufficient moisture could support some areas of locally moderate to heavy rainfall mainly Sunday-Monday. Highest elevations under the upper low may see some snow. Expect precipitation to end from the northwest and become lighter/more scattered as the upper low opens up and the shortwave continues onward. 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. Temperatures should finally trend closer to normal by Wednesday-Thursday. On the other hand, 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 settle more over southern areas while locations over the far northern tier may trend colder than normal by next Thursday. 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