Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Thu Sep 22 2022 Valid 12Z Mon Sep 26 2022 - 12Z Fri Sep 30 2022 ...Heavy rainfall expected across the Panhandle and the coastal Kenai and Chugach Mountains through the upcoming workweek... ...Synoptic Overview... A highly unsettled weather pattern continues across much of mainland Alaska through the upcoming workweek. A large longwave trough will remain anchored in place over western Alaska. As shortwave troughs traverse around the base of the upper level trough, multiple surface lows will develop over the North Pacific and move into the Gulf. This will keep conditions cloudy and unsettled, especially through the Panhandle and along the Kenai and Chugach Mountains of Southcentral. On Monday, a low over the central/eastern Gulf has gotten much stronger in the latest guidance. It will track northward from the central Gulf to near Yakutat Monday afternoon, then continue inland and rapidly weaken over the Wrangell-St. Elias range. As the low lifts north, expect heavy rain to the east of center, as an atmospheric river of plentiful Pacific moisture aims at the northern Panhandle. As that low moves inland and dissipates on Tuesday, a second low tracking eastward following right along the Aleutians will move into the western Gulf by Wednesday. This low will be much slower moving, persisting in the northern Gulf through Friday. This will keep unsettled conditions ongoing across Southcentral. In the vicinity of both these lows, expect the development of a barrier jet along the coast of the Panhandle Monday, and strong gap winds through the Barren Islands and into Turnagain Arm with the second low on Wednesday. This will also accompany heavier rains into the Chugach and Kenai Mountains Wednesday and Thursday to the east of the low center. Across Interior and northern Alaska, a relatively quiet period is expected as those lows make their way inland. Expect plentiful cloud cover as those lows dissipate, but little in the way of precipitation as much of the moisture is left in areas further south. ...Guidance/Predictability Assessment... The 12Z model guidance suite continues to remain in good agreement at least through the first half of the upcoming workweek. While there are some differences with the exact track of the low on Monday that plows headlong into the northern Panhandle, the overall trend has been for a stronger low. As usual, stronger lows tend to track further west than previous weaker forecasts. Thus, the area of heaviest rainfall associated with the atmospheric river has been shifted north and west along the Panhandle, resulting in several additional inches of rain in the northern Panhandle, and conversely, several inches less of rain for southern sections. This does not preclude the southern Panhandle from still picking up an inch or 2 of rain as the leading front lingers over the Gulf as the center moves inland. As regards the low tracking along the Aleutians, the general consensus is for it to move a little bit faster than previous forecasts, but the overall intensity remains largely unchanged. The low looks to follow a similar track as many lows, tracking along the Pacific side of the AKPEN and into the western Gulf, where it will slow to a crawl, resulting in multiple days of rain into the north Gulf Coast. This low won't have quite the tropical connection as the first low, resulting in less intense rainfall into the Kenai and Chugach, but its much slower movement will result in a much longer duration of rain. The WPC fronts/pressures are primarily derived from a deterministic blend through Wednesday, with the CMC and UKMET dropping off sooner. By Thursday and Friday, agreement among the deterministic models rapidly diminishes, so ensemble blends were introduced and rapidly increased by this point. Despite that disagreement in where the Gulf low will track, the overall very sluggish movement of the longwave pattern favors a slow moving low by that point. ...Weather/Hazards Highlights... Heavy rainfall totals are likely across southern portions of the mainland, generally from the eastern side of the Kenai Peninsula to the northern Panhandle region going into early next week, associated with an atmospheric river. Several inches of rain are expected for lower elevations, particularly where there is terrain enhancement, while high elevations will see heavy snow. Elsewhere across the state, periods of rain and breezy conditions are likely for the Aleutians as the next storm system moves through the region around Tuesday and into Wednesday. In terms of temperatures, highs should generally be in the upper 40s to around 50 with a gradual cooling trend going into next week. Highs will likely struggle to get much above freezing for the Arctic Coast. Expect gusty winds with the barrier jet on Monday along the north Gulf Coast east of Prince William Sound, and strong southeasterly winds through the Barren Islands and Kamishak Gap, as well as through Turnagain Arm with the approach of the second low on Wednesday. All of the winds will diminish by Friday as the low that will be somewhere over the Gulf weakens. Wegman Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Thu, Sep 26-Sep 29. - Heavy precipitation across portions of mainland Alaska, Wed-Thu, Sep 28-Sep 29. 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 Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html