Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 525 PM EDT Fri Sep 20 2024 Valid 12Z Tue Sep 24 2024 - 12Z Sat Sep 28 2024 ...Heavy rain for portions of the southern coast and Southeast early next week... ...Overview and Guidance Evaluation/Preferences... The pattern will feature retrogression of a deep layer cyclone from the Gulf of AK across the AK Peninsula into the Bering Sea, as each cyclone moving in from the west either absorbs or boots the more eastern system away. This should lead to some increase in the ridging across the AK interior. Updated 12Z deterministic guidance remains in good agreement early to mid period. There are some depth issues with a system moving near Haida Gwaii mid next week, but a compromise of the deterministic guidance accounts for that nicely. While the 12z GFS is alone in its uniquely deep cyclone approaching the Aleutians at the end of next week, though probably too deep as it exceeds September low pressure records for the AK Peninsula and Aleutians, it does lie within the upper level trough position preferred by much of the deterministic and ensemble guidance and is internally consistent with its more amplified ridge in eastern Siberia, so it can't be ruled out. For the pressures, fronts, winds, and QPF, transitioned from a compromise of the 12z runs of the GFS, UKMET, Canadian, and ECMWF to one that includes 40% ensemble means (NAEFS and ECMWF) late next week. Temperature, dew point, and cloud grids were heavily weighted on the 19z NBM, while PoPs were modified to account for the QPF. ...Weather/Hazards Highlights... A strong low pressure system near Kodiak Island will weaken with time, leading to at least a couple days of very heavy rain favored from around Prince Williams Sound into the AK Panhandle and winds near gale force, particularly in spots that experience gap winds, early next week. A strong influx of moisture from the tropical Pacific should lead to several inches of rain for favored coastal/upslope locations next Monday into Tuesday. Another system lifting northeastward into the Gulf from the Pacific may bring in another surge of moisture and additional heavy rainfall for the latter half of the week to southern portions of the AK Panhandle, though with greater uncertainty on timing and amounts. Winds should reach gale force once more across the Aleutians late next week into next weekend, but their strength remains an open question until the guidance shows some convergence with the incoming low from offshore Japan. For the interior, precipitation amounts should be light as weak troughing gives way to weak ridging aloft. Periods of light snow will become more common for some of the lower elevations as temperatures follow a downward trend through the week. High temperatures will generally range from the 30s north, 40s interior and Aleutians, to 50s along the south-central coastline and AK Panhandle. Lows will generally be in the 30s across mainland AK, with 40s along the coast and Aleutians. Colder temperatures will be seen at elevation. Roth 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