Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 627 PM EDT Fri Sep 06 2024 Valid 12Z Tue Sep 10 2024 - 12Z Sat Sep 14 2024 ...Rain likely along the Southern Coast, with heaviest focus over the Kenai Peninsula to Prince William Sound to Yakutat midweek... ...Overview... Into Tuesday, a fairly amplified upper trough will swing east across the Bering Sea anchored by an upper low atop eastern Siberia/Bering Strait, progressing across the Mainland Wednesday-Thursday. This trough and associated surface lows/fronts will direct a weak atmospheric river promoting moderate to heavy precipitation especially across the Southern Coast. Upper ridging is forecast to push through behind this trough and in response to additional energy and troughing coming into the Bering Sea late week. ...Guidance Evaluation/Preferences... Recent model guidance shows relatively good agreement with the large scale features of note at the beginning of the period. Models agree in principle that an upper low will slowly drift from eastern Siberia toward the Bering Strait while swinging a trough more progressively across the eastern Bering Sea Tuesday and into the Mainland midweek and eastward while weakening. The deterministic ECMWF and GFS and their ensemble means were reasonably agreeable in terms of the timing of the trough moving eastward (and therefore the surface low/frontal system and the QPF). The CMC and UKMET were both slower than the GFS/EC consensus, with the CMC showing some energy focusing on the southwestern side of the trough and slowing it down. The 12Z GFS does have some semblance of energy pinching off on the southern side, but farther south than the CMC, so will continue to monitor trends. Thus early in the period, used a mainly deterministic GFS/ECMWF blend with small portions of the EC and GEFS ensemble means. This worked to allow for a double barrel surface low around midweek, as the northern low moving through the western to central Mainland weakens in favor of the triple point low in/near the Gulf of Alaska. Then for the latter part of the week, attention turns to upstream energies that come into the Bering Sea. Models vary with the individual shortwaves but at least there is some consensus that a deeper upper low should be in place in the western Bering Sea by Saturday. Given the increasing spread, ramped up the proportion of ensemble means to over half by Days 7-8. ...Weather/Hazards Highlights... Notable precipitation is expected to spread into the western Mainland with a forming occluded low pressure system ahead of the upper trough into Tuesday and progress across the state through Wednesday and Thursday. As model guidance has trended down a bit with rain totals over the Alaska Peninsula/Kodiak Island, the heaviest rain totals should focus farther east in the Southern Coast, from parts of Kodiak Island to Prince William Sound to Yakutat, where a weak atmospheric river will focus moisture and a triple point low is forecast to form. Moisture spreading into the Alaska Range in colder temperatures will allow for snow in the higher elevations. Lighter precipitation is expected north of there into much of the Mainland as well. Some precipitation should spread into Southeast by around Wednesday-Thursday, with some uncertainty in amounts depending on the atmospheric river weakening. Generally northern parts of the Panhandle like the Yakutat area are forecast to see the most rainfall compared to southern parts. Another round of precipitation is possible across the Aleutians to western Mainland and Alaska Peninsula by late week, but the details remain uncertain. High temperatures should generally be in the 50s across lower elevations of the state next week. This is generally near to slightly below normal for highs. However, the North Slope with highs in the upper 40s will be above normal. Prevalent clouds and precipitation will favor mostly above normal morning lows, with only portions of the Panhandle and perhaps a few other isolated pockets over the mainland seeing below normal lows. Tate 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