Area Forecast Discussion
Issued by NWS Anchorage, AK
Issued by NWS Anchorage, AK
765 FXAK68 PAFC 241250 CCA AFDAFC Southcentral and Southwest Alaska Forecast Discussion...CORRECTED National Weather Service Anchorage AK 450 AM AKDT Tue Sep 24 2024 .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today through Thursday evening)... Unsettled weather will continue over much of Southcentral through Thursday as a quasi-stationary upper-level trough weakens and becomes negatively tilted over the northwestern Gulf. The extent of cloud cover and precipitation will be largely driven by a series of upper-level waves and mesoscale surface lows / inverted troughs retrograding east to west across the northern Gulf and Prince William Sound. The focus today will be on a weak upper-level wave moving inland over Prince William Sound this morning, bringing with it another round of showers to the coast with lighter periods of rain inland and along the lee side of the Chugach and Kenai Mountains. A secondary wave and attendant mesoscale low will then move from east of Middleton Island northwest into Prince William Sound later this afternoon. There is some uncertainty in the timing and strength of the surface feature, whether it remains a closed low or strong inverted trough, as it pivots westward. Whatever the timing, though, it does appear that coastal locations from Cordova to Whittier will see a second round of showers along with a period of gusty easterly winds. This band of showers will also move inland across the western half of Southcentral for Wednesday morning with yet another round of rainfall from Talkeetna south to the southern Cook Inlet. Given the showery nature of the precipitation, rainfall amounts will be difficult to pin down for many inland locations. However, anywhere from a tenth to a third of an inch is possible from location along Cook Inlet north and east into the Susitna Valley and Copper River Basin through Wednesday evening. By late Wednesday, the various upper-level waves weaken as they continue to track inland and the aforementioned upper-level trough slides south into the southern Gulf. However, most of the area continues to remain within cyclonic flow and thus will continue to see cloudy conditions with most of the showery activity confined to the coast and higher elevations. The aforementioned trough over the Kenai Peninsula may also drift south by Wednesday, bringing another round of light rain to Kodiak Island. The case remains the same on Thursday, although shower coverage looks to be less than Wednesday. With cyclonic flow still present, expect very little to no sunshine on Thursday with mostly cloudy conditions. -TM/DN && .SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3)... Minor changes to the forecast overnight, but otherwise the bigger picture remains largely on track. A broad upper-level longwave trough stretches across much of Mainland Alaska, with weak higher pressure building into the western Bering. Northerly flow upstream of the trough advects cold air across the eastern Bering Sea and Southwest Alaska, elevating winds through the gaps and passes south of the Eastern Aleutians and southern Alaska Peninsula. Winds will gradually diminish from west to east through tonight. Morning low temperatures across Southwest Alaska will continue in the low to mid 30`s, and low 40`s along the Alaska Peninsula and Eastern Aleutians. Through today, upper-level shortwaves rotating around the base of the longwave trough will curve across Southcentral Alaska and back towards Southwest Alaska, producing a deformation band along the Western Alaska Range and Middle Kuskokwim Valley. Compared to the previous forecast package, the development of the deformation band has been delayed from this morning to this afternoon. Still, this band is expected to bring prolonged light precipitation persisting through at least Wednesday. As temperatures drop overnight, precipitation type will likely similarly change each overnight period to snow, returning to a mix of precipitation in the afternoons. Across the Alaska Peninsula, gusty northerly winds are expected to return by Wednesday afternoon as another surge of cooler air drops through the region. Given shifts in the upper level forcing, have reduced forecast wind gusts along the southern side of the Peninsula. For now, expecting gusts tomorrow to be a little weaker than what was seen yesterday afternoon and evening. The next system to enter the fray will be a vertically stacked upper-low entering the northwestern Bering Sea by Wednesday night into Thursday morning. A front along the eastern leading edge of the front will spread across the Bering ahead of the main low, reaching the Pribilofs by Thursday morning. While the front appears fairly transient, the upper low appears to stick into the central Bering continuing the active pattern through the weekend for the Bering, and Southwest coastline. -CL/KC && .LONG TERM FORECAST (Days 4 through 7: Friday through Monday)... Broad longwave troughing will persist over mainland Alaska through the extended forecast period, resulting in unsettled weather leading into the start of next week. The front of a shortwave low in the Bering will move eastward onto Southwest Friday afternoon, providing them with their next round of widespread steady rain.The shortwave low will continue to slowly trek southward before situating itself near Kodiak Island Sunday morning. Strong southerly flow from the system will shoot ample moisture northwards towards the Kenai Peninsula and western Alaska Range, with enhanced amounts of both rain and snow possible in those locations. This will be short-lived however as the shortwave dissipates and makes way for a potentially strong North Pacific low to enter the Gulf of Alaska on Monday. The aforementioned low entering the gulf on Monday has not been picked up well by deterministic models and ensemble guidance, resulting in an uncertain forecast for this part of the forecast area. The intensity, track, and timing of this low has been considerably variable over the last couple of days, so confidence on the impacts of this storm are very low. Continue to monitor for future updates as we get more guidance on this system for early next week. && .AVIATION... PANC...Light northerly winds will persist through the TAF period. VFR conditions are now expected to prevail, likely dropping below 5000 ft at times. Periods of ceilings lowering to MVFR or IFR are possible, but low decks are mostly expected to remain scattered. Intermittent light rain showers are also likely to continue through at least Wednesday morning, after which point a drying trend is possible with lifting ceilings. && $$