Short Term Forecast Southcentral Alaska
Precipitation that has overspread the Kenai Peninsula, Anchorage, and the MatSu this afternoon will continue to spread east into the Copper Valley late this evening and overnight. The surface low, currently centered just west of Kodiak Island, will continue to track northeast along the eastern Kenai as it weakens through Friday while the potent shortwave in the upper levels pushes north through the region tonight. For Friday, expect remnant showery rain as subsequent, weaker shortwaves rotate through the longwave pattern providing continued weak lift coupled with marginal instability.
For Saturday, the pattern becomes highly amplified as an upper low in the southeastern Bering deepens while ridging builds across the AlCan border. This will result in a northward surge of warm Pacific air and moisture from Kodiak Island north into the MatSu Valleys as the frontal system stalls in the Gulf as the ridge to the east holds. This will bring the next round of prolonged, widespread rain to these areas while the Copper Valley looks to stay relatively dry through the weekend. Gap winds will be on and off in a daily cycle most days with a coastal ridge gradually strengthening through the forecast period.
.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3: Tonight through Sunday).
The pattern across the Bering, Aleutians, and Southwest Alaska remains active through the short term, but is on track. Currently, a low pressure system centered over the Alaska Peninsula is sending a stratiform shield of rain across the Alaska Peninsula and Southwest Alaska. Clouds and rainfall as expected have held temperatures down today across the region, with daytime high temperatures topping out in the low to mid 50s for most. This system will be moving east and exiting into the Gulf tonight, but the combination of rainfall and cloud cover will keep overnight low temperatures elevated in the mid to upper 40s.
Aloft, the feature driving weather for the short term is a large, amplified, upper level trough that has consolidated over the Eastern Bering Sea and Western Alaska. With one wave rounding the trough and moving east with the current low, another wave is progged to quickly dig southeastward from Eastern Russia tonight. At the surface, deepening of low pressure is indicated in the global models as the surface low aligns with the shortwave's upper level forcing. Henceforth, there is high confidence in the system strengthening into a gale force low and its precipitation shield/expected rainfall field expanding as it does so. The trend-line over the past few global model runs is for an increasingly stronger system, and widespread gales are expected from Atka to Nikolski and eventually Unalaska and Akutan as Friday morning wears on. This system has also trended wetter as well, so have increased expected rainfall for this forecast package to 0.5 to 1 inch of rainfall from Nikolski to Sand Point from Friday morning to Friday evening.
As this low becomes vertically stacked and occludes as it approaches the Alaska Peninsula, any remaining gales will most likely decrease to small craft winds. Models show a swath of high end small craft winds on the backside of this system that begin to move across the Pribilof Islands Friday evening. As the low weakens further, winds will decrease through Saturday morning for the Pribilof Islands. As the low continues to move eastward and just north of the Alaska Peninsula, Kamishak Gap winds increase Saturday evening, giving locations like Iliamna breezy conditions. This uptick in winds is on track to be short lived as the low backs northwestward and moves north up the Southwest Alaska coastline.