215 FXAK68 PAFC 270109 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 509 PM AKDT Wed May 26 2021 .ANALYSIS AND UPPER LEVELS... A weakening upper level ridge is in place over the northern Gulf with widespread stratus along the northern Gulf coast. To the west, a sharp trough axis extends from Siberia over the northern Bering while to the east, an Arctic trough extends over the Alaska-Yukon border. A frontal boundary over the eastern Bering continues to weaken as it approaches the Southwest coast. Satellite imagery confirms clear skies over much of the mainland with some high level cirrus across the eastern portion of the state. && .MODEL DISCUSSION... Model solutions remain in good synoptic agreement. The biggest forecast challenge is the timing of the Arctic trough and the associated upper level waves moving southward over Southcentral by the end of the week. The interaction of this trough with a North Pacific low will also dictate the weather heading into the weekend. At this time, models are indicating a cooler, wetter pattern for the southern half of the state for the end of the week. && .AVIATION... PANC...VFR conditions will persist. Winds will switch to the south to southeast this evening around 10 kt and then increase some Thursday afternoon as Turnagain Arm winds make it over the airfield. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)... The current pattern has high pressure over the Alaskan Interior with a weak low pressure center near Prince William Sound. This is allowing for sunny skies and warm temperatures across the Anchorage Bowl, Matanuska Valley and Susitna Valley. GOES-17 Day Cloud Phase Distinction RGB shows convective showers moving southeastward across the Copper Basin. The strongest convection is in the southern Copper Basin along the northern edge of the Chugach Mountains as orographic lift helps enhance the convection. In more windward locations across the southern Kenai Peninsula and Prince William Sound, widespread low stratus is causing overcast and cooler temperatures. A pattern change is expected across Southcentral over the next 3 days as high pressure moves to the east and an upper level trough moves over the area. This pattern will allow for waves of upper- level energy to move through the region and increase cloud cover as well as chance of precipitation. This pattern change will occur Thursday afternoon through Friday morning as clouds and precipitation move southwestward across the Copper Basin into the Mat-Su Valley and Anchorage Bowl. The clouds and rain/showers will become more widespread on Saturday. Colder temperatures aloft are making it possible for snow in the higher elevations of the Chugach Mountains this weekend. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Wed night through Saturday)... Mostly clear to partly cloudy and warm conditions will persist through Friday as an upper level ridge breaks down. Today (Wednesday) will likely be the warmest day of the week as upper level clouds move in and limit heating Thursday and Friday. On Friday the pattern will change considerably as a stout trough moves in from northeast Alaska. This will bring in a cooler airmass along with a chance of isolated showers along the Western Alaska Range. Overall this should be a dry airmass down to the surface, so widespread convection is unlikely. As the trough drives south it will tap into moisture extending north of a low over the central Aleutians. This will bring rain to Bristol Bay early Saturday, pushing into the Lower Kuskokwim Saturday afternoon. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Wed night through Saturday)... A strong, compact low will move over the western Aleutians Thursday morning bringing gale force winds and rain to the region before the low begins to occlude midday. The low will move slowly southeast on Friday before it merges with a stronger system moving north out of the central Pacific Friday afternoon. This will enhance the area of likely rain from the central Aleutians to the AKPen Friday night into Saturday morning. By Saturday afternoon the upper level energy will shift south, but widespread southerly flow will likely lead to persistent light rain on the windward (south) facing slopes of the eastern Aleutians and AKPen. Meanwhile, the western Aleutians and Pribilof Islands will enjoy the sunshine as a dry, polar trough moves over the region from the northwest. && .MARINE (Days 3 through 5 - Saturday through Monday)... There is growing confidence that a weakening low pressure system moving south of the Aleutians will have the potential to bring easterly gale force winds along the north gulf coast from Saturday afternoon through Sunday morning. Elsewhere, expect winds less than 35 kts and seas less than 20 ft over the Bering and the Gulf for the Day 3 through 5 period. && Long Term Discussion (Days 4 through 7: Sunday through Wednesday) A broad upper level trough will extend across the Bering/Aleutians and across Southcentral and Southwest Alaska during the extended period. There is a high degree of confidence in this trend as good ensemble agreement exists with respect to sea surface pressure as the spread of ensemble members shows to remain within 0.8 standard deviations. These ensembles all show the same solution which is the strengthening of the Aleutian Low this weekend. The stronger Aleutian Low will support more on-shore flow across Southcentral and Southwest Alaska. The stronger on- shore flow will allow for more cloud cover, cooler temperatures and a greater chance of precipitation. Moreover, a series of southeasterly upper level waves associated with the aforementioned Aleutian Low will move over Southcentral and Southwest Alaska and increasing the chance for precipitation during the extended period. Windward locations (areas on the side of the mountains facing the Gulf of Alaska) will see the highest chance for clouds and precipitation. Leeward locations (areas on the land facing side of the mountains) such as the Anchorage Bowl, will be the areas that will have the least cloud cover and lowest chance of precipitation due to downsloping. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...Gale 177 178. FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...KO SOUTHCENTRAL ALASKA...ED SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ MARINE/LONG TERM...MF/ED