705 FXAK68 PAFC 270020 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 420 PM AKDT Wed Apr 26 2023 .ANALYSIS AND UPPER LEVELS... A large, upper-level low is centered between Cape Newenham and the Pribilof Islands. This low is anchoring a broad longwave trough that is encompassing much of the Southern Mainland, Bering Sea, and Aleutian Islands. Embedded within the trough and rotating around the upper-level low, are multiple shortwave troughs. The first of these is associated with a weak surface trough along the Southcentral coast moving northward this afternoon. An area of showers is developing along the immediate coast in response to these features. High and mid-level clouds are also spreading over interior Southcentral in advance of these features. A second, more potent wave is deepening south of Kodiak Island this afternoon. The strengthening shortwave and upper-level jet streak moving over the western Gulf are both allowing for the development of a mature surface low that is pushing a front toward the western Gulf this afternoon. Easterly winds are on the increase along the eastern coast of Kodiak Island ahead of the front. Farther west, a vertically stacked low is spinning near Cape Newenham with a secondary trough dropping south across the Pribilofs. Scattered shower activity is accompanying both features with snow reported in Saint Paul and a mix of rain and snow along the Southwest Alaska coast. Along the Aleutians, clouds continue to linger along with a cold northerly wind. && .MODEL DISCUSSION... Models remain in good overall synoptic agreement through the short-term, handling the track and evolution of the low currently south of Kodiak well as it moves toward Yakutat for Thursday. There are subtle differences with respect to a weak trough moving through Southcentral for late Thursday into early Friday morning. While all guidance depicts shower activity over Southcentral, the western extent of precipitation is more uncertain. The GFS, with a slower solution, keeps a light, southeasterly cross-barrier flow across the Kenai and Chugach Mountains into Friday afternoon, limiting the amount of precipitation for Cook Inlet and the Anchorage Bowl. Other models, however, orient the wave as to shift these winds more southerly, allowing for moisture and showers to develop for these locations. Discrepancies also arise with the western extent of a surface warm front associated with a low moving northward from the Pacific for Friday in to Saturday. The GFS continues to be the western outlier, bringing precipitation to Anchorage and the Mat-Su Valleys. The GEM is next, but keeping the bulk of precipitation from Prince William Sound east. The NAM and EC are the farthest east, with the bulk of precipitation remaining east of Valdez. Out west, models are coming into better agreement with the development and track of a low as it moves from south of Kamchatka to south of the western Aleutians. All guidance is now showing the development of a triple-point low along its occluded front by Friday with that feature moving to the central and eastern Aleutians by Saturday. && .AVIATION... PANC...VFR conditions will persist. There will be some gusty southerly winds this afternoon that should diminish into tonight. There may also be an isolated shower in the vicinity of the terminal through this evening. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Tonight through Saturday)... A large, upper-level low anchoring a longwave trough located over much of southern Alaska continues to rotate multiple waves of energy around it. As a result, a surface low has developed and is deepening south of Kodiak Island pushing a front toward the western Gulf this afternoon. Precipitation will spread into Kodiak later this evening as the low lifts northward into the Gulf towards Cordova. Tonight through Thursday morning, precipitation will overspread into the Prince William Sound region with the Cordova area mountains receiving moderate precipitation tonight. As warmer air is transfered into the region with the aforementioned low, snow levels will rise resulting in the main precipitation type at sea level expected to be rain. By Thursday the aforementioned surface low will slowly dissipate resulting in a more showery pattern as the broad longwave trough located over much of southern Alaska continues push waves of energy into the region. Overall flow will turn more southerly as upper-level flow is amplified due to a trough digging over the north-central Pacific and a ridge building downstream from the west coast of CONUS up to western Canada. By Saturday, models continue to struggle with the placement of a warm front and its western extent over the region. Most of the models have the precipitation following the more eastern track with the most likely corridor of rain from eastern Prince William Sound northward into the Copper River Basin, but the GFS is holding onto that more western extent and has it extending further into Prince William Sound and lingering the precipitation. Given all of this uncertainty, be sure to check back with the forecast as models come into better agreement and the finer details for later this week are ironed out. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight through Saturday)... A weak vertically-stacked low in the Eastern Bering Sea continues to promote showers across Southwest Alaska. These showers are expected to taper down overnight tonight as upper-level energy departs the area. However, by Thursday morning another shortwave looks to sweep through the area, supporting the return for more showers throughout the day. Additionally, this pattern is expected to be persistent, allowing the potential for rain and snow showers across Southwest into the weekend. Furthermore, the warmer easterly flow is expected to come to an end tonight, as cooler air wrapping around the southern edge of the surface low in the Eastern Bering into coastal Southwest Thursday morning. This flow may be slightly gusty as well due to the incoming temperature shift. Additionally, a shift to primarily snowfall as the precipitation type for the Greater Bristol Bay region. Warmer air yet to be kicked out of the Kuskokwim Delta and Lower Kuskokwim Valley Thursday may keep to rain or a rain/snow mix Thursday. By Friday, the air mass will be sufficiently mixed out and snow will be the predominant precipitation type across Southwest Friday into the weekend. Due to the cooler temperatures continuing to flow in, gusty southerly winds are again expected, mostly near Cape Newenham Friday afternoon. Supporting this unsettled pattern an upper- level low will establish over the Central Bering and will work to spin shortwaves of energy up into Southwest Alaska into Saturday and the early weekend. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Tonight through Saturday)... A weak vertically-stacked low in the Eastern Bering Sea continues to promote showers across the Eastern Aleutian Chain, Alaska Peninsula, and Pribilof Islands. This weakening system will continue to promote isolated and scattered showers through tonight and Thursday. By Friday morning, an upper-level low looks to drop down from the Arctic into the Central Bering Sea, reinvigorating some of the showers across the region. Additionally, this flow from the north will bring cooler temperatures across the Central Bering, depressing the temperatures for the Pribilofs by late this week, heading into the weekend. The temperature shift may promote slightly gustier southerly winds in the southeastern Bering late Thursday through Saturday morning. For the Western Aleutians, mostly benign conditions currently will be slightly disrupted by late Thursday evening as an elongated front originating from a Kamchatka low, stretches south of the Chain. This front is currently expected to bring some rain across the Western/Central Aleutians, however the core of moisture is still expected to keep just south of the Chain at that point. Until around Saturday morning when the front shows potential for spinning up a low at the triple-point which would be south of Adak/Atka and may push more significant precipitation into the Central/Eastern Aleutians by the early weekend. && .LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)... Looking ahead in the extended forecast, the latest model runs and ensemble guidance are in fairly decent agreement with the evolution and progression of the synoptic features. Starting Sunday, most of the precipitation along Prince William Sound and the southeast Panhandle tapers off as a weakening low pressure system exits the region. Meanwhile, a strong upper low over the central Aleutians moves across the southern Bering Sea. As the upper low slowly tracks eastward early next week, an upper ridge extends north across western Canada and into eastern Alaska. As such, a warm, southerly flow pattern develops, allowing a series of low pressure systems to lift northward across Southcentral, with heavy rainfall expected along the coastline. Out west, the aforementioned upper low will also support unsettled conditions across the Bering Sea, Aleutian Chain, and Southwest Alaska through midweek. To the far west, a separate ridge develops across the western Bering as a system pushes its front into the western Aleutians, bringing winds and rain to the area. && $$ SYNOPSIS AND MODEL DISCUSSION...TM SOUTHCENTRAL ALASKA...NS SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL LONG TERM...MF