Area Forecast Discussion
Issued by NWS Anchorage, AK

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000 FXAK68 PAFC 191259 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 459 AM AKDT Mon Apr 19 2021 .ANALYSIS AND UPPER LEVELS... The blocked pattern featuring a prolonged period of warm and dry conditions across much of the mainland continues today. A strong omega-shaped ridge remains anchored in place from the Canadian Rockies to the eastern interior. A weakness in the ridge that allowed upper clouds and moisture to infiltrate parts of Southcentral on Sunday has since filled in, with predominantly clear skies now returning to most of the area. Areas that remained free of clouds farther east yesterday have continued to observe soaring temperatures following peak daytime heating. This included a high of 71 F at the Cordova airport on Saturday, where weak northeasterly winds and subsidence likely aided temperatures locally with downslope warming. To the south of the ridge, a large, stacked low continues to swirl south of the Gulf as it retrogrades northwest towards the southern AKPen. The pressure gradient between the low and the ridging farther north is helping to drive northeasterly small craft to low gale force winds over the western Gulf and across the southern Bering. The low`s occluded front is also beginning to bring rain showers into the southern AKPen and Eastern Aleutians. Conditions continue to be about as low-impact and calm as one could expect elsewhere for this point in the year. && .MODEL DISCUSSION... Models are still in very good agreement at the synoptic level, and overall forecast confidence is high. However, the fate of an Arctic trough dropping into the eastern mainland near the AlCan border on Tuesday and Wednesday remains somewhat contested. The ECMWF/Canadian continue to show a more progressive and north track for this feature, keeping weak ridging intact over Southcentral through midweek. The GFS/NAM show the wave axis drop farther south, ultimately becoming entangled with the easterly wave moving across the AKPen on Tuesday. The trough then pushes more into the Susitna Valley through Tuesday night with this scenario, resulting in some precipitation potential from the Susitna Valley to the Copper River Basin. This could involve some mixing issues, especially for the Copper River Basin where the coldest air would spread into from Tuesday to early Wednesday. && .AVIATION... PANC...VFR conditions will persist. Southeast winds out of the Turnagain Arm of around 10 to 15 kts may impact the terminal at times from this afternoon through early Tuesday. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)... A vertically stacked low situated well south of Kodiak Island this morning will lift a front north into the western Gulf today and eventually into the northern Gulf by this evening. This system will bring rain and higher elevation rain/snow first to Kodiak Island then to coastal Southcentral. For interior locations, another mainly clear and dry day is on tap as the weather over a majority of the state is dominated by an expansive area of high pressure. This ridge, however, is moving east, and that will allow a southerly flow to develop across Southcentral by the afternoon which will allow for some higher clouds to advance across the interior. As the frontal system approaches, its associated upper-level shortwave will also move inland. This feature is expected to help the increasing southeasterly gap winds ahead of the surface front mix down and allow for a southerly wind to develop for places like Anchorage and Palmer this afternoon. The front is expected to stall out and weaken along the coast, keeping a lingering threat for coastal rain showers and higher elevation rain/snow showers in the forecast through late Tuesday. Also on Tuesday, a shortwave and associated cold front will drop from northwest Alaska to the Alaska Range. There are still uncertainties as to timing of this feature and just how far south it digs before exiting to the east. Right now, it looks like an increased chance for mixed rain/snow showers across the northern half of the Susitna Valley, Talkeetna Mountains, and Copper River Basin Tuesday afternoon through early Wednesday. Temperatures Tuesday will be cooler and more seasonal for this time of year as the front ushers in cooler air and increased cloud cover. However, the Rex Block pattern looks to re-establish by Wednesday, allowing for a return to clear and dry conditions along with above normal temperatures for the second half of the week. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)... The upper level high pressure ridge gets shoved off to the east late this afternoon by a longwave trough extending off a polar low. This allows a wave of energy along a trough on the backside of the North Pacific low to move across Southwest Alaska. The main impacts will be increased winds across Bristol Bay, especially through the gaps in the Alaska Range and across the Kuskokwim Delta. A majority of the precip with this system will get caught up along the Aleutian Range. Most of Southwest Alaska is expected to be dry, with relatively clear skies throughout the day Monday and clouds rolling in late Monday night. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)... The front extending off the North Pacific low moves across the Southern AKPen and into the Bering this afternoon, producing rain showers across the Eastern and Central Aleutians. The backside of the low consists of gusty northerly winds across much of the Bering, with areas of gales in the northern Bering and south of the AKPen from Monday night into Tuesday morning. This transition to colder northerly flow allows for some heavy freezing spray concerns along the ice edge in the northern Bering late Monday night and Tuesday morning. As winds begin to diminish Tuesday afternoon, the threat of heavy freezing spray will subside. A secondary North Pacific low develops Tuesday night and rotates around our existing low before merging and moving south of the AKPen. As the two systems become one, the new low strengthens and brings high end gales south of the AKPen and Aleutian Chain. There is the possibility for a period of storm force winds on Wednesday as well, however, there is still some model inconsistency with the higher end winds. Additionally, expect another round of showers to spread across the southern AKPen to the Central Aleutians. && .MARINE (Days 3 through 5: Tuesday through Thursday)... Shelikof Strait and the Gulf of Alaska southeast Kodiak looks active late Tuesday and Wednesday. Expect gusty and enhanced seas for this vicinity. With a tight pressure gradient in place, there is potential for gales to develop south of the Alaska Peninsula, and also the Eastern Aleutians on both the Pacific and Bering side. Tue through Thur...the seas over the Bering should stay below 11 feet. However, seas south of the Eastern and Central Aleutian could be upwards of 17 ft. && .Long Term Forecast (Days 4 through 7: Thursday through Sunday)... This forecast package was composed using a blend of models. There are a few items that jump out. Temperatures in Southcentral and SW AK will be spring like and numerous locations will hit or exceed the 50 F benchmark. There are a couple of systems in the North Pacific that the models don`t have a precise track locked on yet. Widespread subsidence from an upper level high will be the cause of the the warm and dry trend over Southcentral and SW AK. Overall, it will be a mild forecast period. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...Gale Warning: 138,150,155,181,185. Heavy Freezing Spray: 185. FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...AS SOUTHCENTRAL ALASKA...TM SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM MARINE/LONG TERM...PJS is the U.S. government's official web portal to all federal, state and local government web resources and services.