792 FXAK68 PAFC 280114 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 514 PM AKDT Fri Aug 27 2021 .ANALYSIS AND UPPER LEVELS... A trough of low pressure continues to push through Southcentral this evening with rain showers noted over the Kenai Peninsula eastward into Prince William Sound. This activity is occurring along a front and trough axis, which will both press further east through tonight. Rain across the remainder of Southcentral, including the Anchorage and Valley areas are gradually improving, and, overall, drier conditions are noted behind the trough axis stretching across much of Interior Alaska southward into the Alaska Peninsula. Meanwhile, northwesterly winds and widespread cloud cover blankets much of the southern Bering with only a few areas of clearing, with the Bristol Bay area experiencing the most pronounced clearing. The Aleutian range is doing a good job of filtering the cloud layer to the south and much of the Pacific side of the Aleutians. The combination of downsloping along with subsidence (sinking motion) behind the departing trough to the east, are helping to keep conditions more clear. Finally, a tightened pressure gradient over the Alaska Peninsula is contributing to increasing northwest gap winds, which are expected to persist into Saturday. && .MODEL DISCUSSION... Global models remain in surprisingly good agreement through the middle of next week with the development of a ridge of high pressure into the Gulf and broad troughing over the Bering. On the Bering side of the Aleutians, however, surface features become more muddled into early next week. General consensus between numerical guidance is boosting confidence in the development of an upper low over the southern Bering. More questions arise as models bring a smaller wave of low pressure through the Aleutians Monday into Wednesday. The GFS brings this wave northward and along northwest coast of Alaska. The EC and Canadian both split the wave, keeping one area of low pressure on a northward path and a more stationary area of low pressure over the Gulf. The latter solutions would favor a cooler and wetter pattern for Southcentral while the GFS would indicate a stronger ridge over the Gulf along with warmer and drier conditions. Thus, despite high confidence in the larger picture, a lower confidence forecast will prevail in the short term with regard to temperatures and precipitation from Monday into Wednesday of next week. && .AVIATION... PANC...VFR conditions with ceilings at or above 5000 feet will give way to clear skies through Saturday morning as high pressure builds into Southcentral. Lingering showers may persist for the next few hours. Expecting elevated north-northwest winds through about 09z but the extent of the wind gusts remains less certain. Sustained winds in the mid teens appear reasonable, with the potential for some gusts to near 25 knots or greater through the early morning hours tonight. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)... A trough with an embedded shortwave will continue to push across Southcentral and dig southward this evening. Temperatures aloft have cooled dramatically with a pool of cold air air ranging from -6F to -8C (at 700 mb) over the Kenai Peninsula, Anchorage, and Eagle River. Snow was observed at approximately 4,000 feet elevation this afternoon near Eagle River. Meanwhile the temperatures aloft over Talkeetna are ranging between -10 to -12C (at 700 mb). Once these clouds clear out we expect to see a fresh blanket of snow on the Talkeetnas and the Chugach Mountains. The aforementioned shortwave will dive south over the Gulf marrying up with another feature on Saturday. While Anchorage will be dry on Saturday, the Gulf Of Alaska, portions of the Copper River Basin and Wrangell St Elias National Park can anticipate more rain (and snow in the higher elevations) on Saturday. By Sunday, the majority of Southcentral will be dry. In the wake of this system, afternoon temperatures will rebound into the 60s for the Anchorage Bowl and the Mat-Su Valleys. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight through Monday)... In the wake of the exiting upper level trough and associated surface front moving east, northwesterly cold air advection will spill across much of the mainland through the rest of today. A combination of mostly clear skies, calm winds, and an unseasonably cold airmass will allow temperatures to drop down slightly below the freezing mark later tonight across the Kuskokwim Valley. Temperatures are expected to rebound on Saturday as a low- level ridge builds in over the area with westerly flow aloft and a warmer airmass. Along the coast, low stratus trapped underneath the weak ridge will return this evening and will likely persist through the weekend. Areas of patchy fog were added into the forecast for Bristol Bay early Saturday morning, with conditions improving thereafter. A couple of shortwaves will transit the northern half of the state over the weekend, bringing increasing cloudiness and chances of rain over the Y-K Delta coast and Kuskokwim Valley. The ridge will slightly weaken, but will hold firmly in place during that time and keep the bulk of the precipitation remaining north of the our forecast area. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Tonight through Monday)... A ridge of high pressure aloft is helping to maintain low stratus and areas of fog across much of the Aleutians and Bering Sea today. Additionally, small craft northwesterly winds will continue funneling through gaps and passes along the southern AKPEN through Saturday night. As the ridge axis shifts east over the weekend, a deepening trough over the western Bering will send a pair of North Pacific lows near the Aleutian Chain, with one low approaching south of Adak and another low approaching the western Aleutians near Shemya. The associated fronts will bring rain beginning Saturday afternoon and continuing through early next week. Otherwise, winds will remain below gale force with no high seas expected. && .MARINE (Days 3 through 5: Monday through Wednesday)... Expect winds less than 35 kts and seas less than 20 ft for the Day 3 through 5 period. The only potential exception could be some isolated gale force winds in the far northwestern Gulf of Alaska on Wednesday. && .LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)... The overall progression of the large scale pattern over the next several days is in decent agreement across operational models and their ensembles, but smaller scale details remain a fair bit more obscured at the extended timeframe. The start of the period on Tuesday will feature a fairly amplified pattern, with a longwave ridge extending across the Gulf and across much of the southern/western mainland. To the west, a complex upper low with numerous smaller embedded waves will continue rotating across the Bering and Aleutians. On Wednesday, the longwave ridge axis will begin to migrate east across the AlCan border, paving the way for a deep fetch of moist, southerly flow to expand into Southcentral ahead of a negatively-tilted trough axis pivoting from the North Pacific into the western Gulf. A surface low is likely to form out ahead of this trough and lift north across the AKPen as its trailing front and gusty southeast winds trek across the Gulf roughly between Tuesday and Thursday. Models are still diverging markedly on the progression of both of these low level features. The GFS is currently the fastest with the low and frontal progression, bringing the low center all the way up to the Y-K Delta by Wednesday morning. The ECMWF is much slower and does not even bring the low into the Bering, eventually stalling out the center near Kodiak Island by Friday morning. Regardless of the exact timing and progression of the low and trailing front, the eastward advancement of the ridge and upstream trough will likely mark a pattern change from clear/dry to wet and unsettled over Southcentral by the middle of next week. The generally wet and cloudy pattern will likely linger through Friday with broad southerly flow aloft continuing around the ridge that will be drifting further into British Columbia and the Yukon. A generally cool and showery regime will persist during nearly the entire period under the large upper level low across the Bering/Aleutians and Southwest, with periods of more moderate rainfall following the individual embedded waves/lows rotating through the large scale cyclonic flow. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...Gale 130 131. FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...JPW SOUTHCENTRAL ALASKA...PJS SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF MARINE/LONG TERM...MK/AS