582 FXAK68 PAFC 141326 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 526 AM AKDT Wed Oct 14 2020 .ANALYSIS AND UPPER LEVELS... The upper level pattern continues to be predominantly influenced by a longwave trough extending from the southern mainland to the North Pacific. A much smaller low remains embedded in the upper trough as it ambles slowly eastwards over the southern Gulf, and the Pacific jet remains offset from the region just to the south of the small low center. A sharp ridge is now beginning to encroach on the Bering from the west with northerly flow increasing over the remainder of the Bering along with predominantly stratocumulus coverage under the building surface high pressure. Another band of stubborn cloud cover currently stretches from the Kuskokwim Valley to the Tanana Valley and northern Alaska Range. However, the persistent fog in the Southwest that had impacted the area for several days on-and-off appears to have finally abated. To the north, a shortwave is digging southeast from the Seward Peninsula with notable Cold Air Advection (CAA) occurring on the trailing portion of the wave in the low to mid levels. This cold advection is resulting as the push of northerly flow taps into a reservoir of polar air pooling into the northern interior where surface temps in the 10s are common. This colder air mass has yet to make it into the forecast area this morning, and recent observations at lower elevations region-wide remain mostly in the 25-40F range for now. && .MODEL DISCUSSION... Models are now in good agreement with the trough and related pulse of cold air over parts of Southcentral from today into early Friday. One notable shift is that the track of the core of the polar air has moved a bit more northeast in most recent solutions, and this should result in somewhat higher temperatures over the Southcentral and Southwest than what earlier model projections indicated for Thursday and Friday. The arrival of the trough and associated colder air has also been pushed back enough that low level temperatures and upper forcing are no longer expected to be supportive of any meaningful rain/snow mix reaching the valley floors in most of the Mat-Su Valley from late this evening into Thursday morning. Most models now limit any noteworthy snow amounts to areas farther east across parts of the western Chugach Range to the Wrangells. That said, determining exact snowfall amounts/locations and onset of cold outflow winds could still be tricky. && .AVIATION... PANC...VFR conditions and light winds will persist through early this afternoon. However, ceilings my dip to MVFR with rain showers later this afternoon and evening. Conditions will improve with the passage of a front early Thursday morning with ceilings rising to VFR and showers ending. Winds will shift to the North at 10 kts or less with this front for the rest of Thursday morning. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today through Friday Night)... This afternoon and evening's brush with winter is looking less impressive this morning. Some notable changes in the forecast that make the chances of snow less likely at lower elevations include the cold air now expected to essentially entirely remain north of the Alaska Range, the upper level trough that will be causing the precipitation will be weakening as it moves east across the area, and most of the precipitation will occur at the diurnally warmest time of day. Due to these three factors, snow is no longer expected in the lower elevations. The chances of stronger shower activity embedded within the deformation zone resulting in localized snow mixing in in the heavier precipitation have also diminished. There is still likely to be some rain shower activity across Anchorage and the Mat-Su this afternoon and evening. Snow is still expected at the higher elevations today, but is unlikely to amount to more than an inch or so except around Hatcher Pass, where 2 to 4 inches are expected. Another change from previous forecasts is since the upper wave is providing the forcing, once it passes over the area after midnight tonight, the precipitation will end with it. Thus, no precipitation is expected for the Thursday morning commute. As the wave moves east early Thursday morning, an approaching strong upper level low will help invigorate the wave as it crosses over the Copper River Basin. Thus, a few inches of snow are more likely late tonight into Thursday morning from Thompson Pass north to east of Glennallen. The wave will cross into Canada Thursday afternoon, ending all but isolated upslope snow shower activity along the Wrangells. Behind the wave on Thursday, skies will clear from west to east. As mentioned above, very little if any cold air will accompany the high pressure building into the area. Thus, nudged temperatures up area-wide for both highs and lows the next couple days. We have reached the time of year where, absent cloud cover and wind, temperatures fall as a net energy balance from day to day. Therefore, the coldest temperatures of the season are still expected for many areas Friday and Saturday mornings, but that will largely be the result of effective radiational cooling due to clear skies and light winds, and not because a colder air mass as moved in. The exception to this will be the Copper River Basin, where the polar cold air mass will actually move into that area Thursday night. Thus, Glennallen and surrounding low elevation areas could very possibly threaten below zero temperatures by Saturday morning. The cold air moving into this area will also strengthen the gap winds through Resurrection Bay, Valdez, and the Copper River Delta. These too will have warmer overnight lows since the wind will keep temperatures up. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)... A fairly benign but cold weather pattern is expected for Southwest Alaska today and Thursday with high temperatures around the freezing mark in the Kuskokwim Valley to the upper 30sF along the coast. Tonight will be the coldest night of the week as clear skies and light winds will prevail, allowing for ample radiational cooling to take place. Overnight lows will range from the middle 10sF across the Kuskokwim Valley to upper 20sF and low 30sF along the coast. Temperatures will begin to rebound on Friday as winds shift to out of the southwest and bring warmer air and an increase in clouds to the region. High temperatures on Friday will range from the middle to upper 30sF in the Kuskokwim Valley to 40sF across the Kuskokwim Delta and along the coast. Rain chances increase on Saturday as a front nears the western coast. Rain and showers will be the most likely along the coast and least likely across the Kuskokwim Valley. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)... A front will move into the western Bering/Aleutians this morning and will bring an increase in winds and seas to the area. This front will move eastward while gradually weakening. The front will reach the central Bering/Aleutians Friday morning, then become nearly stationary while rapidly weakening over the eastern Bering/Aleutians on Saturday. Widespread gale force winds are expected today through Thursday. Storm force winds are expected in the gaps between islands due to these areas being prone to localized funneling from the south. Gale force winds will become less widespread around the central Aleutians on Friday, though gale force winds are expected in gaps prone to funneling from the south. Winds will then ease to below gale force on Saturday. Seas will build to 20-25ft across the western Bering/Aleutians tonight. These 20-25ft seas will then move to the central Bering Thursday night and gradually abate Friday onward. These seas will remain long in period. && .MARINE (Days 3 through 5: Friday through Sunday)... Gulf of Alaska: Late Friday into Saturday there is significant model disagreement on the track/strength of a developing low in the North Pacific. For now, our forecast calls for this to low to stay weak and far enough south to limit impacts to a slight increase in easterlies across the southern Gulf Friday night into Saturday. However, if this low develops further north and stronger, as some models currently indicate, there could be a brief period of of higher end small craft winds or low end gales in the southern Gulf on Saturday. There is good agreement on lower winds making a return on Sunday. Bering Sea: No significant forecast concerns with the current marine outlook during this period. && .LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)... On Saturday a weakening front will be found across the eastern Bering and western mainland as an upper trough digs into the Bering. There is significant model disagreement as to whether this Bering trough will phase with a cutoff low in the central Pacific later on Saturday, with the GFS showing considerable phasing, while the Euro is more progressive. Our forecast leans towards the less phased European solution, with a shortwave kicking off the development of a low in the North Pacific early Saturday and this storm system just barely scraping the southern Gulf of Alaska before exiting south/east of our area by late Saturday. This yields minimal impacts outside of a brief period of enhanced easterly winds in the southern Gulf early in the weekend. Sunday into Monday there is decent model agreement on a weak low developing south of the Alaska Peninsula tracking towards the Northern Gulf Coast. Impacts from this system look limited to just some slightly increased rain (and higher elevation snow) chances. By Tuesday models a bit of a wreck with the GFS indicating a Siberian low working into the Bering while the Euro builds in a strong ridge for the same period. For now I would just expect increasing storminess across the western Aleutians and minimal weather concerns for the eastern Bering/Aleutians through the mainland. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...Gale: 178. FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...AS SOUTHCENTRAL ALASKA...JW SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED MARINE/LONG TERM...BG