578 FXAK68 PAFC 220055 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 455 PM AKDT Wed Jul 21 2021 .ANALYSIS AND UPPER LEVELS... A broad upper level low is maintaining its hold over the northern Bering and northwestern mainland. A second low has developed over Kodiak Island and its associated front is lifting inland over Southcentral. Cloudy skies and persistent showers have filled in across the Southcentral area and are extending into the northern Susitna Valley. Over Southwest Alaska, some residual showers are slowly pushing toward the northeast. Persistent low level stratus remains over the Bering Sea with generally zonal flow in place. Low level stratus has also remained across the northern Gulf, with communities across Prince William Sound seeing foggy and rainy conditions. && .MODEL DISCUSSION... Model solutions are in great synoptic agreement. There is consensus that the low currently in place over Kodiak Island will drift southward by Friday. Models continue to advertise an easterly wave pushing across the Southcentral region Friday into Saturday. The main forecast challenge with this easterly wave will be the timing of progression from east to west. Overall, there is increasing confidence that there will be moderate to heavy rainfall with this feature for the early part of the weekend. Farther west, models advertise an upper level ridge building over the western Aleutians and central Bering for Friday. This will create a more amplified pattern heading into the weekend. && .AVIATION... PANC...Ample moisture streaming northward over Southcentral will keep intermittent MVFR ceilings and rain at the terminal. Gusty southeasterly Turnagain Arm winds persist, though are expected to remain out of the terminal. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through Day 3: This afternoon through Saturday)... Low pressure has formed near Kodiak Island with an occluded front extending south and east across the Gulf. Several weak shortwaves will rotate around this low and produce rain north of the coastal mountains to include the Anchorage and MAT-SU valleys through Thursday morning. A ridge of high pressure is bringing drier air to the Copper River Basin, so they will see mostly sunny conditions with a few showers and an isolated thunderstorm possible over the Wrangell Mountains through this evening. As we head into Thursday afternoon much of the area will dry out with the exception of Prince William Sound to Kodiak where the low will move into the southern Gulf keeping rain going for those near coastal locations. Expect a bit warmer temperatures from the northern Kenai Peninsula to Anchorage and the MATSU valleys. A few showers may develop on the higher terrain north of the coastal mountains, but most valleys will only have a slight chance of any rainfall. There will be a little bit of instability near the Wrangell Mountains, so an isolated thunderstorm can't be ruled out. Thursday night and Friday morning an easterly wave will move across the Copper River Basin and spread rain east to west as we head into the afternoon. So there will be a very good chance of rain for Anchorage and the MAT-SU valleys by Friday afternoon and evening. This shortwave will keep much of the area under cooler conditions with highs mostly in the 60s. Saturday expect similar conditions to Friday with additional weak easterly waves keeping the area cloudy with periods of rain throughout the day. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)... The longwave pattern features broad troughing from the Central Aleutians to the Gulf Of Mexico. On satellite imagery and radar (Bethel and King Salmon) the remnants of a frontal boundary are discernible. Also on satellite (CIMSS MIMIC Precipitable H2O) there is a swath of modified tropical air as far north at the Point Lay and the Chukchi Sea. This moisture is also over Lime Village, Kipnuk, Pilot Point and Bristol Bay. Expect more showers this evening and tonight. In the wake of the frontal passage there is an upper level low with multiple shortwaves embedded in it. Expect cool and cloudy conditions with intermittent showers for Southwest Alaska the next few days. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)... Showers are lingering over the Eastern Aleutians which are the remnants of a frontal boundary. At the onset of the forecast period the jet is zonal across the Aleutians and at the surface over the Bering. As a ridge builds into the western Bering the jet and the longwave pattern become more amplified late Thursday. At with time there is a ridge axis from the North Pacific..to Attu Island and Kamchatka. Meanwhile the longwave trough will encompass the Eastern Bering, Mainland Alaska and the Gulf Of Mexico. The next 2 systems to impact the Western Aleutians are expected Thursday and Saturday. However, neither storm looks impressive. The Thursday system will be 1016 mb when it nears Attu and the Saturday system will be at 1018 mb when it nears Attu. Both of these systems will bring rain but nothing ominous. && .MARINE (Days 3 through 5: Friday through Sunday)... A West Central Bering low weakens to a trough as it moves towards Norton Sound by Monday. Its trailing front extends across the Aleutians, with gusty winds less than gale force North of the Aleutians through Monday. Local gale force winds occur South of the Aleutians for Sunday. A North Pacific low pushes into the Central Aleutians by Monday. Forecast confidence is good. A Southern Gulf low exits into the North Pacific Saturday. Gusty winds less than gale force with the low Saturday. High pressure builds across the Northern Gulf through Monday. Seas less than 10 feet across the region. && .LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)... The pattern will be highly amplified during the extended period with a Rex Block over the Alaskan mainland and Gulf of Alaska. The upper ridge associated with this Rex Block will extend over the Alaskan mainland while an upper low remains over the Gulf on Sunday. Forecast uncertainty increases on days 5-6 when ensembles try to break down this Rex Block. Ensembles have a history of breaking down blocks too quickly. With respect to sensible weather, this means warmer temperatures, drier conditions and more sun than clouds are expected for Southwest Alaska. Uncertainty is higher across Southcentral as a small shift in the placement of the upper ridge associated with this Rex Block will greatly affect the weather over Southcentral Alaska. Due to this, Southcentral has a better chance of precipitation, cooler temperatures and more cloud cover as easterly upper level waves move near the area. For the Bering/Aleutians, the unsettled but benign weather continues as weak surface lows move southeastward off of Siberia towards the eastern Aleutians. These lows will bring rain and strong winds. For now, winds are expected to remain below gale force. No high seas are expected at this time. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...NONE. FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...KO SOUTHCENTRAL ALASKA...RC SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS MARINE/LONG TERM...KZ/KC/ED