339 FXAK68 PAFC 181323 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 523 AM AKDT Fri Jun 18 2021 .ANALYSIS AND UPPER LEVELS... The upper levels continue to be dominated by a broad, long-wave trough extending from the western Bering and Aleutians east across the North Pacific to the Gulf of Alaska. To the north, an upper-level ridge is centered over the Yukon Territory and extending over the northern half of the state down to the eastern Bering. Embedded within the long-wave trough are a series of upper-level lows and shortwaves. The first of these is an easterly wave that continues to move west across the western half of the state and is bringing scattered showers to the YK-Delta. A second wave is moving west toward the Al-Can border this morning. This feature will be the focus for the next round of scattered showers and isolated thunderstorms across interior Southcentral Friday afternoon and evening. At the surface, a trough moving into the western Gulf is producing an area of rain from southern Cook Inlet to Kodiak Island this morning. Ahead of this trough, and behind the main upper-level wave to the north, a surface ridge has developed across the southern half of the state. Subsidence underneath the ridge has kept conditions generally clear and dry through the overnight hours. A surface ridge over the Bering is also keeping conditions rather tranquil, albeit with plenty of low stratus still lingering underneath the ridge axis. To the south of the ridge, an area of low pressure is spinning well south of Adak. This feature continues to produce gusty easterly winds along and south of the central and eastern Aleutians as it meanders south into the north Pacific. && .MODEL DISCUSSION... Models have come into better agreement with the progression of shortwaves moving across the Southern Mainland through Saturday night. However, subtle differences remain in the track and timing of these waves as they progress across the state. Thus, a broad- brush approach to the forecast was maintained for the morning forecast package. Guidance continues to key in on a stronger surface low tracking from the North Pacific to an area south of Kodiak Island for Sunday morning. The result of this will be an expansive area of rain and gusty northeasterly winds for Kodiak Island and along the Southcentral coast. Models are also in good agreement with a low approaching the western Aleutians by late Saturday with its front moving across the western half of the Chain. Thus, forecast confidence is good through the weekend. && .AVIATION... PANC...Expect another round of southeasterly winds to develop later this afternoon. The pressure gradients do not look overly strong, so this should be a weak to moderate event. The timing could be delayed by an hour or two. Ceilings and vis should remain VFR. As some showers pass through later tonight, expect ceilings to gradually lower. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Fri through Sun)... Cyclonic flow over the area will lead to a mix of sun and clouds over the mainland with continued rain for Kodiak and the Gulf Coast. After a surprisingly nice day on Thursday, conditions should remain dry for many of the population centers (Anchorage, Palmer, Kenai) again today. A stagnant upper level low near Kodiak Island will grab onto another wave of energy from the Eastern Gulf later today. This energy will then get absorbed into the upper level low as it persists near Kodiak. This will serve to reinforce the continued wet weather for Kodiak. It will also start to increase precipitation chances for the Gulf Coast, primarily the southern Kenai Peninsula. Meanwhile, within all of this cyclonic flow, another shortwave will trickle into the area from the Yukon Territory. This feature should help trigger some thunderstorms across the Alaska Range and Wrangell/Talkeetna Mountains this afternoon and evening. As we head into the overnight and Sat morning, all of this energy will start to consolidate over the entire area. This could aid in some of the afternoon convection being able to "survive" past peak heating into the late evening. The general storm motion should be mostly east to west, so expect increasing chances for showers for the Mat-Su Valleys by Sat morning. At this time, it looks as if Anchorage should remain mostly dry, but we will be taking a close look at that today. Gusty southeast winds through the typical gaps (Turnagain/Knik Arms and the Copper River Basin) should easily develop on Sat. By Sat afternoon, a stronger area of low pressure will start to approach the Gulf from the North Pacific. This will finally displace the upper level near Kodiak, forcing it to eject over the southern mainland. This will cause all areas to be vulnerable to showers, with more steady rain expected along the coast. It will also bring another round of possible thunderstorms to the Copper River Basin. The new North Pacific low will bring strong (gale force) winds to the marine areas and as it drives more rain from Kodiak up through Prince William Sound. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Fri and Sat)... A quasi-stationary upper level low will remain parked over the AKPen through the forecast period. This will continue somewhat active weather over the region as easterly waves pass north of the low center. A break in cloud cover over the Kuskokwim Valley and Delta early this morning has lead to some valley fog development across the area. If skies remain clear, some instability will build over the Kuskokwim Valley this morning and into the early afternoon, which may kick off a few brief showers in the afternoon to evening. Instability parameters are low and with only weak upper level waves thunderstorms are not expected. On Saturday expect afternoon thunderstorms to develop as an upper level low currently in the Gulf rounds the northern side of the AKPen low and into Southwest Alaska. There is some uncertainty remaining in the timing of this shortwave and how far north it will move. The uncertainty likely stems from another, stronger low moving east along the 45th parallel today before interacting with the AKPen low Sat morning. There's a chance this stronger low will keep the easterly shortwave from coinciding with the area and timing of maximum instability over the Kuskokwim Valley Saturday. Regardless, there should be enough of a kick from the shortwave for the marginal instability to break the cap and produce a few isolated thunderstorms. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Fri and Sat)... Seasonal weather will persist over much of the Bering through Saturday. Fog, low stratus, and mostly calm winds are expected over the central and eastern Bering due to a weak region of high pressure. The western Aleutians will see a gale force front and rain move in Saturday morning as a Kamchatka low pulls and merges with a stronger system currently in the northwest Pacific. Due to the strong temperature gradient along the front, the storm will strengthen on its way into the region. However, with the region of high pressure over the eastern Bering, the front is expected to stall and weaken over the western to central Aleutians on Saturday. && .MARINE (Days 3 through 5/Sunday through Tuesday)... Forecast confidence has greatly increased with a low in the Gulf of Alaska on Sunday. Now expect widespread small craft advisory winds, with smaller areas of gale force winds along the Gulf coastal waters. This low will rapidly weaken Sunday night through Monday. Meanwhile, a storm out along the western Aleutians will exit westward Sunday, with gales lingering over the western Bering Sea. The next storm of interest looks like it will track northward from the Pacific toward Kodiak Island on Tuesday. The track and intensity of this low are very much in question at the moment, so stay tuned. && .LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)... An amplified Rossby Wave pattern is expected during the extended period due to a deepening of the Aleutian Low combined with upper level ridging extending northward across the eastern Gulf of Alaska. This pattern will result in stronger prevailing onshore flow across Southcentral. This will lead to extensive cloud cover, cooler temperatures and a greater chance of precipitation. A series of easterly upper level waves will move through Southcentral and Southwest during the period. The easterly waves combined with ample moisture is likely to bring rain to the area. Ensemble guidance shows normalized precipitable water anomalies to be 2 standard deviations higher than the mean, suggesting the atmosphere will have a significantly higher moisture content than normal. The ample cloud cover and cooler temperatures will help suppress thunderstorm activity across Southcentral. If any thunderstorms form, they are likely to be confined to the Alaska Range and Talkeetna Mountains due to the strong prevailing onshore flow. Out west, the deepening Aleutian Low will bring strong southeasterly gap winds along the Alaska Peninsula and eastern Aleutians on Wednesday. Across the western Aleutians, cold air advection on the back side of the Aleutian Low will lead to widespread stratocumulus with isolated to scattered showers. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...Gale Warning 178. FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...TM SOUTHCENTRAL ALASKA...MSO SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ MARINE/LONG TERM...ED