047 FXAK68 PAFC 100023 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 423 PM AKDT Tue Oct 9 2018 .ANALYSIS AND UPPER LEVELS... A mature closed low over the southern Bering Sea anchors a high amplitude trough over the Bering Sea and North-Central Pacific. An unstable air mass is associated with this low and is producing showers and gusty winds, especially with a short-wave on the south side of the low passing right along the Aleutian chain. An old occluded front from this low is oriented east-west across the northern Bering Sea, then southeastward and over Southwest Alaska. This is producing areas of rain from Bristol Bay to the Kuskokwim Valley and Kuskokwim Delta. On the front side of the upper trough a weak surface low and associated frontal system are moving slowly northward across the western Gulf of Alaska, with light rain spreading as far north as the eastern Kenai Peninsula. This low is weakening as heights rise ahead of an intensifying low south of the Alaska Peninsula. This second low is in a good spot ahead of a strong short-wave rounding the base of the upper trough and near the left exit region of strengthening jet streak. This low will be the primary weather producer for the southern mainland of Alaska over the next 24 to 36 hours. && .MODEL DISCUSSION... Models remain in good agreement with large scale features over the next couple days. The Canadian solution is an outlier on Friday with phasing of upper short-waves and development of a surface low which it ejects out of the trough and into the eastern Bering Sea. While this solution is plausible will need to see more models get on board before thinking about buying into it. The primary forecast challenge is the very localized details of wind and precipitation across Southcentral and Southwest Alaska tonight through Wednesday as this major storm moves right across the region. && .AVIATION... PANC...Clouds will continue to lower as a weak front approaches from the south this afternoon and evening, but strengthening southeasterly low level flow will keep ceilings safely in the VFR category. Low level wind shear will become more and more likely overnight into Wednesday morning out ahead of a deep low moving up from the Pacific. Surface winds will remain out of the north- northwest as southeasterly flow strengthens above this. The worst period of shear will be just prior to frontal passage - approximately 15Z to 20Z. As the front pushes through, the Turnagain Arm jet will bend strongly into the terminal then shift to up Inlet (southerly) flow shortly later. While downslope flow will maintain dry conditions for much of the period, will probably see some light rain as a strong vorticity- maximum tracks up Cook Inlet between 12Z and 18Z. This could briefly lower ceilings to 5000 feet or lower, but would expect ceilings to remain VFR. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)... A warm front stretching across the northern Gulf will stall just south of the coast this evening. Along the front, weak gales will continue with rain falling primarily along the coasts and western Cook Inlet. A strong southeasterly, cross-barrier flow aloft will act to keep locations in the lee of the Chugach mountains on the drier side through tonight. The well-advertised stronger of the two surface lows continues to trek northeast toward Kodiak Island this evening. ASCAT scatterometer winds are indicating gale-force winds beginning to wrap around the center of circulation as the low continues to intensify. This system will move over eastern Bristol Bay Wednesday morning. The low will then weaken as it heads over the western interior Wednesday afternoon. A secondary triple point low will develop along the decaying low's frontal trough in the vicinity of Shelikof Strait overnight and head up Cook Inlet Wednesday afternoon carrying what is left of the front with it. Strong east/northeast gales to storm-force winds will develop over the western Gulf tonight, especially along both the eastern and western coast of Kodiak Island. Winds will shift to a southerly direction, due to strong pressure rises on the backside of the triple point, Wednesday morning as the front moves inland and the low tracks up Cook Inlet. Very strong gap flows are expected in favored locales, Turnagain Arm, Portage Valley, Matanuska Valley, Knik Arm, and along the Copper River, to name a few. Winds should diminish across most areas Wednesday evening/night. As with the first low, a cross-barrier flow will limit precipitation for areas in the lee of the Chugach Mountains. However, precipitation is expected to increase for thees locations as the low works its way up Cook Inlet Wednesday afternoon/early evening and the overall flow shifts to the southwest. The longwave pattern driving steering these systems will remain in place through the second half of the work week, with high pressure sitting over the eastern Pacific and a low over the Bering. The resulting southwesterly flow will keep clouds and moisture over Southcentral. Another surface low will move from the north Pacific into Bristol Bay late Thursday with more rain and brisk southeasterly winds for Southcentral Thursday night into early Friday. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight through Friday )... A strong low, the extratropical remnants of Typhoon Kong-Rey, is currently centered a couple hundred miles south of Cold Bay. The low center will make landfall along the Alaska Peninsula in the vicinity of Chignik tonight, then track north along the west side of the Alaska Range on Wednesday. This will bring widespread rain across all of Southwest Alaska, along with strong and gusty winds, especially through the gaps. Winds will be southeasterly ahead of the low, then will turn out of the southwest behind it. Those southwesterly winds may cause erosion issues along the Bristol Bay coast from Port Heiden northward during the day Wednesday. As the center tracks northwestward to the southern coast of Norton Sound Wednesday evening, the associated rainfall will gradually end from south to north across Southwest Alaska. Expect gale force winds along the coast until long after the low passes. The low will remain nearly stationary off the coast to the east of St. Matthew Island on Thursday, as a secondary low develops and moves ashore around Kuskokwim Bay Thursday afternoon. This will bring another round of rain with gusty winds along the coast during the day Thursday. Another disturbance tracks northward into Southwest Alaska during the day Friday, though the models are having a difficult time resolving the timing of this feature. Regardless, it will mean a very unsettled weather pattern across the area straight through Friday with any breaks in the precipitation expected to be short. Impact-wise, those along the coast who have south-facing exposures should be especially cognizant of a very persistent onshore southerly wind flow through Friday, which will likely cause localized erosion concerns at each high tide. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Tonight through Friday)... A vertically stacked low currently over the central Bering will act as the focus around which numerous lows and impulses of energy will cause their own localized steady rains and gusty winds. The first such low will be the strongest of the lows to impact the area through the period. It is centered south of Cold Bay and will track into Southwest Alaska tonight, this will lead to broad northwesterly winds across the eastern Bering as the low tracks into mainland Alaska. Meanwhile another low centered off Kamchatka will track southward into the western and central Aleutians on Wednesday, bringing widespread small craft conditions to that area. The low will track into the eastern Aleutians Wednesday night, then north into Kuskokwim Bay on Thursday. These lows will help bring the vertically stacked low further east with time, keeping the central and eastern Bering very unsettled while the western Bering gets a short break Thursday afternoon through Friday morning. Another strong low moves into the western Bering and Aleutians Friday afternoon and evening. && .LONG TERM FORECAST (Days 3 through 7)... The mid to long term begins Friday with the models in overall agreement with a low over the eastern to northern Bering Sea and a ridge of high pressure extending into southeast Alaska. The models have also come into good agreement with respect to the next storm on tap for the western Bering Sea, moving into the western Aleutians late Friday. This storm is taking a track along the Aleutian chain to the western Alaska Peninsula through Monday morning. This is where the models then begin to show more significant differences in the solutions for it. At that point the ensemble and WPC solutions will be used. The current forecast have a slightly faster track for the low along the Aleutians, and we will nudge toward this solution, primarily resulting in adjusting the wind direction to better align with the WPC solution. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...None. MARINE...Gale 119 120 125 131 136 137 139-141 155 160 173-177. Storm 130 132 138 150. && $$ SYNOPSIS AND MODEL DISCUSSION...SEB SOUTHCENTRAL ALASKA...TM SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW LONG TERM...SA