FXAK68 PAFC 220154 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 454 PM AKST Thu Feb 21 2019 .ANALYSIS AND UPPER LEVELS... There is a fairly strong vertically stacked low over the northeast Gulf rapidly moving east. Behind this low a ridge of high pressure is building over southern Alaska, both at the surface and aloft. Strong offshore winds are occurring over and near the Southcentral coast. Most of southern Alaska is dry, but some showers are lingering over the lower Kuskokwim Valley. There is a large upper low over the western Bering Sea. A strong surface low (around 970 mb) is associated with this low, and a front stretches from the western Bering to near the Pribilof Islands, and then south-southwest across the central Aleutians into the north Pacific. High end gale-force winds are associated with this front. && .MODEL DISCUSSION... The numerical models are in very good agreement across southern mainland Alaska through Saturday afternoon. Further west, there are some differences in how the models handle a low moving across the central/western Aleutians Friday afternoon and then northward across the central Bering Friday night and Saturday. The American models (GFS and NAM) are further west, a little faster, and keep the low strong longer than the ECMWF and Canadian GEM. In general, forecast confidence is above normal. && .AVIATION... PANC...VFR conditions and light winds will most likely persist. There is a chance of some fog early Friday morning, but the likelihood is low, so it was handled with VCFG for now. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)... (Tonight through Sunday) A much quieter and less challenging forecast is in store for the region. In general, a ridge of high pressure will develop across southern Alaska on Friday. So while the day with start with gusty outflow winds in the Prince William Sound and Copper River Delta regions, these will be winding down through the day. Then, the ridge builds eastward and repositions its center over the Kenai Peninsula/Anchorage Bowl. This will lead to dry conditions through the period, with the main weather concern being fog development, as a steep low-level inversion develops by the weekend. For most locations, morning fog and low stratus will gradually give way to at least some breaks in the clouds, with the exception to this rule being across parts of the Copper Basin, where the fog may be more dense with less potential for the stratus to scour out during the max heating cycle (afternoon). The one challenging part to the forecast in this pattern will be temperatures. Depending on where fog sets up and how long it lingers through the day can wreak havoc on maximum and minimum temperatures (fog/stratus keeps it significantly warmer at night and colder during the day). && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tonight through Sat)...A vertically stacked low centered over the Bering Strait continues to weaken. While cold dry air from eastern Siberia wrapping into the south side of the low could trigger showers in the Kuskokwim Mountains this evening, most of southwest will continue to see quiet weather and areas of blue skies until tonight. A ridge over the central and eastern Aleutians will maintain dry conditions downstream, with patchy fog in Bristol Bay. Farther north in the Kuskokwim Delta and Kuskokwim Delta Coast, expect a transition to wetter conditions tonight. South flow ahead of a gale force low in the Western Bering will bring increased south winds and possible precipitation in the Kuskokwim Delta tonight into Saturday, with precip types initially snow. Areas of blowing snow are possible along the Kuskokwim Delta coast/Nunivak Island tonight, but considering projected temperatures are just below freezing and low forecast precipitation amounts, do not expect significant impacts other than possible periods of lowered visibility. A secondary low developing in the baroclinic zone in the north Pacific will deepen and push northward into Saturday. This will deepen the southerly/southwesterly fetch of warm, moist air warming conditions to above freezing over most of the mainland by Saturday, though any precipitation should stay offshore. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...A gale force low roughly 250 miles north of Attu will continue to impact the Bering Sea, Aleutians, and Pribilof Islands into Friday. The low is weakening and shifting northward, but a secondary low forming in the associated baroclinic zone in the north Pacific will deepen and shift northward tonight into tomorrow. This will support persistent southerly winds and moisture advection ahead of the low. Precipitation will continue over the central Aleutians into the Bering including the Pribilofs through much of Friday, with snow in the Pribilofs changing over to rain by early afternoon. The low will shift northeastward Saturday and the focus of precipitation will shift to the eastern Aleutians as brief ridging develops to the west. Another potent low will approach the Western Aleutians Sunday. && .MARINE (Days 3 through 5)... Once again, the main area of active weather with strong wind will be over the Bering Sea Sunday through Tuesday. Sunday begins with a ridge extending across the eastern Bering Sea with a large strong storm crossing the western Aleutians. This storm should pack storm force winds on its leading edge with the potential for a brief period of hurricane force winds possible in the Central to Western Aleutians as well as the western Bering Sea offshore zones. By Monday the low weakens but we should still see some storm force winds close near the center of the low over the Central Bering Sea with gales across much of the eastern Bering Sea to the southwest coast. By Tuesday, this storm continues its northward trek as yet another gale to storm force low crosses the western and central Aleutians. Meanwhile the Gulf of Alaska remains under high pressure and we are not anticipating any significant wind or waves for the Gulf region Sunday through Tuesday. && .LONG TERM FORECAST (Days 3 through 7...Sun through Thu)... For the extended period, high pressure over the Gulf of Alaska and southern mainland will result in benign conditions for the southern mainland. The large upper level high will likely lead to the development of inversions through Southcentral and may result in more foggy conditions. Also, we are expecting very warm 850 mb (~5000 ft) temperatures over the southern mainland that could result in some unseasonably warm temperatures along with large diurnal fluctuations for areas that aren't trapped under an inversion. The southwest mainland will see temperatures running 10 to 20 degrees above normal with persistent southwesterly flow through the week. Out west, we have a very active pattern on tap for next week. We start off with a strong storm to hurricane force low over the western bering and western Aleutians that treks north across the Bering as a ridge over the eastern Bering gets pushed east over the mainland. After this low moves out of the Bering, another strong storm force low is on tap for the western and central Aleutians. By the later half of the week the lows are a bit weaker as they trek across the Aleutians and Bering Sea. A weak ridge pokes up across the central Aleutians to the eastern Bering Sea Friday. Lastly, another storm force low will impact the western Aleutians and western Bering Sea Saturday. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...Winter Weather Advisory 131. MARINE...Storm Warning 131 Gale Warning 411-414 165 170 172-185 351 352 119 120 121 127 128 130 132 137 FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...BL SOUTHCENTRAL ALASKA...PD SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM MARINE/LONG TERM...SA/KVP