Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EST Wed Dec 18 2024 Valid 12Z Sun Dec 22 2024 - 12Z Thu Dec 26 2024 ...Stormy pattern into next week with multiple episodes of southern coast into Panhandle precipitation and hazardous maritime conditions over the North Pacific/Gulf of Alaska... ...Overview... Guidance agrees on a persistent mean ridge aloft from western Canada reaching eastern portions of the mainland through the period, while a weak upper low atop the Alaska Peninsula and vicinity should support a lingering surface low over Bristol Bay and direct another surface low northward through the northeast Pacific early next week. Then the pattern becomes more dominated by a large and strong upper/surface low tracking east across the North Pacific/Aleutians into Tuesday-Thursday. These systems will produce focused precipitation along the southern coast while the North Pacific and then the Bering Sea into Aleutians could see periods of strong winds. The forecast pattern will likely support above normal temperatures over most areas aside from parts of the North Slope, as well as perhaps some far western areas toward midweek as Arctic into Bering Sea upper troughing deepens to the west of the mean ridge. ...Guidance Evaluation and Preferences... Model guidance is reasonably agreeable with the overall pattern described above, but detail differences lead to some forecast issues that increase over time. At the start of the period Sunday, the 12Z CMC was a bit displaced from consensus with the couple surface lows in Bristol Bay and the northeast Pacific. A GFS/ECMWF/UKMET blend seemed to work well. Then with the large upper and surface low, models agree in principle with a low track eastward through the northern Pacific near the Aleutians Monday-Tuesday. The 12Z GFS however was odd with its low structure, showing multiple low centers all along what is likely the occluded front, without a consolidated low. While these spinups are possible, the low probability with any small scale low placement, and consensus (including the 06Z GFS, 12Z ECMWF and CMC and AI/ML models) indicating one or perhaps two lows, made it seem unwise to lean the forecast in that direction. Thus favored the 06Z GFS more by the middle of the period along with the 12Z ECMWF. Guidance continues to be strong with this low, into the 940s mb range, though models and ensemble members are showing less tendency to reach anywhere below the upper 930s mb. Though the 06Z GFS was favored for the mid-period, it was not as favorable by midweek as it dove polar energy south into eastern Siberia and oriented an upper low/trough west-east over the Bering by Thursday. The 12Z ECMWF seemed to be the primary deterministic model that agreed well with the ensemble means by that point. Thus the WPC forecast used a blend of the 06Z/12Z GFS, 12Z ECMWF, and 12Z UKMET early in the period. As the period progressed, reduced the proportion of the 12Z GFS and UKMET by Day 6 and the 06Z GFS by Day 7, maintaining some 12Z ECMWF but favoring the ensemble means to half by Day 7 and more Day 8 as spread increased with the low track during the later period. ...Weather/Hazards Highlights... By Sunday, an initial low pressure system will be weakening in Bristol Bay, while another reasonably strong storm system is forecast to move through the northeastern Pacific Sunday-Monday. These should bring a couple of rounds of enhanced precipitation across Southcentral to Southeast Alaska with weak to moderate atmospheric rivers. Additionally, the low will lead to areas of strong winds to the east of the low track offshore of Southeast Alaska, potentially strengthening barrier jets and gap winds over favored areas. Then focus will turn to the potentially deep North Pacific/Aleutians storm that may reach near the Alaska Peninsula by midweek. This system should produce moderate amounts of precipitation across the Aleutians with somewhat greater enhancement eventually possible into the southern coast. Expect a broad area of strong winds as well, including on the north side early in the week followed by northerly flow in the storm's wake from the Bering Sea into North Pacific as well as in leading southerly/southeasterly flow heading into midweek. Currently the Days 3-7 Hazards Outlook depicts no confident areas of heavy precipitation or high winds reaching hazardous thresholds, but either parameter may still be hazardous on a localized scale and will continue to monitor for updates in coming days. The pattern will favor above normal temperatures over most areas, with anomalies for min temperatures tending to be higher than those for highs. Parts of the North Slope may remain somewhat below normal during the period, along with a few pockets in the eastern Interior, though for the most part the Brooks Range should block most of the coldest air. Some far western areas may trend a little below normal by next Tuesday-Thursday as upper troughing deepens just west of the mainland. Tate/Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: The WPC Hazards Outline depicts these threats. https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html