Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Sun Mar 12 2023 Valid 12Z Thu Mar 16 2023 - 12Z Mon Mar 20 2023 ...Overview... Expect an upper low somewhat south of the Alaska Peninsula/Kodiak Island to persist late this week through the weekend, with some weakening and east-southeastward progression possible by early next week. This feature and surrounding flow will support one or more northeastern Pacific surface lows/frontal systems along with multiple days of precipitation from the Panhandle through the southern coast. Day-to-day specifics of this moisture, including the westward extent of locally enhanced activity, remain very uncertain due to the low predictability of important small-scale shortwave details. Away from the upper low, guidance continues to suggest some changes in the pattern. Consensus still indicates that an upper trough/weakness over the mainland should give way to some degree of ridging by Sunday-Monday, while more progressive flow over the Aleutians/Bering Sea may bring a weakening front across the region next weekend and a storm system by next Monday. ...Guidance Evaluation/Predictability Assessment... There is good clustering and continuity for the deep-layer low near Kodiak Island early in the period Thursday into Friday, and the same applies reasonably well to the surface waviness to the south along with a warm front that may lift northwestward over the northeastern Pacific and Panhandle. Otherwise, guidance continues to paint a very confusing picture as to the details of surface low specifics, given the dependence on small scale shortwave specifics that have low predictability several days out in time. At the very least, the ensemble means are suggesting some deepening/consolidation of low pressure a ways south of Kodiak Island during the weekend but with a track not reaching quite as far north/west as the 12Z ECMWF or new 18Z GFS for a portion of the period. The overall pattern of the 06Z GFS compared somewhat better to the model/ensemble mean majority than the 12Z GFS early-mid period. Models also show the possibility of a stray compact wave reaching into the Gulf of Alaska/southern coast late this week and weekend. Given the low confidence in specifics, prefer an intermediate blended solution as the best starting point. The main continuity change from yesterday is for a farther south surface low track around Sunday in light of ensemble mean persistence over the past couple days. The guidance over the mainland is fairly stable during Thursday-Saturday, with a gradually weakening trough that may become a mere shear axis or col region. Exact details become more ambiguous by Sunday-Monday but the models and means generally show a trend toward more ridging aloft from some combination of the northern part of the ridge approaching from the west and a separate western Canada ridge. Embedded/surrounding shortwave energy is still a possibility though, but with very low confidence. Over the Aleutians/Bering Sea, model clustering/continuity are somewhat better than average for a weakening front reaching the area during the weekend. Then a majority of guidance shows what could be a fairly vigorous storm system coming into the picture by next Monday. Latest CMC runs have been on the faster side of the spread though the 12Z GEFS mean is fairly progressive as well. The 12Z ECMWF along with the GFS/ECMWF mean (both of those two trending somewhat faster than their prior 06Z/00Z run, respectively) now comprise the majority scenario. Today's guidance blend started with a split of the 00Z/12Z ECMWF runs plus the 06Z GFS and 12Z CMC/UKMET on day 4 Thursday to represent the best consensus of guidance, followed by adding a little 12Z GEFS/CMCens means on day 5 Friday since those means actually depicted a slightly more defined upper low than the initial model blend at that time. By mid-late period, the forecast eventually phased out the CMC while transitioning GFS input to a split of 06Z/12Z runs (with the later one comparing better over the Aleutians/Bering Sea late) and increasing the total weight of GEFS/CMCens/ECens means to about half. ...Weather/Hazard Highlights... The upper low south of the Alaska Peninsula and Kodiak Island along with one or more northeastern Pacific/Gulf of Alaska surface lows/fronts will support multiple days of precipitation across the Panhandle and along the southern coast from the latter half of the week into the weekend. Expect some rain over southern Panhandle/coastal areas and snow over inland/higher elevation locations. The best guidance signal continues to point toward highest totals on a daily basis over the Panhandle on Thursday, with some local enhancement extending back to the eastern Kenai Peninsula. There is still some potential that low pressure could track close enough to Kodiak Island around next weekend to bring enhanced activity farther westward but current probabilities of this are in the lower half of the spectrum. Over most areas the multi-day accumulations may be a more prominent consideration than extreme amounts on any particular day. Confidence remains below average for finer details of specifics given the guidance spread for surface lows. Expect a drier trend to begin by Sunday or Monday. Elsewhere, the overall pattern will likely support brisk to strong northerly/northwesterly winds over the Bering Sea and Aleutians into late week, along with periods of light precipitation. A weak front may cross the Bering Sea/Aleutians by next Sunday followed by a possible Monday storm/frontal system accompanied by increasing winds and precipitation. The evolving pattern will generally favor a warming trend over the course of the period. Below normal temperatures should prevail over most areas on Thursday, aside from perhaps above-average lows over some areas in the northwestern corner of the mainland and pockets of the Panhandle/Southcentral. By early next week expect greater coverage of above normal temperatures, with some lingering below normal highs over the Panhandle. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: 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