Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 652 PM EST Sat Mar 11 2023 Valid 12Z Wed Mar 15 2023 - 12Z Sun Mar 19 2023 ...Overview... Today's guidance continues to highlight a persistent upper low generally a little south of the Alaska Peninsula/Kodiak Island, favoring at least a couple surface lows evolving near or tracking toward the northeastern Pacific/Gulf of Alaska. This pattern will support multiple days of precipitation along the southern coast and Panhandle with relatively dry conditions over most other areas. Some higher latitude energy feeding down through a mainland mean trough may help to reinforce the upper low by the latter half of the week. The mainland trough could give way to some weak upper ridging over northern areas by next Sunday. ...Guidance Evaluation/Predictability Assessment... Models and ensembles still agree on the large scale pattern featuring the mean upper low just south of the mainland. However, after fairly good agreement and continuity for initial surface low pressure Wednesday-Thursday, guidance has had a difficult time resolving the specifics of potential additional surface lows from Friday through the weekend. There are also some trends and spread for Arctic shortwave energy that comes into the picture north-northwest of the mainland on Wednesday, followed by differences for ridging or shortwaves behind it. During the first half of the period, an operational model composite of more 12Z GFS/ECMWF relative to the 12Z CMC/UKMET provided a good starting point to represent the consensus themes of guidance. Most solutions suggest that some Arctic energy will drop south over the mainland and serve to elongate/reinforce the upper low just off the Alaska Peninsula/Kodiak Island, while the rest continues on to the east/northeast. Multi-day trends suggest somewhat higher heights/ridging could nudge into the northwestern mainland at times relative to earlier forecasts. Meanwhile there is decent agreement for slowly weakening surface low pressure near Kodiak Island and the Kenai Peninsula. From Friday through the weekend, guidance is showing some new trends with respect to northeastern Pacific surface low details. Earlier ECMWF/CMC model and ensemble mean runs had been suggesting strong low pressure could lift northward into the northeastern Pacific/Gulf of Alaska around Friday into Saturday while GFS/GEFS runs had been showing various different ideas for what may happen. Most of today's 12Z runs now cluster toward this specific low tracking more toward Haida Gwaii and weakening (CMC still somewhat northwest) while the new 18Z GFS is even weaker and farther south. Broad spread in the various ensemble systems leads to this system losing definition or merging with another low in the means. While these trends suggest a reduced threat from this particular low, the past 12 hours or so of model runs have started to place greater emphasis on a trailing low that deepens as it wraps back toward Kodiak Island during next weekend. In contrast, the 12Z CMC mean and new 12Z ECMWF mean in particular show the upper low and associated surface reflection drifting off to the southeast. Preference by days 7-8 next weekend leans somewhat toward the 12Z GFS/ECMWF/CMC in principle to maintain better detail but includes 30-40 percent total of the 12Z GEFS/00Z ECMWF means to temper the depth of the GFS/ECMWF in particular. Farther north, this approach yields slowly rising heights aloft over central/northern parts of the mainland and even weak ridging by day 8 Sunday, albeit with low confidence over the relative proportion contributed by separate initial ridges over western Canada and west of the mainland. The chosen blend also reflects the model majority that has a front moving into the Bering Sea/Aleutians by next Sunday. The 06Z GFS had a strong western Bering storm anchoring this front but no other guidance has depicted such a deep system so far. ...Weather/Hazard Highlights... The overall forecast is consistent with respect to the upper low just south of the Alaska Peninsula and Kodiak Island and one or more northeastern Pacific/Gulf of Alaska surface low centers supporting an extended period of precipitation along the southern coast and Panhandle from midweek through 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 extending from the Panhandle to the eastern Kenai Peninsula. Some locally moderate to heavy activity is possible but the multi-day accumulations may be a more prominent consideration than extreme amounts on any particular day. Unresolved details for surface low pressure after Thursday will influence precipitation specifics within the favored areas, as well as determine whether moisture could extend farther west than the Kenai Peninsula by next weekend. Elsewhere, the overall pattern will likely support brisk to strong northerly/northwesterly winds over the Bering Sea and Aleutians, especially mid-late week, along with periods of light precipitation. A weak front may cross the Bering Sea/Aleutians by next Sunday. The expected pattern will support below normal temperatures over many areas through midweek or so, with coldest anomalies likely over parts of the interior and the Panhandle. Anomalies for lows should be less extreme with pockets of above normal readings already possible early in the period over parts of the northwestern mainland and Southcentral/Panhandle. Temperatures will likely moderate with time. By next weekend this trend should bring temperatures above normal for both highs and lows over the North Slope and Southcentral, while the Panhandle is more likely to see above normal lows but continued below normal highs. 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