Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Thu Jan 23 2025 Valid 12Z Mon Jan 27 2025 - 12Z Fri Jan 31 2025 ...Heavy precipitation shifting from the southern coast this weekend into the Panhandle early next week... ...Heavy snow from the Alaska Range across interior western and central Alaska to eastern Brooks Range through the weekend and possibly into Monday... ...Colder trend to well below normal temperatures next week... ...Overview... Guidance continues to show a remarkably abrupt change in regimes from the short term into the extended period that covers next Monday-Friday. Low pressure forecast to be tracking over or near the central part of the mainland as of early Monday and the supporting Arctic into Pacific upper trough to its west will push eastward with time. This will replace the near-term very warm and wet/snowy pattern with a much colder one along with a drier trend to the north of the southern coast/Panhandle. The most significant forecast challenge next week involves the details of upper troughing that settles over and south of the mainland and how this trough may interact with mid-latitude Pacific low pressure Wednesday onward. Current guidance differences in this evolution yield significant spread for precipitation coverage/amounts/duration along the southern coast and Panhandle as well as for winds. Even with these differences, confidence is much greater for the forecast of cold temperatures settling over the state especially by mid-late week. ...Guidance Evaluation and Preferences... Guidance has finally come into decent agreement for the system expected to be tracking over the mainland as of early Monday, recommending a 12Z model composite for that aspect of the forecast. Meanwhile to the southwest, the models are generally all continuing to show at least a modest wave tracking across the Bering Sea with a trailing cold front but with a fair amount of spread for exact track/strength/timing. This spread is related to the diverging details of shortwave energy that will be digging on the back side of the mean trough aloft. Initially a compromise looks reasonable but then other aspects of the overall upper trough and the mid-latitude interaction play a much greater role in the forecast after early Tuesday. With respect to the forecast evolution from later Tuesday onward, dynamical/machine learning models and ensemble members have been showing a lot of spread regarding the amplitude and longitude of upper troughing that reaches into the Pacific. This leads to a corresponding wide envelope for what interaction may occur with a system expected to be over the mid-latitude Pacific early in the week. The old 00Z ECMWF was a flat/eastern extreme with the upper trough to yield no interaction at all and a drier trend even along the southern coast. On the other extreme, recent GFS runs (and to some extent 12Z CMC) are far enough westward with the trough and quick to pull up mid-latitude waviness (ahead of the parent low) that there is a continuation of focused southern coast precipitation from early in the week. 00Z/06Z ML models offered the best general support for the GEFS/ECens/CMCens means that showed the phasing process occurring around the Wednesday-Thursday time frame--whether from the parent low or a leading frontal wave. However some ML models, and seemingly a little more so in the new 12Z cycle, offer potential for an embedded upper low to reach the southern coast/Gulf of Alaska more quickly than suggested by dynamical models and ensemble means (whose new runs do show a better hint of a possible closed low than earlier). As a result a higher number of the new ML models yield the potential for a somewhat more suppressed surface evolution via a mere leading wave (though with some lingering low pressure near the southern coast). As for other forecast details, by Friday recent GFS runs have been tending to dig more Arctic energy into the mainland versus most other guidance. There are some detail differences with western Pacific low pressure/leading frontal system(s) but a model/mean average provides a reasonable depiction of the gradient over the Aleutians from late Tuesday onward. Guidance available through the arrival of the 12Z ECMWF run ultimately favored a 12Z model composite for Monday, a transition to the ECMWF/UKMET/CMC (in order of more to less weight) and 40 percent ensemble mean input (12Z GEFS/CMCens and 00Z ECens) by Tuesday, and then 30 percent ECMWF and 70 percent means for Wednesday-Friday. This reflected latest operational model tendencies for Gulf low pressure to be deeper than the means mid-late week while accounting for the continued potential that significant adjustments with important details could occur in future runs. ...Weather/Hazards Highlights... During Sunday-Monday low pressure tracking northeastward across the mainland will enhance a long fetch of moisture across the state. This cyclone and its interaction with a preexisting front across mainland Alaska will generate bands of heavy snow from the Alaska Range across interior western and central Alaska and into the eastern portion of Brooks Range through the weekend and extending into Monday. Meanwhile, a period of heavy precipitation should initially focus along the southern coast and then gradually push more into the Panhandle by early next week. The Days 3-7 Hazards Outlook depicts the aforementioned heavy snow threat along with the heavy precipitation which should be most focused from the Kenai Peninsula into the northern half of the Panhandle. This system may also produce some strong winds but likely not reaching hazardous criteria. From Tuesday onward, expect most areas to the north of the southern coast to trend drier. Some precipitation should linger over the Panhandle into Tuesday. After a brief lull, some moisture may return to the southern coast/Panhandle for the latter half of the week, depending on upper trough/surface low details. Best potential for additional precipitation will be from the Panhandle to the Southcentral coast. Note that there is a minority scenario that could yield a minimal break in activity along the southern coast. A Bering Sea wave may bring a period of mostly light precipitation to the eastern Aleutians/Alaska Peninsula early in the week. Northerly/northwesterly winds over the eastern Aleutians/Alaska Peninsula should strengthen behind this wave and its cold front, and the tightening surface gradient between low pressure over the Gulf of Alaska and surface ridge over the northern mainland will also raise the potential for strengthening gap winds farther east along the southern coast by the latter half of the week. Specifics will be determined by still uncertain low pressure details, so will continue to monitor over future model runs. The temperature forecast remains on track, with the early part of the week featuring a west-to-east transition from above normal to below normal readings. Well below normal temperatures (by at least 20F over some areas) should cover much of the western-central mainland by Wednesday, with perhaps some lingering pockets of slightly above normal still over the North Slope and southern Panhandle. Coldest anomalies should continue to nudge eastward a bit during Thursday-Friday. While there are continued uncertainties with the surface pattern over and near the Gulf by mid-late week, ensemble guidance offers a very strong signal for this episode of cold weather across much of the mainland. 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