Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Wed Mar 13 2024 Valid 12Z Sun Mar 17 2024 - 12Z Thu Mar 21 2024 ...Heavy precipitation from the Southcentral coast into the northern Panhandle through the weekend, tapering off early next week... ...Overview... Latest guidance continues to advertise a retrogression of the large scale pattern, ultimately shifting the surface low track/precipitation focus from the Southcentral coast and Panhandle this weekend to the eastern Aleutians/Alaska Peninsula and possibly parts of the far western mainland next week. This shift will accompany weakening of initial western Canada upper ridging in favor of a strong northeastern Pacific into mainland ridge that will push a western coast upper low westward across the Bering Sea. There is good agreement on this large scale pattern evolution, which will support a pronounced warming trend over the northwestern half or so of the mainland, but various uncertainties continue for the system forecast to reach the southern coast of the mainland on or before Sunday as well as for trailing North Pacific/Aleutians/Bering Sea systems. ...Guidance Evaluation and Preferences... The weekend southern coast system's timing spread that began to develop with the 12Z/12 ECMWF run has continued into today, with ECMWF runs (and ECens means) continuing to be on the fast side while other solutions remain slower to varying degrees. The difficult aspect of this forecast is resolving exactly how quickly the supporting upper low will open up as the overall system lifts northward, with the early opening of the ECMWF's upper low leading to the faster surface progression. The compact nature of this feature suggests that details have low predictability multiple days out in time. With 00Z ECMWF-initialized machine learning (ML) models displaying enough spread to capture the dynamical guidance envelope, preference sides with keeping a progression slower than the ECMWF and closer to continuity. Immediately behind this system, most guidance shows additional weak shortwave energy which on Monday should reflect at the surface as an elongated area of low pressure much weaker than the first system. By next Monday-Tuesday expect upstream North Pacific low pressure to track into or near the Aleutians as ridging builds to the east. After fairly good clustering into Monday, solutions start to show increasing north-south divergence for the best-defined low pressure by Tuesday and then even more thereafter as various possibilities exist such as continued northward progression through the Bering Sea and beyond (latest ECMWF runs) or becoming incorporated within the overall circulation ahead of the next storm system before reaching beyond the southern Bering Sea. Also of note, the 12Z UKMET becomes a deep extreme by the end of its run early Tuesday. While dynamical/ML solutions do not cluster well enough to have much confidence in a specific scenario, there is a much better signal for the tight surface pressure gradient that should set up over the eastern Aleutians/Alaska Peninsula around late Monday-Tuesday. The next northwestern Pacific system should be approaching the western Aleutians by next Wednesday. Earlier fast/eastward GFS runs have trended back some, while most of the slower guidance from yesterday has nudged a bit faster with a gradually improving signal that this system should reach into the southern/western Bering Sea by early next Thursday. This has left latest ECens runs as a bit of a slow extreme at that time. Some northward drift of this system seems reasonable given the strong upper ridge to the east, and a tight surface gradient may again set up for a time over the Aleutians/Alaska Peninsula. The upper low being pushed westward across the Bering Sea could have some influence on one or both of next week's North Pacific/Aleutians systems, depending on its position at a particular time. The main difference of note is that CMC runs continue to be on the eastern side of the spread after early in the period. CMCens runs have tended to mimic the operational run in slightly weaker form but the 12Z CMCens does eventually gravitate more toward consensus. Preferences for the initial southern coast system led to a day 4 Sunday blend consisting of only 25 percent 12Z ECMWF and the rest GFS/UKMET/CMC. Thereafter, the forecast reverted to more even weight of the ECMWF/GFS and eventually split ECMWF input between the 00Z and 12Z runs as detail differences arose. Lower confidence details in the UKMET/CMC led to reducing their weight by day 6 Tuesday, while the blend incorporated a modest amount of the 12Z GEFS/CMCens and 00Z ECens means by the latter half of the period. ...Weather/Hazards Highlights... The system forecast to track toward the Kenai Peninsula this weekend will bring heavy rain and inland/mountain snow along the Southcentral coast into the northern Panhandle. Guidance still shows significant moisture anomalies with this system and strong onshore flow plus orographic forcing should produce multiple inches of rain or liquid equivalent over some locations. The Days 3-7 Hazards Chart maintains an area from the eastern Kenai Peninsula through the northern Panhandle as being the most likely focus for this heavy precipitation event. Currently the greatest uncertainty is the duration of heavy precipitation. The majority of guidance would extend the meaningful precipitation through Sunday but the faster system progression of the ECMWF would taper off this event earlier, so continue to monitor upcoming forecast updates. A trailing much weaker feature could help lighter activity linger into Monday. Meanwhile some areas of light snow may extend northward over the mainland. The Panhandle through eastern half of the mainland should trend drier during next week as upper ridging builds overhead, while North Pacific/Aleutians low pressure will direct more moisture and brisk winds from the eastern Aleutians/Alaska Panhandle northward, possibly including parts of the far western mainland. There is now enough of a signal for precipitation focus over the eastern Aleutians/western Alaska Peninsula to depict a corresponding area on the Hazards Chart, with some lingering uncertainty for exact location and duration. Another system possibly reaching the Aleutians/Bering Sea by next Wednesday-Thursday could bring more precipitation and brisk winds to the Aleutians/Alaska Peninsula. The forecast pattern evolution will bring a pronounced warming trend to the northwestern half or so of the state. This area will initially see below normal temperatures during the weekend due to an upper low near the western coast as of early Sunday. Then the building Northeast Pacific into mainland upper ridge will lead to well above normal temperatures. By midweek or so, some locations should see temperatures at least 20-30F warmer than Sunday readings. Meanwhile Southcentral and most of the Panhandle should start out moderately above normal, with Southcentral staying above normal through the period but the Panhandle drift more below normal by midweek due to northerly flow aloft to the east of the upper ridge axis along with Arctic high pressure extending into western Canada. 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