Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Tue Mar 12 2024 Valid 12Z Sat Mar 16 2024 - 12Z Wed Mar 20 2024 ...Heavy precipitation from the Southcentral coast into the northern Panhandle late this week through the weekend... ...Overview... Guidance suggests that the large scale pattern will retrograde over the course of the period, ultimately leading to a westward trend for surface low tracks and precipitation focus. A leading system tracking northward to the southern coast this weekend, around the western side of initial Canada/Mainland Alaska upper ridging, will produce significant rain and snow from near the Kenai Peninsula into the Panhandle. A trailing North Pacific system that may interact with a Bering Sea upper low should track into the Aleutians early next week and encourage an upper ridge to build from the northeastern Pacific into the mainland, redirecting moisture back to the eastern Aleutians and Alaska Peninsula/western mainland. The upper ridge will likely persist or even continue to build into next Wednesday as yet another system reaches the western Pacific and possibly the Aleutians. This pattern evolution will support a pronounced warming trend over many areas after colder than average temperatures prevail over about the northwestern two-thirds of the state at the start of the weekend. ...Guidance Evaluation and Preferences... With some typical spread and variability, the guidance average has been fairly stable over the past day for the system tracking toward the southern coast this weekend. Consensus shows arrival around Kodiak Island or the Kenai Peninsula. Among the 12Z runs, the ECMWF strayed faster than its prior run or most other guidance while the UKMET lagged a bit. Interestingly the longer term trend has been a little slower. With the 00Z ECMWF-initialized machine learning models also showing some timing spread (but the majority somewhat slower than the 12Z ECMWF), an intermediate solution looks best. This approach led to good timing continuity versus yesterday's forecast an only a slight eastward nudge for track. The next features/evolution of interest are the upper low forecast to drift westward over the Bering Sea (originating from an initial upper low/shear axis near the western coast of the mainland) and North Pacific system that is most likely to track into the Aleutians by early next week. With time the 12Z CMC/CMCens mean hold the upper low increasingly east of remaining guidance that shows greater retrogression. In addition the CMC/CMCens ultimately direct primary surface low focus well south of the Aleutians, though as a consolation they still reflect the enhanced deep-layer gradient that develops in other guidance from the eastern Aleutians/Alaska Peninsula into the western mainland. GFS runs may be a little too quick to bring the system north into the Aleutians, but in a broader sense the GFS/ECMWF and their ensemble means offer the best clustering in principle. How the Bering Sea upper low and the Pacific system's dynamics interact are an ongoing source of uncertainty for this part of the forecast. Upstream dynamics reaching the western Pacific by early-mid week should bring another system into the western Pacific and possibly the Aleutians late in the forecast period. In varying ways the last few GFS runs have been on the fast/strong side with low pressure tracking into the Aleutians and Bering Sea. Remaining solutions hold low pressure south of the western Aleutians at least into early day 8 Wednesday, with the fastest non-GFS solutions bringing the parent low (12Z GEFS) or frontal wave (12Z ECMWF) into the Aleutians by later Wednesday. Farther east, the late-period pattern should feature strong surface high pressure over the northeastern Pacific and Arctic/northwestern Canada, with a frontal zone over the mainland separating the two (but still with relatively high surface pressures in its vicinity). Guidance comparisons led to starting today's forecast with a blend of the 00Z/12Z ECMWF runs, the 12Z GFS, and somewhat less weight of the 12Z CMC/UKMET early in the period. Day 6 Monday introduced the 12Z GEFS/00Z ECens means to replace the less desirable CMC and then to fill in after the end of the UKMET's run. Total weight of the means reached 50 percent by day 8 with the majority of operational input skewed to the aforementioned ECMWF runs given deteriorating comparisons in the GFS by that time. ...Weather/Hazards Highlights... The system tracking northward toward Kodiak Island or the Kenai Peninsula this weekend will bring heavy rain and inland/mountain snow along the Southcentral coast into the northern Panhandle. Guidance is showing significant moisture anomalies with this system and strong onshore flow plus orographic forcing should produce several inches of rain or liquid equivalent over some locations. The Days 3-7 Hazards Chart depicts an area from the eastern Kenai Peninsula through the northern Panhandle as being the most likely focus for this heavy precipitation event. Some areas of much lighter precipitation may extend northward over the mainland. The Panhandle through eastern half of the mainland should trend drier during the early to middle part of next week as upper ridging builds overhead, while North Pacific/Aleutians low pressure will direct more moisture into the eastern Aleutians/Alaska Panhandle northward through the western mainland. Totals with this area of precipitation should be lower than with the earlier event to the east but could still be meaningful. Another system approaching and possibly reaching the Aleutians by next Wednesday could bring another episode of precipitation to that area. The pattern evolution over the course of the period will bring a pronounced warming trend to most of the state. Expect the northwestern two-thirds of the mainland to see below normal temperatures at the start of the period Saturday, with above normal readings confined to Southcentral and the Panhandle. Then a rapid warming will yield above to well above normal temperatures advancing northwestward with time, ultimately yielding the greatest anomalies over northern/western areas. Portions of the Panhandle may see temperatures drift a little more below normal though. 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