Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Sun May 05 2024 Valid 12Z Thu May 09 2024 - 12Z Mon May 13 2024 ...Wet/snowy pattern along the southern coast and Panhandle late this week through the weekend... ...Overview... Models/ensembles have maintained the general theme of a mean upper low over or near the Alaska Peninsula during the period, likely consisting of one departing low and arrival of a second from the west. This pattern should favor potentially significant precipitation focus along the southern coast and Panhandle. With typical minor adjustments, consensus still shows a leading vigorous system tracking through the Gulf and into the mainland Thursday into Friday, bringing a brief episode of heavy rain/higher elevation snow to areas from the Kenai Peninsula through the Panhandle. Then latest guidance trends are increasing the probability that a trailing system tracking near the Aleutians and over or south of the Alaska Peninsula late week through Sunday-Monday will spread organized rainfall across the Aleutians and then rain/mountain snow from the Alaska Peninsula (which may see a period of enhanced easterly wind/moisture focus) through the Panhandle. Meanwhile the majority of guidance still shows an upper trough axis near the western mainland, possibly transitioning to a col region at times. Also, western Canada upper ridging could periodically extend back into parts of the mainland. ...Guidance Evaluation and Preferences... Over the past day the operational models have remained fairly consistent with the overall forecast of the wave tracking northward through the Gulf of Alaska and into the mainland Thursday-Friday. There is still the typical spread and run-to-run variability for exact strength/track details, with the latest GFS runs and 12Z UKMET a bit on the western side of the envelope along with the 12Z CMCens mean while the 12Z ECMWF/ECens mean are similar to the 00Z run. All but the CMC show a depth under 1000 mb. 00Z ECMWF-initialized machine learning (ML) models continue to be on the weaker side. While the supporting shortwave energy is on the eastern side of the overall upper low circulation near the Alaska Peninsula, there is lingering uncertainty over what influence that upper low may have on this system. The new 18Z GFS even goes beyond this point, dropping the upper low farther south which not only pulls the Gulf system farther west but then creates a separate trailing Gulf wave by Friday as the remaining upper shortwave lifts northeastward. The other primary storm of interest should be over/near the western Aleutians as of early Thursday. 24 hours ago there was wide guidance spread for the latitude of this system as it continues eastward (with the CMC/CMCens even holding it well westward for a time), but now the 00Z/12Z guidance runs have gravitated much closer together and in the direction of the ECMWF/ECens which have generally been on the northern side of the envelope near the Aleutians. The 12Z ECMWF did nudge a tad south of its 00Z run though, while the 12Z GFS/CMC are a bit on the southern side of the narrowing envelope. The ML models have been favoring a track fairly close to the Aleutians. By the weekend into early next week the full array of dynamical and ML guidance diverges for upper/surface low position, ranging between the southeastern Bering Sea and the Gulf of Alaska/Northeast Pacific. Currently the best intermediate track by the latter half of the period would have the upper low over or just south of the Alaska Peninsula with the surface low eventually tracking close to Kodiak Island. The ensemble means cluster well in principle at the surface and aloft, though the GEFS mean is suspiciously weak compared to the ECens/CMCens means. Otherwise, details over the rest of the mainland are somewhat ill-defined or variable aside from a continued general signal toward a weak mean trough most likely aligned over the west, with a col region possibly evolving at some point. There is also continued potential for periodic extension of western Canada upper ridging into parts of the mainland. The first half of the forecast started with 40 percent of the 12Z ECMWF with the remaining weight split evenly among the GFS/UKMET/CMC. This yielded a reasonable intermediate solution for the Thursday Gulf system. While recent trends could argue for even more ECMWF weight for the Aleutians storm, the blend already provided a meaningfully northward adjustment from continuity while waiting for guidance to stabilize regarding specific details. The CMC strayed south of remaining guidance especially around Sunday so it was removed from the forecast at that time. Days 7-8 Sunday-Monday shifted to 40 percent ensemble input (only the 12Z CMCens/00Z ECens due to the GEFS being weak) while the relative GFS/ECMWF weights adjusted to a more even balance than on earlier days. ...Weather/Hazards Highlights... Continue to expect a compact but vigorous storm to track northward through the Gulf of Alaska Thursday and then weaken/broaden as it reaches the Interior by Friday. This system will likely bring a period of heavy rain and high elevation snow to areas along and near the Southcentral coast and Panhandle, with 12Z GFS/ECMWF runs now agreeing on precipitable water (PWAT) values surging to 2-3.5 standard deviations above normal within the moisture shield. Currently expect the highest precipitation totals over the northern Panhandle while the western extent of enhanced precipitation (near the Kenai Peninsula) will depend on exact track of the surface low. This system may produce strong winds over open waters as well, while strong gap winds will be possible near the Alaska Range as the low tracks northward through the southern mainland. Areas of lighter precipitation may extend northward over parts of the mainland, still with uncertain influence on any river flooding due to ice jams and snowmelt. Behind this system, latest guidance trends are helping to increase confidence that another storm may track along or just south of the Aleutians and Alaska Peninsula late this week into the weekend, bringing organized precipitation and areas of brisk to strong winds depending on exact track. As this solution is looking more likely, there is increased potential for a period of easterly low level flow to focus precipitation along the Alaska Peninsula--but with lesser PWAT anomalies than forecast with Thursday's Gulf system. Moisture should continue eastward along the southern coast and into the Panhandle during the weekend with some persistence over at least the eastern areas into Monday. Southern areas will likely see below normal daytime high temperatures through the period due to the unsettled pattern across that region, while the North Slope should see below normal highs as well given a tendency for the surface gradient to produce a cool northerly low level flow. This leaves the Interior as the main region with the potential to see some areas of warmer than average highs. Expect greater coverage of above normal morning lows across the state with mostly localized pockets of below normal readings. 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