Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Fri Feb 03 2023 Valid 12Z Tue Feb 07 2023 - 12Z Sat Feb 11 2023 ...Very deep low pressure likely to bring strong winds to the Aleutians and surrounding waters late next week... ...Overview... A lingering upper trough initially over the western mainland will give way to a brief period of fairly low amplitude progressive flow (with upper heights slowly rising) containing multiple closely spaced shortwaves from the North Pacific/Aleutians/Bering Sea into the mainland. Guidance has continued to have difficulty in resolving the details of these shortwaves and how they will influence the surface pattern along with the distribution of precipitation. Consensus continues to advertise a transition to a regime dominated by larger scale and typically more predictable features late in the week as a very strong storm system moves into the western Aleutians and Bering Sea while a downstream upper ridge builds over the mainland. Models/means still generally maintain the idea of an upper weakness along or just north of the northern coast of the mainland. ...Model Guidance/Predictability Assessment... Behind a departing mainland/northeastern Pacific upper trough on Tuesday, the global models are still having difficulty in figuring out the details of two initial North Pacific/Aleutians shortwaves Tuesday onward. This is causing a lot of between-model and run-to-run spread for surface evolution along with moisture distribution. After a full array of very wet (GFS) to fairly dry (ECMWF) scenarios 24 hours ago, 00Z/06Z guidance started to trend toward a more agreeable "moderately wet" solution along the southern coast with nearby low pressure but now the 12Z suite has essentially split into two camps. The 12Z/18Z GFS and 12Z CMC form a compact wave that tracks over the southern/eastern Bering Sea to bring some moisture into the western mainland while other models and the ensemble means emphasize the better dynamics dropping into the Pacific to keep low pressure well south of the mainland. Either way this keeps at least the western half of the southern coast drier, while the ECMWF/UKMET and latest GEFS/ECMWF/CMC means still have varying ideas for whether the Pacific low pressure could ultimately curl northward into or near the Gulf--bringing some moisture to areas from the eastern Kenai Peninsula to the Panhandle. With the ensemble means all showing higher surface pressures over the southern/eastern Bering Sea versus the 12Z GFS/CMC (though with a signal for a separate weak feature farther back over the Aleutians), today's preference for the first part of the period was with the 12Z ECMWF and 12Z GEFS/00Z ECMWF means. Meanwhile, most GFS/ECMWF and GEFS/ECens runs have been more agreeable and consistent than would typically be the case for a days 7-8 forecast with respect to the very deep storm forecast to track into the western Aleutians and Bering Sea late next week. For at least a couple days the ECMWF/GFS runs have been advertising this surface low to reach into 940s or low 950s mb around Thursday into early Friday before gradually trending weaker. Recent CMC/CMCens runs have tended to stray farther east than the GFS/ECMWF majority cluster. The favorable clustering thus far favors staying close to the operational GFS/ECMWF to depict the strength of this system. Latest guidance shows some spread and variability for the specifics of the upper ridge that builds over the mainland toward the end of the week. The 12Z/18Z GFS have strayed a bit to the strong/western side of the spread due to more pronounced troughing over northwestern Canada, while the 12Z ECMWF trended somewhat weaker/broader than the 00Z run. An average of the 12Z GFS/ECMWF is somewhat stronger than the ensemble means, probably a good place to be given the strength of the Aleutians storm and its likely influence downstream. However consensus would suggest upstream moisture making a little more eastward progress into the western mainland at the end of the period versus the 12Z GFS. Early-period considerations led to a starting blend of half 12Z ECMWF and half total 12Z GEFS/00Z ECMWF mean for days 4-5 Tuesday-Wednesday. With differences over and near the mainland becoming less pronounced later in the week and preference to stay fairly close to operational model depictions for the Aleutians storm, the blend introduced some GFS input by day 6 Thursday followed by a transition to 70 percent total GFS/ECMWF input and 30 percent means for days 7-8 Friday-Saturday. ...Weather/Hazard Highlights... Lingering low pressure near the Kenai Peninsula Tuesday into early Wednesday may still produce some precipitation from the southeastern coast into the Panhandle early in the period. Then the forecast confidence remains lower than average from the Aleutians eastward/northeastward, though at least the overall spread for potential precipitation areas/amounts has decreased somewhat over the past day. Currently the most likely scenario is for some moisture to flow around eastern Pacific low pressure, bringing periods of precipitation to areas from near the eastern Kenai Peninsula to the Panhandle, but still with a lot of uncertainty over precise timing and amounts depending on the evolution/track of low pressure. A lower probability solution could have a separate moderate area of moisture extend from the Bering Sea into the parts of the mainland. Either way, there is now better agreement on drier conditions along the Alaska Peninsula through Thursday. Meanwhile confidence continues to be above average for the deep storm expected to track into the western Aleutians and then the southern Bering Sea late next week. This storm should bring a period of strong winds especially to the western half of the Aleutians and surrounding waters, while a band of less extreme but still brisk to strong winds may accompany the leading front as it pushes eastward across the rest of the Aleutians/Bering Sea and possibly getting close to the western mainland next Saturday. The flow associated with this front may also bring an increase of precipitation to parts of the western mainland/southwestern coast by Saturday. Below normal temperatures over the North Slope on Tuesday should trend warmer with time, while early-period upper troughing may also support below normal temperatures along the west coast of the mainland. Otherwise expect a mix of above/below normal temperatures with somewhat warmer anomalies for lows versus highs through midweek or so. The forecast pattern change with an upper ridge building over the mainland toward Friday-Saturday should support a trend to more above normal temperatures at that time. Rausch Hazards: - High winds across portions of the Aleutians, Thu, Feb 9. 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