Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Sat Dec 16 2023 Valid 12Z Wed Dec 20 2023 - 12Z Sun Dec 24 2023 ...Periods of windy conditions across the Aleutians... ...Snowy/wet pattern over the southeastern coast/Panhandle mid-late week... ...Overview... Latest guidance shows a series of compact surface lows tracking along a corridor from the Bering Sea/Aleutians into the Gulf of Alaska mid-late week, ahead upper trough energy that should weaken as it progresses eastward. These waves will promote a persistent period of precipitation across the southeastern coast/Panhandle though with daily totals tending to be in the moderate range. Meanwhile an upper low should drift back away from the northwestern corner of the mainland. From Friday into the weekend most solutions continue to suggest strong storm development from the North Pacific/Aleutians to just south of the southern coast. There are continued significant differences for details but there is a common theme toward strengthening winds across the North/Northeast Pacific and vicinity along with enhanced precipitation focus along/near the southern coast into the Panhandle next weekend. ...Guidance Evaluation/Preferences... From the Bering Sea into the mainland/Gulf of Alaska, the average of latest models has settled fairly well into the sequence and strength of waves favored in continuity over the past couple days for the mid-late week period. A leading wave should be over the Gulf by early Wednesday, followed by a trailing Alaska Peninsula wave that should reach the Gulf by Thursday and then a final wave another day later. Aloft, today's guidance has trended toward the idea of initial troughing near the western coast briefly filling or moving inland as it weakens before more energy fills in a mean trough position a little west of the mainland by next weekend. With assistance from North Pacific development described below, the upper trough by day 8 next Sunday may take on a negative tilt with some degree of ridging possibly extending from far western Canada into the mainland. Most operational guidance fits into this pattern which the ensemble means depict very similarly at that time. Meanwhile the models and ensembles continue to suggest a potentially complex scenario for storm development across the North Pacific/Aleutians by Friday and continuing east-northeast to the south of the mainland through next weekend. As appeared to be case previously, the challenge of resolving the details rests with the dynamics spinning up on the northern side of the strong Pacific jet--typically an evolution with lower predictability than that of a well-defined large scale feature. Latest operational runs are generally keying on two separate waves as of Friday, the leading one tracking somewhat farther north than the trailing one at that time. ECMWF runs have tended to be on the southern extreme with the leading wave (but with the 12Z run nudging northward). Ensemble means suggest that contributing members have varied in their relative emphasis on each one, with latest CMCens means placing more weight on the leading wave and the GEFS mean strongly emphasizing the trailing one with no reflection of the leading one. In spite of the chaotic forecast even among the means at that valid time, at least the means converge to nearly identical positions for a deep low pressure system just south of Kodiak Island by early next Sunday. This is between the 06Z/12Z GFS runs that were farther west and very deep versus the 12Z ECMWF/CMC that were somewhat farther east. The new 18Z GFS still becomes a deep extreme but with an ending low position more similar to the means. Today's preference reflects latest improved signals toward two separate systems as of Friday and then a trend toward continuity and the ensemble means during the weekend given the uncertainty over how the Friday systems may consolidate/interact. Following an operational model blend of somewhat more 12Z GFS/ECMWF versus UKMET/CMC early in the period reflected the favored consensus well, with day 6 Friday serving as the start of a transition to include some ensemble mean input (12Z GEFS/CMCens and 00Z ECens) while still representing mostly operational model ideas. By days 7-8 Saturday-Sunday the blend reached nearly half weight of the means while the operational solutions generally averaged out near the means for the strong storm south of the mainland. Note that this blend downplayed one or more possible waves that may reach the western/central Bering Sea or Aleutians next weekend, given their compact nature along with poor model clustering and no definition yet in the ensemble means. ...Weather/Hazards Highlights... The mid-late week pattern from the Bering Sea and Aleutians into the Gulf of Alaska may support strong winds lingering over parts of the Aleutians through midweek or so, while a rapid succession of surface waves reaching the Gulf of Alaska will likely spread some precipitation over the far southwest mainland and promote a more persistent snowy/wet pattern over the southeastern coast/Panhandle. Forecasts continue to show totals for snow and coastal/southern Panhandle rain that should not reach hazardous criteria, but could still be meaningful given the multi-day duration. Some light/scattered snow will be possible farther north over portions of the mainland. The Aleutians may see some increase in precipitation and wind by Friday, depending on the strength/track of what is expected to be a leading low pressure system. Then by next Saturday-Sunday the better convergence of guidance toward a strong system reaching just south of Kodiak Island would lead to a broad area of strong winds across the Northeastern Pacific and nearby areas plus enhanced precipitation along/near the southern coast to the Panhandle. The Aleutians would see a period of fairly strong northwesterly winds at that time. A majority of the state should see above normal temperatures during the period. Best potential for some pockets of below normal readings will be for highs on some days over parts of the interior and Panhandle, and perhaps with a little broader coverage over far western areas at times when upper troughing is nearby. 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