Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Mon Oct 09 2023 Valid 12Z Fri Oct 13 2023 - 12Z Tue Oct 17 2023 ...Deep storm likely to bring strong winds and high seas to the Aleutians and surrounding waters late this week... ...Heavy precipitation threat over Southeast Alaska late this week into the weekend... ...Overview... Most guidance has been suggesting that an upper ridge over Canada will extend its influence westward into the mainland from the weekend into next week, pushing an initial western mainland upper low westward and ultimately leaving a mean trough axis near the western coast of the mainland through next Tuesday. Meanwhile the deep storm/upper low over the southern Bering Sea late this week and at least one possible leading frontal wave should reach south of the Alaska Peninsula by the weekend. Early next week this feature may get replaced by a strong mid-latitude storm expected to be the extratropical reflection of current Tropical Storm Bolaven, soon to become a typhoon per the Joint Typhoon Warning Center forecast. Northeast Pacific flow late this week will likely carry along a couple defined surface waves, one reaching the southern coast near Prince William Sound by early Friday and a second that may track a little southwestward. These waves should focus potentially heavy precipitation from the Panhandle back to parts of the southern coast. From later in the weekend into next week, precipitation may become somewhat less concentrated and focused more along eastward-facing terrain along the southern coast given the forecast pattern evolution. ...Guidance Evaluation and Preferences... During the first half of the forecast period, the primary question marks involve the surface waves over the Northeast Pacific and details of the initial Bering Sea storm plus possible leading wave(s). Over the northeastern Pacific, the guidance majority has settled into the intermediate longitude from the past couple days for the leading wave expected to reach the southern coast near or just east of Prince William Sound by early Friday. Details remain more ambiguous for a trailing wave which should track a little farther southwest than the first one. Over this region the 12Z UKMET offers the primary discrepancy, showing a more amplified shortwave extending southeast from the mainland upper low on Friday. This keeps the UKMET surface low track and moisture axis shifted farther east than consensus. Meanwhile recent runs have been inconsistent over how quickly the initial Bering Sea storm will translate into the North Pacific. A slower progression had been in the minority most days but the latest GFS/ECMWF/UKMET runs have clustered to this idea, leaving the CMC and the means as the remaining faster solutions. A blended approach leaning to continuity but including a wave along a trough to account for the new 12Z model cluster provides a reasonable starting point for now. The latitude of low pressure south of the Alaska Peninsula becomes rather uncertain by early day 6 Sunday, so confidence trends lower for how strong developing easterly flow along and near the Alaska Peninsula may become. From Sunday into next week the dominant system of interest will likely be the extratropical reflection of Bolaven as it tracks across the mid-latitude Pacific. Recent guidance has been fairly stable and agreeable that this system should not reach much farther north than 45-47N latitude while tracking south of the Aleutians. Among latest solutions the 12Z CMC leans a bit on the northern side of the spread, leading to a period of brisk easterly winds over the Aleutians (but still keeping associated precipitation to the south) versus more of a northerly component otherwise. Latest GFS runs have been on the south side of the spread. There has also been a remarkable east-west spread for progression over the past couple days, with the ECMWF/CMC and their ensembles generally favoring a fairly fast speed and the GFS on the slow side. From an elongated area of low pressure earlier in the period in the GEFS mean, the past couple runs ultimately come close to catching up to the faster cluster by the end of the forecast to provide some added confidence in an overall non-GFS scenario. This solution would bring the system to 50N latitude at 150-155W longitude by early next Tuesday. By late in the period this system should overwhelm the leading feature expected to track south of the Alaska Peninsula. Farther east over the northeastern Pacific, individual model runs show potential for one or more defined waves to be embedded within the cyclonic mean flow, but with minimal confidence for specifics. Thus prefer to downplay any such wave for the time being during the latter half of the period. Forecast considerations led to the first half of the period emphasizing the 12Z/00Z ECMWF and 12Z GFS/CMC, followed by a transition to half ensemble means (12Z GFS/CMCens and 00Z ECens) and lingering ECMWF/CMC input by day 8 Tuesday. ...Weather and Hazards Highlights... A wave forecast to reach near or east of Prince William Sound by early Friday, along with its leading moisture feed and another wave that may follow to its southwest, should support areas of heavy precipitation from the Panhandle back westward to parts of southern Southcentral late this week into the weekend. The upper level and surface pattern evolution should transition toward favoring more easterly flow along the southern coast by late weekend into early next week. This will likely lead to precipitation becoming somewhat less concentrated and focused more over windward terrain from the Kenai Peninsula southwestward. Lighter totals could still be possible elsewhere. One or more shortwaves embedded within the mean flow aloft could produce periods of snow farther north over the mainland. Deep low pressure forecast to be over the southern Bering Sea as of early Friday should be in process of gradually weakening by then, but will still maintain strong winds and high seas to a potentially hazardous degree over the Aleutians and surrounding waters through Friday. This system and its lingering lighter rainfall along with a possible leading frontal wave should ultimately track south of the eastern Aleutians/Alaska Peninsula during the weekend. Behind this system, confidence is gradually increasing that the extratropical reflection of Bolaven should track across the Pacific at a fairly rapid pace, but at a sufficiently far south latitude (around 45N) to have modest impacts on the Aleutians aside from perhaps a period of more brisk easterly winds (versus more of a northerly component) if the track is on the northern side of the guidance spread. Typical guidance error several days out in time is still large enough to require continued monitoring for any significant changes in the forecast. Expect a steady warming trend during the the period to yield above normal temperatures over much of the state by early next week. Below normal readings, more so for highs, are most likely over the southern half or two thirds of the state late this week. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Sat, Oct 12-Oct 14. - High winds across portions of the Aleutians, Thu-Fri, Oct 12-Oct 13. 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