Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Sat Jul 10 2021 Valid 12Z Wed Jul 14 2021 - 12Z Sun Jul 18 2021 ...Overview... The large scale pattern should gradually become a little less blocky with time as an upper high initially south of the Bering Strait drifts southward and likely opens up by next weekend to yield a ridge axis that reaches the eastern Aleutians/far southern mainland. It remains to be seen what proportion of Bering Sea upper low/shortwave energy to the southwest of the initial ridge will travel to the south of the ridge and into the North Pacific versus around the western side of the ridge and eventually into the mainland. The persistent upper low over the Northeast Pacific should meander toward a position slightly offshore the southern Panhandle/Haida Gwaii. Expect unsettled weather at times over the northern mainland as a mean frontal boundary oscillates from day to day in response to shortwaves embedded within progressive Arctic flow. Meanwhile there is still a decent signal for low pressure to track over Kamchatka early in the period but the ultimate eastward extent of this system remains uncertain. Flow ahead of the system should at least affect parts of the Aleutians. ...Guidance/Predictability Evaluation... Consensus position of the front over the northern/eastern mainland as of Wednesday, in response to an upper trough heading into the Canadian Archipelago, has held up fairly well over recent days. Then over the past 24 hours or so the operational models have been improving their agreement for the details of Arctic flow aloft and associated frontal evolution later in the week. The majority scenario now has a wave and a trailing frontal system helping to lift the initial mainland front northward as a warm front, followed by the trailing front settling back over the northern mainland. This improves confidence in the forecast through about Friday but solutions become more diverse next weekend. Of particular note the 12Z GFS appeared too deep with its upper trough reaching into the northwestern mainland next weekend, with the 18Z run trending less extreme and faster. The 12Z CMC strayed faster than consensus late in the week. Elsewhere, an intermediate solution provides reasonable continuity for the Northeast Pacific upper low. One question mark is how much northern stream energy could reach far enough southward to interact with/displace the initial low. In general the ensemble means have favored a middle ground between western/eastern solutions for upper low position over the past couple days--favoring a blend/compromise approach. Guidance trends have become more mixed for low pressure emerging from Kamchatka. Prior adjustments had been away from the fast side of the envelope but the 12Z CMC leads other guidance already by Friday and then the 12Z ECMWF catches up to it during the weekend as it tracks into the Bering Sea. On the other hand the 00Z ECMWF and recent GFS runs have kept the low to the south/southwest of the western Aleutians through the weekend. Current preference goes to a slightly enhanced version of the ensemble means that are between these two extremes and more stable from run to run. Today's guidance comparisons led to a blend consisting of 12Z operational guidance early in the period followed by removal of an already minimal CMC weight. Later in the period the forecast trended more to the means, reaching 70 percent total 12Z GEFS/00Z ECens by day 8 Sunday. At that time the remaining weight went to the last two ECMWF runs whose differences over some areas averaged out closer to the means. The questionable 12Z GFS forecast over the Arctic led to its removal late in the period. ...Weather/Threats Highlights... The southeastern quarter of the mainland should see organized precipitation on Wednesday followed by a drier trend, while the Panhandle may see periods of light to locally moderate precipitation one or more days depending on the exact path of the Northeast Pacific upper low. Expect the northern mainland to see one or more periods of precipitation in association with the mean frontal boundary likely to meander over or just north of the region from midweek into the weekend. The gradient ahead of low pressure emerging from Kamchatka should bring an increase of precipitation and winds into at least parts of the Aleutians. The eastward extent of moisture by next weekend becomes very uncertain, with possibilities ranging from having it stay over the western Aleutians or reaching as far east as the western mainland (also aided by any lingering Bering Sea energy picked up by flow aloft). The Panhandle and southern coast will tend to see below normal temperatures for most of the period. The North Slope should see below normal readings around midweek followed by some variability in temperatures as the mean front over the region oscillates. In-between expect upper ridging to support above normal temperatures though the gradual weakening/opening of the upper ridge should lead to a cooling trend over western portions of the central-southern mainland. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards 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