Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 635 PM EST Sun Feb 02 2020 Valid 12Z Thu Feb 06 2020 - 12Z Mon Feb 10 2020 ...Overview... The best clustering of latest guidance shows an Arctic upper low settling close to the northwest corner of the mainland late this week before retrograding into eastern Siberia and then lifting northward/northeastward in response to upstream flow. Within a progressive North Pacific/Aleutians stream expect a midweek Aleutians system to continue across the eastern Aleutians/Alaska Peninsula late this week and then dissipate, followed by a stronger system heading into the Aleutians/Bering Sea around Fri-Sat. Ridging aloft that builds into the mainland and northwestern Canada ahead of this system should support an ultimate storm track near the western coast of the mainland. By late in the period shortwave energy filtering through the upper ridge may support a wave near the Panhandle by Mon, while strengthening flow ahead of a developing western Pacific system may overspread the Aleutians and Bering Sea. ...Guidance Evaluation... Model/ensemble trends over the past day have yielded much improved (though not yet ideal) agreement for the Arctic upper low expected to be near the northwest corner of the mainland for at least the first half of the period. After some recent erratic runs, the late-week 12Z ECMWF forecast has arrived at a solution close to the GFS--which itself has adjusted a bit westward from a number of prior runs that had been on the eastern side of the spread. The UKMET is fairly close while the CMC strays westward of the primary model/ensemble cluster. By the latter half of the period the overwhelming majority of guidance--including the 12Z ECMWF mean--supports some retrogression of the upper low in contrast to the 12Z ECMWF that keeps it over/near the northern mainland. The relatively better agreement provides some increase in confidence but the behavior of guidance thus far suggests an ongoing risk for meaningful adjustment of consensus. To the south clustering is improving for track of the mid-late week Aleutians system, with some help from the reduction of spread for flow to the north. Latest ensemble means depict a track over Bristol Bay and into the extreme southwest mainland/eastern Peninsula, most similar to the 12Z ECMWF. Through its 12Z run the GFS continues to be on the southern side of the envelope, south of the Peninsula, though the UKMET combines both ideas with a consensus parent low but also a southern triple point wave. The new 18Z GFS shows a trend toward consensus. Guidance continues to offer varying details for North Pacific low pressure extending into the Aleutians/Bering Sea by Fri-Sat. Operational runs are still showing better defined development versus the means for a leading frontal wave relative to the western parent low, though the means do hint at this evolution. The more eastern position of the Arctic upper low in the 12Z ECMWF ultimately leads to a storm track through the central-eastern mainland in contrast to consensus and continuity that maintain a farther west track. This requires transitioning the ECMWF component of the forecast blend from the 12Z run to the prior 00Z run around mid-period. Also, including some ensemble mean input mid-late period helps to tone down detail uncertainties and maintain elements of continuity. Latest ensemble means show better than average agreement for low pressure developing over the western Pacific and possibly reaching near the western Aleutians by early day 8 Mon. Operational runs show various ideas with the GFS closest to the means in principle. The forecast incorporated a 12Z multi-model blend for day 4 Thu followed by exclusion of the CMC due to its less likely solution for the Arctic upper low. The late Fri through Sat period used both 12Z and 00Z ECMWF runs on the way to only the 00Z run being used Sun-Mon. Some 12Z GFS input continued through day 8 Mon, while total weight of the 12Z GEFS/00Z ECMWF means increased from 20 percent on day 5 Fri to 60 percent by days 7-8 Sun-Mon. ...Weather Highlights... Continue to expect areas along the southern coast and Panhandle to see the highest five-day precipitation totals, while meaningful amounts are also likely over parts of the southern and western mainland as well as across the Aleutians. Most late-week activity should be in the light to moderate range. Flow around low pressure reaching the Bering Sea by Sat may produce some enhancement from the eastern Aleutians to southern coast and Panhandle during the weekend. This system should also spread moisture northward over portions of the mainland. With considerable uncertainty in specifics, developing western Pacific low pressure may bring an area of moisture and brisk winds into parts of the Aleutians/Bering Sea by next Mon. Late this week a front will separate below normal temperatures over northern and western areas from above normal readings over the south/east, with a brief southward push of colder air over western locations. The system tracking into the Bering and possibly the western mainland next weekend should push the front northward and promote an expanding area of above normal temperatures across the 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