Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 642 PM EST Thu Dec 09 2021 Valid 12Z Mon Dec 13 2021 - 12Z Fri Dec 17 2021 ...Overview... Guidance generally shows a progressive North Pacific-Bering Sea stream carrying along multiple lows and frontal systems while more blocky flow exists in the Arctic. As might be expected, recent model runs have been waffling on some details within the progressive flow and near the interface between the two regimes, but for the moment there is decent consensus over the mainland. Early-mid week upper ridging over and then north of Siberia should support an Arctic trough/possible upper low that digs into and across the northeastern half of the mainland. This feature should depart as a Bering Sea upper ridge crosses the mainland mid-late week. The pattern evolution will likely promote some moderation in temperatures over the state by the latter half of the week after much below normal readings prevail early in the forecast period. ...Guidance Evaluation and Preferences... A composite of 12Z operational models represented consensus well for significant features during the first half of the period. For the northern/northeastern mainland upper trough this blend tilted away from the GFS that was a little weaker than other models, and the new 18Z run has adjusted slightly toward the preferred solution. Recent runs have offered some significant differences for what ultimately happens with this energy, so further change is certainly possible--but it is good to see the improved clustering for now. Meanwhile there is good agreement for two strong western Bering Sea lows that will anchor windy frontal systems (especially with the first one) that will cross the Bering Sea/Aleutians/North Pacific. Consensus has trended farther south from yesterday for a wave that breaks off from the leading front by late Monday/early Tuesday, with a track now a ways south of the Alaska Peninsula/Kodiak Island by early Tuesday and continuing southeastward thereafter. For the latter half of the period, fairly rapid divergence of wave details from the western Pacific into the Bering Sea led to a transition of the initial model blend to a model/ensemble mean mix. This included splitting ECMWF input between the past two runs (and keeping the 12Z GFS/CMC) while using the 12Z GEFS/00Z ECens for the ensemble component. The resulting forecast downplayed a possible compact wave that could track into the Bering Sea underneath (and/or interacting with) the main low over the Bering Sea Wednesday-Thursday, while awaiting better model agreement and signal in the means. There is a little more clustering for another system that may reach the western Bering Sea late in the week. Farther east there is reasonable agreement in the general pattern with surface pressures over the mainland trending lower as Bering Sea low pressure approaches, while a triple point wave ends up near the southern coast of the mainland early Friday. By late in the week the model/mean blend helped to balance competing factors, the models possibly leading the means in being stronger with Bering Sea low pressure versus the higher surface pressures over the mainland in the means helping to account for occasional model biases to erode low level cold air too quickly. ...Weather/Hazard Highlights... An initial western Bering Sea storm and leading front will produce strong winds over the north-central and eastern Bering Sea on Monday while bringing organized precipitation through the Aleutians and Alaska Peninsula/southwest corner of the mainland. However those will be the only effects as a frontal wave tracks southeastward from early Tuesday onward. Lighter precipitation and somewhat weaker winds should accompany another strong western Bering Sea storm and leading front extending through the Aleutians Tuesday-Wednesday. Moisture with this latter system will likely reach into western and southern parts of the mainland during mid-late week. A wave expected to reach near the southern coast by early Friday may help to focus precipitation along the southeastern coast and Panhandle around that time. Additional waves may affect the Aleutians late in the week but with low confidence on details at this time. Expect below normal temperatures over much of the state early in the week with best potential for much below normal readings over the southern half of the mainland. The forecast pattern evolution should promote a moderating trend from southwest to northeast during the latter half of the week. This could allow for a few pockets of slightly above normal highs to develop by Friday while coverage of above normal lows could be greater at that time. Rausch Hazards: - High winds across portions of the Alaska Peninsula and Kodiak Island, Sun, Dec 12. - Much below normal temperatures for the southern half of Alaska including the Alaska Peninsula, Sun-Mon, Dec 12-Dec 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