Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 707 PM EDT Mon Apr 13 2020 Valid 12Z Fri Apr 17 2020 - 12Z Tue Apr 21 2020 ...Overview and Guidance Evaluation... Latest ensemble means and most operational model runs remain similar with the large scale pattern evolution during the period but embedded detail uncertainties persist. A strong mean ridge aloft over the Northeast Pacific and extending into the Alaska mainland/western Canada late in the week will move eastward/flatten in response to progression of weakening dynamics associated with the system initially over the Aleutians/Bering Sea. By next Mon-Tue the combination of a North Pacific system and upstream energy tracking from eastern Asia into or near the Aleutians should lead to a general area of low heights (one or more upper lows) centered somewhere between the North Pacific and southern Bering Sea. At the same time an elongated upper ridge will likely extend from the northeastern Pacific/Alaska Panhandle northwestward, perhaps as far as eastern Siberia. From the large scale perspective the greatest uncertainty for specifics is with the North Pacific into southern Bering evolution during the latter half of the period. Smaller scale spread/variability persist for the northern mainland wavy front that should eventually push southward as the initial upper ridge flattens and Arctic flow becomes more broadly cyclonic. Today's forecast follows the approach fairly similar to what has worked well in recent days, an average of 12Z operational models for days 4-5 Fri-Sat followed by a trend toward a model/mean blend as specifics become more uncertain. By the latter half of the period the 12Z GFS and 12Z/00Z ECMWF provided model input while ensembles came from the 12Z GEFS/00Z ECMWF mean runs. Early in the period the consensus blend provided good continuity for the initial Aleutians/Bering Sea low pressure and associated fronts while resolving persistent differences with wave/position details for the front draped across the northern mainland. Of particular note the 12Z ECMWF appeared to make a good adjustment from the 00Z run that had been farther north than most other guidance for the mainland front Fri-Sat. Later in the period the mainland front should sag southward as heights aloft decrease compared to the late week/weekend time frame, but still with low-medium confidence in timing. By the first part of next week there are still considerable uncertainties with the timing/track/evolution of compact but deep upper low energy add surrounding flow tracking from eastern Asia into or a little north or south of the Aleutians, along with how this energy will affect the leading system expected to track across the North Pacific. On the positive side the guidance is getting better defined with the upstream energy as it crosses over or near Kamchatka. However specifics diverge considerably after that point--ranging between shearing underneath the North Pacific system (12Z GFS) to becoming the dominant feature that helps pull low pressure into the Bering (12Z CMC). Latest ECMWF runs shear some of the energy into the leading system while keeping the rest over the Aleutians. The trend to a model/mean blend late in the period accounts for the detail uncertainty while reflecting recent continuity that has emphasized keeping the best-defined low pressure over the North Pacific versus a farther north track. There is still a wide envelope of plausible tracks though. ...Weather/Threats Highlights... Expect a broad area of precipitation to cross the state late this week through the weekend, supported by a combination of initially strong low level flow and dissipating front heading into the western mainland and then weakening upper dynamics initially associated with Aleutians/Bering Sea low pressure. The wavy front persisting across the northern mainland for most of the period will also provide a precipitation focus. Thus far there has not been a coherent signal for which areas should see highest totals during the period, with a number of regions potentially seeing significant enhancement at least on a localized basis. The large scale pattern evolution should promote a drier trend over much of the mainland early next week. Parts of the Aleutians/Alaska Peninsula could see meaningful precipitation/brisk winds depending on the precise surface evolution by the latter half of the period. The temperature forecast looks consistent, with above normal temperatures over most of the state during the period aside from the Panhandle and localized areas over the southern mainland where highs may be modestly below normal. Morning lows should tend to be farther above normal than daytime highs and the northern mainland will likely see the highest anomalies. Multi-day trends have been toward a later southward push of the northern mainland front but it should eventually drift south next week as broadening cyclonic flow aloft over the Arctic begins to have some influence over the northern part of the state. 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 Hazards: - High winds across portions of mainland Alaska and the Aleutians, Thu-Fri, Apr 16-Apr 17. - Much above normal temperatures across portions of mainland Alaska, Thu, Apr 16. 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