Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 645 PM EDT Wed Sep 23 2020 Valid 12Z Sun Sep 27 2020 - 12Z Thu Oct 01 2020 ...Overview and Guidance/Predictability Assessment... Model/ensemble guidance is agreeable and consistent in showing a transition to a very amplified upper pattern, featuring a strong ridge over western Canada and a mean trough (with one or more embedded lows) over and extending well south from the western mainland. This configuration should lead to a significant increase of precipitation over the Panhandle and portions of the southern coast. Guidance behavior thus far suggests well above average confidence in the mean pattern but models have had considerable difficulty with the specifics for individual systems. Forecast preferences essentially fall into three segments with fairly quick transitions between each. The first, centered on day 4 Sun, focuses on low pressure that may track toward the Panhandle. The starting blend emphasizes the 12Z CMC/00Z ECMWF (with 20 percent total 12Z GEFS/00Z ECMWF means) as those two model solutions are the most consolidated and closest to the means. GFS runs are somewhat fast, the 12Z UKMET quite deep/westward, and the 12Z ECMWF more fragmented. Then day 5 Mon is a transition to days 6-7 Tue-Wed which incorporate 70 percent total 12Z ECMWF/GFS (30 percent means) for the low pressure expected to track northward toward Kodiak Island. The GFS may be a tad fast given the ensemble means, thus favoring somewhat more ECMWF input, but the ECMWF does represent a faster trend from the 00Z run. The operational model emphasis helps to maintain a reasonable amount of gradient for what could be a windy system in light of the amplified pattern. After this system arrives the surface details near the mainland rapidly become more ambiguous. An elongated area of Northwest Pacific low pressure (extratropical evolution of T.S. Dolphin) dropping into the mid-latitude Pacific should become the dominant system within the forecast domain by day 8 Thu. At that time detail uncertainties lead to majority weight of the ensemble means in the forecast. ...Weather/Threats Highlights... The general pattern evolution will favor increasing potential for heavy precipitation along the Panhandle and portions of the southern coast next week. The details of timing, magnitude, and coverage are far from resolved though. The most common theme in the guidance is for a leading system tracking into the Panhandle on Sun to focus the most precip over or south of the southern Panhandle, followed by a Mon-Wed system tracking northward toward Kodiak Island--pulling significant moisture into the Gulf of Alaska and the Panhandle/southern coast. The trailing front could contain one or more waves that would produce localized enhancement beyond midweek. Elsewhere, expect periods of mostly light precipitation over the Aleutians/Alaska Peninsula with some scattered activity over the mainland away from the main focus in the south. Below normal high temperatures will prevail over the North Slope (though gradually moderating with time) and southern mainland/Panhandle. Some pockets of warmer than average highs could exist over the south though. Other locations should see above normal highs during the period. Expect widespread coverage of above normal lows with greatest anomalies tending to be over some central/northern areas. Any modestly below normal lows should be localized and limited to the extreme western/southwestern 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 Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Wed, Sep 28-Sep 30. 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