Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 702 PM EDT Mon May 18 2020 Valid 12Z Fri May 22 2020 - 12Z Tue May 26 2020 ...Overview... Most guidance maintains the recent consensus for large scale evolution aloft during the period, with a slowly weakening Aleutians/southern Bering Sea deep-layer low settling over or near the southeastern Bering Sea and a downstream ridge likely closing off an upper high over the Arctic or far northern mainland. Flow aloft to the east side of the Aleutians/Bering upper low should elongate across the Gulf of Alaska to some degree. Confidence remains above average that the forecast evolution will support a period of locally heavy precipitation over/near the Kenai Peninsula and northeastern Alaska Peninsula late this week into the weekend. However low-predictability aspects of important frontal waves over the Gulf lead to lower confidence regarding duration of precip over this area this weekend and early next week as well as for timing/intensity of precip that should reach the southeastern coast and Panhandle. Meanwhile spread persists for some aspects of the upper ridge, and by next Tue uncertainty increases for specifics of features over parts of the North Pacific. ...Guidance/Predictability Assessment... Today's forecast started with a blend of the 12Z GFS/ECMWF and a little 12Z CMC for days 4-5 Fri-Sat. Then it tilted the ECMWF input somewhat more toward the 00Z run relative to the 12Z run from day 6 Sun onward and increased 00Z GEFS/ECMWF ensemble input to 40 percent on day 7 Mon then 70 percent on day 8 Tue. In addition the blend removed the 12Z GFS after day 7. Models and individual ensemble members continue to vary with specifics of individual waves over and near the Gulf of Alaska during Fri-Sun. The small scale of these waves leads to low predictability and confidence for specifics during this time frame. The most common themes in guidance have maintained some continuity though, with one wave initially southeast of Kodiak Island on Fri and then a trailing wave/frontal system arriving by Sun. Confidence in wave location on the latter is quite low by Sun, with potential for it to be anywhere along the front depicted in the manual forecast on that day. Recent trends toward greater eastward elongation of lower heights aloft allow for quicker departure of this feature by next Mon. Toward the end of the period the guidance diverges considerably for potential Pacific development originating from a lengthy mid-latitude wavy front. The 12Z GFS/GEFS means showed a dramatic trend toward stronger/northward development to the south of the Alaska Peninsula by day 8 Tue. Recent GEFS/ECMWF mean trends had actually favored later North Pacific development and longer persistence of the southeastern Bering low, and prior GFS runs were not enthusiastic about a 12Z GFS/GEFS type of system either. The overall pattern did provide room to reflect a much weaker system though. The 00Z GEFS mean was much closer to the majority clustering of guidance, hence its use for the GEFS component in today's forecast. Differences persist with the specifics of the upper ridge that builds over the Arctic and parts of the northern/northeastern mainland. As in the previous couple days the ECMWF/ECMWF mean-based solutions were a little weaker other models/means. In addition the 12Z ECMWF pulled its upper high center farther northwest than continuity/consensus (including ECMWF means). This led to favoring more 00Z ECMWF weight in the blend versus the 12Z run from Sun onward. Most guidance suggests that the overall upper ridge covering the mainland/Arctic may begin to strengthen around or just after day 8 Tue. ...Weather/Hazard Highlights... Continue to expect low level flow ahead of one or more frontal waves to provide a narrow focus of significant precipitation along eastward-facing terrain over the Kenai Peninsula and northeastern Alaska Peninsula late this week into the weekend. Some enhanced activity may also be possible over nearby areas. Confidence is still less than desired for exactly how long this precip focus will last, as well as for specifics of moisture that should reach the southeastern coast and Panhandle for a portion of the period. Over the past day little has changed with respect to guidance showing a wide spread for timing and amounts over the southeastern coast/Panhandle. The surface pattern over the mainland will be highlighted by a persistent east-west trough over central areas where highs will tend to be above normal through the period. Most of the southern mainland and into parts of the Panhandle should see below normal highs and pockets of below normal readings are also possible over the North Slope. Morning lows will be above average in most cases with the greatest anomalies over parts of the northern half of 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 Hazards: - Heavy precipitation across portions of mainland Alaska, Thu-Sat, May 21-May 23. 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