Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Mon Mar 11 2019 Valid 12Z Fri Mar 15 2019 - 12Z Tue Mar 19 2019 Confidence is relatively good with respect to the large scale flow pattern across Alaska and the North Pacific during the extended period, with persistent mean upper-level troughing near the Aleutians/Alaska Peninsula, and an upper ridge axis rebuilding across western/northwestern Canada into portions of mainland and Southeast Alaska. ECENS and GEFS D+8 standardized anomalies provide strong support for the persistence of this large scale pattern, with maximum positive 500 hPa height anomalies centered over west central Canada, and minimum negative anomalies centered between the Aleutians and Hawaii. This setup will keep the preferred storm track generally south of the Aleutians and into the Gulf of Alaska, with persistent southerly flow into Alaska. Models/ensemble members showed relatively good clustering for a low pressure system moving northward across the Gulf toward Kodiak Island late Fri-Sat, which should then quickly weaken as it moves inland late Sat into Sun. Thus, the forecast during days 4-5 (Fri-Sat) was based heavily on a blend of the ECMWF/GFS solutions, which showed relatively good consensus during this early portion of the extended period. While spread is a bit higher given the time frame, models show general agreement that another low pressure system should follow a similar track north across the Gulf on Mon, with yet another following quickly on its heels just beyond the forecast period. Despite increased spread on the timing, intensity, and structure of these systems later in the extended period, ensemble scatter plots reveal some semblance of clustering even through the end of the period. Thus, ensemble means may perhaps provide some realistic semblance of the evolution of these systems and their tracks. Thus, the forecast was shifted gradually toward heavier weighting of ECENS and GEFS ensemble means starting on day 6 (Sun), with deterministic solutions completely absent from the forecast by day 8 (Tue). The continuous track of storm systems across the Gulf into southern Alaska, and the persistent deep southerly flow will make for a period of widespread and occasionally heavy precipitation for many areas from southern mainland Alaska to Southeast Alaska. The air mass will quickly lost its moisture content farther inland, however, with relatively dry conditions expected to persist across the Interior and the North Slope. Additionally, the Fri-Sat system may bring the potential for high wind gusts to southern mainland Alaska, particularly for coastal areas. Ryan Bullet points from the WPC Days 3-7 Hazards chart: https://www.wpc.ncep.noaa.gov/threats/threats.php - Heavy precipitation across portions of mainland Alaska, Thu-Mon, Mar 14-Mar 18. - High winds across portions of mainland Alaska, Thu, Mar 14. - High winds across portions of mainland Alaska, Fri-Sat, Mar 15-Mar 16. WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can 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