Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Wed Sep 26 2018 Valid 12Z Sun Sep 30 2018 - 12Z Thu Oct 04 2018 ...Wet Pattern for the upper Aleutians/southwest Alaska with locally heavy rains on Sunday/Monday... ...Anomalous warmth in store for the region underneath the powerful upper ridge... ...Pattern Overview and Guidance/Predictability Assessment... An impressive ridge with 500-mb height anomalies on the order of over 4 sigma above climatology will be in the process of being forced northward toward the Arctic Ocean on Sunday/Monday. This will be in response to a broad area of lower heights pushing in from the Bering Sea and Aleutians regions. Ultimately the mean upper ridge position should settle across northern Alaska up into the Arctic Ocean while broad upper lows settle to either side forming an omega block late in the period. Before this occurs, a potent shortwave is primed to lift up toward the Aleutians on Monday accompanied by a rather deep area of low pressure. The previous few runs of the GFS and 00Z ECMWF were fairly aligned with the low track while the 00Z UKMET remained off to the south. While this system shears into the downstream ridge, other embedded shortwaves may aid in further cyclone passages later next week. However, these remain of lower confidence given they are in the Day 6-8, October 2-4 realm. Ultimately, the biggest pattern changer in the period will come from the effects of what is now Typhoon Trami. Its extratropical transition depicts a potent surface low with pressures possibly below 950 mb based on some of the latest operational guidance. Given enormous spread with timing and placement, an ensemble approach would be wise. Overall, the forecast path here was initially reliant on a combination of the 12Z/06Z GFS and 00Z ECMWF with inclusion of relevant ensemble means. Gradually removed the operational components of the blend and favored a 3-way combination of the 06Z GEFS and 00Z ECMWF/NAEFS ensemble means from Day 6/Tuesday onward. Given the heavy use of ensembles, the resultant extratropical pressure of Trami is likely being highly underdone. ...Weather/Hazards Highlights... With highly anomalous ridging in place, warm early October temperature are expected. Sunday highs will be in the low 60s across sections of southern Alaska with widespread 50s over central sections of the state. Given the ridge should retreat northward in time, temperatures should come down by around 5 to 10 degrees moving into next week. Regarding precipitation, the vigorous shortwave will drive a steady period of moderate to locally heavy rainfall into the upper Aleutians. The current guidance suggests 12-hour amounts approaching the 1 inch mark in some locations within a well established north-south oriented moisture plume. Gradually the focus for precipitation will migrate eastward into the early/middle part of next week as the robust cyclone emerges out of the north-central Pacific. Given reasonably strong pressure gradients with each low, expect multiple periods of gusty winds over the Aleutians into southwestern/western Alaska. Rubin-Oster WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html