Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Mon Feb 19 2018 Valid 12Z Fri Feb 23 2018 - 12Z Tue Feb 27 2018 Models and ensemble continue to show a progressive pattern continuing across Alaska during the medium range, north of a North Pacific upper ridge. This will allow multiple upper shortwaves originating in an area of negative height anomalies across northeastern Asia to move east toward Alaska, amplifying as they cross the Bering Sea, and eventually moving into mainland Alaska and the Gulf. Predictability has perhaps improved somewhat compared to recent days as models get a better grasp on the progressive pattern. Uncertainties remain, however, largely due to flow interactions between the stream of energy originating in northeast Asia, and additional upper-level flow/smaller-scale shortwave energy originating over the Arctic and affecting Alaska. The forecast period begins early Fri with one upper shortwave digging southeastward over the Gulf, and another crossing the western Aleutians. Overall, the ECMWF and GFS showed similar solutions with respect to the placement of these two features, but the GFS has been fairly persistent with showing a greater degree of amplification with the wave crossing the Gulf on Friday. GFS/ECMWF continued to show similar solutions as shortwave amplifies across the Bering/Aleutians Sat/Sat night. At the surface, there is general consensus on one relatively deep surface low lifting north across the Bering (although the GFS takes the low farther west than the ECMWF), and a new triple point low developing near the Aleutians on Sat, which then quickly moves east into the Gulf on Sun. Yet another shortwave and surface low cross the Bering Sun/Sun night, and spread among the models/ensemble increases quite a bit by that time, with the ECMWF more quickly bring the low east toward mainland AK, while the GFS showed a deeper and slower solution. By the end of the period (Tue) this system lifts north, either to the northern Bering or perhaps northern AK, with an elongated occlusion extending into the northern Gulf as it weakens. The WPC forecast was generally based on a blend of the ECMWF/GFS along with the ECENS/GEFS ensemble means. More weight was placed on the GFS early in the period, as a somewhat more amplified wave crossing the Gulf Fri was favored. Otherwise, ensemble weighting was gradually increased through the period, with majority ECENS/GEFS weighting from Day 6 (Sun) onward. In terms of sensible weather, this will continue to be a fairly snowy pattern for much of Alaska, with periods of relatively high pops even across interior areas of central/northern Alaska due to the progressive flow with multiple streams affecting the state. The Aleutians along with areas along the Gulf of Alaska coast in southern/southeastern Alaska will see periodic rain/snow and gusty winds as the series of systems traverse those areas. Temperatures will be a bit colder than have been experienced recently, as the weakened North Pacific ridge and the Arctic flow affecting much of interior Alaska allow a colder air mass to settle in. Ryan