Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 PM EST Tue Jan 30 2018 Valid 12Z Sat Feb 03 2018 - 12Z Wed Feb 07 2018 A Rex Block comprised of a ridge centered over eastern Russia and the Chukchi Sea and an upper low over the north central Pacific is expected to persist through the weekend before beginning a slight weakening/reorganizing trend, perhaps becoming reestablished a bit farther west by the middle of next week. Models/ensembles show generally good agreement with respect to this large scale flow pattern, but the details become increasingly chaotic at smaller scales. A series of small-scale shortwaves will traverse the flow on the eastern periphery of the upper low, moving into the Gulf of Alaska. From there, solutions vary, depending on the strength of the western North America ridge axis, with some solutions driving waves north into southern mainland Alaska, and others taking the energy more eastward into western Canada or Southeast Alaska. Large scale agreement was sufficient during days 4-5 (Sat-Sun) to justify use of a majority deterministic blend (00Z ECMWF/CMC/12Z GFS) along with the 00Z ECENS/NAEFS means. From day 6 (Mon) onward, the pattern becomes a bit more chaotic as the height anomaly center dipole associated with the Rex Block becomes at least temporarily disrupted. All model solutions show a vigorous shortwave of Arctic origin moving southward around the eastern periphery of the upper high early next week. The degree of weakening of the ridge which occurs will play a significant role in the evolution of this shortwave. The GFS seems to merge the high latitude upper high with the western North America ridge axis, and deflects the aforementioned shortwave energy eastward entirely into Canada. The ECMWF/CMC, on the other hand, weaken the ridge more, allowing the shortwave to amplify and dig into northern/central Alaska. Additional uncertainty arises in the Bering Sea and north Pacific as the upper low present earlier in the period begins to disintegrate a bit and becomes increasingly exposed to westerlies. Most guidance then suggest the upper low becomes reestablished a bit farther west by mid-week. This period of reorganization introduces increased uncertainty into the forecast during the mid to late extended range, and thus the forecast was trended toward heavier/majority ensemble means (ECENS/NAEFS) during the day 6-8 time frame. In terms of sensible weather, this pattern will be a dry one for much of mainland Alaska, with surface high pressure dominant through much of the period. The Aleutians as well as southern/southeastern Alaska will see precipitation chances through much of the period as multiple shortwaves and surface frontal systems pass. Ryan