Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Mon May 14 2018 Valid 12Z Fri May 18 2018 - 12Z Tue May 22 2018 The flow pattern across Alaska during the medium range transitions from a rather complex and blocky pattern late this week to a (somewhat) more progressive pattern by early next week. Confidence is high that an upper ridge axis across western North America will initially extend into eastern/southeastern Alaska, with a broad upper-level circulation (and numerous embedded shortwaves) centered over the Bering Sea. Interactions between shortwave energy emanating from the Arctic with additional energy arriving in the westerlies introduces additional complexity during the early part of the medium range, and models are having a tough time locking onto a consensus. Even at the start of the forecast period (12Z Fri), the past few runs of the ECMWF, GFS, CMC have shown dramatically different solutions with respect to the smaller scale shortwaves, and differing intensities for a blocking ridge over the northwestern Bering. Given the low confidence, the WPC forecast was based on a slight majority ensemble means (00Z ECENS/NAEFS) even for days 4-5. Forecast confidence seems to actually improve slightly by later in the period. The likely culprit being the change to a more progressive flow regime, and the increasing influence of a strong North Pacific jet. There is a general consensus among the guidance that a low pressure system will approach the western Aleutians by Mon-Tue, with perhaps some consolidation of energy across the Gulf of Alaska into an upper low (and an associated occluded surface frontal system). One of the areas of larger uncertainty by early next week is how strong the ridge across western North America will remain, and whether it will continue to influence eastern/southeastern portions of Alaska, or will shift eastward allowing heights across these areas to fall in association with the expected Gulf upper low. The WPC forecast continued some use of deterministic guidance through the entire forecast period, but ensemble weighting was gradually increased further during days 6-8. In terms of sensible weather, this will be a particularly wet pattern through much of the period for southern mainland Alaska and the Alaska Peninsula - the result of persistent and deep southerly flow between the broad upper cyclone over the Bering and the western North America ridge. By early next week, if the ridge is sufficiently broken down to allow height falls into Southeast Alaska, expect a significant increase in precipitation for that area as well. Meanwhile, expect somewhat drier conditions across the interior, with scattered and relatively light precipitation. Temperatures will be near or even a bit above average (especially for areas across the interior that are free of precipitation) under the influence of the aforementioned southerly flow and the ridge axis extending into east central Alaska. Ryan 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 www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html