Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 PM EST Wed Nov 21 2018 Valid 12Z Sun Nov 25 2018 - 12Z Thu Nov 29 2018 For most of the period operational models and individual ensemble members still show significant spread for features within an overall evolution depicted similarity in the ensemble means. As a result today's forecast maintains exclusive focus on the ensemble means for the starting point, using the 06Z GEFS/00Z ECMWF means followed by minor manual refinements as appropriate. This yields a forecast close to continuity. From the Bering Sea into the northeastern Pacific/Gulf of Alaska, combined uncertainties involve the relative path and degree of interaction among separate upper lows over/near the northern Bering Sea and southwest of the Alaska Peninsula as of 12Z Sun. The past couple ECMWF runs through 00Z/21 show the farthest south track of the northern Bering upper low while other models have various depictions that include meandering in place or eventually looping into the central Bering. Differences over the Bering contribute to North Pacific upper low path spread ranging anywhere from the Alaska Peninsula to due eastward progression. Meanwhile there is also a signal from some guidance that one or more compact but strong waves could spin up within the flow ahead of initial northern Pacific system and track into the Gulf of Alaska. Thus far GFS runs have been more enthusiastic about such a feature than the ECMWF. Going with the ensemble mean scenario leads to an evolution that eventually places greater emphasis on the northern-central Bering Sea upper low with a mean trough extending southeastward, while western Canada ridging aloft extends into Mainland Alaska. The deep/massive surface low initially over the North Pacific would gradually weaken as it tracks south of the Alaska Peninsula and eventually just east of Kodiak Island before dissipating after early next Wed. At the same time surface low pressure would take shape over the eastern Bering in response to the nearby upper low. The overall pattern should favor heavy precipitation across much of the southern coast of the mainland and the Panhandle, along with strong winds and high waves over portions of the Gulf of Alaska. Farther westward the guidance generally agrees on a system emerging over the western Pacific by early day 5 Mon but from that time onward individual solutions display a very wide north-south spread and also some timing spread depending on the character of the energy aloft. The ensemble means offer some hint of the latitude difference but are very close for timing through Day 7 Wed. Then the CMC mean falls behind the GEFS/ECMWF means. The next system in the series reaches the western Pacific by day 8 Thu. The ensemble means are similar for this system while operational runs are not surprisingly more diverse. Across the Arctic individual model runs display various ideas for flow details after day 4 Sun. The 06Z FV3 GFS is about the only solution that extends cyclonic flow into the northern mainland by day 8 Thu. Ensemble means advertise a slow increase of heights aloft as western Canada/Mainland Alaska ridging extends farther northwestward. Rausch 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