Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 629 PM EST Sun Dec 29 2019 Valid 12Z Thu Jan 02 2020 - 12Z Mon Jan 06 2020 The best clustering of today's guidance continues to show a mean pattern that features troughing over the mainland and/or Gulf of Alaska depending on the day, a western Pacific trough anchored by an upper low most likely to track west and then south of Kamchatka, and varying degrees of mean ridging in-between. The portion of the ridge over/near Siberia early in the period may ultimately drift into the Arctic and end up to the northwest or north of the mainland by the start of next week. Models/means still show a progressive North Pacific/Aleutians stream extending out from flow around the western Pacific trough. It is this flow that has been producing some of the greatest model spread and variability in recent days. The uncertainty in shape and position of the Siberia/Arctic ridge aloft leads to further challenges in determining specifics over and near the mainland. At the start of the period early Thu there is decent agreement for low pressure to be near the Kenai Peninsula/Kodiak Island, with this low drifting off to the east/east-southeast while the corresponding upper low makes modest progress as well. Recent GFS runs had been eager to track the surface low farther south/southeast than model/mean consensus but the 12Z run compares better. Issues with upstream flow across the North Pacific/Aleutians into the southern Bering arise already by day 5 Fri and become increasingly pronounced thereafter. Operational models have been erratic with individual waves/frontal systems while ensemble spread has been sufficiently broad that their means have been fairly modest with the depth of any wave. At least from Fri onward the 06Z/12Z GFS runs generally fit into the theme of the GFS/ECMWF means, tracking low pressure over the Aleutians/far southern Bering and then into the area of long-term mean low pressure over/near the Gulf of Alaska. Differences between the 12Z ECMWF and its 00Z predecessor are particularly dramatic with systems becoming 180 degrees out of phase by Sun-Mon. At that time the 12Z ECMWF does not compare well to other guidance including its mean so it had to be excluded from the forecast blend after mid-period. Confidence decreases in resolving the character of upper troughing over the mainland by the latter half of the period due to combined dependence on details of the low-predictability North Pacific/Aleutians flow as well as the ridge over Siberia and the Arctic. Currently the most likely scenario is for lingering energy to yield an upper low over the mainland after the early-period Gulf upper low weakens. Recent GFS/GEFS mean runs have shown a gradual trend toward higher heights within the overall trough, so the weaker ECMWF mean could merit some consideration. Late in the period models/means show enough broadening and/or progression of the western Pacific trough aloft to push a strong surface gradient into the Aleutians with a corresponding increase of wind speeds. Based on the above forecast considerations the initial blend started with an average of the 12Z GFS/ECMWF early day 4 Thu. Developing spread in the North Pacific stream required the forecast to begin incorporating the 12Z GEFS/00Z ECMWF means after early Thu, reaching nearly half total weight by early day 6 Sat. Questionable attributes of the 12Z ECMWF led to its removal by days 7-8 Sun-Mon. During those last two days the ensembles reached 70 percent emphasis while the GFS component included both 06Z and 12Z runs to mitigate detail differences. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html