Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 727 PM EDT Wed Mar 25 2020 Valid 12Z Sun Mar 29 2020 - 12Z Thu Apr 02 2020 ...Below to much below normal temperatures to continue over parts of Alaska into next week... ...Overview... Guidance continues to show significant spread and variability for various aspects of the forecast. The most prominent discrepancies involve the evolution of the upper ridge to the west of the mainland and an upper low to the south of the Aleutians at the start of the period early day 4 Sun. Handling of the upper ridge/surrounding flow, the North Pacific upper low, and fairly low-predictability progressive upstream flow will all play a role in the specifics of individual surface lows within a favored area of mean low pressure over the Gulf of Alaska/Northeast Pacific. ...Guidance Evaluation/Preferences... For the upper ridge initially west of the mainland, over the past day the GFS has made a pronounced trend toward recent ECMWF/ECMWF mean runs that have been closing off a high by Sun with this high continuing across the Arctic with time. GEFS mean runs have been closing off a high as well but with a path somewhat farther south/east. Alternative ideas persist, such as the open/progressive ridge in the 12Z CMC/CMC mean and a closed high which then opens up in the 12Z UKMET. Continuity and the current array of guidance would favor the GFS/ECMWF cluster for the upper ridge. Meanwhile even among that cluster (including the past two ECMWF runs) there are significant differences for shortwave/upper low energy that travels to the south of the upper high and upstream energy that flows into the northeastern Pacific from the North Pacific/Bering Sea. Both streams may have a significant influence on Gulf of Alaska/northeastern Pacific surface low evolution. At this time prefer a blended approach to provide a conservative depiction of the energy/possible upper low that could travel over the southern half of the mainland, as well as the additional energy from farther west. As for the North Pacific upper low the short range portion of the 12Z GFS splits the feature in a way not shown by other guidance, leading to the remaining upper low hanging back much farther west. The 06Z GFS run compared better in principle. Still the GEFS/CMC ensembles are quite slow and the CMC/UKMET ultimately drop the upper low far enough south for it to be a non-factor in the Alaska forecast. By Tue-Thu there is still a weak signal for a possible wave crossing the Bering Sea while latest trends from some of the operational runs place more energy on a system tracking south of the Aleutians and Alaska Peninsula. Ensemble means have been less defined and generally keep lowest pressures over the Bering. The 12Z ECMWF mean is starting to hint at a trend toward the operational scenario. Interestingly some of the best agreement among the means during the entire period is for low pressure to consolidate to the south of the Alaska Peninsula by day 8 Thu. ECMWF runs and the 06Z GFS compare well to the means with this aspect of the forecast. The various considerations noted above led to a starting mass field blend that included the 06Z GFS and 12Z/00Z ECMWF runs along with the 00Z ECMWF mean/12Z GEFS mean. Operational model weight was 70 percent early-mid period and 50 percent by days 7-8 Wed-Thu while ECMWF mean input was somewhat higher than that of the GEFS mean. ...Weather Highlights/Threats... Below to much below normal temperatures should persist into next week over a considerable portion of the state, with greatest negative anomalies likely to be over interior and eastern areas. Exact anomalies and any potential moderation (likely delayed somewhat) will depend on very uncertain specifics of flow aloft. Locations over and near the North Slope region may see some above normal readings after the early part of next week. The most coherent signal for highest five-day precipitation totals is confined to the southern Panhandle, in association with multiple surface lows expected over the northeastern Pacific/Gulf of Alaska during the period. Some of this moisture may extend back to the southern coast of the mainland but with low confidence for timing or amounts. The Aleutians should see one or more episodes of precipitation while activity associated with the system currently forecast to be south of the Peninsula next Thu will be very sensitive to exact storm track. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Sat, Mar 28 and Mon-Tue, Mar 30-Mar 31. - High winds across portions of the Alaska Panhandle, mainland Alaska, and the Aleutians, Sat-Mon, Mar 28-Mar 30. - Much below normal temperatures across portions of mainland Alaska, Sat-Tue, Mar 28-Mar 31. - Much below normal temperatures across portions of mainland Alaska, Sat, Mar 28. - High significant wave heights for coastal portions of mainland Alaska and the Aleutians, Sat, Mar 28. 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