Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 224 PM EST Wed Feb 14 2018 Valid 12Z Sun Feb 18 2018 - 12Z Thu Feb 22 2018 Latest guidance continues to show that the pattern will be dominated by an east-central Pacific mean ridge aloft extending into the mainland, though with some gradual (perhaps temporary) weakening of the ridge with time. Expect the strongest systems to remain off to the west, from the northern-central Pacific into the Bering sea, while a couple shortwaves passing through the mean ridge may generate northeast Pacific/Gulf of Alaska systems early and late in the period. Early in the forecast there is still a moderate amount of spread for the evolution of shortwave energy forecast to be around 160-170W longitude as of 12z Sun. A positive trend relative to 24 hours ago is better agreement that some energy will cut through the ridge over the northeast Pacific/Gulf of Alaska while the rest goes around the ridge. However there is still disagreement over depth/track of a possible upper low with GFS/UKMET runs on the deep/northern side of the spread for the feature and associated low pressure versus the weaker/southward CMC/ECMWF. Ongoing spread and typical difficulty in resolving details of energy passing through mean ridges would favor a conservative compromise approach until there is better clustering. Through the 00z/06z cycles there had been decent continuity/agreement for a system tracking into the western-northern Bering Sea Mon onward. The 12z GFS changed toward holding the primary low farther southwest where a smaller number of solutions were suggesting. With the ensemble means more supportive of the 00z ECMWF and 00z/06z GFS idea so far, prefer to hold onto that scenario for continuity. Later in the period some guidance is suggesting that at least some of the energy reaching the northern Bering/eastern Siberia (or surrounding flow) may progress across the mainland. Recent ECMWF runs have tended to be the deepest and/or fastest with the energy aloft. Meanwhile northern Pacific into southern Bering energy may generate a Gulf of Alaska system toward late Wed-Thu. Operational models have varied considerably with the strength and timing of the shortwave aloft and thus for the corresponding surface development. The 00z ECMWF was a slow extreme for this feature while the new run has caught up to the means and recent GFS runs. In both cases there has been sufficient spread/variability thus far to recommend leaning more toward the means by next Wed-Thu. There is a fairly stable signal in the ensemble means regarding a potentially strong system tracking east of Kamchatka by next Wed-Thu while its leading gradient extends into the Aleutians. Most operational solutions over the past couple days have been depicting a significant storm but with sufficient track/timing differences to preclude use in a single deterministic forecast. Today's forecast started with a blend of mostly 00z-06z GFS/00z ECMWF runs to provide the desired compromise with some detail for days 4-5 Sun-Mon. Then the blend gradually increased 06z GEFS/00z ECMWF weight as confidence in specifics decreased, reaching 100 percent ensemble means by day 8 Thu. Rausch