Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Sun Jul 11 2021 Valid 12Z Thu Jul 15 2021 - 12Z Mon Jul 19 2021 ...Overview... Today's guidance continues to suggest that the upper pattern will open up somewhat after late this week. An upper high initially near the western coast of the mainland should gradually lose definition as it drifts southward with the overall ridge dropping down into the southern mainland by the weekend. Another upper ridge, ahead of mean troughing most likely aligning between Kamchatka and the western Aleutians (supporting one or more surface systems), may build into the mainland behind the first ridge. The evolving pattern will eventually push away initial upper low/trough energy over the Bering Sea but with continued uncertainty over what path this energy will take. Mean troughing is likely to persist over the northeastern Pacific with various possibilities for specifics over the course of the period. Northern/eastern parts of the mainland will see an oscillation of surface fronts in response to Arctic shortwaves that will be partially influenced by the precise amplitude of ridging over the mainland. ...Guidance/Predictability Evaluation... The one detail that has relatively lower confidence is the 12Z/00Z ECMWF idea of taking the initial Bering Sea upper low/trough and bringing it through the southwestern mainland next weekend. Consensus that includes latest ECMWF mean would have more persistent upper ridging. Thus it seems more likely that the Bering Sea energy will eject either weaker/northward or to the south (or split between these two options). Meanwhile guidance shows various ideas for how features may evolve to the east of Kamchatka after clustering fairly well at the start of the period on Thursday. Initial low pressure could linger for a while to the south/southwest of the western Aleutians, whether through persistence or redevelopment, or eject to be replaced by another system. Operational models have not been very agreeable or consistent over recent days. So far the ensemble means have not been too keen on the eastern side of the model spread for height falls/low pressure. Even with the uncertainty for individual low centers, there is better agreement for existence of the gradient between the low(s) and mid-latitude Pacific high pressure. Across higher latitudes, like yesterday there is a general theme in the guidance that late this week Arctic energy will provide a northward lift to a northwest-southeast front initially over the mainland as of early Thursday and then a trailing front will take its place over northern areas. At the initial time the front is a bit southwest of continuity due to slightly more amplitude with the upper trough extending from northern Canada. By the weekend and early next week solutions diverge for the shape of Arctic flow with combined uncertainties for the amplitude of mainland ridging at any particular time and details of flow over/north of Siberia. The most common theme is for mean troughing to develop over Siberia, with low pressure lifting the northern mainland front back to the north once again. An operational model blend for about the first half of the period downplayed the ECMWF handling of the initial Bering Sea energy while emphasizing the most common details of guidance elsewhere. The forecast trended more toward the ensemble means by day 8 Monday due to the widening guidance spread east of Kamchatka as well as for Arctic flow. This approach provided fairly good continuity with modest adjustments where agreed upon by enough guidance. ...Weather/Threats Highlights... Organized precipitation over the southeastern quarter of the mainland may extend into Thursday with the initial front over the region. Most of the Panhandle should see a drier trend with time as the upper trough axis moves just far enough east. Northern portions of the mainland will likely see one or more episodes of precipitation with the oscillating fronts over the region. The gradient ahead of one or more low centers emerging from Kamchatka should bring an increase of precipitation and winds into at least parts of the Aleutians and Bering Sea. There is still a lot of uncertainty over how much moisture may extend into the mainland and when/where--not only associated with these systems but also the initial closed low/trough energy over the Bering Sea. The majority of the Panhandle and southeastern coast should see below normal highs during most of the period. The North Slope should tend to see below normal highs too but may see some variability depending on day-to-day frontal position. Elsewhere expect a mix of above/below normal highs, with a tendency for a cooling trend from west to east by the weekend. Min temperature anomalies will tend to be higher, yielding greater coverage of above normal readings. 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