Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Sun Jul 30 2023 Valid 12Z Thu Aug 03 2023 - 12Z Mon Aug 07 2023 ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... Additional adjustments made by guidance over the past day have finally led to better agreement for many aspects of the forecast pattern over the course of the period. The larger scale of important features hopefully improves predictability for the time being. The developing consensus shows an upper ridge initially centered over the southern mainland becoming a little stronger and amplifying northward as it drifts eastward. By late weekend/early next week the ridge axis should extend from western Canada north-northwestward. To the west of this ridge, leading Bering Sea/Aleutians shortwave energy and associated frontal system will quickly weaken with the approach of a larger scale trough anchored by a system expected to track from Kamchatka into the western/northern Bering Sea or Siberia. This system's leading front should weaken as it reaches the eastern Bering Sea, but expect a leading deep-layer gradient supporting southerly flow to set up over or near the far western mainland. Meanwhile a northeastern Pacific upper low will likely begin to drift northward with the changing orientation of the ridge to the north/northeast, possibly opening up late in the period. Regarding trends/comparisons for today's forecast, most guidance has become stronger with the mainland into western Canada upper ridge over the past 24 hours. This trend is most pronounced in the ensemble means. The GFS was one of the earlier pieces of guidance to suggest how strong/northward amplified the ridge might become, though the 12Z run has backed off a bit from the 00Z/06Z runs by day 6 Saturday. The GFS was also a leader among the operational models to cluster with the southward ensemble means for the northeastern Pacific upper low, versus other operational models that had been trying to track the feature somewhat farther northeast/north in some runs. Farther west, most guidance is also trending closer together for the upper trough moving into Siberia and the Bering Sea. GFS runs have been quite erratic for the leading height falls, with the 12Z/29 run being an aggressive extreme to bring lower heights into the far western mainland while the current 12Z run represents a potential overcompensation, extending its upper ridge over/beyond the northwestern mainland more than in other guidance. The new 18Z GFS has come in with a pattern that looks a lot closer to the majority model/ensemble mean cluster. By late in the period, the exact position and shape of the north-south gradient that may support an axis of enhanced moisture along the western coast of the mainland will be an important aspect of the forecast. This will take additional time to resolve, especially given how guidance has behaved up to this point. Upstream shortwave energy may feed into/around the upper trough late in the period, supporting a wave of low pressure into the Aleutians or Bering Sea but with limited confidence thus far. The improved agreement in latest guidance allowed for a more routine starting point from day 4 Thursday into early day 6 Saturday, consisting of input from the 12Z GFS/ECMWF and slightly less 12Z CMC/UKMET. Typical detail spread later in the forecast favored some incorporation of the 00Z ECens and 12Z CMCens/GEFS, reaching 40 percent total by day 8 Monday. The ECMWF/GFS/CMC provided the remaining input from more to less weight. The ECens/CMCens kept the upper ridge a little stronger than the GEFS late, favoring a tilt toward the former two, while the blend yielded a day 8 Aleutians wave much weaker than the 12Z GFS (and in fact quite similar to the new 18Z GFS). ...Weather/Hazards Highlights... Expect most of the mainland to be fairly dry from late week into early weekend. Parts of the southern mainland may see light and scattered activity at times. An initial weakening front may produce a narrow axis of rainfall over the eastern Aleutians/southwestern Alaska Peninsula on Thursday. The front extending from western Bering Sea/Siberia low pressure should spread a broad area of rainfall across the Aleutians and Bering sea from late week into the weekend, with some brisk winds as well. While this front should weaken as it reaches the eastern Bering Sea, the leading deep-layer southerly flow may help to enhance rainfall over some locations near the western coast of the mainland. Low predictability of fine details for the potential moisture axis several days out in time would favor a rainfall forecast between the heavy an light extremes that currently exist. The forecast pattern evolution should bring some increase of rainfall to southern areas late in the period as well, but with less pronounced focus. By that time what is left of the northeastern Pacific upper low may get close enough to contribute a little moisture as well. The gradual strengthening and eastward movement of the upper ridge over the mainland during the period should favor above normal highs especially over the Interior and North Slope, with greatest anomalies tending to drift west to east with time. Some pockets of below normal highs will be possible over parts of the Brooks Range late this week and the southern coast/northern Panhandle on most days, plus the southwest late in the period with the arrival of clouds and possible rainfall. Much of the state should see above normal lows through the period. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook 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