Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Thu Oct 05 2023 Valid 12Z Mon Oct 09 2023 - 12Z Fri Oct 13 2023 ...Atmospheric River event across Southeast Alaska likely to extend into early next week... ...Overview... Most guidance shows a persistent upper low over the western mainland drifting only gradually northward over the course of next week. Flow to the southeast should carry a leading Pacific wave northward into the Gulf of Alaska early-mid week, bringing some enhanced precipitation to the Panhandle/southeastern coast and adding to totals expected in the shorter term. Another wave and associated moisture may lift toward the Panhandle late next week on a track a little farther east than the first system. Compared to yesterday, guidance has trended closer together for a potentially deep storm that may track into the Aleutians/Bering Sea mid-late week with a broad area strong winds and precipitation. Expect below normal high temperatures over a majority of the state aside from far northern areas. ...Guidance Evaluation and Preferences... For the most part there was better than average model and ensemble clustering (relative to the time frame involved) for the western mainland upper low and the two primary waves forecast to track northward toward the southern coast or Panhandle. A number of latest GFS runs provide the main discrepancy with the mainland upper low, straying northward of consensus by the latter half of the week and building in more of a ridge. The 06Z run was the one version that kept the upper low more persistent. The average of other models and means (including the GEFS) would have the upper low reaching only as far north as the far northwestern mainland by day 8 Friday. Over the northeastern Pacific/Gulf, guidance differences for the strength/track/timing of the early-mid week system are generally within the typical range for forecasts 4-6 days out in time--favoring an intermediate approach. Agreement is particularly good by the time low pressure reaches just east of the Kenai Peninsula by early Wednesday. A compromise also looks good for the next wave, initially well south of the Aleutians, expected to track toward the Panhandle by next Friday. Current signals are for this second wave to track somewhat east of the first one. Predictability in specifics is modest that far out in time, so significant adjustments may be possible in future runs. Recent GFS/GEFS trends leading to the 12Z/18Z cycles have improved guidance comparisons for the potentially deep storm forecast to reach the Aleutians/Bering Sea. Earlier GFS runs, especially from 00Z and prior, tended to keep the main low well south of the Aleutians while depicting a separate weaker northern Bering Sea system associated with northern stream dynamics. The GEFS mean had generally tracked along or south of the Aleutians. ECMWF/CMC runs and their means have been showing earlier phasing of dynamics as the storm heads into the western Pacific, bringing a deep low into the Aleutians if not into the southern Bering Sea. Now the 12Z/18Z GFS bring the system into the Bering Sea to a latitude most similar to the latest ECMWF means, with the GEFS/CMCens means just a tad southward. The GFS/ECMWF/CMC all suggest the surface low could be deeper than 960 mb for a time, while the GFS delays gradual weakening toward the end of the week relative to the other models. Even with the improved agreement for the storm in principle, there are a lot of detail uncertainties that arise toward the end of the week and a blend approach helps to resolve these issues. The first half of the period started with a 12Z operational model blend with somewhat more GFS/ECMWF input relative to the CMC/UKMET, in light of good guidance agreement aside from lower-predictability details. The late-week part of the forecast still held onto 70 percent total operational guidance (ECMWF/GFS/CMC) while adding in the 12Z GEFS/00Z ECens means for the remaining 30 percent weight. This yielded a Bering Sea storm depth between the models and means, while representing the non-GFS consensus for the mainland upper low and an intermediate solution for the wave that may track toward the Panhandle. ...Weather and Hazards Highlights... The most active weather early in the week will be over the Panhandle/far southeastern mainland, as a northeastern Pacific system lifts northward just offshore the Panhandle and likely settles into a position near Prince William Sound by early Wednesday. This system should produce enhanced rainfall and mountain snow, adding to precipitation totals from prior days. There may be some brisk to strong winds around this system, as well as to the north over the mainland which will be in the gradient between Gulf low and Arctic high pressure. Another wave expected to track toward the Panhandle late in the week may bring another episode of precipitation whose axis should be oriented a little farther east than with the previous wave. Meanwhile confidence is gradually increasing for a strong low pressure system to track into the Aleutians/Bering Sea during the latter half of the week. This system would spread a broad precipitation shield across these areas, and also strong winds that will require monitoring for potentially reaching hazardous criteria. High temperatures should be below normal over a majority of the state, with areas near the northern coast seeing the best potential for some above normal anomalies. Readings may moderate a little toward the end of the week as the western mainland upper low drifts northward. Low temperatures will tend to have warmer anomalies, leading to a mix of above and below normal readings. Below normal lows are most likely over western/southwestern areas and into parts of the Interior. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sun-Mon, Oct 8-Oct 9. 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