Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EST Sat Nov 11 2023 Valid 12Z Wed Nov 15 2023 - 12Z Sun Nov 19 2023 ...Bering Sea into Mainland Alaska storm likely to bring high winds and heavy precipitation to southern and western parts of the state by midweek... ...Overview... From the large scale perspective, most of the recent models and ensemble means have been fairly agreeable and consistent with the pattern evolution from Wednesday through next weekend. A strong Bering Sea storm will likely track into the southwestern mainland around Wednesday and gradually weaken as it continues northeastward, while multiple waves evolve over and/or track into the Gulf of Alaska/Northeast Pacific through the period. The leading storm will bring a variety of hazardous weather conditions to areas from the Bering Sea/Aleutians into the southern and western mainland. A deep upper trough moving into the mainland behind the storm will bring a pronounced colder trend, while the leading surge of moisture followed by the upper trough/Gulf low pressure will likely promote multiple days of precipitation (some heavy) across the southeastern coast/Panhandle. ...Guidance Evaluation and Preferences... In a continuing theme from recent days, guidance is behaving a lot better for the track versus the timing of the Bering Sea storm. The GEFS/ECens means in particular have been steadiest with a path that would bring the system into the southwestern mainland just south of Norton Sound, with recent GFS/ECMWF/UKMET runs close to this idea. However, even at the start of the period early Wednesday the timing spread is nearly a day apart--between the fast 12Z UKMET/CMCens and slow 12Z ECMWF. In addition multi-day trends of the guidance average have been oscillating, with the past 24 hours of runs adjusting slower after a prior faster trend. The latest trends arise from a combination of adjustments from the initially supporting Bering Sea upper shortwave and then the larger scale upper trough following immediately behind. Based on the 12Z GEFS/00Z ECens, the 12Z GFS/CMC and 00Z ECMWF provided the best operational model cluster for depicting some slower trend for this system while waiting for additional confirmation regarding an even slower adjustment. The 12Z ECMWF does catch up to those runs to some degree by Thursday, though by Friday it holds lower surface pressures over the mainland versus consensus due to becoming somewhat sharper and more negatively tilted with the upper trough over the mainland. Otherwise for the mainland upper trough from late week through the weekend, a blended/ensemble mean approach looks best for the time being. This is due to the operational models waffling with respect to if/where any embedded upper lows may form, along with low predictability details for shortwaves dropping down the west side of the trough and associated waves/fronts. Regarding the Gulf of Alaska/Northeast Pacific, consensus is fairly good with a leading Gulf wave around Wednesday-Thursday as the Bering Sea system tracks into the mainland but the 12Z ECMWF becomes out of sync with details by early Thursday. Details remain uncertain for surface details by Friday. The GFS/CMC/UKMET and 00Z ECMWF continue to show a general theme of a frontal wave and/or trailing North Pacific feature tracking into the Northeast Pacific/Gulf, though the ensemble means are showing less hint of this evolution than yesterday. The means instead maintain the best defined low pressure near the coast as of early Friday. Holding onto a majority of operational guidance into late week maintains continuity in principle, albeit with some detail differences. Note that the 12Z GFS may be too strong with its gradient focused into the southern Panhandle by Saturday, and on the heavy side with its southern Panhandle QPF around the end of the week. The new 18Z GFS has at least adjusted its evolution to reduce the magnitude of heaviest QPF versus the 12Z run. Finally, models and means all show a strong system emerging from Asia with a strong gradient ahead of the associated frontal system reaching into the Bering Sea and Aleutians by day 8 Sunday. Leading into the 12Z cycle, the ensemble means had been a little faster than the operational models overall but the 12Z GFS/ECMWF have trended somewhat faster so a blend of those models and the means is reasonable at this time. The 12Z CMC is on the slow side. Based on guidance comparisons/preferences, today's forecast started day 4 Wednesday with a composite of the 12Z GFS/CMC and 00Z ECMWF. With time the blend split ECMWF input between the 00Z/12Z runs and by day 6 Friday started introducing the 12Z GEFS/00Z ECens. With the typical decrease in detail confidence later in the period, the forecast increased the total ensemble mean weight to 40-60 percent by days 7-8 next weekend. ...Weather/Hazards Highlights... The strong storm system expected to track from the Bering Sea into the southwestern mainland around Wednesday and then continue northeastward continues to bring the potential for areas of high winds over the Bering Sea/Aleutians and into the southwestern mainland, with rough seas and possible coastal erosion/flooding issues (tempered somewhat by the relatively short duration of strongest winds). Elevated tides will be a concern as well. Anomalously high moisture being carried by leading flow may support heavy snow over parts of the south-central and western mainland, with rain mostly confined to areas near the southwestern coast. Expect lesser snow totals as moisture spreads northeast across the mainland, while the leading moisture surge will likely promote a brief episode of heavy precipitation along the southern coast east of the Kenai Peninsula and into the Panhandle during the mid-late week time frame. Within this area, expect rain over coastal/southern Panhandle locations and snow elsewhere. Precipitation should continue over the southeastern coast and Panhandle into late week/weekend given the approaching upper trough and one or more Northeast Pacific/Gulf surface lows, but confidence decreases for amounts given the uncertainty in surface details then. A majority of the mainland should begin to see a drier trend by Friday-Saturday as weak surface high pressure builds in. Over the Aleutians expect brisk winds and mostly light precipitation into Saturday, followed by a calmer conditions as a narrow ridge crosses the region from west to east. Then by next Sunday a front approaching the western Aleutians should spread an area of strong winds and precipitation across the western half of the island chain. Aside from some below normal readings over the southern Panhandle on Wednesday, most of the state will see above to well above normal temperatures through at least Wednesday-Thursday. The deep upper trough approaching/arriving behind the Bering Sea storm will bring a colder trend from west to east during late week into the weekend, with below normal temperatures likely covering much of the state by Saturday. Coldest departures from normal will likely be over western-central parts of the southern mainland. The North Slope will be one area that should remain above normal 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