Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EST Sun Nov 12 2023 Valid 12Z Thu Nov 16 2023 - 12Z Mon Nov 20 2023 ...Overview... Latest models and ensemble means maintain the idea of a significant colder trend over most of the state as a deep upper trough drifts eastward across the mainland during the period, behind a strong storm system forecast to reach the western mainland by early Thursday and weaken/continue northeastward thereafter. The upper trough will likely support a general area of low pressure over the Gulf of Alaska from about Friday through the weekend while waviness along a leading front may enhance precipitation to some degree over the Panhandle Friday-Saturday. The upper trough should reach far enough east by next Monday to push low pressure and leading moisture farther southeast. Also by that time, an upper ridge axis may reach the western coast of the mainland while an upstream frontal system could spread a broad area of brisk winds and precipitation across the Aleutians/Bering Sea around Sunday-Monday. ...Guidance Evaluation and Preferences... There is still some lingering timing spread for the Bering Sea storm expected to reach the western mainland by the start of the forecast early Thursday. The 12Z CMCens mean is the notably fast extreme at and leading up to that time, with an average of remaining guidance providing a good starting point to depict this system as it weakens and continues to track northeastward over the mainland. Behind this storm, guidance has displayed fairly typical spread and variability for details of the upper trough crossing locations over and south of the mainland. Operational model runs have been hinting at the potential for an upper low to close off for a time over the southern mainland, but so far without much agreement among different models from the same cycle. Such details will depend on smaller-scale (thus having low predictability) shortwaves feeding into the trough. For now, depicting an open trough but with a little separation to suggest the possibility of an upper low seems reasonable until there's better agreement. The new 18Z GFS is fairly extreme with its upper low, pulling it farther southeast and maintaining its definition for a longer time than the 12Z GFS. The guidance average has also been waffling somewhat on eastward progression of the overall trough. Most solutions agree fairly well upon on mean low pressure (or at the very least a surface trough) persisting over the Gulf of Alaska during Friday-Sunday, but the forecast continues to be somewhat ambiguous for details of frontal wave evolution to the south and precise effects on the Panhandle. This relates to additional low-predictability shortwave details. The GFS/ECMWF/UKMET show some similarities for the shape of evolution but GFS runs show a track increasingly north of the ECMWF/UKMET by late Friday-Saturday. In addition trends have diverged as the ECMWF/UKMET have shifted southeast in the 12Z run while the GFS has been shifting northward since its 06Z run. Overall prefer an average of the 12Z model runs, given better definition than seen in the ensemble means while tempering opposing trends. This would yield a northern extent of meaningful precipitation between the contrasting GFS and ECMWF. There are two primary forecast issues that arise by days 7-8 Sunday-Monday. Over and west of the mainland, an ensemble mean average provides the most stable timing for the mainland trough and the approaching upper ridge. The 12Z ECMWF strays fast by late in the period, leading to earlier western mainland arrival of the strong surface gradient crossing the Bering Sea/Aleutians. Recent GFS runs have been erratic, with the 18Z/12Z runs closest to the means but the 06Z run slow and the 00Z run fast. The amplitude of the pattern through the weekend seems to support leaning away from the fast side of the spread for now. Over the Pacific the question is how much interaction may occur between the southern periphery of the mainland/North Pacific trough and a system over the mid-latitude Pacific. Recent CMC runs have been most enthusiastic about splitting off a strong wave from the Pacific system and tracking it into or just south of the Panhandle. The 06Z GFS was one other solution remotely close, eventually picking up the entire system into the Northeast Pacific (with effects after the forecast period). ECMWF runs, other GFS runs, and the ensemble means all support the idea of sufficiently progressive and detached northern stream flow to keep any Pacific waviness suppressed along or south of 50N latitude through the end of the period. What low pressure exists over or near the Panhandle by early next Monday would more likely represent the southeastward progression of Gulf low pressure as the supporting upper trough continues eastward. Based on today's guidance the first half of the period started with 70 percent total 12Z GFS/ECMWF and the rest 12Z UKMET/CMC. Thereafter, varying questionable attributes in the CMC and ECMWF along with the typical decreasing confidence in specifics led to phasing out the CMC/ECMWF while increasing 12Z GEFS/00Z ECens weight. The ensemble means comprised 70 percent of the blend by day 8 Monday, with the 12Z GFS claiming the other 30 percent. ...Weather/Hazards Highlights... The strong storm system reaching the western mainland by the start of the forecast period early Thursday should gradually weaken as it continues northeastward late in the week. Snow spreading across the central/northern mainland will trend lighter and more scattered with time, and fairly dry conditions should exist over a majority of the mainland by Saturday-Monday as high pressure builds in. The Panhandle, and to some degree the southeastern coast east of Prince William Sound, will see a longer period of unsettled weather with potential frontal waviness keeping precipitation over the Panhandle especially Friday-Saturday and a Gulf wave persisting Friday-Sunday (but with less moisture to work with). Precipitation should be in the form of snow over most areas aside from some coastal and southern Panhandle locations. Snow levels over the Panhandle should decline late in the period as the mainland upper trough nears. Over the Aleutians expect brisk winds and mostly light precipitation into Saturday, followed by a brief period of calmer conditions as a narrow ridge crosses the region from west to east. Then a front approaching the western Aleutians early Sunday should spread an area of strong winds and precipitation across the Aleutians/Bering Sea Sunday-Monday as it continues eastward. The leading edge of the winds and moisture could reach the far western mainland by Monday. Most of the state will see above to well above normal temperatures into Thursday. The deep upper trough arriving behind the storm crossing the mainland that day will bring a colder trend from west to east during late week through the weekend, with below normal temperatures likely covering much of the state by Saturday. Coldest departures from normal will likely be over the southern mainland, including the Panhandle by next Monday. The North Slope will be one area that should remain above normal through the period. Warmer flow beginning to push in from the west may raise temperatures to near or above normal levels over the far western mainland by next Monday. 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