Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 647 PM EDT Sat Jun 20 2020 Valid 12Z Wed Jun 24 2020 - 12Z Sun Jun 28 2020 ...Overview and Guidance/Predictability Assessment... The guidance remains fairly similar in principle for the large scale pattern. A persistent Arctic upper low will have a trough extending southward into the Bering Sea, with a leading surface front over the northern mainland likely to lift northward and weaken toward the end of the week as the upper trough drifts westward a bit. Latest consensus expects a system initially well south of the Aleutians to track northeastward into the Gulf of Alaska by early day 6 Fri. Farther west there is good agreement on an upper trough reaching/evolving over the central Pacific/Aleutians by the latter half of the period. However there is still considerable uncertainty over the specifics of this trough and the associated surface reflection. Starting with some 00Z guidance and more emphatically in the 12Z cycle, there has been a notable shift toward a more progressive and moderately southern track for the system over the North Pacific as of midweek and now expected to track into the Gulf of Alaska by Fri. Yesterday's 12Z CMC was one solution that was close to this idea 24 hours ago. The key to this change appears to involve a deeper upper trough/possible embedded low that settles for a time over the Bering Sea. The 12Z GEFS/CMC ensemble means are slower than the operational model consensus for the surface system while they are weaker than other guidance with the Bering upper trough. Latest ECMWF means are also weaker than operational guidance with the Bering upper trough but at least shape the flow in a way that is more similar to the operational models--thus leading to a surface progression also more like the models. Additional changes may be in store but at this time it is hard to argue against 12Z operational model consensus. Thus the 12Z models (more GFS/ECMWF relative to the CMC/UKMET) served as the basis for the forecast during the mid-late week time frame. Not surprisingly the operational models continue to vary among each other and from run to run for specifics of the Arctic upper low and surrounding trough while the ensemble means have been a lot more stable. The operational model blend favored during the first half of the period kept the forecast fairly close to the means and then increasing 12Z GEFS/00Z ECMWF ensemble input to 50-60 percent provided stability in the forecast through next weekend. The ensemble means are remarkably similar through day 8 for the general upper trough forecast to reach the central Pacific/Aleutians next weekend, with a modest surface wave tracking into the Aleutians. Operational models still reflect a variety of ideas over how this trough will evolve, depending on specifics of western Pacific energy and northern stream flow. Thus far they suggest decent potential for an embedded closed low that would result in a stronger and possibly slower system. The aforementioned model/ensemble mean blend reasonably accounted for this range of possibilities. ...Weather/Threats Highlights... The mainland will continue to see periods of convection/rainfall from midweek into the weekend, with the front over the northern mainland mid-late week and shortwaves within the upper trough aligned near/just off the western coast of the mainland through the period providing some of the focus. The North Pacific system currently expected to track into the Gulf of Alaska by late week should bring a period of enhanced precipitation at least to the Panhandle and possibly portions of the southern coast if the storm track is sufficiently far north. Some moisture may reach the Aleutians by next weekend depending on specifics of the system forecast to track into the region. Below normal highs will prevail over much of the state from midweek through next weekend. Greatest negative anomalies should be mid-late week over the northwest as well as south/southeast and Panhandle. Expect anomalies for lows to be more moderate with some pockets of above normal lows possible away from the cool northwestern/southeastern areas. 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