Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Fri Jun 04 2021 Valid 12Z Tue Jun 08 2021 - 12Z Sat Jun 12 2021 ...Overview and Guidance Evaluation... In a broad sense the level of chaos in today's forecast for Alaska and surrounding areas has improved somewhat compared to yesterday, but there are still minority solutions that would yield significantly different sensible weather over some areas. The ultimate path of the upper low initially over or near the far western mainland continues to be a dominant aspect of the forecast. Meanwhile there is some degree of continuity in the North Pacific, with a leading wave tracking out of the eastern Aleutians after early Tuesday followed by a western Pacific system that tracks into the Aleutians by late next week. Details continue to vary with higher latitude flow that will affect the North Slope and vicinity. At least early in the period the latest model runs and ensemble means are gravitating toward the idea of the initial western mainland upper low remaining nearly in place instead of the wide range of northward or southward tracks seen in some earlier runs. This seems to be a reasonable adjustment given improved consensus toward weak surrounding flow. After early Wednesday some solutions still depict a southward/southeastward progression (12Z UKMET and then the 12Z CMC as well as 00Z ECMWF). Trends toward slower motion early in the period seem to favor maintaining the slower trend thereafter as well. Compared to the 00Z run, the new 12Z ECMWF mean holds onto a slightly better defined low over the composite operational model location (near Norton Sound), suggesting slower movement in a greater number of members in this run. Farther north a number of models show a shortwave or compact upper low retrograding underneath an Arctic ridge but with considerable spread for the timing and strength of the feature, favoring a moderately conservative depiction for now. Overall the flow aloft at higher latitudes has trended less progressive than yesterday thus reducing the potential for any fronts to reach the northern mainland during the period. Across the North Pacific the best clustering of guidance has a leading eastern Aleutians wave as of early Tuesday (a bit north of continuity) dropping southeastward thereafter while another system tracks from the western Pacific into the Aleutians. Due to a combination of differences for dynamics supporting the Aleutians wave and track of the mainland upper low, the 12Z CMC/UKMET keep the leading wave farther northward after Tuesday and maintain a greater flow of moisture into the southern coast and vicinity by midweek. The 12Z GFS also does not compare well to consensus for some aspects of the pattern from the Aleutians into the northeastern Pacific/Gulf due to yet other shortwave differences. The new 18Z GFS has made a big adjustment toward the favored scenario of the ensemble means and recent ECMWF runs. For the second system that should ultimately track into the Aleutians, the ensemble means provide the most stable template for the forecast--close to an average of the faster 12Z ECMWF and slower 00Z ECMWF. Consecutive GFS runs have been very erratic with this evolution. Today's forecast blend started with a composite of 12Z operational models (though splitting ECMWF input between the last two runs) early in the period to provide the best detail of the upper low that likely persists over the far western mainland. After early Wednesday the blend phased out the UKMET and a bit later the CMC/GFS, due to their different handling of the mainland upper low and/or North Pacific into Gulf pattern. At the same time total weight of the 12Z GEFS/00Z ECens rapidly increased to 60-80 percent by Friday-Saturday, with lingering minority input of the last two ECMWF runs. There were some manual edits to the resulting blend to improve detail. ...Weather/Threats Highlights... The upper low expected to linger over or near the western mainland should promote areas of enhanced precipitation at times, with coverage and intensity gradually decreasing as the upper low slowly weakens. A wave tracking east/southeast from the eastern Aleutians after early Tuesday should bring a period of rain to the Alaska Peninsula and vicinity. The most likely scenario has the southern coast trending drier after midweek as low pressure drops well southeastward but there is a lower-probability potential that a farther north surface low track could bring more moisture into areas along the southern coast. The upstream system emerging from the western Pacific should bring organized rainfall to the Aleutians by next Friday-Saturday. The western mainland upper low will keep highs in its vicinity on the cool side for multiple days, with a gradual moderating trend as it weakens. Many other areas in the state will likely see below normal highs as well with coolest anomalies tending to be over the Panhandle and parts of Southcentral. Portions of the North Slope could see some above normal highs. Warmer anomalies for lows will yield a mix of above/below normal readings, with best potential for below normal values over the Panhandle, pockets within Southcentral, and the Interior. Rausch No significant hazards are expected over Alaska during this forecast period. 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