Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Thu Jun 03 2021 Valid 12Z Mon Jun 07 2021 - 12Z Fri Jun 11 2021 ...Overview and Guidance Evaluation... The full array of guidance suggests a regime of rather low predictability over a significant portion of the forecast area. Models and some ensemble members continue to be all over the place for the ultimate evolution/track of a concentrated upper low expected to be over or near the far western mainland at the start of the week while separate details of central/northern Pacific flow are also problematic. Arctic flow is questionable as well. Thus confidence is below average for some aspects of sensible weather during the period. In order to navigate the current guidance spread and provide some elements of continuity until confidence increases in any alternative scenario, today's forecast started with a 60/40 blend of the 00Z ECMWF/12Z GFS for days 4-5 Monday-Tuesday followed by a rapid trend toward increasing ensemble input so that days 7-8 Thursday-Friday leaned 70-80 percent to the means. For the western mainland upper low, the ECMWF has been the most consistent over the past couple days with a south/southeast track. GFS runs have varied among a southward track (12Z), meandering over the mainland for most of the time (06Z and new 18Z runs), or eventually lifting northward (00Z). 12Z UKMET/CMC runs take a northward track at differing speeds. Recent GEFS/ECMWF means have had a southward tilt but likely weaken the upper low too quickly due to the expanding member spread. The new 12Z ECMWF mean hints at a splitting of energy per divergence of members, with the 12Z CMC mean even more dramatic in that regard. With minimal confidence, preference was to maintain the southward track scenario per the ECMWF runs and 12Z GFS to hold onto some degree of continuity while awaiting better clustering. Meanwhile details across the North Pacific/Aleutians are ambiguous as well. The 12Z ECMWF differs from other guidance in quickly picking up a cutoff low over the central Pacific and producing a rather strong Aleutians storm by midweek (though the CMC mean uses weaker northern stream energy to produce a modest surface low of its own). As a result the 12Z ECMWF could not realistically be used in today's forecast blend. Details of leading North Pacific waviness are also in doubt, with the 12Z CMC on the northern side of the spread among solutions that have a progressive wave. By the latter part of the period the ensemble means and some models develop the general theme of a northeastern Pacific upper low/surface system (from some combination of the mainland upper low and leading North Pacific energy) while a moderate upper ridge builds over the mainland. At the same time a western Pacific system may begin to extend its influence toward the Aleutians. Recent trends in the GEFS mean for the western system have been slower toward the ECMWF mean, while the operational 12Z ECMWF has jumped to a faster timing. The GFS/CMC/00Z ECMWF all compare more favorably to the means. Meanwhile the character of high-latitude flow is not lacking in uncertainty either. The GEFS/ECMWF means both show an evolution from initial Arctic ridging toward greater progression that could ultimately bring a front near the northern coast, but operational models offer different ideas for possible patterns at the surface and aloft. ...Weather/Threats Highlights... If the initial western upper low tracks southward/southeastward as currently anticipated, expect it to produce some areas of focused precipitation over parts of the western and southern mainland during at least the early part of next week. Precipitation would become lighter and more scattered mid-late week if it either continues into the Gulf/Northeast Pacific or instead lifts northward from the western mainland. Persistence for a longer time is yet another potential scenario to be monitored. How much moisture reaches the Aleutians during the period is another question mark. It is fairly likely that at least some rainfall reaches the Aleutians during the first part of the week with leading waviness but there is a lower-end possibility of a drier scenario with a more southward track. Flow ahead of a western Pacific system may bring another area of moisture into the Aleutians during the latter half of the week. Expect below normal high temperatures early next week underneath the western mainland upper low, while locations across Southcentral and the Panhandle should see below normal highs for most of next week. The western mainland will see a moderating trend mid-late week assuming the upper low departs in some fashion, while northern areas will tend to be below normal though possibly with some pockets of above normal highs. Anomalies for lows should generally be somewhat warmer than those for highs, with the Panhandle and Interior most likely seeing moderately below normal lows versus above normal readings elsewhere. 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