Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Wed Aug 04 2021 Valid 12Z Sun Aug 08 2021 - 12Z Thu Aug 12 2021 ...Heavy rainfall threat over parts of southern Mainland Alaska into the Panhandle during the first half of next week... ...Overview and Guidance/Predictability Evaluation... Some of the details remain uncertain but there is a decent signal in the guidance that flow ahead of a Bering Sea into northern Pacific mean trough aloft may promote a period of significant rainfall over portions of the southern mainland and the Panhandle from Sunday into the next work week. Precipitation intensity should decline around midweek or so as the upper trough fills somewhat but the pattern will remain unsettled. Meanwhile the northern part of the mainland should be under a col region in the upper flow into early next week and then transition to flat or broadly cyclonic flow as an Arctic upper low drops down to 80N latitude or so. The most prominent issue early-mid period is what becomes of a Pacific wave (which may be related to current T.D. 15W) that comes into the picture by day 5 Monday. On one side of the spectrum, the 12Z UKMET/ECMWF suggest the upper energy initially over/south of the western Aleutians will promote significant development and a track into the Bering Sea/Bristol Bay, ultimately replacing the system over the Bering Sea as of Sunday. On the other hand the CMC is much weaker/eastward. The 12Z GFS becomes quite fast and somewhat east of the middle of the envelope while the 18Z GFS has adjusted slower. Ultimately preferred a 12Z model blend that led to more reflection of such a wave than seen so far in the ensemble means but with a track that hedged more to the means and ECMWF--providing a more moderate continuity adjustment than would be the case with other solutions. Details of any trailing frontal wave are even more uncertain. Farther upstream, by day 8 Thursday 12Z guidance as a whole is starting to gravitate to a variation of recent GFS runs which have been showing some combination of emerging western Pacific energy and flow on the west side of the Bering/Pacific mean trough separating into an upper low, with potentially significant surface development. Latest GEFS means hint at a system and the new 12Z ECMWF mean has also lowered its surface pressures compared to the prior run. This system may have some initial connection to T.D. 14W. At higher latitudes the primary extreme solution of note is the 12Z CMC which brings its Arctic upper low down to near the northern coast of the mainland instead of 77-80N latitude as indicated by most remaining models/means. The aforementioned 12Z model blend early in the period trended toward a model/mean mix later in the forecast, with a little more than half total input from the 12Z GEFS/00Z ECMWF means and the rest coming from the past two ECMWF runs plus only a small minority 12Z GFS component. ...Weather/Threats Highlights... Currently expect the most significant precipitation to be during Sunday-Tuesday across southern parts of the mainland and into the Panhandle. A leading Bering Sea low and rapidly weakening frontal system will likely bring one surge of moisture around Sunday while a trailing Pacific wave and its associated fronts bring another area of enhanced moisture by Monday-Tuesday. These moisture surges could bring precipitable water values up to at least 2-3 standard deviations above climatology. Some brisk to strong winds could also be possible if the latter wave develops on the more aggressive side of the spectrum. Unfortunately confidence is still lower than desired for the strength and track of this trailing wave. Ensemble guidance has been fairly consistent in highlighting areas from the eastern side of the Kenai Peninsula into the Panhandle, and possibly favored terrain in the Alaska Range, for receiving the greatest five-day rainfall totals but other areas may see significant totals as well. Some locations could see several inches of rain during the period. Precipitation may remain fairly widespread into Wednesday-Thursday but with lesser intensity. Central Pacific low pressure may bring an increase of rain/winds to the Aleutians after midweek. The cloudy/wet pattern over many locations as well as the trend to cyclonic flow aloft over northern areas will support below normal high temperatures across much of the state through the period. Min temperatures will be more moderate with various areas of somewhat above or below normal anomalies. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle and Southcentral mainland Alaska, Sun-Tue, Aug 8-Aug 10. 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