Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Sun Aug 15 2021 Valid 12Z Thu Aug 19 2021 - 12Z Mon Aug 23 2021 ...Overview... Today's guidance continues to show broad cyclonic flow aloft from the Arctic into the North Pacific while a ridge over the eastern Pacific gradually weakens somewhat, with the overall pattern leading to a period of unsettled weather and rather cool daytime highs across much of the state. There is still a fair amount of uncertainty in some specifics for individual shortwaves within the various steams and how the separate streams may interact. ...Guidance/Predictability Evaluation... Overall the most agreeable feature remains the upper shortwave and associated low pressure expected to track into the mainland from the west during the latter half of the week. There is still some spread, especially with the latest GFS runs deeper than most other guidance for the shortwave and deeper/northward for surface low pressure. The 12Z GFS was also at the deep edge of the 12Z GEFS member envelope for the surface system. Yesterday's continuity favoring moderate amplitude of the upper trough has held up well with today's consensus. Predictability and confidence are lower with other aspects of the forecast, albeit with a little more definition for features of interest. Over the past day the signal has increased somewhat for a northern Pacific trough aloft to support a surface wave that could track anywhere from well south of Mainland Alaska (06Z GFS/12Z ECMWF) to the mainland itself (00Z ECMWF) with 12Z/18Z GFS runs between these extremes and the CMC becoming slower than other solutions as it has tended to do in recent runs. Latest ensemble means are starting to hint at this wave and suggest a surface track that may reach near the Alaska Peninsula and continue into the Gulf. A slightly more defined version of an ensemble mean average would provide a reasonable adjustment at this time given how much uncertainty still exists. An upstream feature could drop into the Bering Sea late in the period but with minimal confidence for specifics thus far. Across the Arctic, there is reasonable agreement that one upper low will depart northeast of the mainland to start the period with a trailing compact upper low tracking north of Siberia and possibly reaching close to the northern mainland. The 12Z CMC becomes quite fast versus other guidance for the trailing upper low/trough while the 12Z ECMWF is fastest among non-CMC solutions. Ensemble means generally favor an intermediate timing. A 12Z operational model blend provided a good intermediate starting point early in the period with the main focus being the system tracking into the mainland from the Bering Sea. Transitioning this blend toward an average among the 12Z/00Z ECMWF, 12Z/06Z GFS, and 12Z UKMET (plus starting to include a small 12Z GEFS/00Z ECens weight) by mid-period early Saturday yielded the desired solution for the wave that may track near the Alaska Peninsula while also maintaining somewhat more detail than the means elsewhere. At that time the CMC was removed due to increasingly poor comparisons to other guidance in multiple areas. The rest the period increased ensemble mean input while maintaining minority components of aforementioned GFS/ECMWF runs. ...Weather/Threats Highlights... The Bering Sea system tracking into the mainland during the latter half of the week along with a leading wavy front will spread organized precipitation over much of the central and southern mainland. Some pockets of enhanced activity are possible. There is still some uncertainty over the northward extent of moisture. The most likely scenario keeps the North Slope fairly dry. Precipitation should exit eastern areas by the weekend. Confidence remains low for details of the North Pacific wave whose track will determine the northward extent of moisture and precipitation intensity from late week through the weekend. Current preferences would bring some precipitation into the eastern Aleutians and Alaska Peninsula then eastward along the southern coast and into the Panhandle while some lighter activity could reach a little northward. The full range of possibilities extends from a more suppressed/drier solution to a more northward extent of meaningful moisture. Meanwhile expect some precipitation to accompany a front reaching the northwestern mainland next weekend as the anchoring system tracks over the Arctic. The unsettled pattern will keep much of the state below normal for high temperatures through the period. Lows should be more moderate with best potential for consistently above normal readings over some locations in the southern mainland. 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