Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 600 PM EST Mon Nov 09 2020 Valid 12Z Fri Nov 13 2020 - 12Z Tue Nov 17 2020 ...Guidance/Predictability Assessment... From late this week into the weekend there is good agreement that the upper low initially over the western coast of the mainland will drop southeast into a Northeast Pacific mean trough. Behind this evolution a progressive upper ridge entering the western Bering Sea early in the weekend may decelerate as it eventually reaches the eastern Bering Sea and possibly parts of the mainland. Forecast details become increasingly uncertain with time as recent guidance diverges with how shortwave energy emerging from eastern Asia by the start of the period ultimately evolves. Some of the significant differences become apparent over the Bering Sea already during the weekend (with a leading compact wave) and then increase in scale and eastward effect going into next week. The primary debate in the guidance is whether enough of the southern half of the emerging Asia upper trough energy will separate from the westerlies to allow for incorporation of a lower-latitude Pacific wave. This wave would develop into a deep storm that reaches the vicinity of the western Aleutians/Bering around the start of the week followed by added uncertainty over strength/track as it heads into the mean ridge aloft downstream. Another scenario that some guidance has been advertising is for greater northern stream emphasis, leading to a farther northwest primary low and a mere front crossing much of the Aleutians/Bering, while the southern wave is more suppressed with no effect on the Aleutians. Over the past couple days or so operational GFS/ECMWF runs have varied significantly between each other and from run to run within the broad envelope of possibilities for the western evolution, favoring a measured approach to the forecast. From Fri into Sat a 12Z operational model blend (though for ECMWF input splitting between the last two runs due to questions over some details) provides a reasonable depiction of consensus. Aside from the UKMET whose western evolution was like the old 00Z ECMWF, there is currently more of a signal for flow separation with the 12Z GEFS mean hinting at this idea more than the 00Z ECMWF mean. Thus after Sat the forecast trends to a model/mean blend with more GEFS influence than ECMWF mean into day 7 Mon with a system reaching the western Aleutians/Bering at that time (12Z ECMWF mean adjusted at least as far south as the GEFS mean). However with operational runs diverging even more after Mon, prefer to trend further toward the more stable means by day 8 Tue. This keeps low pressure over the western Bering with progressive leading front. Note that ensemble means converge by day 8 Tue so GEFS/ECens relative weights return to even then. Modest input of individual models helps to sharpen the mean pattern a bit. In addition the blend makes sure to downplay a leading Bering wave in the 12Z ECMWF/CMC around Sun in light of consensus suggesting a fairly strong ridge aloft over the region. Meanwhile there is better consensus toward surface high pressure building over more of the mainland from late weekend into early next week. ...Weather/Threats Highlights... A fairly broad area of scattered precipitation should gradually decrease in coverage with time as the upper low initially near the western coast of the mainland drops southeast into a Northeast Pacific trough. One or more surface lows over the Gulf of Alaska/northeastern Pacific may provide some focus at times along the Panhandle and possibly the southeastern coast. Broad cyclonic flow late this week into the weekend will favor unsettled conditions over the Aleutians/Alaska Peninsula. Low pressure and associated frontal system may bring an episode of enhanced precipitation and strong winds across the Aleutians and Bering Sea around early next week but so far guidance has displayed poor agreement/continuity for the details. The cooling trend forecast to be in progress late this week will continue into the weekend, leading to below normal temperatures over a majority of the mainland and Panhandle by the weekend and early next week. The main exception will be over the North Slope where above normal readings should persist. Some locations near the western coast of the mainland may see modestly above normal temperatures on one or more days. 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