Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 646 PM EDT Fri Jun 19 2020 Valid 12Z Tue Jun 23 2020 - 12Z Sat Jun 27 2020 ...Overview and Guidance/Predictability Assessment... At higher latitudes an Arctic upper low and trough extending to its south should be a persistent feature for most of the period. The trough will extend into portions of the northern/western mainland. Over the past day the ensemble means and some models have trended deeper with the trough, leading to a farther southeastward extent of the leading surface front at least early in the period. The trough should still weaken somewhat later in the week and allow the front to return northward. Thus far the ensemble means have provided the most stable and agreeable forecast in terms of the Arctic upper low and shape/axis of the upper trough while operational model runs continue to vary among each other and in consecutive runs. This recommends a blended/mean approach for this part of the forecast. Farther south the most prominent system will be North Pacific low pressure that should gradually weaken as it reaches the vicinity of Bristol Bay or the Alaska Peninsula late in the week. There is fairly good clustering into day 5 Wed but then solutions display increasing timing/longitude differences. The past couple ECMWF runs are on the slow side of the full spread (including the ensemble means, with the 00Z and new 12Z ECMWF means actually a tad faster than the 12Z GEFS mean) while the 12Z CMC strays to the fast side. The 12Z GFS is the closest operational model solution to the guidance/ensemble mean average though it may be somewhat on the deep side. Ahead of this system there is a majority cluster suggesting that a modest upper low will track just south of the Alaska Peninsula and into the Gulf Tue-Wed, along with a weak surface wave at least into early Wed. This majority maintains greater definition than the 12Z GFS. Then late in the period guidance spread/variability lead to low confidence for forecast specifics over the central Pacific/Aleutians. There is some consensus for an upper trough to evolve over or reach the central North Pacific by Fri-Sat. However models have varied widely on what proportion of energy comes from high latitude flow versus the northwestern Pacific, with corresponding differences at the surface. Overall trends from the past couple days have been toward slower and/or farther south low pressure. Preference is for a conservative starting point until better agreement develops. Days 4-5 Tue-Wed used an operational blend followed by a rapid increase of 12Z GEFS/00Z ECMWF means, with the means reaching 60 percent total weight by days 7-8 Fri-Sat. As for operational inputs, the forecast excluded the 12Z CMC after Wed due to its progressive North Pacific system while using both 00Z and 12Z ECMWF runs through the period due to some very different high latitude trough/upper low details at times along with central Pacific divergence late. The blend included the 12Z GFS through the period and the UKMET through the end of its run. ...Weather/Threats Highlights... The mainland will see episodes of convection/rainfall of varying intensity during the period. Shortwaves within the upper trough extending south from the Arctic low will provide occasional focus, as will the leading surface front that reaches the northern/western mainland and eventually drifts northward. The North Pacific system expected to weaken as it reaches the vicinity of Bristol Bay late this week should spread some moisture to areas from the Aleutians through Alaska Peninsula/southern coast and possibly to the Panhandle. Track and speed are still in doubt so confidence is not yet very high regarding intensity and duration of precipitation. The most likely track may lead to some enhancement over favored windward terrain. The pattern continues to favor below normal highs over a large portion of the state during the period, with greatest negative anomalies over the northwest as well as south/southeast and Panhandle. Anomalies for lows will likely be more moderate and some above normal lows will be possible away from northwestern/southeastern areas. Rausch 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 Hazards: No significant hazards are expected over Alaska during this forecast period. 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