Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 649 PM EST Sun Dec 22 2019 Valid 12Z Thu Dec 26 2019 - 12Z Mon Dec 30 2019 ...Below average temperatures persist over much of Mainland Alaska... ...Stormy pattern over the Gulf of Alaska and a couple strong storms tracking south of the Aleutians... The most pronounced trend in a majority of today's guidance arises mid-late period as it suggests stronger upper-level ridging over and south of eastern Siberia. The combination of this ridge and a deep upper low settling into the North Pacific would lead to a Rex block configuration which should promote a longer duration of deep upper troughing over the mainland and extreme northern Gulf of Alaska--along with persistence of well below normal temperatures across a large portion of the mainland. This troughing will encourage mean low pressure to prevail at the surface over the Gulf (by way of multiple low centers expected to reach the Gulf/Panhandle during the period) with a precipitation focus over the southeastern coast and Panhandle. Strong winds may accompany one or more systems as well. Meanwhile the gradient between Siberia high pressure and North Pacific low pressure systems will be fairly tight at times, leading to a potential for strong winds over the Aleutians. For specifics of the forecast the starting blend used the 12Z ECMWF/GFS/UKMET for day 4 Thu and then transitioned toward a combination of 12Z and 00Z ECMWF runs along with ensemble input placing more emphasis on the 00Z ECMWF mean than 12Z GEFS mean. The total ensemble weight reached no higher than 50 percent in order to maintain a reasonable degree of detail while downplaying questionable specific ideas from one operational run or the other. Over the Gulf of Alaska and vicinity early-mid period, there is decent clustering for the first (and potentially strongest) storm at the start of the period early day 4 Thu. Then low pressure should weaken in place just off the southern coast into Fri. By late Fri/early Sat multiple ECMWF runs and the 12Z CMC are fairly similar in bringing another system into the Gulf/Panhandle. The UKMET is slower/southward The latest ECMWF means only provide a faint hint of this system though. GFS runs have been more erratic and 06Z/12Z runs are westward of other solutions. Farther west recent GFS runs have been on the northern side of the spread with initial low pressure south of the Aleutians. The 12Z GFS at least trended away from the 06Z run that actually tracked the low to the Aleutians but remains farther northwest and then much farther west than other models and ensemble means (including the GEFS). By the latter half of the period the operational GFS runs appear too fast with progression of the second and potentially very deep North Pacific storm in light of remaining solutions trending toward a well-defined Rex block configuration aloft. On the other hand the 12Z ECMWF holds the deep storm well westward of the last few ECMWF runs or ECMWF/GEFS/CMC ensemble means and 12Z CMC--hence the greater emphasis on the 00Z ECMWF/ECMWF mean in the initial blend for later in the period. The 12Z ECMWF mean maintains a similar longitude. Recent trends for the track have been southward in accordance with the stronger trend for upper ridging to the north. The specifics are quite hazy at this point but one or two more systems could reach the Gulf/Panhandle by Sun-Mon, one that may be supported by energy from or ahead of the first system to the south of the Aleutians and another by some combination of energy ahead of the second North Pacific storm and/or within the southern periphery of the mainland upper trough. 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 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