Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Mon Oct 07 2019 Valid 12Z Fri Oct 11 2019 - 12Z Tue Oct 15 2019 The latest model guidance generally agrees on a progressive flow in the polar westerlies, with a series of systems moving across the Aleutians, through the Gulf of Alaska and into the panhandle region. To the north across much of interior Alaska, upper ridging will predominate and result in mostly dry weather during this medium range period. There are two main forecast challenges today. The first is the timing and strength of the systems moving through the polar stream, and the second is with the extratropical transition of super typhoon Hagibis and its potential impacts on the state later in the period. Right at the beginning of our period, the models are struggling a bit with the amplifying trough over the Gulf of Alaska on Thursday-Friday. There has been a fair amount of run-to-run inconsistency in exactly how this feature evolves, but there is good agreement on its impact. Strong southerly flow ahead of the developing surface low and cold front will draw deep moisture northward and bring heavy precipitation (on the order of 1-2 inches into south-central Alaska and the panhandle late Thursday through Friday. The main difficulty is with the timing of the QPF, but right now am favoring the 12Z GFS, which is a compromise between the latest ECMWF and Canadian. Upstream, another closed low will push across the Bering Sea Friday-Saturday and then through the northern part of the Aleutian chain later Saturday into early Sunday. Models all are in pretty good agreement with this feature until it gets in the Gulf of Alaska later Sunday, and examining the latest guidance, it appears that the differences with this system are at least partly tied to the handling of Hagibis. All of the models show a very robust surface system as the storm makes the extratropical transition, with the Canadian and GFS showing a sub 950mb low tracking through the Bering Sea. The Canadian is considerably faster and farthest west with the system, and amplifies a short wave ridge of the Aleutians and Gulf of Alaska early next week. This has the effect of pushing the downstream upper low well southward into the eastern Pacific. The GFS and ECMWF track the system farther south, and allow the initial closed low over the Gulf of Alaska to combine with another rapidly moving short wave that is tracking ahead of Hagibis. Considering the uncertainty with the evolution of Hagibis, the forecast confidence is relatively low for Days 6-8 over the Aleutians, south central and southeast Alaska is low. We generally took a blend of the latest GFS, the 00Z ECMWF ensemble mean, and the 06Z GEFS mean for Days 6-7, and mostly a 50/50 split between the two ensemble means by Day 8. What does seem likely is that a very strong low will spin up as Hagibis transitions and could have sigificant impacts with respect to winds and waves over the Bering Sea late this weekend through early next week. Klein 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 Alaska Hazards... Heavy rain is expected across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Oct 10-Oct 11. 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