Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 800 PM EDT Fri Oct 13 2023 Valid 12Z Wed Oct 18 2023 - 12Z Sun Oct 22 2023 ...Synoptic Overview... There will be three main storm systems of interest going into the extended forecast period. The first will be the extratropical low associated with what is currently Typhoon Bolaven, and this low is forecast to be over the Gulf of Alaska by the middle of the week. An atmospheric river event on the eastern side of this low will likely produce heavy rainfall for the Southeast Panhandle region. The second system will be a strong low approaching the Aleutians from eastern Siberia that could produce gusty winds and higher waves for the Islands. The third will be an amplifying arctic upper trough that will envelop much of the mainland by the end of the week, and this will herald the arrival of colder weather. ...Guidance Evaluation and Preferences... The 12Z model guidance suite is generally in good agreement on Tuesday across the Alaska domain, although the UKMET is faster with the low crossing the Gulf. The models have consolidated more on the expected evolution of the arctic trough, although the ECMWF is stronger with the 500mb heights and the CMC is stronger with a surface low over the northwestern mainland by Wednesday night. Looking ahead to Friday, the CMC is a little to the north of the model consensus with the low pressure system originating from eastern Siberia, although the guidance is in agreement in taking this low steadily towards the southeast and away from the mainland. The CMC then becomes an outlier solution with a trailing low over the Bering by next Saturday that is not supported by the ensemble means. The WPC forecast was primarily derived from mainly a GFS/ECMWF compromise along with some CMC and UKMET through mid-week, and then greater incorporation of the ensemble means going into Friday and beyond, with the GEFS mean more favored over the earlier 00Z run of the ECENS. ...Weather and Hazards Highlights... A prolonged moderate to heavy rainfall event is expected to make weather headlines Wednesday into early Friday as an atmospheric river event likely becomes established on the eastern side of the extratropical low from Bolaven with an enhanced moisture flux directed towards the southeast Panhandle region. Rainfall totals on the order of 3 to 6 inches, locally higher, can be expected for this region. This should abate going into late Friday and the weekend with the low steadily weakening and moving inland. Temperatures will also become noticeably colder across most of the state as the strong cold front associated with the arctic trough sweeps across the state from northwest to southeast. Highs will likely remain below freezing across most of the Interior and even getting close to the southern coastal areas to close out the week, and probably some sub-zero overnight lows for the normally colder locations of the North Slope and the Yukon Flats. Hamrick Hazards: - Heavy rain across portions of the Alaska Panhandle and southeast mainland Alaska, Wed-Thu, Oct 18-Oct 19. 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