Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 641 PM EDT Sat Oct 14 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 Wednesday across the Alaska domain, and the UKMET has trended more in line with the consensus compared to yesterday with the low crossing the Gulf. The models have consolidated more on the expected evolution of the arctic trough, although the CMC is slower with arrival of height falls compared to the GFS and ECMWF. Looking ahead to Friday, the GFS is more amplified with the trough over the eastern mainland, but there is sufficient ensemble support for this and thus considered a plausible solution. For the storm system over the weekend across the North Pacific, the GFS/GEFS is generally southwest of the CMC/ECMWF/ECENS, and would have even less impact on Alaska. The WPC forecast was primarily derived from mainly a GFS/ECMWF compromise along with some CMC and UKMET through Thursday, and then greater incorporation of the ensemble means going into Friday and beyond, with the GEFS mean slightly 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 initially develops on the eastern side of the extratropical low from Bolaven with an enhanced moisture flux directed towards the southeast Panhandle region, and this will be followed by instability type showers with locally higher rainfall rates, and heavy snow in the higher terrain. Rainfall totals on the order of 3 to 6 inches, locally higher, can be expected for this region. Winds could also be an issue with this event for southeastern Alaska with wind gusts of 40-50 mph possible near the coast. 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 near freezing for highs including the greater Anchorage metro area, to close out the week and going into the weekend, and probably some sub-zero overnight lows for the normally colder locations of the North Slope and the Yukon Flats. Hamrick 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