Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EDT Fri Sep 29 2023 Valid 12Z Tue Oct 03 2023 - 12Z Sat Oct 07 2023 ...Heavy rainfall potential for parts of southern and southeast Alaska mid to later week... ...Pattern Overview and Guidance Evaluation/Preferences... Troughing and surges of energy over the Mainland when the period starts Tuesday should get suppressed northward as a strong and blocky ridge builds over the northeast Pacific and into the Panhandle. This should also force a separate upper low northward towards the Alaska Peninsula, with the associated warm front acting to direct a possible atmospheric river towards Southeast Alaska around the middle to latter parts of next week. The next trough/upper low should move through the Aleutians middle of the week with likely phasing in some way of this trough and the aforementioned upper low moving north towards the AKPen. Another upper low may move towards/south of the western Aleutians next weekend as well. Uncertainty in the overall pattern over Alaska is high, especially regarding potentially high impact weather into southeast Alaska Wednesday-Friday. The ECMWF, along with the UKMET and ensemble means, support a steady northward progression of the low pressure system towards the Alaska Peninsula, with a warm front lifting through the Gulf of Alaska. The GFS, and to a lesser degree the CMC, build in a much stronger and blockier ridge which keeps the upper low meandering well to the south. This supports a more robust heavy rain event across much of Southeast Alaska. This also affects any possible phasing of this system with the next one moving through the Aleutians and shortwave details over the Mainland. The WPC forecast leaned heavily on the ECMWF the first half of the period, blending in the fairly agreeable ensemble means later on. ...Pattern Overview and Weather/Hazards Highlights... Southern to Southeast Alaska should remain rather unsettled during the first half of next week as shortwave energy moves through the Gulf. This may also support some lingering gusty gap winds through Southcentral Alaska and vicinity, aided by northern stream flow. Beginning on Wednesday, potential is increasing for a strong surge of moisture (possible atmospheric river) toward Southeast Alaska as a warm front lifts through the Gulf. This leads to a fairly decent signal for heavy rainfall impacting the Panhandle Thursday into Friday. The warm front should continue to lift northward with rainfall extending into into parts of the southern Mainland by next weekend as well. There remains a fair amount of uncertainty in exact rainfall amounts, but ensemble guidance suggests the signal is there. With the exception of the North Slope, much of Mainland Alaska should be below normal, with daytime highs as much as 20-25 degrees below normal across southern parts. As the upper ridge builds in, temperatures should moderate and trend warmer later next week. Santorelli Hazards: - Heavy rain across portions of the Alaska Panhandle and southern coastal Alaska, Wed-Fri, Oct 4-Oct 6. 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