Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 659 PM EST Sun Dec 03 2023 Valid 12Z Thu Dec 07 2023 - 12Z Mon Dec 11 2023 ...Overview... The main feature of note late this week into early next week will be a northern stream upper low meandering over the southwestern Mainland, leading to below normal temperatures centered in the Lower Kuskokwim region and gradually expanding. Some weak surface low reflection will promote light to moderate precipitation and northwesterly flow across the Alaska Peninsula, while some heavier precipitation totals are likely farther east for Southcentral and Southeast Alaska, especially by the weekend as a southern stream surface low pivots across the northern Pacific. ...Guidance Evaluation and Preferences... Model and ensemble agreement with the 12Z cycle was reasonably good with the upper low tracking southeastward late week and hovering over the state into early next week. The 12Z CMC was a little farther west with the centroid of the upper low at the start of the period Thursday than consensus, but caught up to other guidance for late week. A multi-model deterministic blend was able to be used for the early medium range period. There starts to be more divergence of model solutions by Saturday/Day 6 as the established upper low pattern varies with shortwaves coming through and the resulting surface lows, as well as how an upstream upper trough will interact. After what looked to be a rogue run from the 12Z GFS yesterday that had a strong surface low near Southeast Alaska on Saturday while other models had the low farther west in the Gulf, more guidance has trended toward an eastern solution, including some ensemble members of all suites (in the new 12Z EC ensemble more than the 00Z) and many deterministic model runs as well. Thus the WPC forecast trended east with this surface low but it may spin/combine with lows into the Gulf early next week. The GEFS mean seemed to be a reasonable proxy for the low position since the 00Z EC ensemble mean available at the forecast production was still favoring a western position. Meanwhile some additional energy and upper troughing will come across/extending south of the Aleutians over the weekend, upstream of the upper low. The EC and CMC combine this more with the preexisting low while GFS runs keep a separate low in the southern stream. Ensemble means are more combined but this may just be a result of blending members with different trough positions. Overall the WPC forecast trended toward using more ensemble means as the forecast progressed and spread increased, reaching over half of the blend by Day 8. ...Weather/Hazards Highlights... Precipitation could linger from Prince William Sound into northern parts of the Panhandle on Thursday, possibly locally moderate to heavy in the Yakutat area. Meanwhile as surface and upper lows move southeast through the eastern Bering/western Alaska, they will spread rain/snow to the Aleutians and Alaska Peninsula, but with totals generally on the lighter side. Some enhanced northwesterly winds are possible there and for the coastal western Mainland, but do not seem terribly strong. As low pressure shifts into the Gulf, this will renew precipitation chances for Southcentral and Southeast for late week. Precipitation amounts could increase into the weekend as another surface low is drawn north through the northern Pacific potentially approaching Southeast and then reestablishing over the Gulf. Farther north, some accumulating snow is possible across western parts of the Mainland late this week and then across the eastern Mainland early next week. A colder trend is likely as the medium range period progresses, especially across the Alaska Peninsula into much of the Mainland where the core of the cold upper low sets up. Below average temperatures by 10-20F for highs will expand from the Lower Kuskokwim region and Alaska Peninsula late week farther into the Interior early next week. High temperatures look to be within a few degrees of average in the eastern third of the Mainland and the Panhandle. The North Slope may maintain above average temperatures through the period, but cooler than the short range period with smaller above normal anomalies. Tate 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