Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Mon Dec 04 2023 Valid 12Z Fri Dec 08 2023 - 12Z Tue Dec 12 2023 ...Overview... The main feature of note late this week will be an 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 rounds of heavier precipitation are likely farther east for Southcentral and Southeast Alaska as surface lows periodically enhance totals. ...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. Shortwave energy rounding the southeastern side of this low should support a surface low with which model guidance is now clustering better, showing an eastern position near the Panhandle on Saturday/Day 5. However, the 12Z GFS seemed like an outlier in terms of how strong and north the low was, as other guidance and the 06Z GFS run were weaker. There is still some spread in the low's depth shown by the ensemble members though, so this will continue to be monitored. The track of the surface low and whether it could maintain itself going into Southeast Alaska is somewhat questionable too. In coordination with the Alaska regional and local offices, it seems most likely (and common climatologically) that the low would weaken and drift into the Gulf rather than barrel into the Panhandle maintaining its strength. Then some additional energy and upper troughing will come across/extending south of the Aleutians over the weekend, upstream of the upper low. Meanwhile the upper low/trough looks to become somewhat southwest to northeast oriented over the weekend. There is still some uncertainty with when and how the upstream energy will combine with the preexisting Mainland upper low. The trend in recent guidance has been to keep a southern stream upper low separate for a little longer through Sunday before eventually combining to create a north-south oriented trough across the Mainland into the northern Pacific sometime Monday, but with plenty of uncertainty in the details. Surface lows are likely in the northern Pacific and Gulf under this pattern, but again are questionable in depth and placement. The 12Z GFS and ECMWF actually happened to agree quite well in a surface low position near Kodiak Island on Monday (which could also cause heavy precipitation), but would like to see some persistence in the model runs before believing that this agreement could last, given that ensemble members still show ample spread. A blend of deterministic guidance, mostly the 12Z runs but including the 06Z run of the GFS, was used for the WPC forecast early in the medium range period. The forecast trended toward using more ensemble means as the forecast progressed and spread increased, reaching half of the blend by Days 7-8. ...Weather/Hazards Highlights... Surface low and frontal systems in the northern Pacific and mean southerly to southwesterly flow will produce rounds of precipitation across Southcentral and Southeast Alaska late week into early next week. Enhanced amounts are possible across Southeast particularly on Saturday with a surface low in the vicinity. Then another low moving northeast across the northern Pacific and Gulf could spread possibly heavy precipitation into Southcentral and perhaps into the Panhandle early next week, but the details are likely to change in future forecasts. After some snow showers are possible farther north into the Interior late this week, available moisture may become more widespread across the Alaska Range and even farther north early next week for some possible accumulating snow. Meanwhile farther west, northwesterly to northerly flow will be common on the backside of the low across the Aleutians to Alaska Peninsula, with some rain/snow that should be generally on the lighter side. 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 (in moderated form) 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