Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Wed Aug 09 2023 Valid 12Z Sun Aug 13 2023 - 12Z Thu Aug 17 2023 ...Heavy rainfall caused by a low pressure system and atmospheric river could last into Sunday over Southeast Alaska... ...Overview... An upper/surface low will move through the Gulf of Alaska toward Southcentral/Southeast Alaska this weekend, spreading heavy rain into those areas that is likely to last through Sunday when the medium range period begins. This feature will help erode a west-east oriented ridge axis initially in place across northern Alaska, leading to a troughier pattern especially in conjunction with a persistent upper low meandering atop the Aleutians and southern Bering Sea into next week. The positively-tilted trough may shift slightly east by the middle of next week, but will have to battle a strengthening high offshore of the Pacific Northwest. ...Guidance Evaluation/Preferences... Model guidance is mostly in broad scale agreement with the pattern described above, with fairly typical detail differences. By Day 4/Sunday models show good consensus on a surface low position over the Gulf of Alaska with just some minor west-east differences, with the 12Z UKMET perhaps the main outlier showing the farthest west position. With the Aleutians upper low, GFS runs have been indicating its center a bit farther east by around Monday-Tuesday than the more agreeable ECMWF/CMC. This was especially true through the 06Z run, which ended up ejecting the low farther east much quicker than other guidance by midweek, but the 12Z run trended favorably closer to consensus. Even excluding the 06Z GFS though, there is still some variability on the timing and movement of the trough/low. The positioning will depend on potential for energy to split or stay consolidated, the strength of a high to the east over the eastern Pacific, and perhaps the position of an Arctic low edging northwest of Alaska and if it has influence on the pattern farther south. The timing and position will also have impacts on any additional surface lows coming in and possibly increasing rainfall totals in southern Alaska at times next week. These aspects are the most uncertain part of the forecast, but at least there is general agreement in models and ensembles for this trough pattern. The WPC model blend favored the 12Z deterministic models, especially the ECMWF/GFS/CMC, early in the period, and gradually increased the proportion of GEFS and EC ensemble means with increasing lead time, ending up at 60 percent of the blend by Day 8. ...Weather/Hazards Highlights... Heavy rain will be ongoing across Southeast Alaska especially in and around Yakutat at the start of the period Sunday as a surface low and an atmospheric river take aim at the region. The surface low is likely to be weakening, lessening the potential for high winds that could focus in maritime and perhaps coastal areas near the Gulf of Alaska particularly on Saturday. For Monday into the middle of next week, moist southwesterly inflow could lead to additional rounds of rain that may be enhanced along the Southern Coast and possibly in the terrain of the Alaska Range, with details dependent on additional weak surface lows. At this point, significant rainfall does not look to be as heavy or widespread as over the weekend. Lighter rain amounts could stretch farther north across the Mainland, while rain chances continue across the Aleutians to Alaska Peninsula as well given the upper low overhead. High temperatures should mainly stay a bit cooler than normal in the southern part of the state given the rain chances and cloudiness. However, parts of Southeast may warm to above normal as the week progresses, influenced by a ridge/high. Northwestern parts of the state could see warmer than normal temperatures into early next week before that ridge aloft retreats/breaks down, allowing temperatures to trend closer to normal. 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 - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat, Aug 12. 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