Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon May 08 2023 Valid 12Z Fri May 12 2023 - 12Z Tue May 16 2023 ...Heavy precipitation likely for southeast Alaska Friday before large northern Pacific upper low weakens/exits... ...Overview... Late this week, the first weather feature of note will be a large and deep layered mean low pressure system atop the northern Pacific, stalled from the short range into the medium range period due to a potent upper ridge in Canada. This pattern will cause unsettled weather across the southern coast of Alaska as impulses wrap moisture around the maritime low, with enhanced onshore precipitation and gusty winds especially for Southeast Alaska. This low should gradually weaken into early next week as another surface/upper low pressure system moving through the Bering Sea is likely to lead to a round of rain across the Aleutians/Alaska Peninsula and the west coast of the mainland. Meanwhile ridging and warmer temperatures are forecast to come into Southeast Alaska, and perhaps farther north into at least eastern parts of the Interior early next week with less certainty, which could increase the rate of river breakup and potentially flooding threats. ...Guidance Evaluation/Predictability Assessment... Recent model guidance agrees reasonably well for the overall pattern late week. Stacked upper/surface lows over the northern Pacific shows good consensus in their placement, and a brief shot of ridging comes in behind this feature over the Aleutians toward the Alaska Peninsula, with good agreement. This upper low looks to weaken/elongate/shift southeast a bit with time, though with some lingering model variations, but differences were not too extreme for the lead time. Models diverge somewhat more with the next initially positively-tilted upper trough/embedded low coming across the Kamchatka peninsula into the Bering Sea, especially by early next week. GFS runs have been taking more energy crossing the Aleutians into the Pacific, creating a quite negatively tilted trough and allowing for ridging to build from western Canada into much of the mainland, for much warmer temperatures for the Interior, by Monday-Tuesday. This was not favored for now, since the ensemble means were agreeable in maintaining an upper low over the Bering Sea. While the ECMWF and CMC looked more in line with the means, they do have some wave timing differences within the trough that will likely take additional time to resolve. Thus the WPC forecast transitioned from a blend of the 12Z deterministic models at the start of the period toward including increasing proportions of the ensemble means through the period to over half by days 7-8. This led to some ridging building into Southeast with some height rises in the Interior, but not nearly as dramatic as GFS runs would indicate. ...Weather/Hazard Highlights... The Gulf/northern Pacific low will draw Pacific moisture into southern parts of the state for late week, with enhanced precipitation totals looking most likely for Southeast Alaska on Friday. Some showers may linger there through Saturday before a drying trend with ridging at the surface and aloft coming in early next week. Some light to moderate precipitation is also possible for Southcentral Alaska. By the weekend, another low/frontal system moving across the Bering Sea should spread rain and perhaps some gusty winds to the Aleutians and the Alaska Peninsula. This moisture is looking more likely to bring precipitation chances to the western part of the mainland by Sunday-Monday. Some showers may be scattered across much of the mainland by early next week as well, but are not expected to be too heavy. The mean low dominating the northern Pacific late this week should lead to below normal highs across the southern coast of Alaska, but lows should be closer to or tending a bit above normal. However, much of Southeast Alaska could see increasing temperatures early next week as part of the Canadian upper ridge builds in. Farther north should see a mix of above and below average temperatures with not too considerable anomalies through this week. But temperatures may trend above normal for at least the eastern Interior next week, which would serve to help rivers break up and possibly increase the threat of flooding there. It will take additional time to say how much above normal temperatures could get as it will depend on the positioning and strength of the ridge. Tate Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Fri, May 12. 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