Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 549 PM EDT Sun Apr 13 2025 Valid 12Z Thu 17 Apr 2025 - 12Z Mon 21 Apr 2025 ...Potent storm to offer lingering heavy precipitation and high wind/wave threats for the southern tier of Alaska and the northern Gulf of Alaska into Thursday... ...Guidance/Predictability Assessment... Prefer to maintain continuity with a composite blend of best clustered guidance of the 12 UTC ECMWF/UKMET/Canadian models valid for Thursday and Friday. The overall pattern evolution still seems to offer slightly better than average predictability despite more prominant than usual shorter range phasing differences leading to development of a main upper trough along 160W into Thursday. The 12 UTC GFS is an outlier here, but is still plausible given flow amplitude. Opted to pivot to a composite blend of the still compatible 12 UTC GEFS/ECMWF/Canadian ensemble means by next weekend amid slowly but steadily growing forecast spread. WPC product continuity was well maintained with this plan. ...Weather/Hazards Highlights... Expect development and northward track of a deep low from the Northern Pacific towards the eastern Aleutians and Alaska Peninsula/Bristol Bay amd vicinity Tuesday into Thursday, with weakening system energies and precipitation propagating and reforming over the northern Gulf of Alaska and vicinity Friday through at least next weekend. The deepened main surface system evolution remains a bit chaotic given short range uncertainties with upper flow phasing, but should eventually consolidate given amplified upper low/trough support aloft and coupled downstream upper ridging to act to slow eastward progression. Mean moist long fetch southerly flow wrapping near and east of the low will focus a multi-day period with locally heavy precipitation potential from southern Alaska into portions of Southeast Alaska, mainly Tuesday into Thursday. In this period of deepest low strength and pressure gradients, northerly winds and gap winds on the backside of the low both over the eastern Aleutians and the Alaska Peninsula as well as with leading winds up through Kodiak Island and southern Alaskan coastal areas through especially the Alaska Range may prove quite strong and gusty. The low will also present a well organized maritime hazard to monitor. While additional influx of upper through energies will meanwhile work up into/over an unsettled Interior around the western periphery of downstream upper ridging, moisture will be more limited. Accordingly, only expect localized pockets of enhanced precipitation, mainly over Southwest Alaska in closer proximity to the main system. Meanwhile, colder high pressure over the Arctic Ocean will slowly work down into the North Slope and the eastern Interior. Meanwhile, there remains a solid guidance signal supporting another deepened upper trough/low approach well upstream to the Aleutians and southern Bering Sea heading into next weekend and into early the following week that offers a threat for organized rains and enhanced winds/waves to monitor in the coming days. Schichtel 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