Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 733 PM EDT Fri Aug 02 2024 Valid 12Z Tue Aug 06 2024 - 12Z Sat Aug 10 2024 ...Multi-day Heavy Rainfall/Runoff Threat from West/Southwest Alaska through the Alaska Peninsula/Kodiak Island and SouthCentral Alaska next week... ...Overview... The bulk of guidance remains consistent in showing an amplified and highly unsettled/windy Bering Sea into North Pacific upper trough and northwestern Canada to Eastern/Southeast Alaska upper ridge during most of next week. This configuration will support multiple days of enhanced rainfall and the threat of runoff issues from the Alaska Peninsula/Kodiak Island and SouthCentral as well as northward up through portions of Southwest and Western Alaska, especialy into southward facing terrain and coastal areas. The axis of best focus generally drifts eastward with time. Wavy fronts ahead of the upper trough axis, as well as a leading North Pacific system being incorporated into the flow early next week, will also contribute to the moisture focus. This pattern will also support much above normal summertime temperatures and drier conditions over the North Slope and from the central to eastern Interior down across Southeast Alaska. ...Guidance Evaluation/Preferences... Guidance solutions are well clustered for Tuesday-Thursday in a pattern with above average predictability and good cycle to cycle continuity. Prefer a composite of the 12 UTC GFS/ECMWF/UKMET/Canadian models for detail for this time period. Switched preference to a model and ensemble blend at longer time frames, increasing emphasis upon the more compatible ensemble means amid growing embedded system uncertainty. It remains the case that most of the latest dynamical and machine learning (ML) guidance offers good agreement in principle and continuity for the overall upper trough-ridge pattern and associated surface evolution. With some lingering differences for specifics, most solutions suggest the upper trough will achieve its greatest depth/amplitude around Tuesday-Wednesday followed by a northward retreat of the trough's anchoring upper low. Guidance remains more varied with the surface system and supporting dynamics expected to reach the Aleutians/Bering Sea by late next week. The most coherent signal in guidance is toward an average of recent GFS/ECMWF runs and the ensemble means. ...Weather/Hazards Highlights... Most aspects of the forecast are still fairly consistent regarding next week's multi-day rainfall event and windy conditions affecting locations from the Alaska Peninsula/Kodiak Island and gradually drifting eastward along the southern coast, with some of this moisture and wind extending into favored southward-facing terrain across parts of the southern and western mainland. Strong deep-layer southerly flow channeling between the Bering Sea upper trough and northwestern Canada to eastern Alaska upper ridge will maintain the flow of moisture into the southern coast and southern/western mainland terrain. Precipitable water values will also likely increase by mid-late week, but maximum rainfall totals should begin to decline toward the end of the week as the surface gradient relaxes. The Days 3-7 Hazards Outlook depicts areas for heavy rain over parts of the eastern Alaska Peninsula and extending across Kodiak Island and the Kenai Peninsula to locations near/just east of Prince William Sound Monday-Thursday, with the heaviest rainfall shifting slowly eastward with time. While not sufficiently large in scale to represent very broadly on the Hazards Outlook, locally heavy totals will also be possible over favored terrain a little farther inland from the southern coast and northward across the western mainland, especially with enhanced coastal inflow. Gradual cooling late in the week could eventually change some precipitation to snow at highest elevations. Continue to monitor forecasts for surface/upper level detail refinements that will help to resolve the specifics of location and magnitude of heaviest rainfall. Some more modest precipitation may spread northward into the remainder of the eastern mainland late next week. Farther to the west, a system may reach the western Aleutians/Bering Sea by next Friday, but with below average confidence in specifics. 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