Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Thu Aug 01 2024 Valid 12Z Mon Aug 05 2024 - 12Z Fri Aug 09 2024 ...Wet pattern from the Alaska Peninsula/southern coast into parts of the southern and western mainland next week... ...Overview... Most guidance is consistent in showing an amplified Bering Sea into North Pacific trough and northwestern Canada upper ridge during most of next week. This configuration will support multiple days of enhanced rainfall from the Alaska Peninsula/southern coast into southward-facing terrain over parts of the southern and western mainland, with the axis of best focus generally drifting slowly eastward with time. A wavy front ahead of the upper trough axis, as well as a leading North Pacific system being incorporated into the flow by the start of the period early Monday, will also contribute to the moisture focus. Most of the state should see a period of above normal temperatures, aside from southern tier areas that may continue to see below normal highs and western locations late in the week as the Bering Sea trough approaches the western coast. With low confidence in details, a system may reach the western Aleutians/Bering Sea by late next week. ...Guidance Evaluation/Preferences... 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 with rising heights (albeit still mean troughing) to the south. At the surface, most guidance shows a complex area of surface low pressure with multiple centers from the Bering Sea into the North Pacific at the start of the period early Monday, including the dissipating remains of a leading short-term North Pacific system being incorporated into the larger scale trough. Low pressure should consolidate over the Bering Sea by Tuesday and then weaken/lift northward with time corresponding to the evolution of upper dynamics, though expect the trailing front to have one or more weaker embedded waves. Most guidance shows the greatest moisture anomalies over the state around mid-late week with a gradual eastward drift, though the surface gradient into the southern coast becomes a lot weaker by next Friday. The 12Z/18Z GFS runs offer the primary discrepancy today, showing greater southward trough elongation/closed low formation after Tuesday with corresponding southward low pressure evolution and slower eastward progression. Consensus has trended a little stronger with the downstream ridge over the past day, but thus far the overwhelming majority of dynamical/ML solutions would recommend leaning away from the 12Z/18Z GFS handling of the upper trough by Wednesday-Friday. Since yesterday the guidance has become a lot more varied with the surface system and supporting dynamics expected to reach the western 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, with the ML models averaging out to a similar idea. This solution yields a delay of about a day for the system to reach this region compared to yesterday's forecast. Guidance comparisons led to starting the early part of the forecast with a composite of the 12Z ECMWF, GFS, and UKMET/CMC in order of more to less weight. The 06Z GFS compared better to consensus than more recent runs Wednesday onward, so that run comprised the GFS input from Wednesday onward. In addition by days 7-8 Thursday-Friday the forecast split ECMWF input between the 00Z/12Z runs while incorporating 20-40 percent ensemble means (12Z GEFS/00Z ECens). ...Weather/Hazards Highlights... Most aspects of the forecast are still fairly consistent regarding next week's multi-day rainfall event and potentially breezy 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. Into Monday, incorporation of a North Pacific system into the broader flow will initially help to enhance the low level gradient into the Alaska Peninsula. Then the strong deep-layer southerly flow between the Bering Sea upper trough and northwestern Canada 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 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. 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 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 at this time. The forecast pattern will support above normal temperatures over most of the state, especially early-mid week. Typically warmest locations in the eastern Interior could see highs reach into the lower half of the 80s from Monday through Wednesday or Thursday. Primary exceptions to the warmth will be the southern mainland and northern Panhandle for high temperatures which may remain below normal next week. Gradual approach of the Bering Sea upper trough should lead to a cooler trend from west to east after midweek, with most of the western half of the mainland seeing near to slightly below normal highs by next Friday. Rausch 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