Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Wed Jul 31 2024 Valid 12Z Sun Aug 04 2024 - 12Z Thu Aug 08 2024 ...Wet pattern setting up from the Alaska Peninsula/southern coast into parts of the southern and western mainland... ...Overview... Today's guidance offers added confidence in the large scale pattern trending toward an impressively amplified upper trough-ridge configuration that would support a multi-day rainfall event, focusing along the Alaska Peninsula and to some degree eastward along the southern coast as well as into favored southward-facing terrain over portions of the southern and western mainland. The trough will likely contain an embedded Bering Sea upper low while the ridge building over the mainland on Sunday should shift its axis into northwestern Canada by Tuesday-Thursday. A wavy frontal boundary between the two, as well as a leading North Pacific system being incorporated into the flow around the start of the week, will also enhance moisture focus to their east. Meanwhile most of the state should see a pronounced warming trend, aside from southern tier areas that may continue to see below normal highs. ...Guidance Evaluation/Preferences... Over the past day the CMC model has finally completed its adjustment to join what had already been a fairly robust consensus for most of the important large scale aspects of the forecast. Latest dynamical and machine learning (ML) models still depict some smaller scale detail differences that have lower predictability and will likely take some time to resolve. The first detail question mark involves the incorporation of an initial northward-drifting North Pacific upper low into the larger scale flow. There has been some waffling for timing of this evolution, with 12Z runs generally trending slower than the 00Z guidance. ML models on average favor the slower timing. This returns the majority cluster toward what had been depicted in yesterday's 12Z data. Also of note, latest ECMWF runs have favorably trended toward deflecting the upper low's energy around the ridge (per most other guidance) instead of having it cut through the ridge. Within the amplifying trough west of the mainland, most of the 12Z guidance hints at a weaker/more elongated depiction of the core for a brief time early in the week with some ML models reflecting that idea as well. Then the upper low should regain definition, with some latitude spread. Some GFS/ECMWF runs momentarily drop the upper low center as far south as the eastern Aleutians/far southwest Alaska Peninsula before rebounding north while a minority of ML models approach this southern side of the spread as well, while preference holds onto a compromise for latitude. Consensus shows the trough starting to weaken a little with the axis approaching the western coast of the mainland by next Thursday as upstream dynamics aloft and associated surface system (with good guidance clustering thus far) reach the western Aleutians/Bering Sea. While the 12Z GFS fits within the general theme of guidance aloft, it does appear to shift its moisture axis over Alaska a little farther east than most other guidance by the latter half of the period. The new 18Z GFS run looks better in that regard. Just ahead of the trough, operational runs suggest that a frontal wave may track from the North Pacific into eastern Bering Sea around Tuesday-Wednesday. Such details have lower predictability given dependence on smaller scale shortwave specifics, but are consistent with the overall theme of frontal waviness over the past couple days of runs. The CMC's adjustment to existing consensus allowed for today's forecast to incorporate a composite of the 12Z GFS/ECMWF and slightly less weight of the UKMET/CMC during the first half of the period. Sufficient agreement existed after early Tuesday to support retaining 70 percent total weight of the GFS/ECMWF/CMC for the rest of the period, with the 12Z GEFS/00Z ECens means comprising the rest. ...Weather/Hazards Highlights... With only modest adjustments, the forecast remains on track for a multi-day rainfall focus and breezy conditions directed at the Alaska Peninsula and gradually nudging 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. During Sunday-Monday, incorporation of a North Pacific system into the broader flow will 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, with precipitable water values possibly increasing during Tuesday-Thursday. 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 Prince William Sound Sunday-Wednesday, with western areas seeing the earliest onset. Hazard areas also include some terrain over the southern/southwestern mainland. There will likely be areas of rainfall continuing into Thursday. Continue to monitor forecasts for surface/upper level detail refinements that will help to resolve the specifics of location and magnitude of heaviest rainfall. Continue to expect a pronounced warming trend over a majority of the state as a strong upper ridge initially builds over the mainland and then shifts into northwestern Canada. Typically warmest locations in the eastern Interior could see highs reach into the upper 70s/low 80s by the early-middle part of next week. Portions of the southern mainland and northern Panhandle may not see much of a warming trend, at least for high temperatures which may remain below normal. Gradual approach of the Bering Sea upper trough may also lead to slightly cooler highs over the western mainland toward Wednesday-Thursday. 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