Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Mon Jun 05 2023 Valid 12Z Fri Jun 09 2023 - 12Z Tue Jun 13 2023 ...Slow moving eastern Aleutians into Bering Sea deep layer low likely to maintain a wet pattern across much of Alaska through at least the weekend... ...Overview... Expect an upper low with associated surface system initially near the eastern Aleutians to wobble northward into the Bering Sea, maintaining definition through the period but displaying a very slow weakening trend. The flow around the eastern side of this feature should support a persistent flow of moisture and shortwave energy into the state, leading to episodes of enhanced precipitation not only along the southern coast/Panhandle but also farther north across the mainland. Over the past day or so models have latched onto a mid-latitude Pacific system that may bring a better defined precipitation focus to the southern coast around Sunday-Monday as it tracks into the Gulf of Alaska. Over the Arctic, high pressure north of the mainland will likely track eastward into northern Canada by late in the weekend. ...Guidance Evaluation/Preferences... Overall the guidance has lately been fairly agreeable and consistent with the Aleutians/Bering Sea upper low and surface system, not surprising for a dominant large-scale feature. There are still lingering differences due to individual shortwaves that can make a particular solution temporarily stray from consensus for track of the upper level and/or surface lows, such as seen in varying ways in the 12Z and 18Z GFS runs. A multi-model average helps to mitigate these smaller-scale issues. The other main forecast issue of note is a mid-latitude Pacific wave which the past 24-36 hours of operational model runs have latched onto, with the majority cluster bringing the system into the Gulf of Alaska by around early day 7 Monday. The 12Z UKMET strays a bit strong/west of other models by the end of its run early Sunday, while the 12Z/18Z GFS, 00Z/12Z ECMWF, and slightly faster/weaker 12Z CMC reflect decent clustering at this time. Thus far the GEFS/CMCens means have been lagging the models in depicting this wave, likely due to spread of individual members. The past couple ECens mean runs have depicted this wave but farther west, though the 12Z run has trended noticeably closer to the operational models. An operational model composite provides a reasonable starting point to reflect this system. The scale and evolution of supporting shortwave energy suggest that models may still vary with some aspects of this system over the coming days, so monitor forecasts for potential changes. Guidance comparisons led to favoring a nearly even blend of the 12Z GFS/ECMWF/CMC from day 4 Friday into day 7 Monday. This solution reflected consensus for the Aleutians/Bering Sea system and the majority scenario of Pacific low pressure tracking just east of Kodiak Island and reaching the southern coast around the Kenai Peninsula. Typically increasing detail uncertainty and a low-confidence 12Z GFS upper trough dropping into the mainland from the Arctic led to the day 8 Tuesday forecast adding in 30 percent total 12Z GEFS/00Z ECens while also splitting the GFS contribution with the 06Z run that did not have the trough in question. ...Sensible Weather and Potential Hazards... Some locally focused precipitation (mostly rain) may persist along the along the southern coast late this week into the weekend, though likely with lesser totals than with a wavy front forecast to approach and weaken Thursday-Thursday night, just before the start of the forecast period. The moisture as well as shortwave energy aloft should lift northward and bring a potential for locally moderate to heavy precipitation over parts of the central and northern mainland, mainly late this week into the weekend. Most guidance shows that precipitable water values could peak at 2-3 standard deviations or so above normal over some locations in this time frame. Interior/northern areas should see activity trending less heavy by early next week. Meanwhile, a wave forecast to track into the Gulf of Alaska by early Monday may bring another episode of significant precipitation to the eastern Alaska Peninsula, Kodiak Island, Kenai Peninsula and to some degree farther eastward/northeastward. Details of this event will depend on the ultimate track and depth of the system. Expect the Aleutians to see a prolonged period of unsettled weather with mostly light rainfall given the proximity of the deep layer low. Widespread clouds and precipitation will support below normal high temperatures across much of the state during the period, with readings tending to trend gradually cooler with time and coolest anomalies likely to be over southern areas. Lows should start out mostly above normal and then trend toward a mix of moderately above/below normal anomalies. Rausch Hazards: - Heavy rain across portions of mainland Alaska, Thu, Jun 8. 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