Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Sun Jun 25 2023 Valid 12Z Thu Jun 29 2023 - 12Z Mon Jul 03 2023 ...Overview... Guidance continues to show a rather cloudy and unsettled period over a majority of the state, with many areas seeing the potential for varying amounts of rainfall over the course of the period. However the models and ensembles have been having a difficult time in determining the specifics of upper level and associated surface features. The upper level features include a low initially over/near the Alaska Peninsula, trailing energy dropping through the Bering Sea/Aleutians during the latter half of next week, and Arctic flow dropping into Siberia and the Bering Sea from Friday onward. Along with the forecast challenges for these features, there are significant differences for the orientation of flow aloft over the mainland at times, complicating the moisture/rainfall forecast over some areas. Today there is a new feature of interest as latest model runs bring a well-defined storm system into the western Aleutians and vicinity by day 8 Monday. ...Guidance Evaluation/Preferences... Lately, each 12-24 hours of model runs have offered new clustering for how the upper low initially over the Alaska Peninsula will behave. Most solutions from today's 12Z cycle are essentially a compromise between the fairly progressive scenario from two days ago and a stationary southeastern Bering Sea low that happened to be in the majority yesterday. Aside from the slightly southward and then increasingly more progressive 12Z GFS (but the 18Z GFS holding it back closer to other guidance), most models/means suggest a modest eastward drift into the weekend followed by some combination of weakening/shearing/progression. West of this feature, the global models show eventual North Pacific sharpening of energy initially dropping down through the Bering Sea/Aleutians but with varied ideas for progression/path of the resulting shortwave and whether there may be an embedded upper low. Needless to say, associated surface solutions across the North Pacific and into/near the Gulf of Alaska vary considerably as well. Meanwhile recent guidance has also varied considerably for the orientation of flow aloft over the mainland, between the westward extent of Canadian ridging that reaches westward into the mainland/Arctic versus the degree of eastward influence of mean troughing expected to set up/reload to the west. GFS runs have generally been on the stronger side of the spread for the ridge and resulting mainland easterly flow aloft, though it is worth noting that the 12Z CMC shows more ridging than some earlier runs that showed greater influence of the trough. Latest consensus is signaling that an upper high could position itself a ways northeast of the mainland by next weekend/early next week. The 12Z GEFS mean shows a little closer proximity of this high than most other solutions. There has also been a lot of spread and variability for how digging Arctic flow evolves within an expected trough over Siberia and the Bering Sea. 06Z/12Z GFS runs were fairly extreme in how much energy digs into the western Aleutians while the 18Z GFS and remaining models/means generally support an upper low somewhere between eastern Siberia and the northern-central Bering Sea. Farther south/southwest, recent operational model runs have been showing a fairly strong system reaching the western Aleutians by day 8 Monday. At this time the details appear to have fairly low predictability, with supporting dynamics coming from flow underneath a blocky regime in place over eastern Asia north of Japan and possible input of some flow on the western side of the Siberia/Bering Sea trough. Individual ensemble member spread is broad enough for the corresponding means to show only a weak signal for this system thus far, with the past couple ECMWF means showing a little more definition than the GEFS/CMCens. The varied forecast issues led to favoring an operational model blend early in the period and then a transition to a model/ensemble mean solution (spread among the 12Z GEFS-CMCens and 00Z ECens) reaching 60 percent ensemble weight by day 8 Monday. This reflected the best average of guidance for the initial Alaska Peninsula upper low and an intermediate solution for mainland flow to the north, while gradually trending to a more conservative solution over the northern/northeastern Pacific by late in the period when guidance spread is greatest there. Model clustering for the potential late-period Aleutians system was sufficient, even with tempered confidence, to favor a surface depiction between the fairly deep models and very weak means. ...Weather/Hazards Highlights... Ongoing adjustments in the guidance keep confidence lower than desired for determining the rainfall specifics during the period. Based on latest solutions, the upper low initially over the Alaska Peninsula should promote some rainfall focus along the southern coast and Panhandle from late this week into the weekend but with most activity of no more than moderate intensity. Northeastern Pacific detail uncertainties at the surface and aloft from the weekend into early next week yield low confidence in rainfall distribution and intensity during that time frame. Meanwhile the potential remains for pockets of enhanced moisture/rainfall over parts of the northern and eastern mainland during the period, but moisture specifics will be sensitive to upper flow details that have yet to be resolved. The northern coast and for a time the western coast may have the best potential to see drier conditions relative to other areas. Finally, the forecast late-period system would bring an increase of rain and wind to the Aleutians. Continue to expect widespread cloud cover to keep the diurnal temperature range more narrow than usual, with lows tending to be near or above normal and highs generally below normal. Expect the most extreme anomalies to be with cool highs along and just north of the southern coast, including the Panhandle as well. Some pockets of above normal highs will be possible late week into the weekend over portions of the northern coast and the Seward Peninsula. Coverage of slightly below normal lows may increase by the weekend and early next week. 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