Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Sat May 20 2023 Valid 12Z Wed May 24 2023 - 12Z Sun May 28 2023 ...Overview... The best clustering of latest guidance shows a system near Kamchatka at the start of the period early Wednesday tracking into the Bering Sea while a leading system near the Alaska Peninsula moves across the southern Gulf of Alaska and over or south of the southern Panhandle during the latter half of the week. There is some debate over the ultimate eastward extent of the Bering Sea system, and disagreements also exist regarding flow aloft over the mainland. The overall signal is for mean ridging between northwestern Canada and north of Siberia but with the potential for a break in this ridge. The forecast pattern evolution should favor best precipitation chances from the Bering Sea/Aleutians through the southern mainland and Panhandle. ...Guidance Evaluation/Preferences... For the most part, an average among the 12Z GFS/ECMWF/UKMET provided a good representation of the majority scenario across the forecast domain during the first half of the period (early Wednesday into early Friday). The 12Z CMC differed from this cluster in two respects. It had a mere open trough over the Alaska Peninsula at the start of the period, emphasizing its closed low farther north, and waited until late Thursday to close off a low with the southern feature. Also the model became a progressive extreme with the Bering Sea system. Before the 12Z cycle, operational models were generally faster with the Gulf system than the ensemble means (before the latter became ill-defined) but the 12Z CMCens/ECens means both trended toward the operational runs in definition and timing. Meanwhile the 12Z ECMWF (perhaps a bit on the deep side by day 6 Friday) slowed a bit versus the 00Z run (leaving the 12Z GFS a bit on the faster side), so ultimately a compromise among the non-CMC models still appears reasonable. There is still north-south track uncertainty later in the week as well, with models currently shuffling around enough to minimize confidence in that regard. GEFS/ECMWF means generally support their operational counterparts for the modest upper trough expected to cross the northeastern mainland around Thursday-Friday. A blended model/ensemble mean solution became increasingly appealing by the latter half of the period with solutions diverging for the Bering Sea system, one or more separate trailing waves (both potentially influenced by upstream energy that has low predictability), and flow details over the mainland. The means provided a good central position for the Bering Sea system, with the 12Z GFS straying eastward some while the past two ECMWF runs and 06Z GFS were on the western or northwestern side of the spread. The manual forecast depicted a weak frontal wave, but to reflect the potential for its existence rather than any confidence in its specifics. Over the mainland, the steady increase of ensemble means helped to nudge the forecast toward the idea of some ridging aloft extending from northwestern Canada through the eastern/northern mainland and beyond. Individual model runs depict various other possibilities though, ranging from Arctic-origin shortwave energy continuing to break the ridge beyond late week to flow around the southeast side of the Bering Sea upper low lifting into the mainland. The favored blend had total ensemble weight reaching 60 percent by day 8 Sunday with a little more 00Z ECens than 12Z GEFS due to earlier trends that had favored a slightly farther west Bering system position (but not necessarily west of the ECens). ...Sensible Weather and Potential Hazards... The system expected to track from near the Alaska Peninsula toward or a little south of the Panhandle should spread precipitation from the southwestern mainland and nearby areas to the Panhandle during the mid-late week time frame. Most totals will likely be in the light to moderate range but some localized enhanced activity is possible depending on the exact strength and track of the system. Meanwhile parts of the eastern mainland may see mostly light precipitation mid-late week in association with an upper shortwave. The system moving into the Bering Sea, along with one or more associated fronts, will likely spread an organized area of rainfall across the Aleutians and Bering Sea especially during Wednesday-Thursday. Some moisture may extend into the western/southern mainland as well from late week through the weekend with the weakening fronts reaching the area, while lighter precipitation may continue over the Aleutians. For temperatures, highs will tend to be below normal across southern and some western areas in contrast to above normal readings over the eastern interior plus northwest coast. Lows should be above normal over most areas, with only scattered pockets of near to slightly below normal average temperatures. Southwestern areas could see a bit of a cooling trend by next weekend if the Bering Sea upper low and its surrounding flow extend their influence far enough eastward. 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