Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Mon Sep 04 2023 Valid 12Z Fri Sep 08 2023 - 12Z Tue Sep 12 2023 ...Heavy rainfall likely over portions of the Alaska Panhandle late week into the weekend... ...Overview... Guidance shows the general theme of a mean trough aloft from the mainland southward into the northeastern Pacific from late this week through the weekend, with the axis possibly retrograding back toward the western coast of the mainland by next Tuesday with the arrival of upstream energy. Today's trends/clustering add confidence to the heavy rain potential for the Panhandle around Friday-Saturday with the combination of southerly flow aloft and a focusing frontal wave just offshore. However there is still a lot of uncertainty with the ultimate track of a late week Bering Sea system, keeping confidence lower than desired for sensible weather effects along the southern coast from the weekend into early next week. Another surface low/frontal system, likely weaker than its predecessor, may cross the Bering Sea early next week. The overall pattern evolution appears to favor precipitation across parts of the southern coast and Panhandle from late weekend onward but the details will take time to be resolved. ...Guidance Evaluation/Preferences... Interestingly, most of today's guidance shows decent agreement in principle early in the period, followed by significant divergence for various important details, and then returns to better consensus at least aloft by day 8 next Tuesday as energy crossing the Bering Sea leads to a trough/upper low along or just offshore the western coast of the mainland becoming the dominant feature. From Friday into Saturday there is finally somewhat better clustering among models/means with respect to short range Arctic energy (from some combination of an initial upper low or nearby flow) dropping into the mainland by the start of the period and becoming a part of the overall mean trough. Meanwhile to the south there is a greatly improved consensus for a frontal wave now expected to lift northward along a track that would be just offshore the Panhandle as of early Saturday. The 12Z UKMET is on the deep side of the envelope, with other models generally within a few mb of each other for depth. Meanwhile guidance exhibits fairly typical spread for Bering Sea low pressure from Friday to about early Saturday, but for the second day rapidly diverges for the low track after that time. Ensemble means have been most consistent thus far, showing a track just south of the Alaska Peninsula/Kodiak Island. GFS runs have been very erratic, switching between having the parent low tracking into/lingering over the eastern Bering Sea near the western coast of the mainland (most recently the 06Z and new 18Z runs) and a farther south track (12Z run), with some runs well south of the means. The UKMET switched from a northern track to a southern one between the 00Z and 12Z runs, the last two 12Z ECMWF runs drop well southward (though returning north farther east in today's run) while the 00Z run was closer to the means. Current preference is to stay closer to the ensemble means for stability until more pronounced clustering offers some confidence in a significant adjustment. Precise details of the upper trough/low initially over the mainland will also likely have an influence on this system. Also of note, the 12Z/18Z GFS runs are faster than other guidance to eject the mainland trough/low northeastward. The general signal of upper dynamics and an associated system crossing the Bering Sea early next week has held up fairly well since yesterday. In contrast to the significant differences over and south of the mainland around mid-period, the upper pattern becomes remarkably agreeable in most of the models/means by day 8 next Tuesday with a trough/possible embedded low reaching the eastern Bering Sea/Aleutians or western coast of the mainland. The days 4-5 Friday-Saturday part of the forecast started with a blend of the 12Z GFS/CMC and 00Z/12Z ECMWF runs to reflect the most common elements of guidance early in the period. In addition to the UKMET being strong with the wave off the Panhandle early Saturday, it drops down a higher latitude Arctic upper low not seen in other guidance, so that model was excluded. The forecast quickly increased 12Z GEFS/CMCens and 00Z ECens input to 40 percent by day 6 Sunday and 60 percent by days 7-8 Monday-Tuesday. This kept the blend close to the ensemble means for the system expected to track south of the Alaska Peninsula/Kodiak Island, with just a slight southward nudge versus yesterday, while holding onto some operational models (but with small enough weight for low-confidence details to cancel out) helped to add a little definition to the means by the end of the period when clustering aloft improves. ...Weather/Hazards Highlights... Latest guidance is converging upon the idea that southerly flow ahead of a northeastern Pacific upper trough and a frontal wave that should reach just offshore the Panhandle as of early Saturday will likely help to focus an axis of heavy rainfall into the Panhandle during Friday-Saturday. Expect a fairly vigorous system to spread a broad area of precipitation and brisk winds across the Bering Sea and Aleutians late this week. However the track of the Bering Sea system beyond early Saturday, and associated weather effects farther eastward, remain more uncertain than average. Taken as a whole, guidance signals appear to suggest a slightly farther south track underneath the Alaska Peninsula/Kodiak Island around Sunday-Monday, which would at least keep most of the southwestern mainland on the drier side and perhaps lead to a bit of a delay in leading moisture reaching the southern coast and Panhandle. Minority potential exists for some precipitation to reach into the southwestern mainland though. Other complicating aspects of upper flow details may affect precipitation specifics as well, but in general the pattern evolution should favor some increase of precipitation along the southern coast and additional activity over the Panhandle. A weaker system may spread lighter rainfall across the Bering Sea and vicinity early next week. Below normal high temperatures should prevail over most areas during the period, with the coolest readings likely to be late this week into the weekend when the deepest upper troughing should be over the state. Low temperatures will tend to average slightly warmer then normal but with some western mainland areas eastward into the interior seeing a couple days of below normal readings late week into the weekend. Lows may trend somewhat warmer early next week. Rausch Hazards: - Heavy rain across the northern portion of Alaska Panhandle, Fri-Sat, Sep 8-Sep 9. 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