Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Sun Sep 15 2024 Valid 12Z Thu Sep 19 2024 - 12Z Mon Sep 23 2024 ...Overview... Most guidance suggests a stormy period for the Aleutians, with a couple systems that may produce some enhanced rainfall/winds which then may reach into the Alaska Peninsula and the southern coast/Panhandle by the weekend and early next week. Over the mainland, expect a lingering diffuse mean trough aloft as of Thursday (supporting weakening Gulf of Alaska low pressure) after supporting a stronger storm beforehand. Then it becomes more uncertain how flow will evolve over the mainland, with current/recent solutions varying considerably over how much ridging or troughing may evolve. This forecast problem will also play a role in the evolution of the expected Aleutians systems as they track eastward. ...Guidance Evaluation/Preferences... Across the North Pacific and adjacent areas, as of Thursday expect low pressure to be organizing between Kamchatka and the western Aleutians while a wavy front extends well eastward across the North Pacific. By Friday-Saturday, latest GFS runs are essentially on their own in developing well-defined low pressure along this front and tracking it to near the Panhandle. The overwhelming cluster of most other dynamical and machine learning (ML) guidance favors weaker/more sheared dynamics that would keep any frontal wave weak and suppressed, likely south of Haida Gwaii. The 12Z GEFS mean does not have a defined surface low either, though it lifts its overall front farther northeastward for a time (though joined to some degree by the UKMET by the end of its run early Saturday). With typical spread, consensus shows the leading upstream system tracking near or just south of the Aleutians Friday-Saturday--but perhaps a little weaker than 00Z/12Z ECMWF runs per other dynamical/ML models. This system may stay defined into Sunday but then the question mark becomes whether it continues into the Gulf of Alaska or gets merged into the rapidly approaching second Aleutians system. Most guidance aside from ECMWF runs would suggest the second system could be a little deeper than the first. Meanwhile a fair amount of track/timing spread develops with the second storm. There is a relative majority of solutions clustering around the Alaska Peninsula (Bristol Bay to south/east of Kodiak Island), while minority options are farther northwest (latest CMCens means, ECMWF AIFS means, small number of ML models) or slow (GFS runs). The mainland forecast starts out rather agreeably with a broad and diffuse mean trough aloft, but models/means gradually diverge with time. Recent ML guidance generally favors some degree of southeast-northwest oriented ridging that reaches into at least the southwest mainland by Saturday, which is closest to ECMWF/CMC runs and the ECens mean, versus GFS runs that drop troughing or even an upper low into the the southwestern mainland. However there is a lot of spread beyond that, as 00Z ECMWF/CMC runs and the 12Z CMCens mean end up being quite strong with the upper ridge in contrast to 12Z ECMWF/CMC/UKMET runs that favor more troughing over the northeastern half or so of the mainland. In fact, the 12Z ECMWF/CMC ultimately drop the northern tier upper low into the southwestern mainland to yield a feature by next Monday close to the position depicted by some other guidance bringing in the upper low from the Bering Sea/Aleutians. Thus, broadly speaking there is somewhat more confidence in an upper low being near the southwestern mainland by next Monday than in how it gets there. Given the spread and variability of guidance, prefer an intermediate solution that offers some mainland troughing per continuity but also brings ridging into the southwest for a time as seen in the majority cluster. Guidance preferences away from various aspects of the GFS led to a Thursday-early Saturday blend starting with half ECMWF runs (00Z/12Z) with the remainder composed of the 12Z CMC/UKMET, followed by a shift to 40 percent ensemble means (12Z GEFS/00Z ECens) and lingering input from the two ECMWF runs and 12Z CMC by Sunday-Monday. At that time the operational model component was split among half 00Z ECMWF and the rest 12Z ECMWF/CMC, to temper the influence of mainland troughing in the latter solutions. This approach yielded some aspects of continuity along with refinement for the Aleutians storms as well as mainland flow aloft. Today's guidance shows a better signal for a cold front to develop/push into the mainland with portions eventually becoming stationary late in the period. ...Weather/Hazards Highlights... Weakening low pressure over the Gulf of Alaska should support lingering light to moderate precipitation over the Panhandle into Thursday. Meanwhile areas of rain and higher elevation snow will be possible over much of the mainland late this week as some degree of upper troughing persists. From Friday onward, two storm systems may track along or near the Aleutians to produce areas of enhanced rain/winds/waves with effects from these systems possibly extending into the Alaska Peninsula and then the southern coast/Panhandle by the weekend and early next week. The best signal for highest precipitation totals by Sunday-Monday is over the Southcentral coast and Panhandle. Note that there is still uncertainty for storm track, especially with the latter system. A less probable but still possible farther north track would likewise spread significant weather farther northward than currently forecast. Also worth noting, GFS runs would bring an earlier surge of moisture to the Panhandle toward the end of the week but for now there is minimal support for that scenario. Precipitation details over the rest of the mainland from the weekend into early next week will depend on uncertain specifics of flow aloft. Southern tier locations including the Panhandle will tend to see below normal highs during the period. Areas to the north may see more of a mix of above/below normal anomalies, but with a possible cooling trend north and west depending on how much upper troughing evolves over the mainland by the weekend. Expect morning lows to be more above normal aside from some modestly negative anomalies progressing from the southwest through the Interior late week into the weekend. 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