Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Fri Oct 25 2024 Valid 12Z Tue Oct 29 2024 - 12Z Sat Nov 02 2024 ...Stormy pattern next week into the weekend with multiple threats of heavy precipitation/high winds across southern coastal Alaska and the Aleutians/Bering Sea... ...Overview... Guidance continues to show an active pattern with multiple significant storm systems. During Tuesday-Wednesday a Gulf of Alaska storm will affect the Southcentral coast and Panhandle with enhanced winds/precipitation while a deeper low will track into the far western Aleutians/Bering Sea with a leading front sweeping through the rest of the Aleutians. An embedded wave should form and track into the Gulf after midweek while the slowly weakening low to the west reaches the central Bering Sea. Then there is increasing consolidation of guidance pointing toward the possibility of another very deep storm (similar to the western Bering one) reaching the North Pacific/Aleutians around Friday and the southeastern Bering Sea or eastern Aleutians/Alaska Peninsula by Saturday. Mainland flow aloft should trend from initial troughing to a ridge passing through around Wednesday-Thursday, and then more ambiguity depending on details of dynamics approaching from the west and possible influence of energy retrograding from the Canadian archipelago. ...Guidance Evaluation/Preferences... Dynamical and machine learning (ML) guidance is trending toward better clustering for the Tuesday-Wednesday Gulf storm. In particular, latest ML runs agree much more closely with the dynamical majority comprising the latest ECMWF/UKMET and GEFS/ECens means which show an upper low just west of the Panhandle by early Wednesday, with similarly positioned surface low. The 12Z CMC/CMCens are over the Panhandle aloft but are still offshore at the surface. At the moment there is minimal support for the 12Z/18Z GFS runs that drop the upper/surface low so far south by Wednesday. The 06Z GFS compared much better to the majority. Over the past day the consensus has trended toward slightly later deepening of the surface low, likely reaching its strongest around late Tuesday. The ECMWF continues to maintain a deeper low into Wednesday but the 12Z run is not quite as strong was the prior version. An operational guidance average maintains fairly good continuity for depth and track of the strong storm reaching the western Aleutians/Bering Sea by Tuesday-Wednesday (central pressure likely dropping below 960 mb for a time) and weakening somewhat as it tracks into the central Bering Sea thereafter. There is some lingering timing spread for the leading front and developing waviness forecast to reach the Alaska Peninsula and vicinity by Thursday, with GFS runs still a bit on the faster side but the 12Z UKMET is similar. Thus a compromise timing appears reasonable. By Friday there is still a fair amount of spread regarding surface low position and its definition, depending on the strength and track of the late week North Pacific/Aleutians storm. A deeper/northern incoming storm would make the leading system lose definition earlier. While there is still a fair amount of spread and variability for the potentially deep North Pacific/Aleutians storm by next Friday-Saturday, guidance has dramatically improved its overall theme compared to 24 hours ago. Since that time, dynamical/ML solutions have unanimously clustered toward this system being a purely extratropical development from progressive energy emerging from Asia while Tropical Cyclone Kwan-rey remains over the far western Pacific. In spite of the lack of tropical system input beyond moisture that the developing storm may pick up to the northeast of Kwan-rey, one or more runs of the GFS/ECMWF/ICON show the storm reaching a depth between the mid 930s and 940s mb at some point during Friday-Saturday. The most common track among dynamical/ML models and means is just south of the central Aleutians by early Friday and over the eastern Aleutians/Alaska Peninsula/southeastern Bering Sea by Saturday. There is a minority track (CMC and one ML model) that stays over the North Pacific. It is notable that the ML model average strength that far out in time is nearly as pronounced as what those models have been showing for the earlier-term western Bering Sea storm. At least relative to typical ensemble mean forecasts, the ECens/CMCens are quite strong as well, but the GEFS mean is questionably weak (perhaps reflecting a greater spread such as 6-hourly GFS runs have been exhibiting over the past day or so). Guidance agrees in principle that upper ridging should replace the trough over the mainland as of Tuesday. Then guidance diverges for how much energy may reach into the mainland from the leading side of the initial Bering Sea upper low, while CMC/CMCens guidance shows broader southerly to cyclonic flow, and the ECMWF/ECens offer potential for some retrograding Canadian archipelago energy to reach the North Slope. Preferences for dominant storm of interest late in the period led to an elongated shortwave lifting northward Friday-Saturday. The early-mid part of the forecast started with the 12Z ECMWF/06Z GFS/12Z UKMET/12Z CMC in order from more to less weight, in order to downplay the lower confidence 12Z GFS scenario for the Gulf system. By Friday-Saturday the blend quickly transitioned to include 30-40 percent total weight of the 00Z ECens/12Z CMCens means along with somewhat more 12Z ECMWF than 12Z GFS for the operational model component. ...Weather/Hazards Highlights... The storm evolving over the Gulf by Tuesday-Wednesday will spread heavy precipitation and strengthen winds along/offshore the Southcentral coast and Panhandle. Gap winds also remain a concern behind the surface low over the Alaska Peninsula plus Kodiak Island and the Kenai Peninsula. The Days 3-7 Hazards Outlook continues to depict areas for high winds along much of the southern coast/Panhandle as well as heavy precipitation from the eastern Southcentral coast through the Panhandle. Meanwhile the deep western Aleutians/Bering Sea storm will bring an area of high wind potential into the Aleutians by Tuesday-Wednesday along with enhanced rainfall focus into the eastern Aleutians/Alaska Peninsula by Wednesday (both depicted in the Hazards Outlook), with the rain focused by a leading front crossing the region. With some lingering uncertainty in timing and track, an evolving Gulf wave originally embedded within the aforementioned front should push precipitation farther eastward across the southern coast and into the Panhandle later in the week. This wave should be weaker than the preceding Gulf storm and produce less extreme precipitation totals. Guidance signals are trending more pronounced for a potentially very deep North Pacific/Aleutians storm late in the week, which would bring another episode of high winds and heavy rain to the Aleutians and surrounding waters plus the Alaska Peninsula. The Days 3-7 Hazards Outlook reflects this high wind and heavy rain threat for next Friday. Precipitation/wind effects over the southwestern mainland by Saturday will be very sensitive to surface low track, while there is relatively higher confidence in moisture reaching the southern coast. Expect temperatures to be mostly above normal over the western mainland and North Slope, with anomalies for morning lows likely to be somewhat greater than for highs on some days. The eastern mainland and Panhandle will be more persistently below normal for max/min readings through the period, though the eastern mainland may moderate a little for high temperatures after midweek. 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