Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 652 PM EST Fri Nov 29 2024 Valid 12Z Tue Dec 03 2024 - 12Z Sat Dec 07 2024 ...Wet/snowy pattern for the southern coast and Panhandle next week... ...Overview... Today's models and ensembles reasonably maintain the theme of the large scale pattern evolution. Expect a blocky configuration with a mean upper low wobbling over the Aleutians/North Pacific while Panhandle/western Canada upper ridging extends farther northwest through the mainland and possibly beyond after midweek, replacing an initial mainland col region/shortwave axis. The main upper low and surrounding shortwave impulses will support multiple surface lows between the Aleutians/North Pacific and the western Gulf of Alaska, but with widely varying amounts of guidance spread depending on the system. At least the overall pattern will likely support a prolonged period of wet/snowy weather along the southern coast and Panhandle, with some unsettled conditions extending back into the Aleutians, and a pronounced warming trend over much of the state. Most of the mainland should be within the gradient between high pressure over the Arctic and the surface lows to the south. ...Guidance Evaluation/Preferences... During Tuesday-Thursday, most guidance clusters around three systems of interest. One is the southern Bering Sea/Aleutians deep-layer low dropping into the North Pacific. Recent GFS runs have been persistent in being a deep extreme with the upper low and overdone with surface low depth (12Z GFS by at least 10 mb). At least the model has been trending toward established consensus for the path of the system, so a minority weight as part of a multi-model solution is an option today. The new 18Z GFS run has finally trended closer to other guidance with a more moderate strength of the surface low as of early Tuesday. Farther east, there continues to be a majority signal for a system tracking northward toward the Alaska Peninsula/western Gulf of Alaska from early Tuesday into early Wednesday with a frontal system extending to its east. The 12Z CMC looks too slow while the 12Z CMCens/00Z ECens better depict this system than the 12Z GEFS mean. Recent machine learning (ML) models offer reasonable support for the majority scenario. Then a trailing mid-latitude Pacific system may track northward to a position near or a little south of the eastern Aleutians by early Thursday. Dynamical and ML guidance generally agree with this scenario. Meanwhile there is a better signal today that the initial Aleutians low may maintain definition into Thursday as it rotates underneath the just aforementioned system. This part of the forecast incorporated a blend consisting of 40 percent 12Z ECMWF, 20 percent each 12Z GFS/UKMET, and 20 percent total 12Z CMCens/00Z ECens. From later Thursday through Saturday, dynamical/ML models increasingly diverge for exact upper level details and surface systems within a somewhat more agreeable mean pattern consisting of an axis of relatively low heights near the Aleutians (a combination of earlier upper low/shortwave energy and eventually additional dynamics that arrive from the west) and elongated ridging that persists over the mainland. There is also an emerging western Pacific system likely to track south of 50N by Friday-Saturday. A considerable dynamical/ML majority keeps this system far enough south to have minimal influence on the Aleutians. The increasing detail differences led to splitting the 40 percent ECMWF component between the 12Z/00Z runs plus increasing the total ensemble mean weight to 40 percent (with some 12Z GEFS included at this point). This solution emphasizes the most agreeable larger scale themes, and in particular the Thursday system near the eastern Aleutians weakening thereafter while another system may track toward the eastern Aleutians/Alaska Peninsula Friday into Saturday. As for other considerations spanning these two parts of the forecast, the slow northward solution of the CMC with the early system leads into other northern stream energy combining with it to yield more upper troughing near the western coast and more associated surface low pressure. Some GFS runs before 12Z had hinted at such a trough axis but ML models favor a pattern more like the ensemble means and other dynamical guidance fitting their solution. Also of note, today's guidance consensus is somewhat stronger with the upper ridge extending across the mainland after midweek. ...Weather/Hazards Highlights... The overall pattern continues to look favorable for episodes of enhanced precipitation from the Alaska Peninsula to the Panhandle next week, with generally southerly mean flow aloft between an upper low/trough over the Aleutians-North Pacific and an upper ridge over the Panhandle/western Canada (eventually through the mainland). Multiple surface systems directed by the upper flow should help to focus the activity, with the most likely surface low tracks directed toward or near the Alaska Peninsula/eastern Aleutians with associated fronts extending to their east. Coastal and nearby low elevations should tend to see rain, with snow elsewhere. Milder conditions over the southern Panhandle will likely yield broader coverage of rain with snow confined to higher elevations. Currently most guidance suggests that a majority of this precipitation will remain below hazardous thresholds, though multi-day accumulations may still end up being significant. There is a somewhat better defined signal for focused precipitation over the southern Panhandle on Tuesday with the first system initially south of the Alaska Peninsula, so the Days 3-7 Hazards Outlook depicts a heavy precipitation area for that day. Nearby time frames could also have enhanced totals but with guidance a little more mixed than seen at this time yesterday. Rainfall may produce enhanced runoff and rises on area rivers. Expect unsettled conditions but with lighter precipitation over the Aleutians. Most of the mainland north of the Alaska Range should remain dry next week. One or more of the surface systems could generate areas of brisk to strong winds. The northern three-fourths of the state will be on the cold side with below normal readings into Tuesday, including some overnight lows in the -20s over far eastern interior areas. Then aside from valley locations where cold air typically persists longer, the overall pattern will support a pronounced warming trend with time. Most areas should see above to well above normal temperatures by the end of the week with anomalies for min temperatures generally a little greater than those for highs. 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