Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EST Thu Jan 02 2025 Valid 12Z Mon Jan 06 2025 - 12Z Fri Jan 10 2025 ...Heavy precipitation and strong winds along the southern mainland coast early next week with potential for another significant storm after midweek... ...Overview... Guidance agrees upon an amplified pattern for most of next week, consisting of a North Pacific into Bering Sea upper low/trough in the mean, with western Canada into Mainland Alaska ridging. The pattern may flatten a least temporarily around Friday as upstream dynamics and one or more surface systems reach the Aleutians and vicinity by the latter half of the week. Flow between the upper low/trough and ridge will likely direct a couple strong storm systems into or just west of the Gulf of Alaska, bringing episodes of heavy precipitation and strong winds, though guidance still differs on some of the specifics. Aside from below normal temperatures over the North Slope and parts of the eastern Interior early in the week, the forecast pattern will promote above to well above normal temperatures over much of the state through the week. ...Guidance Evaluation and Preferences... Within the agreeable large scale pattern favoring a very wet/snowy regime along the southern coast and Panhandle, individual models continue to have difficulty in resolving specifics of individual storm systems due to important small to medium scale upper level details that have lower predictability. Guidance is mixed for the relative strength/sequence of systems early in the week (but with greater clustering for the second one being stronger) as shortwave energy lifts northward between the North Pacific-Aleutians-Bering Sea upper low and the ridge to the east. Then there is a decent signal for strong low development by Thursday as the North Pacific upper troughing rotates around and possibly closes off an upper low (with surface details sensitive to the upper low versus open trough depiction). Farther west, details are messy regarding system details over the North Pacific and Aleutians late in the period with the general theme of one leading system and possibly a second coming into play by around next Friday. Based on the full array of dynamical and machine learning (ML) guidance, the first half of the period favored a split of the 06Z/12Z GFS runs and minority input from the 12Z UKMET. Then the forecast shifted to maintaining about one-third 12Z GFS weight along with 20-25 percent 12Z CMC and 40-45 percent ensemble means (12Z GEFS/CMCens and 00Z ECens). Even at the start of the forecast period early Monday, guidance has diverged significantly for surface low details from the southeastern Bering Sea southward into the Pacific. Recent ECMWF/CMC runs have been more defined than most other guidance for a leading wave that may reach the vicinity of the southeastern Bering Sea or Alaska Peninsula by 12Z Monday, with latest ML models favoring the less defined majority. The past couple ECens means hint at a system between the leading ECMWF one and the trailing system for which there is better ML model support, and is most likely to reach a position somewhere between the southeastern Bering Sea/Bristol Bay (or a tad north) and the Kenai Peninsula. Recent GFS runs and the 12Z GEFS/CMCens means cluster closest in principle to the ML models by Tuesday, just a fraction slower. The 12Z UKMET/ICON are close to this idea as well. Meanwhile the 12Z CMC is a slow extreme with this system. Latest ECMWF runs are notably weak/east, leading to significantly less QPF over western parts of the coast versus consensus. Relative emphasis on the 12Z/06Z GFS with a little UKMET reflected the majority scenario for track and timing, while not being quite as strong as those models individually is a nod to the ongoing uncertainty and ML guidance average. While the likely Tuesday system weakens and lifts into the eastern/northern Bering Sea, there is a slowly improving signal in the guidance for northeastern Pacific development by Thursday in response to upper trough energy swinging eastward, with a subsequent track toward the southern coast. Surface specifics will be sensitive to evolution of upper dynamics, with current solutions ranging between an open trough and a brief upper low (before opening up as the pattern flattens Friday). The past couple ML model cycles have shown better than usual agreement relative to typical day 7 forecasts in depicting a strong system southeast of Kodiak Island (most in the 960s-970s mb) early Thursday, weakening some by the time it reaches the southern coast by Friday. Ensemble means agree well in principle (GEFS/CMCens stronger than the ECens) with a track slightly west of the ML model average. The 06Z GFS seems to stray too fast, favoring removal from the blend in favor of remaining 12Z run emphasis, while the 12Z CMC wobbles a bit east for a time but is okay as part of a compromise. The 12Z ICON is strong as well with track/timing that fits the majority cluster. ECMWF runs have been a little slower to spin up the system but eventually catches up somewhat over the northern Gulf. The ensemble means have been consistent in principle for a general area of low pressure to reach the Aleutians and vicinity by next Thursday-Friday, albeit with some differences in timing that arise from differing relative emphasis of a leading system versus potential influence from a second one approaching from the west. ML models show various detail differences like the operational models, suggesting a model/mean blend provides a reasonable starting point while waiting for details to get better resolved. ...Weather/Hazards Highlights... Expect a couple systems in close succession to bring a threat of heavy precipitation (coastal/low elevation rain and snow elsewhere) to areas from the Alaska Peninsula to the northern Panhandle early in the week, with strong winds also possible over the Gulf of Alaska and extending into areas along the southern coast of the mainland. The first storm may reach a position between the Alaska Peninsula and southeastern Bering Sea by early Monday (with uncertain definition at this time) while another one may reach into or near the Alaska Peninsula by Tuesday. The Days 3-7 Hazards Outlook reflects the heavy precipitation threat during Sunday-Tuesday along with the best potential for strong winds on Monday. An area of lighter snow may spread northward across the mainland during Tuesday-Wednesday while activity tapers down some around midweek along the southern coast/Panhandle. With typical uncertainty for some details, guidance is suggesting decent potential for another strong storm to track into the Gulf of Alaska by Thursday-Friday. This storm may bring another period of heavy rain/snow along with strong winds, so continue to monitor forecasts for potential changes and/or improvement in specifics. The Aleutians will see somewhat unsettled conditions early-mid week but with only scattered light precipitation. One or more surface lows and frontal systems may bring an increase in winds/precipitation to the region by next Thursday-Friday. The North Slope may see below normal temperatures for most of next week and pockets of the eastern Interior should start out next week somewhat below normal. Otherwise expect the pattern to support a trend to above or well above normal temperatures over much of the state with the greatest anomalies likely to be around midweek. Some flattening of the upper pattern by next Friday may promote moderating temperatures late in the week, though still at above normal levels. 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