Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Sat Feb 04 2023 Valid 12Z Wed Feb 08 2023 - 12Z Sun Feb 12 2023 ...Very deep low pressure likely to bring strong winds to the Aleutians and surrounding waters Thursday-Friday... ...Overview... During mid-late week the guidance has been showing a low-predictability regime from the Aleutians/Bering Sea through the mainland and south into the Pacific mid-late week, with multiple shortwaves whose ultimate evolution will determine the surface pattern and distribution of moisture. Today there is improving agreement toward the idea of consolidation/phasing of this energy that will help to pull surface low pressure up into the Gulf of Alaska by Thursday-Friday, producing a period of enhanced precipitation most likely over the Panhandle. Farther west, models/ensembles maintain the forecast of a very strong storm tracking into the western Aleutians and Bering Sea Thursday into the weekend while some degree of upper ridging builds over the mainland and northwestern Canada. Expect this system to produce a broad area of strong winds while the leading wavy front and possibly a trailing Pacific wave may spread focused precipitation eastward across the southern coast and into the Panhandle from Friday through the weekend. ...Model Guidance/Predictability Assessment... At least in relative terms, the most agreeable aspect of the mid-late week regime that models have had difficulty with over recent days is that the overall complex of shortwaves from the Aleutians/Bering Sea eastward as of Wednesday should consolidate and sharpen into an overall upper trough reaching near 140W longitude by around early Friday. Latest models have gravitated closer to recent ensemble mean ideas of this evolution supporting the northward progression of surface low pressure into the Gulf, with some spread still evident for the surface low. The 12Z CMC was the main extreme with a farther south track so it was not considered for the forecast blend. Meanwhile the southeastern Bering Sea pattern is still in question. The 12Z UKMET showed the strongest low and the 12Z ECMWF had a much weaker one, while other models generally downplayed such low pressure. Ensembles are not enthusiastic either. Some GEFS members had a low (west of the ECMWF) while 12Z CMC/00Z ECens members had none. Thus prefer to downplay such a defined wave. Overall the models have actually trended a little deeper with the strong storm tracking into the Aleutians/Bering Sea. Latest GFS runs are on the deep side, reaching around 940 mb by early day 6 Friday (within the GEFS ensemble spread) while the 12Z ECMWF trended a few mb lower than its 00Z run to 944 mb. The 12Z UKMET was 952 mb at the end of its run at that time. It is worth noting that February sea level pressure records for the western Aleutians range within the 940s mb. Based on the range of solutions, the manual forecast depicts a depth in the mid-upper 940s mb early Friday and gradually weakening into the upper 950s mb over the southern Bering Sea by early Saturday. In general the GFS/ECMWF and their ensemble means have been fairly well clustered and agreeable, however the 12Z GFS strayed north of this consensus. Therefore the preferred solution incorporated the 06Z run of the GFS for that model's input from late week into the weekend. The 18Z GFS has returned closer to the majority scenario. The CMC, which had been an eastern extreme in recent days, was initially a northern extreme in today's 12Z run and then came back to the majority by Friday. By Friday-Sunday the main forecast challenge involves one or more waves along the storm's leading front and possibly a trailing Pacific wave, along with strength of the downstream upper ridge. The combination of these potential waves would help to focus precipitation from the Alaska Peninsula through the Panhandle. Such features have much lower predictability than the large scale storm/overall front themselves, so confidence in specifics is quite low at this time. Recent model/ensemble runs have been waffling some for the relative strength of shortwave energy versus the ridge, though the means have weakened the ridge a bit over the past 24 hours. The manual forecast incorporates the idea of a couple waves to represent their potential existence rather than confidence in their exact strength or track. Continuing the recent tendency for the CMC to offer very stray solutions relative to the majority, the 12Z run had a separate wave that lifts through the eastern Bering and into the far northwestern mainland. Preferences early in the period (days 4-5 Wednesday-Thursday) led to blending the 12Z/00Z ECMWF and 06Z/12Z GFS to represent the most common aspects of guidance while downplaying less confident features from any individual model run. Then the dominant focus on keeping as much detail as possible for the deep Aleutians/Bering Sea storm (and sufficient agreement along the southeastern coast/Panhandle by early Friday) favored using a compromise among the 06Z GFS and 12Z ECMWF for days 6-7 Friday-Saturday. Then by day 8 Sunday the forecast incorporated 30 percent total of the 12Z GEFS/00Z ECens means along with the 06Z GFS/12Z ECMWF in light of the increasing detail uncertainty. ...Weather/Hazard Highlights... The southeastern coast and Panhandle may see lingering light precipitation on Wednesday before initial low pressure near the Kenai Peninsula dissipates. Then the latest guidance consensus has low pressure moving northward over the northeastern Pacific into the Gulf Thursday into early Friday, bringing a brief period of enhanced precipitation. Best potential for highest totals is over the southern two-thirds of the Panhandle, with some continued uncertainty over coverage and amounts over the northern Panhandle and far southeast corner of the mainland. Confidence remains above average for the deep storm forecast to track into the western Aleutians and then the Bering Sea after midweek and into the weekend. This storm should bring a period of strong winds especially to the western Aleutians and surrounding waters, especially around Thursday-Friday. A band of less extreme but still brisk to strong winds may also accompany the leading front as it pushes eastward across the rest of the Aleutians/Bering Sea and possibly getting close to the western mainland toward the end of the week. The flow associated with this front as well as any embedded waves and even possibly a trailing wave may help to enhance precipitation that should progress from the Alaska Peninsula through the Panhandle over the course of Friday and the weekend. Some of this activity may be locally heavy, though it will take additional time to resolve wave/frontal details that will help to refine the timing and magnitude of precipitation. Below normal temperatures will tend to prevail over the North Slope and far western parts of the mainland during Wednesday-Thursday while other areas see a mix of anomalies. In general, anomalies should be a little warmer for min temperatures relative to highs. The expected change in large scale pattern is likely to support a pronounced warming trend to broadly above normal temperatures by next weekend, with pockets in the eastern mainland and southern Panhandle probably being the last to experience this trend. 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