Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 735 PM EDT Wed Apr 28 2021 Valid 12Z Sun May 02 2021 - 12Z Thu May 06 2021 ...Overview and Guidance Evaluation... The best consensus of latest guidance shows a leading shortwave and surface system tracking south of the Alaska Peninsula and eventually weakening after they reach near the southern Gulf of Alaska. Behind this feature, expect a fairly deep upper low initially over the Bering Sea (with its own surface system) to drop southeastward and energize a mid-latitude Pacific wave that should reach a ways south of Haida Gwaii by early next Thursday. Models/ensembles are more ambiguous with details from the mainland into the Arctic but with a tendency for upper ridging from western Canada into the mainland and possibly a mean low aloft over high latitudes of the Arctic. Meanwhile low pressure emerging over the western Pacific and its leading frontal system will likely affect areas from the North Pacific into the Bering Sea after Monday. This pattern will focus most precipitation from the Aleutians through the Alaska Peninsula, and perhaps the southern coast and Panhandle for a period of time. A composite of latest guidance accounts for remaining detail differences for the leading wave that tracks south of the Alaska Peninsula and eventually weakens. Guidance is coming into better agreement for the ultimate evolution/track of the Bering Sea upper low, with a strong consensus now dropping most if not all of the upper low's energy southeastward into the Pacific. Prior model/ensemble runs had varied between keeping it over the Bering (especially some earlier ECMWF/ECMWF mean runs that had been fairly far north) and somewhat farther south/southeast motion or interaction with additional northern stream flow. At the moment only the 12Z CMC keeps a greater proportion of upper low energy over the Bering after early Tuesday. Otherwise comparisons show that the 12Z ECMWF pulls the upper low farther south than other solutions by the latter half of the period. For the system tracking into the Aleutians or vicinity, models and ensembles show a fair amount of north-south track spread (12Z ECMWF farthest north) and differences in evolution but overall are providing a consistent signal for the system in general. Based on guidance comparisons, the part of the starting blend focused on the 12Z operational runs and with more ECMWF/GFS weight relative to the UKMET/CMC. Lower confidence in the aforementioned more extreme attributes of the 12Z ECMWF led to shifting ECMWF input a little more toward the prior 00Z run by mid-late period. Also the 12Z GEFS/00Z ECMWF means provided a reasonable component of a multi-guidance compromise for primary features later in the period (half models/half means by day 8 Thursday). The newly arrived 12Z ECMWF mean provides reasonable support for the preferred blend. ...Sensible Weather... From late weekend into early next week expect Bering Sea low pressure and its associated frontal system to focus precipitation over and just north of the Alaska Peninsula. There may be some localized enhancement within this area of precipitation. Precise evolution of this system and the leading wave weakening over or near the southern Gulf will determine how much moisture may reach Kodiak Island and the southern coast. Also the flow ahead of the leading wave may bring a period of mostly light to moderate precipitation into portions of the Panhandle. The eastern Aleutians may see some light precipitation on the back side of the initial Bering Sea system. Then from Monday night or Tuesday onward the western Pacific system coming into the picture will likely spread precipitation eastward across a broad area from the North Pacific into the Bering Sea. There may be some areas of enhanced winds as well. However confidence in system details is fairly low at this time so it will take additional time to resolve specifics of precipitation coverage and totals as well as winds. Southern parts of the mainland along with the Panhandle will likely see below normal highs for most of the period, with other areas seeing a mix of above/below normal readings. Anomalies for morning lows should be somewhat higher, leading to greater coverage of above normal min temperatures and more localized pockets of below normal values--mostly in parts of the north-central mainland, Southcentral, and Panhandle. Rausch No significant hazards are expected over Alaska during this forecast period. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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