Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Thu Jul 07 2022 Valid 12Z Mon Jul 11 2022 - 12Z Fri Jul 15 2022 ...Heavy Precipitation Threat for parts of the Alaskan Southern Tier... ...Overview... The ensemble means are agreeable and consistent in showing that upper level flow over the mainland should transition from a northeast-southwest ridge on Monday toward increasing coverage of broadly cyclonic flow with gradually lowering heights. Individual models differ with specifics of this change though. Meanwhile a northeastern Pacific upper low/surface system should track over or near the southern Panhandle by around midweek while a trailing Bering Sea system will likely track into the northeastern Pacific with a somewhat more uncertain evolution/track generally near or south of the leading system. The forecast evolution will support a cooling trend from initially very warm temperatures over interior areas, while persistently above normal moisture will support the potential for periods of locally heavy precipitation over portions of the southern mainland. The leading northeastern Pacific system could also enhance amounts for a time over the southern Panhandle. A west-central Pacific system may drift far enough north to affect the Aleutians after midweek. ...Guidance/Predictability Assessment... Consensus ideas for the largest-scale aspects of the pattern have held on fairly well over the past day. These include the transition toward more cyclonic flow over the mainland with time, the northeastern Pacific system tracking near the southern Panhandle, the Bering Sea system tracking into the northeastern Pacific, and continued separation of the west-central Pacific system from the westerlies (keeping it south of the Aleutians until late in the week). As for the details, guidance has trended noticeably faster and somewhat southward for the Bering Sea system tracking into the northeastern Pacific. Latest ensemble means support this trend with the GFS/ECMWF in the middle of the current spread between the faster CMC (which trends so far suggest could be a possibility) and slower UKMET. The intermediate timing is already a significant adjustment from yesterday so preference is to start there and make further trends if new guidance make additional trends. The 12Z GFS was a bit on the weak side for the upper low with the new 18Z run correcting to consensus. Confidence in specifics is lower as the system tracks over the Pacific with possibly peripheral influence on the southern Panhandle. Solutions rapidly diverge over the mainland after Wednesday, with the 12Z GFS actually bringing in a shortwave ridge by day 8 Friday (06Z and new 18Z GFS runs look better) while the CMC becomes fairly extreme with deeper troughing than consensus. Latest ECMWF runs are much closer to the means in principle, albeit with typical differences in the trough axis on a day-to-day basis. Predictability remains low for individual shortwaves/surface reflections within the mean flow. Meanwhile the GFS/GEFS trends over the past day have yielded better relative clustering for the system that may reach the western-central Aleutians late in the week. The old 00Z ECMWF had an infusion of northern stream energy that led to a rather deep surface low but the 12Z run is back to a system of more modest depth, comparable to most other guidance. Based on the array of guidance available at time of forecast preparation, the starting blend consisted of the 12Z ECMWF/GFS/CMC/UKMET (in order from more to less weight) for about the first half of the period. Spread with flow over the mainland after midweek led to a rapid transition toward more 12Z GEFS/00Z ECMWF mean input and phasing out the GFS/CMC. By day 8 Friday the forecast consisted of 70 percent means and 30 percent 12Z ECMWF. ...Weather/Hazard Highlights... Moisture on the northern periphery of the initial northeastern Pacific/southern Gulf of Alaska upper low (likely to track over the Panhandle by around midweek), and then persistence of this moisture as mainland heights gradually decrease, will contribute to a multi-day threat for moderate to locally heavy/terrain enhanced precipitation over parts of the southern mainland. Significant totals may be the result of varying combinations of more intense activity or accumulation of more moderate precipitation over several days. A more focused band of moisture on the southeast side of the initial Pacific upper low may produce heavy precipitation over the southern Panhandle early next week. Meanwhile, the Bering Sea upper low/surface system tracking into the northeastern Pacific should bring a period of rainfall to the eastern Aleutians and Alaska Peninsula during the first half of the week. Recent southward trends have reduced the potential for this system's moisture to reach the southwestern mainland. Continue to expect leading shortwave energy in the Arctic stream to bring a cold front into the northern part of the mainland early next week, with a wavy front (stationary or sagging a bit southward) likely to persist through late week. This front and upper level shortwaves, along with some moisture reaching the mainland from the west, could focus light to moderate precipitation across parts of the northern/central mainland. Locally heavier pockets of precipitation are a possibility, including over the northeastern mainland, but confidence in specifics is still quite low. The slowly moving system over the west-central Pacific may drift far enough north to bring its moisture into the Aleutians after midweek. Expect a northeast to southwest band of above normal high temperatures over the interior portion of the mainland on Monday, corresponding to the initial upper ridge. Transition of the upper pattern toward broad cyclonic flow will ultimately lead to near normal or perhaps even below normal highs over interior areas. The front pushing into the North Slope early in the week will bring below normal highs that should persist through late week and clouds/precipitation will also tend to keep Southcentral and the Panhandle below normal for highs. Warmer anomalies should prevail for lows, with modestly below normal readings likely to be confined mostly to the North Slope and southeastern Coast/Panhandle. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle, Sun-Mon, Jul 10-Jul 11. - Much above normal temperatures across portions of mainland Alaska, Sun-Mon, Jul 10-Jul 11. 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