Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 746 PM EDT Sun May 10 2020 Valid 12Z Thu May 14 2020 - 12Z Mon May 18 2020 Pattern Overview ________________ There has been relatively good continuity regarding the synoptic scale pattern evolution across the Alaska domain and adjacent maritime regions over the past day. One of the main features will be an anomalously strong Arctic high that will extend to the Brooks range, and over time this ridge is forecast to retrograde to the west. The westward progression of this high will introduce height falls from northwest Canada to support a trend toward cooler mainland temperatures for much of the Interior and Arctic Coast region. There will also be two noteworthy oceanic lows of interest, with the first tracking across the Gulf of Alaska during the first half of the forecast period but likely remaining well south of the mainland. The second storm should pass south of the Aleutians by next weekend. Guidance/Predictability Assessment __________________________________ The 12Z model suite is in relatively good agreement on the synoptic scale at the beginning of the forecast period Thursday, with the main difference being the CMC farther north with the low tracking across the Gulf of Alaska through the end of the week. One of the main forecast challenges involves the height falls with the retreating Arctic ridge and the building upper level trough from western Canada, with the 12Z GFS much more pronounced and the ECMWF indicating a considerably more subtle change. In regards to the second low that tracks south of the Aleutians, the models appear to handle the downstream ridge well, but differ on the timing and intensity of the low. Both the GFS and ECMWF are suggesting a triple point low that develops and becomes the dominant feature, whereas the CMC is stronger with the initial low over the western Aleutians and does not have as much signal for secondary low development. By the end of the forecast period, the 12Z GEFS mean is strongest with the trough across central and northern Alaska, whereas the CMC mean is more broad with more of an upper ridge axis across west-central Canada. For the Thursday and Friday time period, a blend of mainly the ECMWF/GFS/UKMET was used as a baseline, along with a little of the CMC on Thursday. By Saturday, there was a greater weighting of the ECMWF and its ensemble mean given the differences with the GFS across northern Alaska, and greater percentages of the means by Sunday and next Monday. About 10 percent of the previous WPC forecast was maintained through day 7 since there were no major changes in continuity. Weather/Hazard Highlights _________________________ There is reasonable confidence in the general trend toward cooler temperatures over a majority of the mainland over the upcoming week. Above normal readings should decrease in coverage and become more confined to southern/western areas of the state while the pattern will gradually become more favorable for below normal temperatures over northern and eastern areas. Expect significant precipitation to be fairly limited in coverage during the period owing to the southern extent of the oceanic lows and the surface high governing the overall weather pattern for central and northern sections. The front settling across the southern mainland may focus some activity at times while light precipitation may reach northeastern areas next weekend with the arrival of upper troughing. As the Gulf of Alaska system moves eastward, expect its track to remain far enough south for at most a little moisture to reach the southeast Alaska Panhandle. The system tracking over the western Pacific may bring some moisture into the Aleutians late in the weekend but with limited confidence on placement and intensity. Hamrick 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