Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 745 PM EDT Sun Mar 08 2020 Valid 12Z Thu Mar 12 2020 - 12Z Mon Mar 16 2020 ...Overview and Guidance/Predictability Assessment... As a whole latest guidance continues to be more assertive with upper ridging forecast to build over the northeastern Pacific and at times extend into the mainland/parts of the Bering Sea, with strong shortwaves riding around the northern periphery of the ridge. The corresponding storm track would have low pressure systems passing through Siberia and then into the Arctic. Some elements of guidance are more agreeable than yesterday but there is still significant spread and run-to-run variability with the amplitude of shortwaves rounding the upper ridge. The forecast is fairly consistent with the leading strong storm tracking through Siberia and into the Arctic late this week though some timing divergence arises after early Fri. To the southwest of this system guidance develops a split regarding the amplitude of shortwave energy crossing the Bering Sea and mainland--between the faster/stronger GFS-UKMET and weaker/slower ECMWF-CMC. Current preference is two-thirds toward the ECMWF-CMC idea given the recent trends toward stronger ridging but tempered a bit by the 12Z ECMWF backing away from its 00Z run a bit for heights over the mainland. However note that by early Sat the 12Z ECMWF is stronger than its previous run with the core of the upper high to the south. It has turned out that the earlier 12Z/07 CMC provided some leadership in the evolution toward a strong upper ridge. Upstream the guidance is starting to develop some clustering for the next progressive system tracking near Kamchatka around Fri-Sat but with increasing spread thereafter. The ECMWF shows the biggest reversal as the 12Z run is more amplified with the associated shortwave versus most other solutions while the prior run was extremely strong with the upper ridge, deflecting the surface system much farther northward. An intermediate solution with a decent operational model component (more total GFS/CMC input versus the ECMWF) appears best in light of operational guidance suggesting the system should not lose definition as quickly as seen in most of the ensemble means. Another question mark lurking in the forecast from Fri onward involves to what extent shortwaves within the western Pacific-Bering stream may interact with energy/moisture flowing around the northern periphery of a broad upper low over the central Pacific. A composite of 12Z model runs would suggest decent potential for some moisture to reach the Aleutians, with specifics of the mean ridge aloft ultimately determining how much of this moisture could ultimately extend into the western/northern mainland. Early in the period the initial blend emphasized nearly half weight of the 12Z ECMWF and the other half a combination of GFS/CMC with a small remaining component reserved for the UKMET. By the latter half of the period aforementioned issues with the 12Z ECMWF led to reducing its weight some while increasing weight of the 00Z ECMWF mean. The forecast maintained inclusion of the GFS/CMC. ...Weather Highlights/Threats... The strong storm tracking through Siberia and the Arctic during the latter half of the week will bring a period of strong winds from the Bering Sea through the northwestern mainland/Arctic with an episode of significant terrain-enhanced precipitation over the northwestern mainland as well. Another system may also track through Siberia and the Arctic during the weekend into Mon. This system is likely to be weaker than the first one, so accompanying winds and precipitation should be less extreme than with the late-week event. Interaction of western Pacific/Bering shortwaves and moisture around the northern side of a large system over the central mid-latitude Pacific should bring some light-moderate precipitation into the Aleutians Fri onward. Some of this moisture could eventually reach the western/northern mainland. Cold high pressure supporting below normal temperatures over about the southeastern two-thirds of the mainland on Thu will quickly give way to incoming warmer flow ahead of the strong late-week Aleutians/Arctic storm. Then expect the pattern into early next week to maintain above to well above normal readings over most areas with highest anomalies over the northwest third of the mainland. Some localized pockets of below normal temperatures may persist over the southeastern mainland and Panhandle. Rausch 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