Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Sun Apr 12 2020 Valid 12Z Thu Apr 16 2020 - 12Z Mon Apr 20 2020 ...Overview and Guidance Evaluation... Guidance is consistent in showing a strong mean ridge aloft prevailing over the northeastern Pacific into the Alaska mainland and western Canada through late this week. Then most solutions advertise some flattening/progression of the ridge in response to weakening shortwave energy associated with the system reaching the Aleutians Thu-Fri. Detail uncertainties with this energy and nearby Siberia/Arctic flow continue to keep confidence fairly low for wave details along a front likely to extend from the Bering Sea across/near the northern mainland for most of the period. Relative to typical late-period forecasts there is better than average clustering for the next system which consensus shows tracking across the North Pacific, though precise effects of this storm on the Aleutians are still dependent on details not easily resolved several days out in time. By next Mon this system may promote a downstream ridge from the Northeast Pacific across the southwestern mainland into the Bering Sea. Broadly speaking today's guidance offered better agreement today versus 24 hours ago. In addition operational models have been consistent for the general evolution of the North Pacific storm during the latter half of the period. Thus the initial model blend started with an average of the 12Z operational models and then maintained about two-thirds operational input (with the rest consisting of the 12Z GEFS/00Z ECMWF means) through day 8 Mon. Continuity/consensus are good for the system reaching the Aleutians Thu-Fri. It is after Fri when detail uncertainties arise downstream. There has been some trending of guidance toward the GFS idea of Aleutians/Bering Sea low pressure (or redevelopment) tracking into the northwestern mainland and beyond by next weekend. However the 12Z GFS is again on the deep/northward side of the spread for this surface low with the 06Z run closer to the consensus pattern by mid-late period. The 06Z GFS also looks better for eastern Asia through Arctic flow aloft in this time frame. Therefore GFS input used the 06Z run instead of the 12Z version after day 5 Fri. Other models including the ECMWF exhibit meaningful run-to-run changes with the specifics of weakening shortwave energy, favoring splitting ECMWF input among the last two runs after Fri. Guidance has been inconsistent with the southern periphery of cyclonic flow over the Arctic as well, with the combination leading to variability in position of the wavy front over the northern mainland. For the second day in a row the consensus forecast has backed away somewhat on southward progression of the front during the latter half of the period. Thus far there has been enough consistency/clustering in the operational models to support an operational tilt to the forecast of the Sun-Mon system south of the Aleutians. Latest ECMWF means are closest to the operational scenario. The GEFS mean shows more potential for interaction with lingering Bering Sea energy to draw the surface low somewhat farther north. There is now a stronger signal that a compact but deep bundle of energy, possibly an upper low for a time, may emerge from eastern Asia (tracking over/near Kamchatka during the weekend). Models increasingly diverge on the speed of this feature, with some fast enough to merge with the North Pacific system by next Mon and others considerably slower with less direct influence. Issues with this upstream flow suggest guidance could experience a period of greater spread than what is currently the case. ...Weather/Threats Highlights... One area of widespread precipitation through the late short-range period (Wed) should progress eastward with some activity possibly lingering into Thu over the extreme southeast mainland and Panhandle. Expect another episode of precip across much of the state from late week into the weekend with moist flow ahead of the late week Aleutians/Bering Sea system and then waviness along a Bering Sea into northern mainland front plus remaining shortwave energy aloft progressing eastward. Best potential for significant totals will be over southern and western portions of the mainland. Rain/snow should become lighter and more scattered by late weekend into early next week. Precipitation coverage/intensity along the Aleutians and Alaska will be sensitive to exact details of the storm forecast to track over the North Pacific by next Sun-Mon. Continue to expect most of the state to see above normal temperatures during the period, with the exception of some modestly below normal highs over the Panhandle and localized pockets of the southern mainland. In most cases morning lows should be more anomalously warm than the highs, while the northern mainland will likely see the most extreme departures from normal. Recent trends toward delayed southward progression of the wavy front over the northern mainland would allow for greater persistence of the much above normal temperatures. 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