Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Wed Apr 17 2024 Valid 12Z Sun Apr 21 2024 - 12Z Thu Apr 25 2024 ...Overview... For the start of the forecast early Sunday, today's models and ensembles continue to show a fairly strong upper ridge extending from western Canada through the mainland while an elongated upper low prevails over/south of the Alaska Peninsula. Meanwhile a compact upper low should be dropping south over the western Bering Sea at that time. Latest guidance suggests the Alaska Peninsula upper low could split with most surface low pressure emphasis tracking well south of the mainland. A more pronounced large scale trend seems to be taking shape to the west, with solutions gravitating toward the idea of a longer term mean upper low becoming established over the Aleutians/Bering Sea. This mean low would be comprised of the initial western Bering Sea low plus input from one or more emerging western Pacific features and possibly some additional energy dropping south from Siberia. Consensus suggests that mean ridging aloft may persist over most of the mainland, but with uncertainty over its shape/orientation and possible weak embedded shortwave(s) through the course of the period. ...Guidance Evaluation and Preferences... The full array of guidance is not very well clustered regarding exactly what will happen with the initial upper low over and south of the Alaska Peninsula as of early Sunday. The 12Z GFS/UKMET are the most pronounced in taking what splits off from the northern side of the feature and carrying it eastward across the mainland. Most other solutions, including a majority of 00Z ECMWF-initialized machine learning models (MLs), either keep a more consolidated low farther south or at least keep the northern energy west of the GFS/UKMET. The smaller scale details involved with this evolution lead to low predictability for specifics. An intermediate latitude provides a reasonable starting point given the north-south spread with the primary surface low, with consensus showing an eastward track well south of the mainland after Sunday. Over the past day, most of the dynamical guidance has trended strongly toward prior signals from the MLs for the evolution over the Aleutians and vicinity. Thus the latest consensus advertises the western Bering Sea upper low dropping just south of the western Aleutians and developing fairly strong surface low pressure already by late Sunday or early Monday. This low may be strongest around Tuesday, with the 12Z GFS/ECMWF reaching as low as the upper 960s mb (though the new 18Z run has tempered it to the mid 970s mb). MLs are split between greatest depth in the 970s or 980s mb. Toward Wednesday-Thursday the typical divergences develop regarding details of the upper low (due to different sources of Pacific and Siberia energy potentially feeding into the circulation) along with one or more surface lows. Latest GEFS/CMCens/ECens means are remarkably well clustered for the low at the surface and aloft by this time, favoring a decent weight of their ideas late in the forecast. Minus any effects from weak shortwaves surrounding or embedded within the mainland upper ridge, the approaching upper low may lead to somewhat more of a north-south orientation of the mainland ridge late in the period (latest GFS/ECMWF runs) or broader southwesterly flow (GEFS/CMCens mean). The new 12Z ECens mean essentially becomes a hybrid between the early-period configuration and the later GFS/ECMWF cluster. Combining the majority scenario among dynamical guidance and MLs plus locally available data, the first half of the forecast started with a 50/30/20 blend of the 12Z ECMWF/00Z ECMWF/12Z CMC. Then the forecast transitioned toward 30-50 percent input from the 12Z GEFS/CMCens means with the operational model component comprised of the two ECMWF runs and 12Z GFS replacing the CMC (as some details of the latter strayed more from the majority). ...Weather/Hazards Highlights... The circulation around low pressure to the south of the Alaska Peninsula as of early Sunday should continue to produce some precipitation over the Peninsula and Kodiak Island plus the Kenai Peninsula--and perhaps a little farther inland over the southwestern corner of the mainland--into early next week. However uncertainty over important but low-predictability details will keep confidence low for specifics in terms of precise magnitude and location. Overall totals should trend lighter relative to earlier days. Meanwhile confidence is increasing with respect to the potential for fairly deep low pressure to develop near the western half of the Aleutians early next week, with greatest strength most likely to be around Tuesday. This system may produce strong winds and there will be ongoing evaluation to see if wind speeds may approach hazardous criteria as guidance stabilizes for the storm's evolution. Surface low details become more uncertain by Wednesday-Thursday due to potential influence of one or more bundles of energy feeding into the overall circulation aloft. Thus the initial system may persist or another surface low may track into or develop over the region. Either way it looks like a multi-day stormy pattern with episodes of precipitation extending from the Aleutians into the Alaska Peninsula with the leading edge of moisture possibly extending into other parts of the western mainland late in the period. The majority of the state should see above normal temperatures during the Sunday-Thursday period. As for exceptions, some erosion of the northern part of the initial east-west upper ridge across the mainland still appears likely and should allow a front to settle over the northern-central mainland, with the North Slope possibly trending toward near or slightly below normal readings next week. Also, localized parts of Southcentral and the northern Panhandle could see one or more days of temperatures close to normal. Rausch 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