Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 733 PM EDT Mon Mar 18 2024 Valid 12Z Fri Mar 22 2024 - 12Z Tue Mar 26 2024 ...Much warmer than normal temperatures expected especially for northern and western parts of the Mainland... ...Active pattern with precipitation and winds for the Aleutians/Alaska Peninsula to the west coast of the Mainland... ...Overview... By late week into the weekend, an amplified pattern will be well established across Alaska, as a strong northeastern Pacific to Mainland ridge is overhead. Warmer than normal temperatures are expected across the Mainland, with the highest anomalies in northern and western areas. To the west of the ridge axis, active weather is expected with mean southerly flow in the North Pacific/Aleutians/Bering Sea. Bouts of moderate to heavy precipitation and windy conditions are likely across the Aleutians to Alaska Peninsula as well as farther north across the western coastal areas. Meanwhile the northerly flow on the east side of the ridge axis should produce a cool and dry pattern for Southeast Alaska. The ridge is forecast to gradually weaken and shift east early next week, but maintaining the general pattern. ...Guidance Evaluation and Preferences... Model guidance has consistently shown persistent upper ridging through late this week into the weekend, with an omega block possible as troughing comes into the southwestern edge of the ridge. Confidence is relatively high on this larger scale. The main uncertainties during the period include details of the flow to the west of the ridge axis, so from the Aleutians to Bering Sea to West Coast, as shortwaves and embedded upper lows within the general troughing pattern show some spread. Models are still indicating relatively good agreement in indicating a shortwave over Bristol Bay/Alaska Peninsula and vicinity early Friday and tracking north across the West Coast toward the North Slope by Saturday, providing a little more confidence in one of the smaller-scale features impactful to sensible weather. Farther west, the details of energy are still murky even for late week, as a couple of surface lows over the northern Pacific that look distinct on Friday potentially interact, with models showing various solutions for whether they merge by Saturday and their placement. The EC and CMC show both lows maintaining some depth through Saturday, while GFS runs merge the two quickly. Ensemble means are not much help since they average the low positions together regardless of whether the members have multiple lows. Either scenario seems possible but at least on the large scale, a large low pressure system (with possibly multiple centers) crossing the Aleutians and setting up in the Bering Sea over the weekend looks likely. By early next week, increasing spread is seen with this surface low track. Questionable evolutions of incoming energy from the Arctic and Pacific do not help matters. There is a general signal that the relentless bouts of energy into the ridge should gradually push the ridge axis east and perhaps reorient it southeast-northwest while weakening. Considering these factors, the WPC forecast utilized a multi-model blend of deterministic guidance early on. Gradually reduced the amount of deterministic guidance as the period progressed in favor of the ensemble means, reaching over half the blend by Days 7-8 amid the increasing spread. ...Weather/Hazards Highlights... Expect the most active area for precipitation to be across the Aleutians and Alaska Peninsula and the western coast of the Mainland, where rounds of low pressure and frontal systems will help focus moist Pacific southerly flow through late week into early next week. These features could also be accompanied by some enhanced winds. During late week, a shortwave and onshore flow should produce some enhanced precipitation for western areas of the Mainland in general, and particularly for favored southern parts of the Seward Peninsula and southwestern Alaska. Both rain and snow are possible depending on the area and elevation, considering the warming temperatures. While multiple rounds of precipitation are likely for the Alaska Peninsula, currently the amounts look to be maximized there Friday into Saturday with a favorable surface low placement to the west. The details of the flow pattern make the precipitation totals and placement more uncertain into next week, but generally there is some signal that energy should press a bit east and bring precipitation to parts of the Kenai Peninsula and Southcentral. Meanwhile, the northerly flow east of the upper ridge axis should keep Southeast Alaska mostly dry, though with perhaps some increasing chances for light amounts by Monday-Tuesday. Much warmer than average temperatures will likely last through the entire period for much of the Mainland due to the ridge aloft and warm mean southerly flow underneath/on the western side. Parts of the North Slope to Brooks Range can expect temperatures over 30 degrees above normal with some above freezing readings becoming more likely especially on Friday. Temperatures look to gradually moderate after that but remain well above normal. Temperatures that are above average but to a lesser extent are likely across the rest of the Mainland too, with highs reaching the 40s on average. On the other hand, Southeast Alaska can expect near to slightly below normal temperatures initially due to northerly flow aloft to the east of the upper ridge axis. Tate 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