Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 723 PM EDT Mon Sep 09 2024 Valid 12Z Fri Sep 13 2024 - 12Z Tue Sep 17 2024 ...Overview... As the medium range period begins Friday, upper and surface low pressure will provide support for lingering moderate to heavy precipitation to Southeast Alaska before moving southeast over the weekend. Upper ridging is forecast to push through behind this trough and in response to additional energy and troughing coming into the Bering Sea late week. These upper and surface lows in the Bering gradually moving east will promote increasing precipitation chances across the Aleutians late week, spreading into western and central parts of the Mainland into the weekend, and affecting the Panhandle into early next week. ...Guidance Evaluation/Preferences... Recent model guidance continues to show general agreement with the large scale features of note. A stacked upper/surface low is pretty agreeable offshore of Yakutat Friday with a southeastward shift into Saturday, so a multi-model blend worked well. As this upper trough exits, overall ridging looks to press east from the eastern Bering Sea/western Mainland Friday and across the Mainland Saturday. Possible shortwaves moving around the ridge are uncertain with their timing and strength, but without much clustering around any particular solution yet, which is typical for these smaller scale features. Meanwhile, broad energy and troughing are likely across the Bering Sea, gradually tracking east as the period progresses. The non-NCEP models including the AIFS indicate one relatively strong upper low within the trough pattern moving from east of Kamchatka toward the Bering Strait over the weekend. This supports a deep surface low, with the EC and UKMET in the 950s mb west of the state. However, the 06Z and 12Z GFS runs show a split upper low solution where the first (east of the non-NCEP consensus position) weakens in favor of the secondary one (west of the other models' position), and the surface reflection is similar with double lows. The 18Z GFS trended in the direction of one stronger upper low though. At least the deterministic models become agreeable with a trough axis over the western Mainland by Monday, though with differences in the energy distribution within the trough. Guidance shows increasing differences in how the trough moves east by next Tuesday and potentially forms a closed upper low in the Gulf per GFS runs, with the pattern affected by a possible additional upstream low as well, so there is considerable uncertainty by Day 8. The WPC forecast was based on a blend favoring the 12Z deterministic models early, with increasing proportions of the GEFS and EC ensemble means as the period progressed due to increasing spread, reaching over half means in the blend by the end of the period. ...Weather/Hazards Highlights... The first upper trough and surface low near the Yakutat region into late week could continue to spread notable precipitation into northern parts of the Panhandle into Friday, though gradually trending down compared to the short range period. The highest totals should be over the typically favored upslope regions like Mt. Fairweather/Glacier Bay National Park. Lingering snow is possible on the backside of the low into the Alaska Range, with lighter precipitation to the north in central/eastern parts of the Mainland. Then as the broad upper trough and surface low pressure system get established in the Bering Sea, precipitation is likely to affect much of the state, moving from west to east with time. Precipitation is forecast for the Aleutians on Friday, moving into the Alaska Peninsula/western coast into Saturday, and continuing east to focus in Southcentral Alaska for the weekend and then into Southeast Alaska early next week. The current forecast is for moderate to locally heavy rain amounts for coastal/lower elevation areas and some snow in the higher elevations. Today's forecast trended up for inland precipitation totals, including the Alaska Range to western/central parts of the Interior to the Brooks Range. With low pressure in the northern Bering, there is some chance for high winds and potentially some minor coastal flooding along the western coast, but model guidance waffling on the strength of the surface low as it approaches the state creates uncertainty in the level of impacts. WPC will continue to keep an eye on those potential threats. High temperatures should mostly be in the 50s across lower elevations of the state through the forecast period, so within a few degrees of normal in most locations. However, the North Slope with highs in the 40s and even low 50s will be above normal this weekend into early next week. Prevalent clouds and precipitation will favor mostly above normal morning lows, aside from a round of slightly below average lows tracking across the Interior Friday-Saturday. Southeast Alaska may see a mix of below and above average lows depending on the day but with mostly below normal highs. 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