Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 724 PM EDT Sat Aug 24 2024 Valid 12Z Wed Aug 28 2024 - 12Z Sun Sep 01 2024 ...Multiple surface lows will bring moderate to heavy rainfall to parts of southwest and southern Alaska... ...Overview... An initial system over Southwest Alaska on Wednesday will slide eastward as another strong upper low swings across the Aleutians late this week. By the weekend though, an upper ridge building over Southeast Alaska will turn the pattern more blocky, forcing the deep upper low (and associated surface reflection) to slow significantly and meander across the Bering Sea. Precipitation is possible with both systems, but the second system has greater potential for significant rainfall and some gusty winds as it is much slower to move out. ...Guidance Evaluation/Preferences... The latest guidance continues to have a good handle on the large scale pattern during the extended range, but plenty of uncertainty in the details, especially pertaining to the second deep low as it moves east across the Aleutians and slows into the Bering. Guidance agrees this system should be modestly deep, but a lot of run to run variability on the exact placement of the low which is going to have large implications for exactly where and how much rainfall some locations get. The 12z ECMWF for today was quite a bit faster/farther south than its 00z run and did not fit in as well with the better consensus. The Canadian and UKMET models are also a bit south (slower than the 12z ECMWF) while the 12z GFS was notably north and even slower. Given the uncertainty though, nothing seemed out of the realm of possibility so this system will need to continue to be closely watched and will likely continue to wobble in its placement with time. The WPC blend for today was able to utilize the deterministic guidance Days 4 and 5, but trended towards half ensemble means (with the 00z ECMWF and 12z CMC) by the latter half of the period to try and maintain a more middle ground solution in terms of placement. This resulted in a slightly slower forecast than previous WPC continuity. ...Weather/Hazards Highlights... A cold front through the Gulf on Wednesday associated with the initial system over Southwest Alaska will progress moderate to heavy rainfall from west to east across the Southern Coast and into the Southeast/Panhandle region on Thursday. Some rainfall may be locally heavy, especially as enhanced by the Alaska range (where snow may fall in the highest elevations), along with gusty winds. Lighter amounts are possible farther north into western and central parts of the Mainland mid to late this week. Precipitation will also accompany the next cold front as it slides east across the Aleutians on Friday. By next weekend, as the cold front slows or possibly stalls in response to a building ridge over Southeast, this will set the stage for an atmospheric river to take aim at portions of the Alaska Peninsula into the Southern Coast. Several inches of rainfall is possible, but the exact location and extent eastward along the coast is highly dependent on exactly where the upper low tracks. Regardless, given multiple rounds/days of heavy rainfall across this region, this may increase the threat for run off or flash flooding. Gusty gap winds are possible too next weekend associated with this system. Temperatures should trend warmer with time during the period across East and Southeast Alaska with the building ridge. Elsewhere, temperatures should reach above normal for low as warmer southerly flow comes in ahead of upper troughs, but general unsettled weather and precipitation should keep daytime highs still a bit below normal. Santorelli 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