Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Tue Oct 22 2024 Valid 12Z Sat Oct 26 2024 - 12Z Wed Oct 30 2024 ...A deep surface low will cause high wind concerns across the eastern Gulf to Southeast Alaska this weekend... ...Overview... Upper and surface lows will track from the southeastern Gulf of Alaska toward the Panhandle this weekend, with high winds likely across maritime areas and perhaps spreading over land, with some moderate precipitation. Mean troughing aloft should prevail over most of the mainland, anchored by an Arctic upper low, while a weak ridge is forecast to move across the Aleutians to Bristol Bay and surrounding land areas. Into the workweek, the ridge is forecast to move east across the Southern Coast, but meanwhile energies over the Bering Sea will combine to likely create an upper trough just upstream, which is expected to move across the Alaska Peninsula or so by Tuesday and likely deepen as it moves into the Gulf next Wednesday. A surface low should accompany this trough but with variations in the details. Another upper/surface low that could be strong may enter the Aleutians by next Wednesday. ...Guidance Evaluation/Preferences... As the period begins Saturday, deterministic models continue to agree that the surface low in the southeastern Gulf will be fairly strong, with pressures in the 950s. Guidance does show the low weakening into the 970s or 980s as it moves inland Sunday. There remains some scatter for position--nothing out of the ordinary, but the exact positions may matter for sensible weather impacts like winds and precipitation. Overall could use a blend of the 12Z GFS/ECMWF/CMC and a bit of the UKMET for the early part of the period, as this also worked well for the other features of the pattern including upper troughing extending into the Mainland from a closed upper low north of the state. The Arctic closed low shows model spread increasing gradually throughout the first half of next week. GFS runs migrate to the west while CMC runs move to the east, while the ECMWF drops southward into the Mainland. EC-based AI/ML models show similar placement to the ECMWF or (some considerably) farther east. Ensemble means show the spread with broadening heights as the period progresses. The WPC forecast kept the low position north of the North Slope at this point, similar to yesterday's forecast. Farther south, energy moving across the Bering Sea is forecast to bring some weak troughing along with surface fronts across the western Mainland to the Alaska Peninsula Monday-Tuesday and then deepen as it moves eastward midweek. The trough axis may eventually stretch to the Arctic upper low. Most guidance is indicating this troughing, except for the 00Z ECENS mean (latest available at forecast generation time) that was flatter/showed weak ridging through Monday-Wednesday. The 12Z EC mean has fortunately trended toward troughing like the other guidance. There are still some timing and placement differences though that affect the QPF. The 12Z GFS was farther north than the deterministic model consensus with the surface low and brings more QPF into Southcentral. However, ensemble means are not too far off with this farther north position, so will continue to monitor. The 12Z GFS is also slower to bring the QPF toward the Panhandle by maybe half a day compared to the EC/CMC. At the tail end of the medium range period, models are agreeable in principle that an upper low accompanied by a surface low may both be relatively strong as it moves near or over the Aleutians, with some timing and placement differences that are certainly within the typical spread for Day 8. After the early period multi-model deterministic blend, trended the forecast blend to half ensemble means (mostly the 12Z GEFS mean since the 00Z EC mean was not favored) by Day 7 and 70 percent means by Day 8 given increasing spread. ...Weather/Hazards Highlights... Moisture inflow possibly in the form of a weak atmospheric river rounding the surface low will provide support for precipitation across Southeast Alaska over the weekend. This precipitation is likely to remain moderate and below hazardous levels, though with relatively higher totals in southern parts of the Panhandle than northern parts. Winds and waves should be the bigger threat with this system given the depth of the low especially offshore, but some high winds will possibly come onshore for parts of Southeast. Light snow may wrap back into Southcentral and southeastern parts of the Mainland over the weekend. Southeast Alaska should trend drier by Sunday or Monday as low pressure weakens and moves eastward. Meanwhile some light snow may reach coastal parts of the North Slope near the Arctic upper low. The Aleutians can expect rounds of precipitation this weekend into next week, with increasing support that precipitation will spread into the Alaska Peninsula and the western (especially southwestern) Mainland early next week. Model spread is higher with how much precipitation reaches Southcentral Alaska or stays in the Gulf, but models are more agreeable that moderate to heavy precipitation should move into Southeast Alaska once again by Tuesday and possibly last into next Wednesday with the upper trough and surface low. Gap winds may be a concern behind the low near Kodiak Island and the Kenai Peninsula. Stronger winds and heavier precipitation could affect the Aleutians midweek as well. The trough atop most of Alaska through most of the period will promote generally below normal temperatures in most places, with some exceptions. Warmer southerly inflow into southwestern Alaska should lead to above average highs and lows there into early next week. The North Slope may also see temperatures that are warmer than average. 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