Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 751 PM EDT Sun May 07 2023 Valid 12Z Thu May 11 2023 - 12Z Mon May 15 2023 ...Large Gulf of Alaska storm pattern all this week... ...Overview... Through much of this week into early next week, the first weather feature of note will be a large and deep layered mean low pressure system that should settle across the Gulf of Alaska/northern Pacific and stall due to a potent upper ridge in Canada, eventually weakening but remaining troughy. This pattern will cause unsettled weather across the southern coast of Alaska as impulses wrap moisture around the maritime low, with enhanced onshore precipitation and gusty winds from Southcentral spreading into Southeast late week. Then by next weekend, another surface/upper low pressure system moving through the Bering is likely to lead to a round of rain across the Aleutians/Alaska Peninsula and perhaps the west coast of the mainland by early next week. ...Guidance Evaluation/Predictability Assessment... Recent model agreement remains quite good through much of the period, for above normal forecast confidence and continuity. Models indicate a persistent upper low over the Gulf/northeast Pacific, and even show reasonably good consensus for surface low positions through the latter half of the week that could include lows spinning in a Fujiwhara effect interaction before perhaps merging by Friday. The 12Z UKMET was perhaps a touch displaced with especially the low position just south of the Alaska Peninsula. Other than that minor outlier, a blend of the deterministic guidance of the 06/12Z GFS, 12Z ECMWF, and 12Z CMC was able to be used for the first part of the medium range period given the good agreement. This blend also worked for a quick bout of upper ridging forming atop the Aleutians, and the overall col pattern over the mainland. The latter part of the medium range period next weekend/early next week shows more spread arising, though nothing too extreme for this lead time. The positioning of the Pacific upper low diverges a bit in both the models and ensembles, though models do agree in principle in a southeastward shift while weakening. CMC runs have been on the slow side while GFS runs have been fast/east, with the ECMWF in the middle and each ensemble mean generally following its deterministic model. Tended to prefer the middle to slower solutions, away from the GFS, given common model biases. A model blend away from the GFS/GEFS also worked for the upstream upper low feature crossing the Bering Sea that shows some timing and track differences. GFS runs have more quickly split energy with some tracking south of the Aleutians and some to the north over the Bering as both move east, while ECMWF/CMC have a phased trough shifting east across the Bering Sea/Aleutians until perhaps next Monday. Developing low pressure seems possible near/south of the Aleutians by then. Thus the latter part of the medium range forecast was based on increasing proportions of the EC ensemble mean in particular as the forecast progressed and models diverged, with the means half of the blend by day 8. ...Weather/Hazard Highlights... The upper low pattern in the Gulf/northeast Pacific will provide opportunities for surface lows and frontal systems to move through and spread moisture into southern parts of the state. Additionally, especially early in the period Thursday, gusty winds are possible, wrapping as gap winds with a northern component behind the low for the Aleutians and AK Peninsula, with some enhanced easterly winds possible into Peninsula and Kodiak Island into Southcentral Alaska. The latter will favor precipitation enhancement for terrain in those areas, leading to some high multi-day precipitation totals in the form of higher elevation snow and coastal rain as rounds of precipitation periodically occur through the latter part of this week into early next week. Farther east, precipitation chances should increase across Southeast Alaska late this week in particular as a couple of low pressure systems draw in Pacific moisture on their eastern side. By next weekend, another low/frontal system moving across the Bering Sea should spread rain and perhaps some gusty winds to the Aleutians and the Alaska Peninsula. This moisture could bring precipitation chances to the western part of the mainland by Sunday-Monday. Some showers may be scattered across much of the mainland by early next week as well, but are not expected to be too heavy. The mean low dominating the northern Pacific should lead to below normal highs across the southern coast of Alaska, but lows should be closer to or tending a bit above normal. However, the southeastern part of the Panhandle could see increasing temperatures early next week if part of the Canada upper ridge builds in. Farther north should see a mix of above and below average temperatures with not too considerable anomalies. This means highs in the 50s and 60s for the Yukon Flats and Interior, serving to help rivers break up, but fortunately not at a terribly fast pace without well above average temperatures and with little precipitation. 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