Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 659 PM EST Thu Feb 08 2024 Valid 12Z Mon Feb 12 2024 - 12Z Fri Feb 16 2024 ...Heavy precipitation and strong wind potential early next week for the Alaska Peninsula into the southwestern mainland... ...Overview... Latest guidance maintains the general idea of early week upper troughing from the Bering Sea into the North Pacific while a downstream ridge steadily builds over the northeastern Pacific/Panhandle/western Canada into the mainland, possibly closing off an embedded high by around Wednesday. With greatly increased spread for specifics versus 24 hours ago, the flow between these two large scale features should direct a leading North Pacific system northward into the Bering Sea early in the week, with potential for one or more waves along a trailing front into midweek. This pattern will bring a threat of heavy precipitation and strong winds to the Alaska Peninsula/southwestern mainland and possibly nearby areas during the first half of next week. Upper ridging will likely expand westward after midweek while a broad area of deep low pressure settles over the eastern half of the Pacific, with the gradient between western Canada/Arctic (possibly into the mainland) surface high pressure and Pacific low promoting easterly flow over the North Pacific after midweek. ...Guidance Evaluation/Preferences... The most significant forecast challenges involve continued spread for the details of one or more North Pacific low pressure systems tracking northward early-mid week, along with newly increasing guidance divergence for the shape and location of upper ridging after midweek. Early in the forecast period, there were competing preferences for an ensemble mean approach from the North Pacific into the Bering Sea to maintain relative stability given a lot of spread and run-to-run variability for surface low details versus an operational model cluster to depict a stronger downstream upper ridge than the means. This ultimately led to a starting blend consisting of the 06Z/12Z GFS, 00Z/12Z ECMWF, 12Z CMC, and a minimal 12Z UKMET component for days 4-5 Monday-Tuesday. Leading Aleutians/Bering Sea low pressure was adjusted a little deeper to compromise between the blend and continuity while trailing frontal waviness was kept fairly weak given very low confidence in specifics. Then the forecast steadily increased 12Z GEFS/00z ECens mean input mid-late period, reaching 70 percent total weight by day 8 Friday. At that time, the remaining input came from the ECMWF runs and CMC. Perhaps foreshadowed by divergence among previous ECMWF-initialized machine learning model runs, over the past day the operational models have significantly diverged among each other and consecutive runs for depicting leading low pressure that has been forecast to track from the North Pacific into the Bering Sea early in the week, and then for details of one or more waves along this system's trailing front. Some recent GFS runs and the UKMET have become somewhat diffuse with the evolution of leading low pressure (or with varying east-west differences for track) while the ECMWF has become very inconsistent with northward timing of its deep low. There is also a myriad of different possibilities for track/timing of the trailing frontal wave(s) that could brush the southwestern mainland--or alternatively, not even reach beyond the North Pacific if the upper ridge builds westward quickly enough. The ensemble means provide better continuity for track/timing of the leading surface low with a position near the eastern Aleutians early Monday and then continuing north-northwest into the Bering Sea, with the aforementioned multi-model/run blend yielding a solution close to this desired result. This blend also provided a signal of trailing frontal waviness into Wednesday, a compromise between the pronounced strength of some dynamical models and minimal suggestion in the means. By Wednesday, recent trends by the GEFS/ECens means and machine learning models have increased confidence in the potential for a closed upper high to form somewhere in the vicinity of the eastern mainland/northwestern Canada. However after that time solutions diverge considerably, with 00Z-12Z GFS runs considerably dampening the northern part of the ridge (with greater westward extent versus consensus) in contrast to ECMWF runs that elongate the ridge well northward into the Arctic (with run-to-run longitude differences). The new 18Z GFS has adjusted closer to preferences. The ensemble means have been much more consistent and agreeable in the depiction of the ridge, so preference goes strongly in their direction after midweek. ...Weather/Hazards Highlights... Details continue to be rather uncertain for North Pacific into Bering Sea low pressure and possible trailing waves (with the western coast of the mainland included on the far eastern side of the envelope) during the early-mid week period. However persistence of the general pattern is improving confidence in the potential for a period of heavy precipitation and strong winds directed at the Alaska Peninsula/surrounding areas plus some locations extending northward across the southwestern mainland. Thus today's Days 3-7 Hazards Chart depicts heavy precipitation areas for Tuesday the 13th over the eastern Alaska Peninsula and in favored terrain north of Bristol Bay, with these locations seeing the best overlap of hazardous precipitation potential among the varied guidance possibilities. Wind strength in relation to hazardous criteria will be more sensitive to surface details and so for now no high wind area is indicated. Lesser precipitation amounts should extend farther east toward the Kenai Peninsula and west into the Aleutians, as well as northward over the western mainland. The forecast remains on track for increasing dominance of mid-latitude Pacific low pressure by the latter half of the week, which combined with northwestern Canada into Arctic high pressure, should turn low level winds more easterly over the Alaska Peninsula and surrounding areas by midweek. This flow will likely maintain some precipitation enhancement but with lighter totals. Expect locations from Prince William Sound through the Panhandle and northward through the eastern mainland to be relatively dry next week. The forecast pattern will favor above normal temperatures over a majority of the state with the highest anomalies over western and northern areas during the first part of next week. High pressure becoming established over northwestern Canada should promote colder/below normal readings over portions of the Panhandle along with some pockets of near to somewhat below normal readings over the eastern mainland. The rest of the mainland should see warm anomalies gradually become less pronounced later in the week as Pacific influence decreases. Rausch 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