Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 744 PM EDT Thu May 04 2023 Valid 12Z Mon May 08 2023 - 12Z Fri May 12 2023 ...Overview... The main feature of note across Alaska next week will be a large low pressure system extending from the upper levels to the surface, expected to shift from the southeastern Bering Sea early week across the Aleutians and setting up atop the northern Pacific around midweek and stalling given a potent upper ridge in Canada. Precipitation chances and winds are likely with the track of the mean low, and the highest precipitation totals are likely to be across the Kenai Peninsula, with uncertainty in amounts farther east. ...Guidance Evaluation/Predictability Assessment... Recent model guidance continues to show fair agreement with the large scale pattern but with plenty of detail differences. The upper/surface lows set to track southeast from the Bering Sea to northern Pacific show some differences between models even as the period begins Day 4/Monday regarding the timing of its track. GFS and GEFS runs through 12Z were on the faster side to track the lows east around Monday-Wednesday compared to the ECMWF and CMC model suites, with the 12Z UKMET in between. Overall, model guidance was a bit faster compared to yesterday's forecast. Leaned more toward the EC/CMC slower solutions for this forecast to be somewhat closer to continuity and because model biases of the GFS/GEFS suite are often in the fast/east direction. Models also continue to struggle with the exact interactions of the eastern part of the low and the ridge to the east, dependent on the shape and location of the low. These are fairly minor in terms of typical predictability at the medium range timescale, but end up affecting the flow pattern for incoming moisture and thus the precipitation forecast for Southeast Alaska. Additionally, there may be additional energy coming in from the north or from the west to reinforce or reorient or eventually push the low out late next week, but there remains considerable uncertainty for now. Given these considerations, the WPC forecast was based on a blend favoring the ECMWF, CMC, then GFS and UKMET (most to least inclusion) early in the period, gradually reducing the GFS in particular and of course the UKMET as it phases out in favor of the EC ensemble mean and eventually a small proportion of the GEFS mean. By day 8 the ensemble means were more than half of the forecast blend because of the increasing model differences with time. ...Weather/Hazard Highlights... By Monday, low pressure crossing the Aleutians should lead to a broad area of rainfall along with brisk to strong winds there and into the Alaska Peninsula. Then most model guidance shows that precipitation (likely higher elevation snow and coastal rain) should begin across the Kenai Peninsula around Monday and lasting for much of the week. Some easterly component of the flow would provide precipitation enhancement along favored terrain of the Kenai Peninsula and Alaska Peninsula. While the exact amounts may remain uncertain, there is certainly a threat for heavy precipitation especially since it could last multiple days with the stalling low. Farther east, precipitation chances may increase across Southeast Alaska by the latter half of the week, but showers are not forecast to be as heavy there at this point, but this will depend on low-predictability shortwave details. Confidence is also not great for specifics of areas of precipitation across southern and eastern areas of the mainland lingering into Monday with an initial shortwave. 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 normal. Farther north should see a mix of above and below average temperatures. This means highs in the 50s and 60s for the Yukon Flats and Interior, serving to help rivers break up. Tate Hazards: - Heavy precipitation across portions of mainland Alaska, Tue-Wed, May 9-May 10. 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