Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Sun Jun 09 2024 Valid 12Z Thu Jun 13 2024 - 12Z Mon Jun 17 2024 ...Overview... Today's guidance continues to show a blocky regime from the latter half of this week into early next week, with upper ridging initially over the Arctic settling over the mainland. However there are various uncertainties regarding the shape/extent/strength of the ridge as well as for a mean deep-layer low forecast to be over the North Pacific/Aleutians. An upper low expected to track northward from the northern Bering Sea could ultimately have some influence on the northwestern mainland but with low confidence in that idea so far. The warm upper ridge will increase the potential for fire weather concerns over portions of the mainland while the mean low near the Aleutians should support unsettled weather over that region. ...Guidance Evaluation and Preferences... Dynamical and ECMWF-initialized machine learning (ML) guidance agrees fairly well with the Arctic upper ridge dropping down into the mainland later this week but spread steadily increases for the shape, extent, and center of the feature with time. There are also meaningful differences for the latitude of the mean low over the North Pacific/Aleutians, with resulting effects on the Aleutians. Most of the ML solutions eventually offer potential for somewhat greater westward elongation of the ridge across the central/north-central Bering Sea versus the average of dynamical guidance (12Z ECMWF/ECens means closest to the ML theme), along with potential for the upper high center over the mainland to be displaced more southwest or west (at least as far as the 12Z CMCens mean). At the same time the ML models depict the mean low near the Aleutians--composed of an initial feature and then another one that moves in from the west--over or within 2-4 degrees latitude just south of the Aleutians for most of the period. Predictability appears fairly low regarding how these two individual features may interact. Depending on the valid time, various averages of the ensemble means and 12Z GFS/ECMWF/CMC are not too far from the overall ML model themes for the mean low. One other lurking forecast issue is with the upper low forecast to be over the northern Bering Sea as of early Thursday. Recent consensus had been to lift it steadily northward with minimal influence on the mainland or other aspects of the forecast. However the 00Z ECMWF had it drifting over the Bering Sea most of the period (not supported by any other guidance), and sporadic model runs such as the 00Z CMC and 12Z ECMWF/UKMET have brought it close to or into the northwestern mainland. ML models show very low potential for that scenario, favoring higher heights aloft over the northern mainland. With minimal effect on the Panhandle aside from a period of easterly flow aloft, a northeastern Pacific upper low should track eastward just south of Haida Gwaii. There is still some timing spread by late week but a slower trend in the 12Z ECMWF has helped clustering somewhat. Today's guidance is showing some potential for trailing energy to form another upper low over the northeastern Pacific late in the period. The start of the forecast period employed a nearly even blend of the 12Z GFS/ECMWF/CMC, with the UKMET not comparing was well to the majority cluster for various aspects of the forecast. With time, some aspects of the ensemble means become preferable whether to downplay uncertain dynamical model details or based on general themes of the ML models. Thus the blend gradually increased 00Z ECens/12Z GEFS-CMCens input during days 5-8, reaching 70 percent total weight at the end of the period. ...Weather/Hazards Highlights... Expect the upper ridge settling over the mainland to bring a warming trend toward multiple days of above normal temperatures, especially over much of the Interior. This will increase fire weather concerns with specifics depending on winds and moisture, as well as any scattered convection. Some scattered areas of rainfall will be possible over southern areas and the North Slope, with these regions plus parts of the Panhandle seeing pockets of below normal high temperatures. There should be broader coverage of above normal min temperatures. The mean low near the Aleutians (composed of one or more separate systems) will likely support unsettled weather over the region, with coverage and amounts of rainfall at any particular time depending on the exact shape of surface low pressure and upper dynamics. 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