Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon Jul 31 2023 Valid 12Z Fri Aug 04 2023 - 12Z Tue Aug 08 2023 ...Very warm to hot temperatures likely over parts of central/eastern Mainland Alaska... ...Overview... Expect southern mainland upper ridging late this week to strengthen and amplify northward into the Arctic as the ridge axis shifts into western Canada, leading to a period of well above normal temperatures that should be most pronounced over central/eastern parts of the mainland. Meanwhile the leading edge of Bering Sea upper troughing, anchored by a strong storm tracking over the western Bering Sea/eastern Siberia, should set up a tight south-north gradient that would lead to an axis of enhanced rainfall focused over the far western mainland and eastern Aleutians/western Alaska Peninsula. After this band of moisture arrives, shortwave energy within more progressive North Pacific/Aleutians flow may support waviness that could shift the rainfall focus more to the southwestern mainland/southern coast early next week. Confidence in details decreases by that time though. Meanwhile a northeastern Pacific upper low should drift northward, possibly opening up by the time it nears the Gulf of Alaska by early next week. ...Guidance Evaluation/Preferences and Predictability Assessment... For the second day in a row, most models and means have locked in on the forecast pattern rather well for the late week into weekend time frame, with only one or two stray solutions that would appear to have lower probability. Today the 00Z/12Z CMC runs are in the minority, depicting somewhat more of a southwesterly (versus southerly) orientation of flow aloft from the Bering Sea into the mainland. This leads the CMC to produce a broader coverage of heavier QPF over the mainland. Consensus has held up well for the slow northward drift of the northeastern Pacific upper low. Guidance diverges noticeably after day 6 Sunday. The 00Z ECMWF and latest GFS runs pinch in the upper ridge from the east by way of a northern-central Canada upper low, shifting the ridge axis back west over the mainland. However the GEFS/ECens/CMCens means are consistent and agreeable in maintaining the strength/width of the upper ridge, keeping the axis over and north from western Canada. The 12Z ECMWF/CMC agree in principle with the ensemble means. Progressive and low amplitude North Pacific/Aleutians/southern Bering Sea flow provides another element of uncertainty to the forecast, with embedded specifics having low predictability several days out in time. Some operational model runs have been trying to close off an upper low somewhere between the eastern Aleutians and northeastern Pacific/Gulf of Alaska. At least the 12Z ECMWF's upper low fits within negatively tilted upper trough depicted in the ensemble means by late in the period, and ECMWF means in particular offer support for a weaker version of the Alaska Peninsula/Kodiak Island surface low pressure seen in the 12Z ECMWF. Based on the above guidance comparisons, the Friday into early Sunday part of the forecast started with the 12Z ECMWF/GFS/UKMET in order of more to less weight. Then the blend quickly transitioned to add 12Z GEFS/CMCens and 00Z ECens mean emphasis (and supported by the new 12Z ECens), so that by day 8 Tuesday ensembles provided 75 percent of the input while the 12Z ECMWF made up the rest. ...Weather/Hazards Highlights... The amplifying upper ridge shifting from the mainland into western Canada will promote well above normal temperatures, especially over the Interior and North Slope regions and likely peaking this weekend. Of particular note, highs may reach hazardous levels of the upper 80s/low 90s over parts of the eastern Interior while the North Slope could see readings get as high as the low-mid 80s. Expect temperatures to moderate somewhat from west to east early next week. The southern half or more of the Panhandle should see above normal highs as well. On the other hand, the southwestern mainland and the southern coast will likely see a trend toward below normal highs. Low temperatures should be above normal nearly statewide with the greatest anomalies over the North Slope and a few scattered areas farther south. Guidance continues to show a broad area of rainfall of varying intensity spreading across the Bering Sea and Aleutians on Friday, ahead of a front anchored by a strong storm tracking over the western Bering Sea. Then during the weekend this moisture should focus into the far western mainland as a band of deep-layer southerly flow sets up between the advancing eastern Bering Sea front and upper ridge farther east. Locally moderate to heavy activity is possible over favored terrain, with the Seward Peninsula currently showing the best potential for relatively higher totals. Depending on how upper dynamics/surface waviness may evolve near the Alaska Peninsula early next week, rainfall emphasis may shift to that region and farther east along and near the southern coast. This part of the forecast currently has fairly low confidence, with additional time needed to resolve important details. Southcentral may see scattered light rainfall from late week into the weekend. The upper low expected to open up as it lifts northward toward the Gulf of Alaska by early next week could spread a little moisture/light rainfall into the southern coast and Panhandle by Sunday-Monday. Rausch Hazards: - Hazardous heat across portions of eastern interior mainland Alaska, Sat-Mon, Aug 5-Aug 7. 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