Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Wed Sep 08 2021 Valid 12Z Sun Sep 12 2021 - 12Z Thu Sep 16 2021 ...Heavy rainfall likely late weekend into early next week from the Peninsula region to the Panhandle with accumulating snow possible in the higher elevations... ...Pattern Overview and Model Guidance... Guidance continues to show that an upper low progressing from the Bering Sea into Mainland Alaska should be the dominant feature affecting the state from the weekend into the first half of next week. The upper low and surrounding energy should support two defined surface systems, a low that tracks from the eastern Bering Sea into the mainland as well as a southern wave that tracks from just south of the Alaska Peninsula northeastward into the southern mainland (with trailing front that crosses the Gulf). The combination of these features will support a significant precipitation event across parts of the far southern mainland/Peninsula and into the Panhandle. By midweek some uncertainty develops over how/when the upper low may open up, as well as for the character of a system emerging from the northwestern Pacific. This system will most likely track across the Aleutians/southern Bering Sea and Alaska Peninsula. At higher latitudes expect upper ridging to extend from northwestern Canada to portions of the northern coast for a time, followed by cyclonic flow encroaching from the north or northwest. For about the first half of the period a blend of the 12Z GFS/ECMWF/UKMET continues to provide the best starting point over and near the mainland. There are still fine-scale detail question marks that will take into the short-range time frame to resolve. One item of note is that the GFS strays somewhat faster than consensus with the front that crosses the Gulf--resulting in a shorter duration of enhanced precipitation along the southeastern coast/Panhandle. GFS weight was low enough to favor the majority timing. After early Tuesday the ensemble means suggest that the 12Z ECMWF may become fast to eject the mainland upper trough/low. Meanwhile guidance diverges considerably for specifics of low pressure emerging from the northwestern Pacific. Some stream interaction appears likely near Kamchatka early in the week and other sensitive details of surrounding flow may come into play thereafter so confidence in any specific solution is not very high at this time. The new 12Z ECMWF mean briefly trended slower in deference to the trend in the operational run but otherwise the means have been maintaining decent progression, more in line with the 00Z ECMWF. GFS runs have tended to be on the slow side but with some inconsistency. The 12Z run was the closest to the means in general while the 18Z run adjusted slower again. Adding to the uncertainty, the 12Z CMC is elongated and suppressed enough to be a complete miss for the Aleutians. Guidance comparisons during the latter half of the period led to a transition toward half models (more 00Z ECMWF than 12Z ECMWF along with some 12Z GFS) and half means (12Z GEFS/00Z ECens) by day 8 Thursday. The blends through the period provide a reasonable balance of continuity and adjustments for ideas from latest guidance. ...Sensible Weather... Over the past day most aspects of the forecast have been consistent for the episode of significant precipitation expected across southern areas from late weekend into the first part of next week. Highest totals are likely to extend from the eastern Alaska Peninsula/southwestern mainland through the southern coast and Panhandle. Favored terrain over the southern mainland could also see enhanced totals. In addition lowering snow levels may bring accumulating snowfall to some of the mountains across southern Alaska. Flow ahead of the upper low tracking from the Bering Sea into the mainland should spread moisture northward across the remainder of the mainland but most associated precipitation should be lighter. Confidence remains lower than desired for specifics of another system that could bring an episode of rain and brisk winds across the Aleutians, Alaska Peninsula, and nearby areas Tuesday onward. The most likely scenario is for this system to track over the Aleutians/southern Bering Sea and Alaska Peninsula but there is a small probability it could miss to the south. Much of the southern mainland and Panhandle will see below normal high temperatures through the period. Northeastern areas will start out with above normal highs but trend cooler with time as below normal anomalies gradually spread northeastward, corresponding to the arrival of the Bering Sea upper low. Clouds and moisture will maintain warmer anomalies for low temperatures over most locations, with greater coverage of above normal readings. Some areas in the southwest may still see modestly below normal lows and coverage of negative anomalies may expand somewhat northeastward. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sun-Mon, Sep 12-Sep 13. 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