Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 652 PM EST Thu Dec 21 2023 Valid 12Z Mon Dec 25 2023 - 12Z Fri Dec 29 2023 ...Heavy precipitation possible for parts of Southcentral and Southeast Alaska early next week... ...Overview... A mean upper trough/low will settle across the junction of the eastern Bering Sea/southwestern Mainland/Bristol Bay (centered around Nunivak and St. Lawrence Islands) for the first half of next week. A surface low is forecast to track through the northeast Pacific and pivot early in the week, helping to cause some potentially heavy precipitation for parts of the Kenai Peninsula eastward into Southeast Alaska. The upper trough looks to get pushed west and weaken late next week as upper ridging builds in from Canada. ...Guidance Evaluation/Preferences... Model guidance is again reasonably agreeable on the large scale with the upper low placement during the first half of next week, but shortwaves rounding the base of the trough are another story, and affect the surface low positions. Even at the beginning of the forecast period Monday, recent deterministic models have diverged with the surface low position in the northeast Pacific. The ECMWF seems to be the best match to the more agreeable ensemble means, with the CMC not too bad either. But the GFS runs are much faster/northeast with the surface low and conversely the UKMET is much slower. Another surface low coming in behind this one may complicate matters Monday into Tuesday, and the GFS runs show the next low as faster too. Favored the ECMWF and ensemble means with some CMC input during the early part of the forecast period that seemed to be the best middle ground. By Wednesday-Thursday, the ensemble means still keep the upper low atop the northern Bering Sea, but the deterministic guidance varies. The 12Z ECMWF that was preferred for the early part of the period ends up drifting the upper low center farther north into the Chukchi Sea, where there is some signal for a relative high. Other models do have some of the energy within the trough tracking north though, and these splitting energies can be complex. So the evolution of the upper low somehow splitting/weakening/breaking down to give way to possible southeast to northwest oriented ridging is quite uncertain. Thus the forecast blend gradually lessened the amount of deterministic guidance with increasing lead time in favor of the ensemble means reaching around half by days 6-7 and more by day 8 given the model spread. ...Weather/Hazards Highlights... As a deep surface low causing heavy precipitation to the southern coast and strong maritime winds to the northern Pacific phases into the short range period, another (weaker) low pressure system is forecast to track through the northeast Pacific. There is some uncertainty in the track and timing of the low, but overall it could direct a weak atmospheric river and produce heavy precipitation for areas of Southcentral to Southeast Alaska. This system will likely have snow as the precipitation type for inland areas of Southcentral, and more of a mix of rain and snow in Southeast Alaska, which is more typical there. The precipitation looks to be heaviest on Monday (Christmas Day) but with some precipitation on either side of the holiday. After that it generally looks like an unsettled pattern for southern parts of the state, but amounts are expected to be much lower. Elsewhere, the Aleutians and vicinity will see unsettled conditions for most of the period with frequent light precipitation. A round or two of lighter snow may be possible for the Interior during the first half of the week, though with some continued uncertainty over whether much of the moisture reaching the south will reach farther northward. Uncertain additional surface lows may cause some windy conditions in maritime areas of the northern Pacific. Another area that winds could be enhanced is across the Bering Strait, with some northerlies on the backside of the stacked Mainland low. The mean trough across the western part of the state will promote below normal temperatures for the Aleutians, Alaska Peninsula, and western Mainland for the first half of next week. Conversely, mean southwesterly flow should lead to above normal temperatures on average for the eastern part, lingering longest in the northeastern part through around midweek. Anomalies for lows should generally be greater relative to those for highs. By Thursday-Friday both the above and below average anomalies look to ease, with temperatures moderating closer to normal. Tate 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