Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 705 PM EDT Fri Mar 15 2024 Valid 12Z Tue Mar 19 2024 - 12Z Sat Mar 23 2024 ...Temperatures will rapidly warm up next week across the Mainland, especially northern and western areas... ...Active pattern with precipitation and winds for the Aleutians to Alaska Peninsula next week... ...Overview... Model guidance is rather agreeable that a significant pattern change will take place next week. By the start of the period Tuesday, a strong northeastern Pacific ridge will be building into the Mainland and persisting through the rest of the week. Thus a pronounced warming trend is likely especially across the northwestern half or so of the Mainland. To the west of the trough axis, an active pattern is expected with mean southerly flow in the North Pacific/Aleutians/Bering Sea. Bouts of moderate to heavy precipitation and windy conditions are likely across the Aleutians in particular, with more uncertainty farther north in how much precipitation reaches the western coast at times. The northerly flow on the east side of the ridge axis should produce a cool and dry pattern for Southeast Alaska. ...Guidance Evaluation and Preferences... Model guidance has consistently shown expanding ridging from the northern Pacific into the Mainland by Tuesday, and maintain the ridge throughout the rest of the week. Confidence is relatively high on this larger scale. The main uncertainties through next week include details of the flow to the west of the ridge axis, so from the Aleutians to Bering Sea to West Coast, as shortwaves and lows with a general pattern of tracking northeast show some spread that impacts sensible weather. These details are harder to determine, but thought the 12Z ECMWF was a reasonable position and strength with a shortwave around St. Lawrence Island early Wednesday, while the CMC was rather strong and GFS runs are displaced a bit with timing. Additional shortwaves into Thursday also show variability. These impact the amount of precipitation that makes it into the western coastal areas. The ECMWF suite is still showing precipitation reaching farther inland than the GFS/GEFS but perhaps not as far inland as yesterday. By late week, larger scale troughing with embedded low(s) looks to form across the Bering Sea likely from Arctic and North Pacific energy combining. This may set up an omega block pattern for late week. Embedded lows and shortwaves are even more uncertain with this, but at least the general flow pattern is in place in model guidance, and promotes a possibly large surface low pressure system coming into the Aleutians. The 12Z GFS seemed to be an outlier in surface low positions by Friday-Saturday compared to the ensemble mean consensus, while the ECMWF matched more closely to the ensemble means. For the WPC forecast, used a multi-model blend of deterministic guidance favoring the ECMWF early on. Gradually reduced the amount of deterministic guidance (especially those other than the ECMWF) as the period progressed in favor of the ensemble means, which reached just over half the blend by Days 7-8 to compensate for increasing model spread. ...Weather/Hazards Highlights... Expect the most active area for precipitation to be across the Aleutians and Alaska Peninsula, where rounds of low pressure and frontal systems will help focus moist Pacific southerly flow through much of next week. These features could also be accompanied by some enhanced winds. Neither winds nor precipitation look to be high enough to reach criteria for the Hazards chart at this point. Though by late week, heavier precipitation and stronger winds may increase in coverage into the Alaska Peninsula as larger and deeper low pressure approaches. Less certain is the amount of precipitation reaching farther north across the Lower Kuskokwim/Yukon areas into the Seward Peninsula at times next week. Models still have some variability in how much may focus off or onshore. This will continue to be monitored, especially since with the warming temperatures some precipitation may be in the form of rain. Meanwhile, the northerly flow east of the upper ridge axis should keep Southeast Alaska cool and dry. The forecast pattern evolution will bring a pronounced warming trend to the northwestern half or so of the state. The building Northeast Pacific into Mainland upper ridge will lead to well above normal temperatures. By midweek or so, some locations should see temperatures at least 20-30F warmer than short range readings. Meanwhile Southcentral and most of the Panhandle should start out near normal, with Southcentral staying near to above normal through the period but the Panhandle drifting more below normal by midweek and beyond due to northerly flow aloft to the east of the upper ridge axis along with Arctic high pressure extending into western Canada. 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