Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Sat Jul 09 2022 Valid 12Z Wed Jul 13 2022 - 12Z Sun Jul 17 2022 ...Heavy Precipitation Threat at times for parts of the Alaskan Southern Tier... ...Overview... With typical uncertainty in the details, models/ensembles still show broad cyclonic flow over and west/northwest of the mainland during mid-late week. Embedded impulses will influence waves along a mean surface front expected to be draped across the central mainland. Guidance is suggesting that an upper low initially near the North Pole may wobble southward to some degree, most likely reaching a position to the north or northwest of the mainland around 75-80N latitude during next weekend. Mean troughing may gradually deepen with an axis setting up just northwest of the mainland in response. Farther south, a North Pacific system should track into the Aleutians/southern Bering Sea by late week/next weekend. A leading system to the south of the Alaska Peninsula early Wednesday should ultimately track toward Haida Gwaii, a bit south of yesterday's consensus and thus with less of an effect on the southern Panhandle. There is still potential for one or more episodes of locally significant precipitation across southern parts of the mainland. Sources may include anomalous moisture persisting from the short range period, northern stream shortwaves/leading moisture moving in from the west, and then possibly from flow ahead of the late week into weekend Aleutians/Bering Sea system. It should be a fairly cool period in terms of daytime highs across most of the state. ...Guidance/Predictability Assessment... Today's forecast started with a composite of 12Z operational models early in the period, introduced a small weight of 12Z GEFS/00Z ECMWF means by day 6 Friday, and then quickly transitioned to half means and half models (12Z ECMWF/CMC) for days 7-8 next weekend. The mainland forecast is fairly consistent early, with the leading northern stream shortwave in the process of crossing on Wednesday. Then latest guidance shows some detail adjustments for incoming energy later in the week, suggesting a weak leading impulse followed by a stronger shortwave whose character diverges in the models with time. Another shortwave (with leading surface front) may approach the northwestern mainland next weekend. The progression from operational consensus to model/mean blend with time reflects the typically better resolution of details in the models early and then the lower confidence in specifics late. For the North Pacific into Aleutians/Bering Sea system, the ensemble means agree quite well for nearly the entire period. An average of the operational models is fairly close to the means during the first half of the period. Then some model runs diverge from each other and among consecutive runs, due in part to differences in how northern stream flow may ultimately interact. The 12Z ECMWF/CMC could be a tad slow by next weekend, but ultimately deemed those runs to be a more compatible blending companion with the means than the significantly faster 12Z GFS and old 00Z ECMWF. The new 18Z GFS has adjusted about 12 hours slower early Sunday, at least adding support for leaning away from the fast side of the spread. Late-period specifics of this system's evolution have moderate to low predictability due to the uncertainty involved in stream interaction, but will be important for determining how much moisture reaches the southern mainland. The current spread leaves open a full range of possibilities between moisture missing to the south and significant amounts reaching into the southern mainland. Guidance continues to display some day-to-day adjustments for the system initially south of the Alaska Peninsula early Wednesday. Today the upper low is a tad weaker, and farther eastward the system ultimately tracks a bit farther south than yesterday's consensus. This brings it more toward Haida Gwaii than the southern Panhandle. The CMC is still north of this majority cluster though. ...Weather/Hazard Highlights... The combination of anomalously high moisture persisting over the southern mainland into midweek, and one or more incoming sources of moisture/dynamics, may contribute to episodes of locally moderate to heavy precipitation over parts of the southern mainland. During mid-late week the primary influence on activity will be northern stream shortwaves crossing the mainland along with leading moisture. Then the uncertain details of how flow ahead of the Aleutians/Bering Sea system (which will bring rain to that region during the latter half of the week) may direct associated moisture will become the primary focus. Confidence is currently fairly low for how much of this moisture will reach the southern mainland. Farther north, moisture levels should be less extreme but the wavy frontal boundary over the central mainland through late week as well as the upper dynamics could still produce some precipitation at times. Multi-day totals from the guidance have generally trended slightly lower than yesterday over the northern half of the mainland. The wavy front may lift up as a warm front during the weekend as another front approaches the northwestern mainland. Precipitation over the Panhandle should be fairly light mid-late week, though there is still a low-end potential for somewhat higher totals if the system crossing the northeastern Pacific tracks north of the latest consensus. Then coverage/intensity of precipitation will depend on details of dynamics reaching the mainland (deeper would increase precip potential) as well as flow ahead of the Aleutians/Bering Sea system. Both have low confidence at this time. The forecast continues to show below normal highs over a majority of the state from midweek through next weekend. Coolest anomalies should be over the North Slope and Southcentral/Panhandle. While staying below normal, northern areas may see temperatures rebound somewhat next weekend as flow ahead of a front approaching from the northwest may push the initial mainland wavy front northward. Any above normal highs should be confined to areas near the western coast. Expect warmer anomalies for lows in most cases. Moderately below normal lows should be confined mostly to the North Slope, though possibly extending into the central mainland for a time, and over the southeastern Coast/parts of the Panhandle. 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