Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Wed Jul 07 2021 Valid 12Z Sun Jul 11 2021 - 12Z Thu Jul 15 2021 ...Heavy rain threat for the Panhandle and coastal Southcentral this weekend... ...Overview... An upper low with associated surface system will drift over the Gulf of Alaska/Northeast Pacific during the period, with the greatest moisture likely to extend into the Panhandle and coastal parts of Southcentral through the weekend before tapering down somewhat with time. Behind this system, a Bering/Siberia upper ridge will build over portions of the mainland while a trailing weak Bering upper low/trough exhibits some combination of slow northward and/or eastward motion. Guidance shows very fast and low-amplitude flow across the Arctic during the period. A mean frontal boundary on the southern side of the upper jet should meander near the northern coast from the weekend into the first part of next week and then possibly drop a little farther south by midweek or so. This front and any waves may produce periods of precipitation. ...Guidance/Predictability Evaluation... Among today's guidance the ECMWF along with the 12Z CMC/UKMET generally clustered the best among each other and the ensemble means early in the period. Then by days 7-8 Wednesday-Thursday a transition toward the past two ECMWF runs, 12Z CMC, and 12Z GEFS/00Z ECMWF means represented the most common themes of guidance while downplaying details that have lower predictability. The 12Z GFS differed from the majority for the shape/evolution of the Bering into mainland upper ridge and this eventually caused the low south of the mainland to become more progressive. The UKMET strayed a bit north/northeast with the upper low near the end of its run so its weight was reduced at that time. As for trends, over the past couple days there has been somewhat of an eastward tendency in forecasts valid early in the period but then latest models and means offer more of a mix between slower and faster trends--supporting the blended/compromise approach. Meanwhile the fast and low-amplitude nature of Arctic flow lends itself to low predictability for specifics over northern areas. The GFS has been particularly erratic lately, with the 12Z run bringing lower heights into the northern mainland versus other models/means while the 06Z run had ridging that deflected flow a lot farther northward. The last couple ECMWF runs and 12Z CMC compared fairly well to the ensemble means in principle. ...Weather/Threats Highlights... Expect the Gulf system to bring the highest precipitation totals into the Panhandle during Sunday-Sunday night, with 1-2 inches possible during that time. Locally higher totals would not be out of the question depending on exact track and strength of the system. Some meaningful precipitation may extend into the southeastern corner of Southcentral as well. Precipitation intensity should decrease to light-moderate from Monday onward but could persist over at least the Panhandle for one or more days. High temperatures will be below to well below normal over the Panhandle and parts of Southcentral during the period. Snow levels will likewise be lower than average for the middle of meteorological summer, especially during the weekend. Northern areas may see one or more episodes of precipitation with the mean frontal boundary that should be near the northern coast early in the week and possibly drop southward by Wednesday-Thursday. Thus expect northern areas to see some above normal readings early in the week followed by a cooling trend that may bring highs down to below normal levels. Interior areas are likely to see near to below normal highs trending to mostly above normal by Tuesday as an upper ridge builds in, followed by slight moderation as the ridge weakens a bit. Rausch Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, Jul 10-Jul 11. Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards 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