Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Thu Jun 17 2021 Valid 12Z Mon Jun 21 2021 - 12Z Fri Jun 25 2021 ...Heavy rainfall threat for portions of coastal Southcentral next week... ...Overview and Guidance/Predictability Evaluation... An upper low will slowly move eastward along and just south of the Aleutians next week, forced in between a trio of strong upper ridging: over British Columbia, roughly 750-1000 miles to the northwest of the Papahanaumokuakea National Monument (west of the Dateline, in the subtropics), and over Siberia. This will cause an incoming system to slow its pace and bring in modest to locally heavy rain for portions of coastal Southcentral next week on southerly flow. The models and ensembles were in generally good agreement on the synoptic evolution but lacked in the smaller wavelength details. Aside from the 12Z UKMET, a blend of the 12Z GFS/ECMWF and Canadian offered a reasonable starting point despite their timing differences both over the Aleutians and across southeastern areas. A middle ground solution was preferred despite a slower trend overall, owing to detail differences that may or may not play out as advertised. Increased ensemble weighting for the last few days of the forecast (next Wed-Fri) with increased uncertainty in the shape of the upper low and its vortices over the Arctic/Beaufort. This could cause a cold front to drop southward, at least to the Brooks Range, should enough height falls move southward. ...Weather/Threats Highlights... Exiting upper low and weak surface system over southeastern areas will promote diminishing showers for the Panhandle Monday into Tuesday as heights slowly rise. Bering/Aleutians system will slip eastward via possible triple point redevelopment late Mon into Tue as the pattern becomes increasingly amplified. By Wed into Thu, southerly flow will increase into coastal Southcentral from Kodiak across the southern Kenai (and west side of Cook Inlet) into Prince William Sound as precipitable water values increase to over one inch to perhaps 1.25" which is about 3-4 standard deviations above normal for these coastal areas. Rainfall may exceed 1-3" per day over parts of the region as the axis slowly moves eastward. To the north, additional areas of showers and perhaps a storm may fire over northwestern areas eastward along a surface trough in response to daytime heating. Temperatures over the state will be near to below normal over southern coastal areas from the Aleutians across Southcentral to the Panhandle due to ample clouds and over the North Slope with lower heights. Areas of near to above normal temperatures will be across parts of the central/southern Panhandle under the ridge and over the Interior with more generous sunshine. Fracasso Hazards: - Heavy rain across portions of mainland Alaska, Wed-Thu, Jun 23-Jun 24. 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