Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Mon Aug 06 2018

Valid 12Z Fri Aug 10 2018 - 12Z Tue Aug 14 2018

...Threat for heavy rainfall and gusty winds with extratropical
transition of Typhoon Shanshan...


At the onset of the forecast, a wide array of shortwaves encompass
Alaska and surrounding bodies of water. The ultimate pattern
change into the weekend will be the extratropical transition of
what is currently Typhoon Shanshan. Strong height falls and the
accompanying deep area of low pressure will force significant
ridging downstream across the northeastern Pacific up into eastern
Alaska by early next week. Based on the 12Z GFS output, the
trough/ridge couplet will feature 500-mb height anomalies in the 2
to 3 sigma range. Overall confidence in the pattern has increased
over the last 24 hours as models have handled the extratropical
transition more readily. Previous issues were with the GFS runs
taking the cyclone into Japan while attempting to form another
system downstream. With all guidance on board with the pattern,
further operational influences could be placed onto the map. With
that said, the 00Z ECMWF and ensemble mean solutions most closely
matched the Joint Typhoon Warning Center forecast points.
Maintained this combination through Day 6/Sunday before taking a
split between the ensemble means. This pattern will favor plenty
of rainfall chances given strong forcing coupled with abundant
moisture transport. The guidance, particularly the 12Z/06Z GFS
runs favor 1-2" 12-hour amounts late Sunday/early Monday reaching
west-central Alaska. This would be where some of the stronger
height falls would track. The 12Z GFS depicts 12-hour falls around
12 dm which is pretty impressive for the time of year. Besides the
rainfall threat, there could be lengthy period of strong gusts as
the tight pressure gradient sets up over the Bering Sea. Based on
the 12Z ECMWF, 6-hour maximum wind gusts could reach 50 to 60
knots across the Aleutians into the Bering Sea and west-central
Alaska. Details will likely change plenty but an impressive system
does lurk for this weekend onward.


Rubin-Oster



WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html