Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Fri Oct 12 2018

Valid 12Z Tue Oct 16 2018 - 12Z Sat Oct 20 2018

...Multi-day heavy precipitation likely across
southern/southeastern Alaska next week...

...Guidance and Predictability Assessment...

Highly amplified flow will prevail over the region with deep
troughing over the Aleutians while a positive height anomaly sits
along western North America. On Tuesday, the parent surface
cyclone spinning over the far eastern Bering Sea will be in the
process of filling with very little downstream movement.
Eventually the focus shifts to a potent shortwave primed to lift
toward the Gulf of Alaska by the Day 6/7, October 18/19 time frame
as shown by recent guidance. Unlike the preceding system near the
eastern Aleutians, a lot of run-to-run spatio-temporal differences
are evident which suggests more ensemble based approaches. Some
operational runs are quite aggressive indicating the possible
strength of this resultant cyclone. Most notably, the 12Z GFS
settles in with a central pressure in the 960 to 969 mb range.
With mean upper ridging sitting over western North America, lower
heights should prevail over Alaska and its periphery for much of
the period. Regarding the forecast preference, found enough
operational clustering to utilize modest amounts of the 12Z/06Z
GFS and 00Z ECMWF through Day 5/Wednesday before shifting toward
ensemble means beyond this period.


...Weather Highlights/Hazards...

The big story will be the threat for several inches of
precipitation across the south-central Alaskan coast down toward
the panhandle region. While not constant, there will be multiple
periods of moisture surges toward the coastline facilitating a
length period of precipitation. The 96-hour totals shown by the
high-resolution 12Z GFS/ECMWF in the 4 to the 8 inch range on a
rather broad scale. If the next system were to deepen as much as
some of the guidance, 850-700 mb moisture flux anomalies may reach
the 3 to 4 sigma above climatology range. Additionally, tightening
pressure gradients would support augmentation of local wind
fields, more so than what is shown by the more ensemble based
solutions. Besides the precipitation and wind, a cooling trend is
in store for the state moving toward the end of the week.
Temperatures fall by roughly 10 degrees during the period of
northern Alaska which brings highs into the 20s by next weekend.


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