Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Sat Oct 20 2018

Valid 12Z Wed Oct 24 2018 - 12Z Sun Oct 28 2018

...Continued threat of heavy precipitation for
southern/southeastern Alaska...

The large scale pattern across Alaska will remain relatively
unchanged through the extended period, with a broad mean upper
trough extending from western mainland Alaska south to the North
Pacific, and multiple smaller scale shortwaves traversing the
larger scale trough. This pattern will result in a sustained
period of southerly flow into Alaska from lower latitudes, with
embedded systems enhancing precipitation potential. While models
show agreement on the larger scale, there continues to be
disagreement on the specific details of individual systems. Models
do show general agreement on a multi-day heavy precipitation
threat for portions of southern and southeastern Alaska with
multiple inches (liquid equivalent) of precipitation likely,
especially at windward locations near the coast. A persistent
upper-level low well northwest of Alaska over the Arctic will
meanwhile inhibit the southward expansion of arctic air across
Alaska.

Models spread has reduced quite substantially during the day 4-5
time period (Wed-Thu) with respect to an expected deep low
pressure system in the Gulf of Alaska. The ECMWF/GFS/CMC all show
a quickly deepening surface low moving north into the Gulf Wed
night/Thu, with pressures falling to near 960 hPa, possibly lower.
At this point, deterministic solutions show relatively small
timing/intensity differences. To ensure a realistic forecast for
this system, chose to rely heavily on the deterministic ECMWF
solution for days 4-5 (as opposed to including a few deterministic
solutions), which was very well centered within the ensemble
spread and has been relatively stable for several runs. Two
additional significant systems of note appear later in the
forecast period. Yet another surface low appears likely to enter
the Gulf late Sat. Given the time frame, model spread is larger
for this system. Additional shortwave energy is expected to cross
the Aleutians next weekend as it amplifies with another low
pressure system potentially passing near/south of the Aleutians.
The details of this system are quite murky given the time frame
and possible interactions with a tropical system (as shown by the
GFS/GEFS). Given the increased spread, boosted ECENS/GEFS ensemble
mean weighting from day 6 (Fri) onward.

Ryan


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