Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
237 PM EDT Sun Oct 21 2018

Valid 12Z Thu Oct 25 2018 - 12Z Mon Oct 29 2018

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

While the upper level flow will become a bit more progressive
across the North Pacific during the extended period, the large
scale pattern across Alaska will remain relatively unchanged, with
a broad mean upper trough extending from western mainland Alaska
south to the North Pacific, ridging across eastern Asia and
western Canada. This pattern will result in a sustained period of
southerly flow into Alaska from lower latitudes, with embedded
systems enhancing precipitation potential. This pattern will
promote continued rounds of potentially heavy precipitation for
portions of southern and especially southeastern Alaska. 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 remains relatively low during the day 4-5 time
period (Thu-Fri) with respect to an expected deep low pressure
system entering the Gulf of Alaska. The ECMWF/GFS/CMC all show a
quickly deepening surface low moving north into the Gulf by Thu,
with pressures falling to near 960 hPa. 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 systems of note
appear later in the forecast period. Yet another surface low
appears likely to enter the Gulf late Sat, although models vary
quite a bit on the timing and intensity of the system. Additional
shortwave energy is expected to amplify south of the Aleutians
next weekend with an associated low pressure system. The details
of this system are quite murky given the time frame and possible
interactions with tropical cyclone 31W (as shown by some GEFS
members). Given the increased spread, boosted ECENS/GEFS ensemble
mean weighting from day 6 (Sat) 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