Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Sun Sep 15 2019

Valid 12Z Thu Sep 19 2019 - 12Z Mon Sep 23 2019

Guidance continues to show an active northern Pacific stream that
will feature two dominant storm systems tracking along or just
south of the Aleutians and then into or near the Gulf of Alaska. 
These systems will likely bring some enhanced rainfall to the
Aleutians/Alaska Peninsula and support much heavier precipitation
at times over the southeastern coast/Panhandle.  Over at least the
past couple days or so the consensus of model and ensemble
guidance has been consistent in highlighting the southeastern
coast/Panhandle region for highest five-day precip totals. 
Meanwhile an upper low (consisting of one or more individual
impulses) should meander somewhere between the northwestern
mainland coast and eastern Siberia.

The first Pacific system should track from just south of the
central Aleutians into the Gulf of Alaska Thu-Sat.  It originates
from an area of disturbed weather south of Japan (98W) and quickly
becomes extratropical under the influence of mid-latitude
dynamics.  An operational model consensus represents the storm
well into day 5 Fri with a stronger depiction than the means. 
Thereafter latest CMC runs stray northwest of consensus (the 12Z
run less so than the prior 00Z version) while the 12Z GFS tracks
south of consensus.

The next system will likely reach near the Aleutians by day 6 Sat
and then continue tracking east/northeast.  There are more detail
uncertainties that persist for this storm, including the relative
emphasis of parent low pressure associated with the upper dynamics
that emerge from eastern Asia versus leading waviness that lifts
up from the lower latitudes of the western Pacific.  A
blend/ensemble mean approach best depicts this system given the
questions with specifics, while holding onto some operational
guidance makes up for the recent tendency for ensemble means to
lose definition for a time between the central Aleutians and
western Gulf.  Through the arrival of the 12Z GFS, the 00Z GFS and
12Z/14 ECMWF provide the best overall comparison to the ensemble
means.  The 12Z/06Z GFS are generally slower than established
consensus while the 00Z ECMWF eventually tracks the system well
south of preference.  The new 12Z ECMWF is much closer to the
preferred track.

For the upper low near the northwestern mainland, the latest
guidance average as well as the ensemble means provide a position
fairly close to yesterday's forecast--after yesterday's 12Z model
runs had suggested potential for retrogression.  There is still
plenty of room for changes in the forecast given that the overall
low may consist of multiple individual impulses.

The early part of the forecast incorporates parts of the 00-06Z
GFS and 00Z ECMWF/UKMET/CMC.  The mid-late portion of the period
adds the 06Z GEFS/00Z ECMWF means while adjusting the remaining
operational model components more toward the 00Z GFS and 12Z/14
ECMWF.  The resulting blend and minor manual adjustments yield
fairly modest changes/refinement versus continuity.

Rausch


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