Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EST Mon Nov 25 2019
Valid 12Z Fri Nov 29 2019 - 12Z Tue Dec 03 2019
Today's guidance continues to display some important detail
discrepancies within a large scale evolution that has been fairly
agreeable and consistent. The greatest uncertainties revolve
around important specifics near the southern coast during the
first half of the period and one or more low pressure centers
tracking northeast from the mid-latitude Pacific Sat onward.
During the late short-range into early extended time frame, recent
trends have been toward the western GFS with respect to western
mainland shortwave energy aloft and associated surface
wave/boundary that are likely to produce a very concentrated area
of heavy precipitation along and near the Kenai Peninsula. Some
of the latest guidance also shows more of a hint of some
interaction with the strong system initially south of the Alaska
Peninsula--which would further enhance precip totals. While
consensus has gravitated more toward the GFS in principle over the
past couple days, nearly all other guidance is weaker with the
mainland shortwave energy. Thus confidence remains low in the
heavy interior southwestern mainland activity seen in the GFS runs
even though trends are increasing the potential for a longer
duration of significant precip near the Kenai Peninsula.
Upstream expect a fast-moving front to cross the Bering
Sea/Aleutians Fri into Sat, anchored by strong low pressure which
consensus has been tracking over or near the extreme
western-northern Bering. Guidance clustering has been gradually
improving toward the idea of triple point development (possibly
incorporating a leading wave near Kodiak Island) reaching the Gulf
of Alaska by day 6 Sun. This area of low pressure will eventually
extend precipitation eastward along the southern coast and into
the Panhandle.
Along the trailing front models and ensemble members continue to
show diverse ideas for timing and track of developing low pressure
lifting northeast from the central Pacific from Sat onward. In
particular the past couple days of ECMWF runs have been persistent
with a progressive system tracking into the Gulf while GFS runs
(and at least the past couple CMC runs) have tended to be well to
the west/northwest. At least during most of the weekend latest
GEFS/ECMWF ensemble means offer support for some variation of the
faster ECMWF wave while the slow GFS/CMC are on the western side
of the full ensemble spread. However after day 6 Sun the GEFS
mean shows lower surface pressures over the eastern Bering in a
nod to the GFS while the 00Z ECMWF mean supports a Gulf wave to
the north of operational ECMWF runs. Note that eastern Bering low
pressure by Mon could be supported by some combination of the
upper trough/embedded low crossing the Bering and/or the trough
crossing the western-central Pacific. Overall it seems
appropriate to reflect a blend of these model/ensemble mean ideas
to yield the best flexibility in the future--a leading wave that
is similar to but weaker than the ECMWF but also an upstream
system that may be forced by Bering/Pacific energy aloft. For the
moment this intermediate approach seems to be validated by the 12Z
ECMWF mean trending toward the GEFS mean by Sun-Mon. The latter
system may reach the southern coast/northern Gulf by day 8 Tue as
the overall upper trough approaches the western mainland.
Coverage and intensity of precipitation along the
Peninsula/southern coast and areas to the north will be very
sensitive to specifics of surface evolution.
Based on the preferred strategy, the initial forecast blend
incorporated ideas from the 12Z GFS/ECMWF and 12Z GEFS/00Z ECMWF
means, with 12Z/06Z runs combined for GFS input mid-late period
(to tone down the strong 12Z GFS) and ensemble mean weight
increasing from 50 percent early to 70 percent late. Mass
fields/wind grids included some manual adjustment due to the
greater than usual weight of the means for the initial blend.
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
Hazard:
- Heavy precipitation across portions of mainland Alaska, Thu-Sat,
Nov 28-Nov 30.
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