Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
707 PM EDT Mon Apr 13 2020
Valid 12Z Fri Apr 17 2020 - 12Z Tue Apr 21 2020
...Overview and Guidance Evaluation...
Latest ensemble means and most operational model runs remain
similar with the large scale pattern evolution during the period
but embedded detail uncertainties persist. A strong mean ridge
aloft over the Northeast Pacific and extending into the Alaska
mainland/western Canada late in the week will move
eastward/flatten in response to progression of weakening dynamics
associated with the system initially over the Aleutians/Bering
Sea. By next Mon-Tue the combination of a North Pacific system
and upstream energy tracking from eastern Asia into or near the
Aleutians should lead to a general area of low heights (one or
more upper lows) centered somewhere between the North Pacific and
southern Bering Sea. At the same time an elongated upper ridge
will likely extend from the northeastern Pacific/Alaska Panhandle
northwestward, perhaps as far as eastern Siberia. From the large
scale perspective the greatest uncertainty for specifics is with
the North Pacific into southern Bering evolution during the latter
half of the period. Smaller scale spread/variability persist for
the northern mainland wavy front that should eventually push
southward as the initial upper ridge flattens and Arctic flow
becomes more broadly cyclonic.
Today's forecast follows the approach fairly similar to what has
worked well in recent days, an average of 12Z operational models
for days 4-5 Fri-Sat followed by a trend toward a model/mean blend
as specifics become more uncertain. By the latter half of the
period the 12Z GFS and 12Z/00Z ECMWF provided model input while
ensembles came from the 12Z GEFS/00Z ECMWF mean runs.
Early in the period the consensus blend provided good continuity
for the initial Aleutians/Bering Sea low pressure and associated
fronts while resolving persistent differences with wave/position
details for the front draped across the northern mainland. Of
particular note the 12Z ECMWF appeared to make a good adjustment
from the 00Z run that had been farther north than most other
guidance for the mainland front Fri-Sat. Later in the period the
mainland front should sag southward as heights aloft decrease
compared to the late week/weekend time frame, but still with
low-medium confidence in timing.
By the first part of next week there are still considerable
uncertainties with the timing/track/evolution of compact but deep
upper low energy add surrounding flow tracking from eastern Asia
into or a little north or south of the Aleutians, along with how
this energy will affect the leading system expected to track
across the North Pacific. On the positive side the guidance is
getting better defined with the upstream energy as it crosses over
or near Kamchatka. However specifics diverge considerably after
that point--ranging between shearing underneath the North Pacific
system (12Z GFS) to becoming the dominant feature that helps pull
low pressure into the Bering (12Z CMC). Latest ECMWF runs shear
some of the energy into the leading system while keeping the rest
over the Aleutians. The trend to a model/mean blend late in the
period accounts for the detail uncertainty while reflecting recent
continuity that has emphasized keeping the best-defined low
pressure over the North Pacific versus a farther north track.
There is still a wide envelope of plausible tracks though.
...Weather/Threats Highlights...
Expect a broad area of precipitation to cross the state late this
week through the weekend, supported by a combination of initially
strong low level flow and dissipating front heading into the
western mainland and then weakening upper dynamics initially
associated with Aleutians/Bering Sea low pressure. The wavy front
persisting across the northern mainland for most of the period
will also provide a precipitation focus. Thus far there has not
been a coherent signal for which areas should see highest totals
during the period, with a number of regions potentially seeing
significant enhancement at least on a localized basis. The large
scale pattern evolution should promote a drier trend over much of
the mainland early next week. Parts of the Aleutians/Alaska
Peninsula could see meaningful precipitation/brisk winds depending
on the precise surface evolution by the latter half of the period.
The temperature forecast looks consistent, with above normal
temperatures over most of the state during the period aside from
the Panhandle and localized areas over the southern mainland where
highs may be modestly below normal. Morning lows should tend to
be farther above normal than daytime highs and the northern
mainland will likely see the highest anomalies. Multi-day trends
have been toward a later southward push of the northern mainland
front but it should eventually drift south next week as broadening
cyclonic flow aloft over the Arctic begins to have some influence
over the northern part of the state.
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
Hazards:
- High winds across portions of mainland Alaska and the Aleutians,
Thu-Fri, Apr 16-Apr 17.
- Much above normal temperatures across portions of mainland
Alaska, Thu, Apr 16.
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