Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
702 PM EDT Mon May 18 2020
Valid 12Z Fri May 22 2020 - 12Z Tue May 26 2020
...Overview...
Most guidance maintains the recent consensus for large scale
evolution aloft during the period, with a slowly weakening
Aleutians/southern Bering Sea deep-layer low settling over or near
the southeastern Bering Sea and a downstream ridge likely closing
off an upper high over the Arctic or far northern mainland. Flow
aloft to the east side of the Aleutians/Bering upper low should
elongate across the Gulf of Alaska to some degree. Confidence
remains above average that the forecast evolution will support a
period of locally heavy precipitation over/near the Kenai
Peninsula and northeastern Alaska Peninsula late this week into
the weekend. However low-predictability aspects of important
frontal waves over the Gulf lead to lower confidence regarding
duration of precip over this area this weekend and early next week
as well as for timing/intensity of precip that should reach the
southeastern coast and Panhandle. Meanwhile spread persists for
some aspects of the upper ridge, and by next Tue uncertainty
increases for specifics of features over parts of the North
Pacific.
...Guidance/Predictability Assessment...
Today's forecast started with a blend of the 12Z GFS/ECMWF and a
little 12Z CMC for days 4-5 Fri-Sat. Then it tilted the ECMWF
input somewhat more toward the 00Z run relative to the 12Z run
from day 6 Sun onward and increased 00Z GEFS/ECMWF ensemble input
to 40 percent on day 7 Mon then 70 percent on day 8 Tue. In
addition the blend removed the 12Z GFS after day 7.
Models and individual ensemble members continue to vary with
specifics of individual waves over and near the Gulf of Alaska
during Fri-Sun. The small scale of these waves leads to low
predictability and confidence for specifics during this time
frame. The most common themes in guidance have maintained some
continuity though, with one wave initially southeast of Kodiak
Island on Fri and then a trailing wave/frontal system arriving by
Sun. Confidence in wave location on the latter is quite low by
Sun, with potential for it to be anywhere along the front depicted
in the manual forecast on that day. Recent trends toward greater
eastward elongation of lower heights aloft allow for quicker
departure of this feature by next Mon.
Toward the end of the period the guidance diverges considerably
for potential Pacific development originating from a lengthy
mid-latitude wavy front. The 12Z GFS/GEFS means showed a dramatic
trend toward stronger/northward development to the south of the
Alaska Peninsula by day 8 Tue. Recent GEFS/ECMWF mean trends had
actually favored later North Pacific development and longer
persistence of the southeastern Bering low, and prior GFS runs
were not enthusiastic about a 12Z GFS/GEFS type of system either.
The overall pattern did provide room to reflect a much weaker
system though. The 00Z GEFS mean was much closer to the majority
clustering of guidance, hence its use for the GEFS component in
today's forecast.
Differences persist with the specifics of the upper ridge that
builds over the Arctic and parts of the northern/northeastern
mainland. As in the previous couple days the ECMWF/ECMWF
mean-based solutions were a little weaker other models/means. In
addition the 12Z ECMWF pulled its upper high center farther
northwest than continuity/consensus (including ECMWF means). This
led to favoring more 00Z ECMWF weight in the blend versus the 12Z
run from Sun onward. Most guidance suggests that the overall
upper ridge covering the mainland/Arctic may begin to strengthen
around or just after day 8 Tue.
...Weather/Hazard Highlights...
Continue to expect low level flow ahead of one or more frontal
waves to provide a narrow focus of significant precipitation along
eastward-facing terrain over the Kenai Peninsula and northeastern
Alaska Peninsula late this week into the weekend. Some enhanced
activity may also be possible over nearby areas. Confidence is
still less than desired for exactly how long this precip focus
will last, as well as for specifics of moisture that should reach
the southeastern coast and Panhandle for a portion of the period.
Over the past day little has changed with respect to guidance
showing a wide spread for timing and amounts over the southeastern
coast/Panhandle.
The surface pattern over the mainland will be highlighted by a
persistent east-west trough over central areas where highs will
tend to be above normal through the period. Most of the southern
mainland and into parts of the Panhandle should see below normal
highs and pockets of below normal readings are also possible over
the North Slope. Morning lows will be above average in most cases
with the greatest anomalies over parts of the northern half of the
mainland.
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:
- Heavy precipitation across portions of mainland Alaska, Thu-Sat,
May 21-May 23.
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