Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Wed Oct 28 2020
Valid 12Z Sun Nov 01 2020 - 12Z Thu Nov 05 2020
...Guidance/Predictability Assessment...
Over the past day guidance has noticeably trended toward a
slower/deeper upper low and surrounding trough over the southern
mainland and Gulf of Alaska from Sun into the first half of next
week. This trend leads to more persistent surface low pressure
near the southeastern coast. Upstream Arctic/northeastern
Siberian energy will likely drop into the mean trough by around
the middle of next week, quickly followed by an amplified upper
ridge approaching the mainland from the west. This late-period
evolution should help to push low pressure to the southeast after
midweek. Meanwhile a system should track south of the Aleutians
early in the period, followed by some uncertainty over whether
some of its energy could reach the Gulf/Panhandle or remain
farther south.
From day 4 Sun into early day 6 Tue the 12Z ECMWF/CMC/UKMET and
06Z GFS offered the best clustering for the overall forecast. The
12Z GFS differed from consensus in being faster already at the
start of the period with the system south of the Aleutians,
ultimately leading to a storm track into the Gulf instead of south
of the Panhandle (with varying ideas for specifics). The 12Z GEFS
mean had a more subdued depiction of a frontal wave that merges
into the overall circulation. Interestingly the 12Z ECMWF mean
(arriving after forecast preparation) trended toward the GEFS
scenario. Forecast preference favored the dominant operational
model cluster led by the 12Z ECMWF given the tendency for
operational runs to reflect important details somewhat better than
the means early in the extended period. The ECMWF mean trend does
temper confidence a bit though. Meanwhile there was good
agreement for the deep upper trough/low settling over the southern
coast and vicinity from Sun into the first part of next week,
while a blend also provided a good depiction of the upstream
energy expected to drop into the mean trough. Late day 6 onward
added in some of the 12Z GEFS/00Z ECMWF means but still no more
than 30 percent total as blending the ECMWF/CMC/GFS already toned
down uncertain details within an agreeable pattern. Day 8 Thu
split the ECMWF influence among the 12Z and prior 00Z runs as the
new 12Z run strayed to the aggressive side of the spread with the
strength of the upper ridge building into the mainland. An
intermediate position for the upper ridge axis by day 8 looked
reasonable, given mixed trends including the 12Z ECMWF and recent
GEFS means trending somewhat faster versus the consistent and
slower ECMWF mean as well as the 06Z/12Z GFS holding back on the
extremely fast 00Z GFS timing. Guidance agreed well with a strong
gradient reaching the western Aleutians and spreading across the
Bering Sea next Wed-Thu ahead of a front forecast to push into the
western Bering by next Thu.
...Weather/Threats Highlights..
The dominant focus for heavy precipitation will be over the
Panhandle through Sun. Some moisture/precip should linger into
next week near persistent northeastern Gulf low pressure but with
less extreme totals. Parts of the southeastern third of the
mainland may see periods of mostly light precipitation. Shortwave
energy originating from northern Russia and the Arctic may bring
some light activity to the western mainland as it drops into the
mean trough aloft by Tue-Wed. The western/central Aleutians may
be in the northern periphery of the moisture shield associated
with North Pacific low pressure early in the period. By next
Wed-Thu expect a broad area of precipitation and enhanced winds to
spread across the western half of the Aleutians and an increasing
portion of the Bering Sea, ahead of a deep low forecast to track
near Kamchatka with associated frontal system extending into the
western Bering.
The state will see a wide spectrum of temperature anomalies during
the period. Anticipate much below normal readings over the
southern/southeastern parts of the mainland, with this cold air
eventually reaching the Panhandle. On the other hand the North
Slope will likely see above to much above normal temperatures with
more modest positive anomalies extending over portions of the far
western mainland. By next Thu the upper ridge approaching from
the west should promote a warming trend over western/central
areas.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Sat-Sun, Oct 31-Nov 1.
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Sat-Tue, Oct 31-Nov 3.
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