Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
647 PM EDT Thu Oct 29 2020
Valid 12Z Mon Nov 02 2020 - 12Z Fri Nov 06 2020
...Guidance/Predictability Assessment...
Today's model and ensemble guidance shows good agreement and
continuity for the large scale pattern evolution aloft during the
period. A deep upper low along the southern coast of the mainland
for about the first half of the week will finally depart after
upstream Siberia/Arctic energy drops southward over the far
western mainland and into the overall mean trough. The resulting
trough should continue southeastward as an upper ridge building
across the Bering Sea through midweek continues its eastward push
into the mainland by Thu-Fri. To the west of this upper ridge a
strong Kamchatka/Siberia upper low will support a similarly
energetic surface system whose leading circulation will spread a
broad area of moisture and enhanced winds across the Bering Sea
and parts of the Aleutians, and eventually into portions of the
mainland.
One persistent uncertainty in the surface forecast regards the
system originating from the short-range system near the Aleutians.
Operational models have been varying with the timing of upper
support and relative emphasis of the parent low versus leading
triple point wave. Current 12Z majority cluster has this system
tracking over or a little north of Haida Gwaii on Tue. The UKMET
diverges from the other models, splitting into two waves with the
leading one most similar to other solutions but weaker and on a
more suppressed track. The GEFS/ECMWF means imply a somewhat
faster evolution toward the Panhandle and ultimately merging into
mean low pressure over the northeastern Gulf. Typically better
refinement of important details in operational guidance during the
first half of the extended period continues to favor the
operational runs but persistence of the ensemble mean scenario
maintains some doubt in the forecast. Meanwhile there is still
spread and variability with specifics of low pressure just north
of the mainland through midweek, with an intermediate solution
providing a reasonable starting point. The same is the case for
the wave that drops southward over the far western mainland around
Tue, with some improvement in definition since yesterday.
Finally, during the latter half of the period clustering is better
than average for the strong Kamchatka/Siberia surface low along
with its associated leading frontal system and broad area of
strong winds.
Based on guidance comparisons the forecast started with a 12Z
operational model blend for day 4 Mon and then subtracted the
UKMET while otherwise maintaining operational model focus for days
5-6 Tue-Wed. Once again operational models compared favorably
enough to the means to require only minority input (up to 30
percent) of the ensemble means by days 7-8 Thu-Fri.
...Weather/Threats Highlights...
Heavy precipitation over the Panhandle through the weekend should
trend less extreme early next week but with some activity
continuing. Uncertainty regarding Pacific low pressure that may
track near or south of the Panhandle around Tue lowers confidence
for precip specifics at that time. Expect a drier trend during
the latter half of the week as mean low pressure progresses
southeastward away from the area. Meanwhile mostly light
precipitation should accompany the wave dropping rapidly southward
over the far western mainland around Tue. Arctic low
pressure/frontal system could promote some light precip as well.
By mid-late week the broad circulation ahead of strong
Kamchatka/Siberia low pressure will bring an episode of
precipitation and strong winds to the western half of the
Aleutians, much of the Bering Sea, and then parts of the
mainland--especially western areas. Favored terrain in the
west/northwest could see significant totals by Thu.
Over most areas the temperature forecast has been fairly
consistent since yesterday. During Mon-Wed the pattern should be
fairly stable with much below normal readings prevailing over the
south/east and eventually extending into the Panhandle, while the
North Slope sees above normal temperatures and the far west sees
more modest positive anomalies. As the pattern becomes more
progressive after midweek expect a warming trend to commence from
west to east Thu-Fri. By Fri only about the southeast third of
the state should be below normal while the North Slope and western
areas see further warming relative to normal.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Sun, Nov 1.
- Heavy precipitation across portions of the Alaska Panhandle,
Sun-Tue, Nov 1-Nov 3.
- Flooding possible across portions of the Alaska Panhandle, Sun,
Nov 1.
- Much below normal temperatures across portions of mainland
Alaska, Sun-Thu, Nov 1-Nov 5.
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Tue-Thu, Nov 3-Nov 5.
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