Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Wed Sep 08 2021
Valid 12Z Sun Sep 12 2021 - 12Z Thu Sep 16 2021
...Heavy rainfall likely late weekend into early next week from
the Peninsula region to the Panhandle with accumulating snow
possible in the higher elevations...
...Pattern Overview and Model Guidance...
Guidance continues to show that an upper low progressing from the
Bering Sea into Mainland Alaska should be the dominant feature
affecting the state from the weekend into the first half of next
week. The upper low and surrounding energy should support two
defined surface systems, a low that tracks from the eastern Bering
Sea into the mainland as well as a southern wave that tracks from
just south of the Alaska Peninsula northeastward into the southern
mainland (with trailing front that crosses the Gulf). The
combination of these features will support a significant
precipitation event across parts of the far southern
mainland/Peninsula and into the Panhandle. By midweek some
uncertainty develops over how/when the upper low may open up, as
well as for the character of a system emerging from the
northwestern Pacific. This system will most likely track across
the Aleutians/southern Bering Sea and Alaska Peninsula. At higher
latitudes expect upper ridging to extend from northwestern Canada
to portions of the northern coast for a time, followed by cyclonic
flow encroaching from the north or northwest.
For about the first half of the period a blend of the 12Z
GFS/ECMWF/UKMET continues to provide the best starting point over
and near the mainland. There are still fine-scale detail question
marks that will take into the short-range time frame to resolve.
One item of note is that the GFS strays somewhat faster than
consensus with the front that crosses the Gulf--resulting in a
shorter duration of enhanced precipitation along the southeastern
coast/Panhandle. GFS weight was low enough to favor the majority
timing.
After early Tuesday the ensemble means suggest that the 12Z ECMWF
may become fast to eject the mainland upper trough/low. Meanwhile
guidance diverges considerably for specifics of low pressure
emerging from the northwestern Pacific. Some stream interaction
appears likely near Kamchatka early in the week and other
sensitive details of surrounding flow may come into play
thereafter so confidence in any specific solution is not very high
at this time. The new 12Z ECMWF mean briefly trended slower in
deference to the trend in the operational run but otherwise the
means have been maintaining decent progression, more in line with
the 00Z ECMWF. GFS runs have tended to be on the slow side but
with some inconsistency. The 12Z run was the closest to the means
in general while the 18Z run adjusted slower again. Adding to the
uncertainty, the 12Z CMC is elongated and suppressed enough to be
a complete miss for the Aleutians. Guidance comparisons during
the latter half of the period led to a transition toward half
models (more 00Z ECMWF than 12Z ECMWF along with some 12Z GFS) and
half means (12Z GEFS/00Z ECens) by day 8 Thursday. The blends
through the period provide a reasonable balance of continuity and
adjustments for ideas from latest guidance.
...Sensible Weather...
Over the past day most aspects of the forecast have been
consistent for the episode of significant precipitation expected
across southern areas from late weekend into the first part of
next week. Highest totals are likely to extend from the eastern
Alaska Peninsula/southwestern mainland through the southern coast
and Panhandle. Favored terrain over the southern mainland could
also see enhanced totals. In addition lowering snow levels may
bring accumulating snowfall to some of the mountains across
southern Alaska. Flow ahead of the upper low tracking from the
Bering Sea into the mainland should spread moisture northward
across the remainder of the mainland but most associated
precipitation should be lighter. Confidence remains lower than
desired for specifics of another system that could bring an
episode of rain and brisk winds across the Aleutians, Alaska
Peninsula, and nearby areas Tuesday onward. The most likely
scenario is for this system to track over the Aleutians/southern
Bering Sea and Alaska Peninsula but there is a small probability
it could miss to the south.
Much of the southern mainland and Panhandle will see below normal
high temperatures through the period. Northeastern areas will
start out with above normal highs but trend cooler with time as
below normal anomalies gradually spread northeastward,
corresponding to the arrival of the Bering Sea upper low. Clouds
and moisture will maintain warmer anomalies for low temperatures
over most locations, with greater coverage of above normal
readings. Some areas in the southwest may still see modestly
below normal lows and coverage of negative anomalies may expand
somewhat northeastward.
Rausch
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sun-Mon, Sep 12-Sep 13.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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