Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Sat Jun 05 2021
Valid 12Z Wed Jun 09 2021 - 12Z Sun Jun 13 2021
...Overview and Guidance Evaluation...
If anything, today's guidance has taken a step backward in the
process of trying to resolve the various contentious aspects of
the forecast. Models continue to show very different
possibilities for what may become of the upper low that should
start the period over or near the far western mainland, connected
to wide spread for details of upstream/surrounding flow.
Specifics of the North Pacific/Aleutians stream are still in
considerable doubt as well, though with the most common element
being that a system will affect the Aleutians in some fashion
after midweek. Leading low pressure initially south of Kodiak
Island as of early Wednesday should progress quickly southeastward
and away from the area but there is a low-probability potential
for other energy (whether from the mainland upper low or flow
underneath it) to produce some residual surface low
pressure/troughing in the vicinity.
Given the ongoing wide spread in the guidance for the mainland
upper low, preference is to maintain the scenario of it weakening
over the western mainland versus the variety of other tracks to
the north (12Z ECMWF/CMC) or south (12Z GFS). The 12Z UKMET and
06Z GFS, along with a split of the past two ECMWF runs (before the
new run significantly diverges), provided the best template early
in the period. Highlighting the continued randomness in the
guidance, the new 18Z GFS has it wobbling over the far northwest
mainland for the entire period. The new 12Z ECMWF mean reflects a
significant change in showing more upstream troughing,
incrementally increasing the potential for northward ejection per
the 12Z ECMWF/CMC. At the very least the idea of the upper low
dropping southward and maintaining low pressure near Kodiak Island
for a longer time per the 12Z GFS is looking like a low
probability, with the UKMET/CMC trending away from a similar
scenario over the past day. Meanwhile there is a growing signal
for some energy within northwesterly Bering Sea flow to stream
underneath the mainland upper low and drop into the eastern
Pacific mean trough. The 18Z GFS eventually uses this feature to
develop an extra low near Kodiak Island but upstream
Pacific/Aleutians flow in that run compares very poorly to other
guidance.
Thus far the ensemble means have been the most consistent over
recent days for the system expected to track into the Aleutians
and possibly the Bering Sea. Operational models are showing what
could be a more complex evolution with an initial western Pacific
upper low supporting the initial surface system and then upstream
troughing/upper low from Siberia/Kamchatka producing some
interaction, with the possibilities of two distinct waves or some
form of merger/reinforcement. Again favoring as many elements of
continuity as possible, the forecast trends the early-period blend
more toward the 12Z GEFS/00Z ECMWF means by days 7-8 next weekend.
As soon as earlier-period features become less of an issue, there
are some elements of the 12Z GFS/CMC and 00Z ECMWF that fit the
general Bering Sea/Aleutians pattern so the blend incorporated a
small minority of those runs with the means late in the period.
The 12Z ECMWF mean has trended more phased with the overall upper
trough, speeding up its surface low, while the 18Z GFS becomes
suspiciously fast with the leading wave.
...Weather/Threats Highlights...
Around midweek the combination of the upper low over the western
mainland and farther south an upper shear axis/inverted surface
trough extending from low pressure initially south of Kodiak
Island should produce areas of precipitation. Expect highest
totals near the eastern Alaska Peninsula and Kenai Peninsula but
localized significant totals could also be possible farther north
depending on specifics of the upper low. After Wednesday it is
most likely that coverage and intensity of precipitation will
gradually decrease as the upper low either weakens in place or
departs and low pressure tracks southeastward. There is still a
small chance for more persistent and heavier precipitation along
the southern coast. Upstream Pacific low pressure should bring a
period of rain and possibly locally brisk winds to the Aleutians
after midweek into the weekend. Some of this moisture could reach
the far southwestern mainland by next weekend.
A majority of the state should see below normal highs during the
period though with a tendency for some moderation to move in from
the southwest by next weekend. For lows, the north and southwest
have the best potential for above normal readings while other
parts of the west through some portions of the Interior and
Panhandle should be below normal.
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
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