Flood in Alaska, August 10 to 11, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
306 PM AKDT Sun Aug 9 2020

.SYNOPSIS...A gale force low in the northern gulf tracks
northeast onshore just east of Yakutat tonight. An associated
front moves ashore of the Panhandle this evening. A ridge of high
pressure builds across the Inner Channels through Tuesday.

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.SHORT TERM...Some wind damage reports have trickled in this
afternoon as windy conditions have been reported all afternoon
from Sitka north to Juneau and other areas of the central
Panhandle. Because of the low's track slightly south of forecast,
the northern Panhandle has escaped wind thus far, although rain
has efficiently pumped into Upper Lynn Canal as other spots.

Winds should begin calming down into early Monday morning, except
for Skagway where winds may continue through Monday due to
ridging behind the system. While the rest of the Panhandle may not
experience strong winds Monday, we do expect breezy conditions to
persist. Showers will hold sway through Monday night but slowly
begin to diminish.

Few changes were needed as we heavily coordinated with RFC. We did
make some important changes to the northern Inner Channels, mainly
covered in the marine section.

.LONG TERM.../Tue through Sun as of 10 pm Sat/The forecast begins
with weak ridging over the panhandle. Onshore flow will keep
cloudy skies and chance showers in the forecast for most
locations, however anticyclonic flow should help minimize the
chances for rain. These conditions look like they will prevail for
most of the week. A short wave passing south of the panhandle on
Tuesday night into Wednesday may help to raise some PoP during
that time frame, however it's influence over the region varies
greatly depending on which model you choose to believe. For the
Tuesday/Wednesday time frame, favored a blend between the GFS and
ECMWF with a slow mix in of WPC guidance.

A late week front was where some decisions needed to be made. As
it was with the model suite yesterday, there was a lot of
disagreement on resolving this system; namely, that the GFS
wanted to bring it in on Friday, while the Euro wanted to bring it
in on Saturday and maybe even have it pass mostly harmlessly to
the south. Today's models line up a bit better with the GFS
slowing it down and Euro being fairly consistent with it's timing
from yesterday. Still a fair amount of uncertainty regarding the
exact track but this feature should have some impact on the
panhandle next weekend. While this may be "wishcasting", decided
to lean more toward the Euro to modify the later portions of the
forecast, not simply because of the continuity, but because this
forecaster was more pleased by the resultant forecast. This choice
left the forecast with declining PoPs Wednesday night through
Friday where they will not be zero, but will be in the 20s for
most of the day. This also presents the real possibility of
getting some sun during the day. This would be ideal, because I
have some overdue yardwork to accomplish. Going into Friday night,
cloud cover would begin to fill back in and PoPs will begin to
increase again over the panhandle ahead of the approaching front.
While still quite early to make any overly useful speculations
about what this front and the associated low pressure system will
bring to the panhandle, early looks suggest that it should not
have a lot of wind with it, and precipitation appears focused to
the south with decreasing PoPs the further north you go. Plenty of
time to re-evaluate the situation before it arrives though.

&&

.AVIATION...TAFs begin with mainly MVFR conditions with plenty of
southwest wind shear. Some tempos were added that take things to
IMC during heavier showers early on. Wind is mainly a concern for
the north.

&&

.MARINE...The low tracked a little farther south (in line with
past trends in the last 48 hours) than predicted. These slight
changes have greatly impacted the wind forecast for the northern
Inner Channels. First, we delayed the westerly onset of Cross
Sound/Icy Strait corridor. We also held back the strongest winds
of northern Lynn Canal until late tonight. Confidence is not good,
but we could not ignore northerly gradients until late tonight.
It should be noted that subtle differences in low track and speed
of its onshore landing will result in big forecast changes

&&

.HYDROLOGY...With the most favored model (the NAM) indicating
high chances of rainfall amounts exceeding 10-year recurrence
intervals between the 00Z and 12Z as well as the 06Z and 12Z time
blocks, we felt the need to issue a Flood Watch for Juneau area
rivers and streams. Mendenhall Lake gauges have been steadily
climbing all day. And with such a large basin, we elected to
extend the flood watch through all of Monday morning. Should minor
flooding develop, the most likely window of impacts will be after
midnight to noon Monday.

The low itself has farmed a deep slug of moisture from the
atmospheric river that earlier blew to our southeast and brought
Ketchikan and the rest of the Panhandle such heavy rain earlier
this morning. CIRA Layer moisture indicates an air mass across
the gulf poised to enter the northern and central Panhandle with
deep moisture all the way up to 300 mb. With strong south to
southwest flow interacting with the mountains we are expecting
some efficient rain producers. In addition, MIRS rain rate product
indicate a band of heavy rain from Sitka northwest to Yakutat
along the coast falling at 3 tenths per hour. Greater rates could
occur as these bands interact with the terrain in such a
perpendicular fashion. IVT values with this system continue to
push past 500 kg/m/s, mainly from Pelican to Sitka over to Juneau
this evening.

A Special Weather Statement covers other area rivers and streams
across the north. While incorporating the wind threat, the SPS
notes bankfull conditions for smaller rivers and streams across
the north tonight.

Should trends of rainfall jive with predictions this evening, we
will issue a Flood Advisory as confidence rises.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Flood Watch through Monday morning for AKZ025.
     Strong Wind through late tonight for AKZ025.
     Strong Wind until 1 AM AKDT Monday for AKZ022.
     Strong Wind until 1 AM AKDT Monday for AKZ020-023.
     Strong Wind from late tonight through Monday afternoon for
     AKZ018.
     Strong Wind from 1 AM AKDT Monday through late tonight for
     AKZ019.
MARINE...Gale Warning for PKZ012-013-021-022-031-032-042-043-051-052.
     Small Craft Advisory for PKZ033>035-041-053.

&&

JWA/JDR

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE JUNEAU AK
259 AM AKDT MON AUG 10 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0145 PM     NON-TSTM WND DMG 1 S MENDENHALL GLACIER  58.40N 134.55W
08/09/2020                                      AK   UTILITY COMPANY

            POWER OUTAGE IN THE MENDENHALL VALLEY DUE TO TREE IN THE
            LINES NEAR GLACIER SPUR ROAD.

&&

EAL