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River Forecast Center

Public Information Statement - River Forecast Center 


NOAK48 PAFC 262254

Public Information Statement
National Weather Service Anchorage AK
254 PM AKDT Mon Oct 26 2020

...Public Information Statement...

What happened to all of the snow? It appears to have been a
combination of circumstances that resulted in the meager snow
totals most of Southcentral saw last night into this morning.
First and foremost, there was a distinct lack of cold air anywhere
in the area. Normally when a low this strong takes the track it
did, it is able to pull in cold air already over the area, or can
move it southward out of the Susitna Valley, or in extreme cases,
from the far side of the Alaska Range. For this particular event,
all stations on both sides of the Alaska Range were in the mid-
30s for the entire event. For much of Sunday night, winds across
all of Southcentral were northerly. Had cold air been present,
this would have changed the rain into snow. In this case, the
northerly winds might actually have worked against there being
snow, as any cooler air from melting or just overnight radiational
cooling was moved away and replaced with more air that was above

Second, this was a very moisture-laden event. At the Anchorage
Forecast Office, and Ted Stevens, almost exactly an inch of liquid
fell during this event. Had it all been snow, that could have
resulted in 6 to 10 inches of snow, which was a bit above the
highest forecast. On Sunday evening, a period of heavier
precipitation was expected. Despite the lack of cold air, heavy
snow can work to bring melting levels down. This is through the
melting process, which takes heat from the surrounding atmosphere,
absorbs that heat into the snowflake, which in turn melts it, but
also cools the surrounding air. With successive forecast model
runs leading up to this event, the overall trend was for total
precipitation to increase with each run, which once again, favors
more areas seeing snow at lower elevations. So the forecast trend
was for more snow. However, once again, the combination of no
outside cold air and the wind blowing away the cooled air due to
melting appear to have been enough to keep the precipitation
mostly rain.

Third, the low itself brought in a significant amount of warm air
with it and was quite strong as it reached the north Gulf Coast.
Ahead of the low, there was a period of downsloping southeasterly
flow which worked to warm the atmosphere ahead of the low, and
once again there was no cold air for the "cold" side of the low.
Thus, the low`s transporting warm air ahead of the storm track
likely required more cooling from melting or radiational cooling
than the atmosphere could handle, despite the Cook Inlet region
being on the "cold" side of the low from start to finish. Strong
lows that move into Prince William Sound frequently cause
downslope drying in Anchorage, the western Kenai Peninsula and the
Mat-Su. With this storm, the downslope drying happened early, but
died off quickly as the abundant moisture overwhelmed the
weakening downslope winds, which never completely died off, but
shifted to mountain top level.

Fourth, this morning, the low moved into Prince William Sound, and
a good amount of that energy has now moved into the Copper River
Basin. The low center tracking north of our latitude allowed the
return flow behind the low to turn south to southwesterly along
the Kenai Peninsula and Anchorage. Once again, under normal
circumstances, the southerly flow is bringing a fresh cold air
mass from Southwest Alaska into Southcentral. However, since
there was no cold air there either, that south to southwest flow
warmed as it tracked up the Inlet, where water temperatures this
morning are in the mid 40s. Thus, when that southerly wind reached
Anchorage, the snow early this morning changed back over to rain
and the temperature warmed 2 to 3 degrees in 30 minutes.

In summary, this storm will definitely be one for the case study
archives. Thanks to the many of you who submitted Spotter
observations to help in this regard! It was a great lesson in
learning if a complete absence of cold air, abundant moisture, and
changeable low level winds are enough to cause large amounts of
snow. For most of Southcentral, the answer is a resounding no.
Confidence in precipitation type was never above the "low"
category, and for several days leading up to the event, the
resounding question is how do you decide between a 31 degree snow
and a 33 degree rain? The range of options for this storm at one
point was all rain to 20 inches of snow, and everywhere in
between. Cold air reigns supreme.