Scientific Forecaster Discussion
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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
410 am akdt Tue Apr 25 2017
Analysis and upper levels...
a vertically stacked low centered over the central Aleutians is
providing the steering flow for much of the weather occurring over
southern Alaska this morning. Another wave of moisture
circulating around the low is over the Gulf and moving towards the
north Gulf Coast. Downsloping has largely kept the usual
downslope shadowed locations dry, but above mountain top level,
overcast skies are preventing much nighttime cooling. Most
locations in the southcentral are holding in the 40s. Temperatures
across southwest Alaska are slightly cooler, but a similar
scenario is playing out, with widespread cloud cover limiting
cooling. Scattered shower activity is present across Bristol Bay.
Over the Bering, once again it's the low over the central
Aleutians causing a slow swirl of the abundant cloud cover, with
off-and-on showers for the entire area.
with a stagnant weather pattern in place,
the models are all in good agreement with the movement (or lack
thereof) of the large- scale weather systems. The NAM was the
preferred model for its higher resolution. However, because any
shower activity over the area, especially southwest Alaska and the
Bering is being driven by even smaller scale processes, timing
where and when each shower will produce rain at the surface is
nearly impossible. The NAM however, gives the best approximation.
Besides precipitation chances, other forecast challenges in this
pattern include determining the strength and location of the
Turnagain Arm jet, which typically sets up with cross-barrier flow
through the mountains. Broad-scale forecast confidence is high
with local scale confidence somewhat lower.
panc...VFR conditions will continue to prevail although brief
periods of MVFR clouds are possible at times this morning. Turnagain
Arm winds may Bend north towards panc this afternoon for bringing
an increase in southeasterly winds.
Short term forecast southcentral Alaska (days 1 and 2)...
southeasterly flow aloft over south central Alaska will continue
today and then weaken this evening. Another "surge" of moisture
will push into the area today, with isolated to scattered showers,
more numerous along the Kenai Peninsula. This moisture will weaken
and move south tonight as weak high pressure builds into the
Mainland from the east. On Wednesday and Wednesday night the high
pressure will retreat northward. With limited moisture, most of
the showers will be in the higher elevations, with just isolated
showers for the inland valleys.
Short term forecast southwest Alaska (days 1 and 2)...
rain showers will be a little more prevalent today across the
Mainland. A shortwave propagating through the flow from the Gulf
will keep showers going this morning, mostly across the Bristol
Bay area. During the afternoon and evening hours, activity will
spread to the Kuskokwim Delta and valley. Both the colder air
aloft associated with the shortwave, and the warmer springtime
diurnal temperatures will enhance the shower activity today making
for some of the first convective activity of the season. Rain
showers will be much less widespread on Wednesday, but still
scattered in nature. Otherwise, expect mostly cloudy conditions
with high temperatures in the 40s to low 50s and generally light
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the broad low pressure system spinning over the central Bering
will make little movement over the next couple days. Scattered
rain showers and light winds will remain the norm. Areas of fog
can be expected from the Pribilof Islands and north/eastward over
Long term forecast (days 3 through 7)...
southwest flow will persist through the weekend and will keep
moderately unsettled weather across the region. Thursday and
Friday, a low pressure system will slide north and into Southeast Alaska.
In its wake will be an abundance of moisture and cloud cover
across southcentral Alaska. Friday and Saturday a closed Bering low
becomes an open wave over the southwest with a trough axis
swinging over southcentral which will bring increased chances for
rain across the southern Mainland.
Weak ridging will build in across the southern Mainland into the
weekend. Come Sunday, Southeast Alaska is expected to see a similar
scenario pan out as what will be happening weds/thurs. Numerical
guidance begins to struggle in day 5 and later frame with run
consistency issues and large ensemble spread across the Bering.
There are similarities in timing of an elongated frontal/trough
passage across the chain this weekend, but wildly different
solutions on how strong and where the attached low will be.