Scientific Forecaster Discussion
fxak68 pafc 231316
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
516 am akdt Sat Jun 23 2018
Analysis and upper levels...
the persistent Bering Sea trough is finally beginning to weaken
and retrograde westward this morning. Weak upper level southerly
flow continues this morning over southern Alaska, but this is no
longer connected to the moisture stream, so showers and rain have
largely ended with only low level clouds remaining. Water vapor
satellite imagery depicts a rather impressive looking low
amplitude shortwave lifting north along the alcan. This shortwave
will eventually become a weather maker for southcentral this
weekend in the form of an "easterly wave". Near 50n south of the
Gulf of Alaska, an impressive cyclone is moving toward the
Panhandle while a warm front is moving toward the far western
the numerical weather models are in excellent agreement with the
general synoptic pattern heading deep into the next week. However,
they are performing terribly with a series of easterly waves
which are now expected to impact most of interior southcentral
through the weekend. The trend in the numerical models has
continued to be stronger which each set of runs, although there
are still disparities in the individual models. Net effect,
expect more clouds and precipitation chances inland this weekend,
especially over the Copper Basin and into the Susitna Valley. A
mix of all the models was favored for this forecast to account for
the uncertainty on the track/strength of the easterly waves
moving across southcentral this weekend into early next week.
panc...MVFR stratus is going to float in and out of the terminal
this morning before likely scattering back out in the afternoon.
Shower threats will move into the terminal overnight and into
Sunday morning, but there is low confidence on this happening at
this time. Winds will be variable and less than 10 knots.
Short term forecast southcentral Alaska (today through monday)...
showers moving over the south Mainland from a series of easterly
waves will be the primary weather makers this weekend. The first
round of showers is already developing over the Copper River
basin, which will continue developing over the course of this
morning as it spreads westward this evening. For the most part
though, conditions should remain generally cloudy but dry other
than a stray shower or two from Kenai Peninsula northward into the
Anchorage bowl/matsu valley's until late this evening. The models
have all trended stronger with the first easterly wave beginning
late tonight into Sunday, allowing for increasing chances of
showers to the west of the talkeetnas into Sunday. At this point
it appears that the lions share of the wetting rains should remain
over the Susitna Valley on Sunday, but it wouldn't be out of the
question for this to spread slightly further south into the
Matanuska Valley or even the Anchorage bowl as the NAM is
suggesting. Forecast confidence then begins to fall further on
Monday as the models are all over the place with the timing of
weak easterly waves that will continue pushing through the south
Mainland, so the pop forecast after this weekend is painted with a
Chances for thunderstorms will also be increasing as the weekend
progresses and instability steadily gets stronger with each
afternoon. Isolated thunderstorms were added to the forecast for
the wrangell's/Alaska Range this afternoon, which will then spread
westward into the talkeetnas for Sunday. Marginal instability
then persists with the continued easterly flow on Monday, so a
slight chance of thunderstorms was also added to the Copper River
basin with the morning forecast package.
Short term forecast southwest Alaska (today through monday)...
weak low pressure over the southwest Mainland will begin to
recede as some slightly higher pressure moves into the area
through the start of next week. This will likely set up a stagnant
pattern over the area, consisting of mostly cloudy conditions and
some showers and isolated thunderstorms along the Alaska Range
each afternoon and evening this weekend. The next front will
approach from the Bering Sea early next week, but will be
weakening as it approaches the southwest coast. Expect
temperatures slightly above normal through the weekend, and light
winds for the period.
Short term forecast Bering Sea/Aleutians (today through monday)...
a sub-gale-force frontal system tracking into the western and
central Aleutians today will continue to weaken as it pushes
further eastward through the weekend. Rain will spread across most
of the Aleutian chain and southern Bering Sea through the
weekend, shifting to the Alaska Peninsula by early next week.
Long term forecast (days 3 through 7)...
the long term forecast begins Monday night with main feature a
low in the eastern Bering Sea near the Pribilof Islands. The
deterministic models, ensemble runs, and wpc are in agreement with
this low position and intensity through the week. The models,
ensemble and wpc remain in good agreement with the resolution of
the ridge in the Gulf of Alaska being squeezed and pushed to the
east as a frontal low from the Bering Sea low pushes into the Gulf
late Wednesday. As we move out into the latter part of next week
the main low moves over the Mainland as a ridge builds in behind
it over the Bering Sea.
This all translates to a continuation of the cool cloudy wet
pattern over the southern Mainland. As the frontal low pushes
into the Gulf Wednesday, more rain and clouds can be expected
along the Gulf Coast to Kodiak Island.