Scientific Forecaster Discussion
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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
432 am akdt Thu Jun 21 2018
Analysis and upper levels...
The large scale synoptic pattern has not changed much from the
previous four days with a persistent upper level trough still
over the eastern Bering Sea. A large ridge remains over the Yukon
and western Canada, and this ridge is now amplifying into
northern Alaska. Water vapor satellite imagery depicts a broad
shield of cirrus moving over the state ahead of a shortwave trough
which is moving north-northwest into the Gulf of Alaska. The low level warm
front is moving into southwest Alaska this morning with a very
extensive shield of rain moving into that region. Kodiak Island is
also directly under the front which is bringing widespread rain
to the island. Rain/showers are also starting to Blossom over
parts of southcentral, and this will continue to increase heading
through the day.
the numerical weather models are in excellent agreement with the
general large scale synoptic pattern through the weekend. They
continue, however, to struggle with a series of low amplitude
waves which are expected to move into the western Aleutians
beginning Saturday, then merge with a pre-existing Bering Sea low.
However, the details of the surface low development do not really
get Muddy until Monday and honestly, the impacts of these
differences are rather small. Otherwise, the bigger short term
challenge will be showers and a slight chance of isolated
thunderstorms this afternoon and again Friday as the upper trough
moves through. The NAM and GFS were favored for this forecast,
which shows more shower threats through Friday morning along with
isolated thunderstorms in northern interior valleys and mountains.
panc...a front moving inland this afternoon will increase the risk
for showers and will also enhance gusty southeast turnagain winds
through the terminal. These winds will persist through the late
evening before diminishing. Showers will continue through Friday
morning, but VFR conditions are anticipated to prevail.
Short term forecast southcentral Alaska (days 1 and 2)...
a shortwave associated with the nearly stationary upper low over
the eastern Bering will cause rainfall over the western Gulf to
spread northward into southcentral for this afternoon. The
expected timing of this shortwave has gradually slowed over the
past day or so, and is now slow enough to bring weak instability
to much of southcentral, and especially in the Copper River basin
where there is outside chance of seeing a strike or two along the
Wrangell Mountains this afternoon. This was, however, left out of
the forecast for now with extensive cloudcover in the area that
should limit daytime heating to to prevent thunderstorm
Over the Anchorage bowl and Matanuska Valley these showers are
initially not expected to be very heavy or long lived given deep
southeasterly flow along with gusty winds through Turnagain Arm
and the Knik River Valley ahead of the shortwave. This will,
however, be a different story further south over the Kenai
Peninsula which should expect widespread rains this evening,
which will then spread northward towards the Anchorage bowl/mat
valley overnight and into Friday morning. Most of the area should
then begin to see slowly clearing skies by Friday afternoon as
the shortwave lifts northward into the interior, though with
continued cyclonic southerly flow the risk of a stray shower or two
will persist for most areas into Saturday.
Short term forecast southwest Alaska (days 1 and 2)...
weak low pressure over the eastern Bering will redevelop over the
northern Bering tonight as it phases with the wave currently
tracking through the southwest Mainland. Under the cyclonic flow of
the redeveloping low, the southwest Mainland will transition from a
rainy southeasterly flow pattern to a showery southwesterly pattern
by tonight. The showery conditions will continue through Friday,
with a secondary peak in coverage this afternoon and evening. Shower
chances then taper off to more of an isolated to scattered coverage
on Friday as the upper low begins to move away. Isolated
thunderstorms will be possible in small portions of the forecast
area each afternoon and evening through Friday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
weak low pressure over the eastern Bering will redevelop over the
northern Bering tonight, as it phases with the wave currently
tracking through the southwest Mainland. This system will remain the
primary weather feature through Friday as it broadens and shifts
gradually west into the central Bering. A Pacific frontal system
will reach the western Aleutians late Friday night and central
Aleutians Saturday. This system will bring rain, gusty winds, and
some warmer temperatures to the western and central Aleutians/Bering
for the start of the weekend.
Long term forecast (days 4 through 8)...
Models continue to be in good agreement with the overall synoptic
pattern through the long term, especially over the northern Gulf
and southcentral Alaska. A low in the northeast Pacific will
continue its trek toward the Alaska Panhandle for the weekend as a
ridge of high pressure slides into the northern Gulf. Over the
interior, a thermal trough will set up roughly along the Alaska
Range as an upper-level ridge retreats northward. This will also
establish a weak offshore flow, allowing for a general warming
trend across southern Alaska. A series of upper-level shortwaves
will move across southern Alaska around both the cyclonic
circulation over the Bering and the low over the Alaska Panhandle.
This will aid in the instability and result in scattered afternoon
and evening clouds and showers throughout southern Alaska. The
ridge begins to break down Tuesday as the next low advances from
Over the Bering, the occluded low will continue to slowly break
down and retreat to the northwest as high pressure spreads across
southwest Alaska over the weekend. The weak, cyclonic flow aloft,
however, will still provide enough moisture and instability for
the build-up of late day clouds and showers across interior
locations. A low moving into the southern Bering from the western
Aleutians on Sunday will help reestablish the upper-level low
across the region. This trough will slide east back over the eastern
Bering on Monday as a surface low tracks toward the akpen for
Tuesday. There is still uncertainty in the timing and placement of
the surface low and its upper-level companion with the GFS favoring
a more progressive solution over the ec. GFS placement of the trough
axis is also negatively tilted and farther south than the ec,
bringing more moisture into southwest Alaska by late Monday.
Nonetheless, the developing pattern favors unsettled weather
returning to the Bering and Aleutians by the start of next week.
public...flood advisory 145.