Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
435 PM akdt Friday Apr 24 2015
Analysis and upper levels...
the main low is anchored in the eastern Bering Sea with a trough
that extends to the Kodiak Island area into the Gulf of Alaska. A
weak ridge of high pressure is building into south central
Alaska. Another trough extends from the low center into the
interior. This is resulting in circulation around this low
rotating into southwest Alaska to Kodiak Island and the southern
Cook Inlet region. The stronger cross barrier flow across the
Aleutians mountains and Alaska Range is resulting in the typical
rain shadowing on the Lee side of the mountains. The jet stream
remains well south of the Aleutians. Overall this will continue to
result in the showery weather across much of the Bering and
Aleutians to southwest Alaska. The southern Mainland east of the
Alaska Range will remain in a relatively benign pattern with the
exception of increased fire weather concerns.
the models remain in excellent agreement with the system moving
from the Bering Sea into the Gulf through this weekend. The next
system on tap for the Bering Sea is not handled as well. We are
running with the ec/NAM theme. The main issue with the next
system moving into the Bering has to do with the low center
toward Sunday night. This will be discussed in the extended section
of this discussion.
Short term forecast southcentral Alaska (days 1 and 2)...
for the upcoming weekend dry conditions will prevail across the
southern Mainland with wetter conditions across the Gulf...including
Kodiak Island and the eastern/southern Kenai. The initial concern
Friday into early Saturday is heightened fire weather danger
across the Matanuska Valley and Anchorage bowl. Dry conditions
across southcentral tonight is driven by a ridge passing from the
northern Gulf to the Copper River valley. Relative humidity values
will be slow to recover with weak ridging staying in place until
the next surface low advects moisture from the Gulf. The red flag
warning for the Matanuska Valley has been extended for the
Matanuska Valley through Saturday afternoon due to the prolonged
low relative humidity. The red flag warning for strong wind across
the Anchorage area was left intact and is set to expire at 10 PM.
Winds are expected to taper down as the ridge axis shifts to the
Copper River basin.
The second concern this weekend is focused over the Gulf as the
next surface low develops tonight...pushing a front into the
southern Gulf early Saturday. This front slowly progresses north
through the day as the upper level system shifts from the Alaska
Peninsula to the western Gulf. Gusty gale winds are expected along
the front along with rain. The northern extent of this system is
still uncertain and may bring stronger winds for a longer duration
across the northern Gulf. Precipitation should remain along the
Gulf and coast areas...however isolated activity may work inland
along weak upper level waves rotating around the main low center.
Short term forecast southwest Alaska (days 1 and 2)...
the low centered just east of the Pribilof Islands will slide off
to the east-southeast tonight and Saturday. The continued south
and southwest flow is pumping moisture into southwest Alaska
feeding the showery regime. These showers are predominantly rain
though some snow may mix in overnight at higher elevation
locations. The southeasterly winds will shift toward the north and
a drying and warming trend will begin over this weekend.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a ridge building in behind the low vacating the eastern Bering is
pushing gusty west and northwest winds keeping any fog from
forming. Showers are wrapping around the low center into the
eastern Aleutians across the Alaska Peninsula. Expect the showers
to persist in the short term in the northwest wrap around flow
across the eastern Aleutians and Alaska Peninsula.
A low is expected to move into the western Bering Sea Saturday
night leading to another round of wind and rain across the western
critically low humidity is expected across the Matanuska Valley
this afternoon and evening. Dry air around three to five thousand
feet is expected to persist past midnight holding back the
humidity recovery overnight until near the early morning hours. A
ridge building in along the north Gulf Coast will result in
increased east-southeast wind across the Chugach Mountains. Dry
conditions will occur again Saturday afternoon and evening. Low
humidity combined with the increased southeast wind across the
Anchorage area will result in near critical thresholds being
reached as the humidity will drop to around 16 to 25 percent
across most of Anchorage. The hillside will likely see wind gust
approaching 30 miles per hour late this afternoon and evening. Humidity
recovery will be slow across the area as dry low level air will be
in place well after midnight. The Copper River basin will remain
generally cloud free and thus will be warm and dry again today.
Minimum humidities will again approach critical values this
afternoon and evening. Humidity recoveries will be slow and not
occur until close to the early morning hours.
Long term forecast (days 3 through 7)...
good agreement continues concerning the Gulf low with a consensus
that it will remain in the southern Gulf into early next week.
The instability caused by this low over the southcentral will
allow opportunity for light showers. Moving into the middle of
next week discrepancies develop as to what to do with the low.
Some models try to move it inland while others keep it in the
Gulf. Either way instability will remain resulting in more light
Out west the models are coming into better agreement with the low
moving across the Bering on Monday but some differences remain
with its exact position. Either way it is expected to be a weaker
low bringing showers to the Aleutians as it moves through. By middle
next week there are much larger discrepancies between the guidance
so the ensembles were preferred which keeps low pressure in the
marine...gale 170 172-178
fire weather...red flag warning 101 111.
Synopsis and model discussion...surface observation
southwest Alaska/Bering Sea/Aleutians...surface observation