Scientific Forecaster Discussion
fxak68 pafc 271247
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
447 am akdt Tue Jun 27 2017
Analysis and upper levels...
A vertically stacked low spinning across the central Bering near
the pribilofs is the driving feature for much of the weather
across southern Alaska this morning. East of the low, broad
southerly flow is driving wave after wave into southern Alaska
from off the Pacific. Each wave is bringing more moisture and
precipitation to the area. The next wave is approaching Kodiak
Island this morning with widespread 15-20 kt winds across the
coastal waters. Elsewhere, nearly all of the rest of southern
Alaska remains under overcast skies.
The models are in good agreement through Wednesday night for much
of southern Alaska. The primary Point of disagreement is with
regards to the next low moving in the southwestern Gulf late
Wednesday night. The NAM and ec are further north, but the NAM
elongates the low, while all of the other models keep a deeper
closed low. The GFS and Canadian are slightly further south and
because they are nearly identical with positioning and strength,
were the models of choice. Forecast confidence is high.
Panc...VFR conditions will persist through Wednesday morning. The
Turnagain Arm jet will also persist south of the Airport with
variable strength through Wednesday morning. While the core of the
jet is not expected to reach the Airport, there is potential,
particularly after 05z, for stronger associated gusts as the jet
bends closer to the Airport. If the core does reach the Airport,
gusts could exceed 30 knots, but the possibility also exists that
the winds never get into west Anchorage. Scattered showers will
also be in the area through the morning, with increased
downsloping expected to reduce the rain chances through the day.
Short term forecast southcentral Alaska (days 1 and 2)...
a weather front will push into Kodiak this morning, to the Gulf
Coast by midday, then inland this afternoon. This will bring
another round of rain and/or showers to the area. The highest
rainfall amounts will be along the coast and coastal mountains.
Less precipitation will fall inland, with only some isolated
showers in the the Copper River basin. The system will move north
of the area by late Wednesday night, with just some some showers
lingering along the coast by Wednesday afternoon. The next system
will move into the Gulf Wednesday afternoon, reaching the Gulf
Coast Wednesday night. This next system will develop into a closed
upper low, and have a defined surface low as well. This will push
the flow aloft to more from the southeast rather than the south,
allowing inland areas to be a bit drier.
Short term forecast southwest Alaska (days 1 and 2)...
A front draped across Mainland southwest Alaska is slowly pushing
north bringing widespread rain to most of the area today. The
front will continue to weaken through the day as it pulls out of
the area it loses upper level support, leading to a brief break in
the widespread precipitation overnight. Another shortwave
rotating around the parent low in the Bering Sea comes ashore
Wednesday morning bringing more rain to the area, especially to
the Kuskokwim River Delta and the Bethel area. Easterly low level
winds wrapping around the system will be funneled through terrain
gaps in the northern akpen, including Kamishak Bay across lake
Iliamna, leading to gusty winds in those areas through Wednesday.
Multiple systems wrapping around a longwave upper level trough in
the Bering will bring continued cool and wet conditions to the
area through at least the end of the week.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
A broad low over the central Bering Sea, currently centered near
the Pribilof Islands, will slowly lift northwest through the
Bering Strait. Rain showers will impact most of the central and
eastern Aleutians and pribilofs through Tuesday night. Cyclonic
flow and the associated winds will weaken as the surface low
fills. A weak low pressure system will form just south of the
western Aleutians by Wednesday night and track to the east
bringing increased chances of precipitation to the Central Island
chain through the end of the week.
Long term forecast (days 3 through 7)...
Friday through the weekend remains in a progressive wet pattern
across the Bering and the Gulf of Alaska as the jet stream along
50n Ushers low pressure along a fairly strong jet streak. Multiple
short waves are expected to move inland through the weekend
bringing higher chances for rain. Winds will generally be under
small craft for area marines as the upper level systems move
quickly and lack surface organization. Operational models struggle
to handle the fast moving shortwaves in the extended forecast;
therefore only minor edits were made for the upcoming weekend
into early next week. Wpc data used update the current package is
a fairly even blend of the GFS and European model (ecmwf) including the ensembles.