Scientific Forecaster Discussion
fxak68 pafc 280111
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
436 PM akdt Sat may 27 2017
Analysis and upper levels...
water vapor and infrared satellite imagery shows a deep stream of
moisture advecting north across the Gulf of Alaska between an
upper low center southwest of Kodiak Island and high pressure
along the northeast Pacific. Clouds overspread the southern
Mainland this morning with a few isolated showers across the
region as weak disturbances moved ahead of the upper low. This
system is currently stacked with a 1006 mb surface center pushing
a front into the southern Gulf this afternoon. Marine winds are
primarily southeast in the small craft range along the Gulf, with
lighter east flow along the Kenai Peninsula and inlet. Inland,
winds are generally out of the south this afternoon with a few
gusts along the Knik Arm and across the Copper River.
To the west...scattered showers are developing across the
southwest this afternoon as a short wave moves inland from the
Bering. A ridge parked over the central Bering is trapping low
clouds over the central and eastern waters with a front pushing
along its western periphery.
the main challenge in the forecast through Monday is the timing
of the front moving up from Kodiak Island. Models are in good
agreement with the general synoptic pattern through the short
term, however there are some minor differences on how the low
pressure system near Kodiak lifts north on Sunday and Monday. Only
minor changes were made as trends continue to bring the main
frontal passage across southcentral late Sunday night into Monday
morning. The most noticeable changes in the inherited forecast
were wind adjustments for southcentral as the ridge building along
the northeast Gulf progressed slower than anticipated today.
A few changes were also made along the western Bering for winds
as all models came into agreement on a surface low center tracking
along the western waters to the central Aleutians Sunday into
mid level clouds will continue to stream in from the Gulf as the
next front moves up from Kodiak Island. A few showers are expected
to develop in the vicinity, however VFR conditions are expected to
prevail through the next taf package as southeasterly flow
downslopes rain chances over the terminal. Winds are going to be
light until the front begins to push north. There is still
uncertainty of the timing of the front, however all models are
currently showing the frontal passage late Sunday night.
Short term forecast southcentral Alaska (days 1 through 3)...
An upper trough is digging into the npac southwest of Kodiak
Island this evening. A plume of warm air advection and a leading
surface warm front are moving into the north Gulf, and this will
increase moisture and precipitation rates along the coast. Cross-
barrier flow will be rather efficient at downsloping most regions
downwind of the coastal mountains and the chugach/Kenai Mountains,
but some light showers will be possible given the mostly southerly
flow aloft and deep layer warm air advection through early
tomorrow morning. Heavy precipitation in excess of 2-4" will
pummel the coast through Sunday night before tapering off Monday
morning as the upper trough lifts north.
Monday will be more of a challenge as the shortwave trough lifts
north into southcentral. The main feed of deep layer moisture will
cutoff, but so will downsloping. Forcing for mass ascent along
the trough axis will help support some inland rain/showers for a
brief period Monday, but timing of the upper low is still slightly
uncertain at this time.
Winds with this system will help accelerate increasing southeast
gap winds, especially through favored areas like Turnagain Arm and
gaps in the chugach/Kenai Mountains. Winds will also increase to
25 to 30 knots through Prince William Sound, so small craft
boaters should prepare wisely. The gradient will weaken late
Short term forecast southwest Alaska...
a stationary trough extending south through southwest Alaska will
keep an unstable environment over the area allowing for showers to
continue through the weekend. With this setup there is also a
chance for thunderstorms to develop around the Kilbuck Mountains
this afternoon/evening. As an upper level low south of the Alaska
Peninsula tracks north over southwest Alaska more thunderstorms
are possible in the corridor from Bristol Bay to the lower
Kuskokwim valley Sunday afternoon/evening and Monday
afternoon/evening. As the low diminishes and continues north,
showers will diminish around southwest Alaska Tuesday.
Short term forecast Bering Sea/Aleutians...
ridging over the central Bering will slowly track east through
Monday keeping low clouds over the area and also keeping precip
off to the west. The ridging will block a low over the western
Bering from pushing east which will cause it to stall near the
western Aleutians into Monday night bringing rain to the area.
Long term forecast (days 3 through 7)...
The unsettled period of weather that appears rather fall-like in
appearance will likely continue through next week into next weekend.
Although models are having a hard time pinpointing individual
features, the trend of the models continues to keep much of Alaska
under some form of cyclonic flow whether it be a trough or an upper
level low. There is some encouraging news in the extended as 850 mb
temperatures are expected to warm through Wednesday which will help
stabilize the atmosphere and likely cause a break of a day or two
without too much widespread shower activity. However, by Thursday
models prog another cold air surge advected back into southern
Alaska as a low in the Bering Sea dives south into the Gulf of
Alaska. This will likely increase shower coverage and intensity
Friday and through the weekend. The European model (ecmwf) is by far the most
optimistic model by trying to bring longwave ridging into Alaska
next weekend but unfortunately the trends of the models have
continued to be wetter so persistence seems like the right way to go
until the models start agreeing on a synoptic pattern change.