Scientific Forecaster Discussion
fxak68 pafc 200143
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
443 PM akst Tue Nov 19 2019
Analysis and upper levels...
a 964 mb low is positioned just southeast of Nunivak Island this
afternoon, with a fetch of tropical moisture feeding into
southwest and southcentral Alaska, with origins near Hawaii. The
warm air just above the surface has made it to the Nushagak hills,
eastward towards the Anchorage bowl, and then along the Prince
William Sound coast. South of that line, areas of freezing rain
have been observed, with significant icing reported near
The low near Dillingham is one of main lows passing through the
base of a trough that extended from the central being through the
central Aleutians. The model analysis indicates additional vort
maxes will round the base of this trough, allowing for additional
the models in the short term have struggled significantly over
the last 12 to 24 hours, with even hourly aviation and
precipitation type guidance varying from hour to hour with their
forecasts. This is due to the battle between the warm airmass
trying to push north through southwest and southcentral both at
the surface and 850 mb. This battle continues into the evening
hours for both locations, making who gets what type of
precipitation, where, and how much a major challenge.
The model solutions stabilize some for the Wednesday through
early Sunday period, before some discrepancies arise in their
panc...VFR conditions will continue through Wednesday morning,
then lower to MVFR in rain showers Wednesday afternoon. Some low level wind shear
is expected late this afternoon. Low level wind shear will end this evening as
gusty southerly winds kick in. Gusty winds are then expected
through Wednesday afternoon.
Short term forecast southcentral Alaska (days 1 and 2)...
a warmer air mass will keep temperatures moderated across much of
the region overnight through Thursday. However, the winter
weather warning for heavy snow across the Copper River basin,
Susitna Valley and Valdez were continued with the afternoon
package as temperatures will remain cooler in these areas to
support snow and/or a mix of precipitation types. The overall
weather pattern remains active as an upper level low currently
over the eastern Bering is replaced by another on Wednesday. There
is still a bit of uncertainty on the evolution of this second
system. Overall, expect a wetter pattern through Friday with gusty
conditions along the coasts.
Short term forecast southwest Alaska (days 1 and 2)...
an occluded front associated with a vertically stacked low
centered around 100 miles south of Nunivak Island has cleared
through much of southwest Alaska, and is currently oriented along
the Yukon Delta. Strong northeast winds and blizzard conditions
persist over Nunivak Island and along the northern Kuskokwim Delta
coast (hooper Bay and chevak), but conditions should improve
later this evening. Cold air aloft wrapping around the base of the
low will transition precipitation to showers later tonight. The
low will swing another cold front into Bristol Bay late tonight
into early Wednesday, which will increase precipitation rates.
There is some uncertainty with precipitation type in the lower
elevations of the Bristol Bay area. But, temperatures are
currently in the low 40s with widespread cloud cover, thus expect
precipitation to mostly be rain or a rain/snow mix. Precipitation
over the southwest Mainland will diminish Wednesday afternoon.
Another system will approach from the west Thursday and bring an
influx of warm, moist air from the south. As of now, there isn't
much concern for freezing rain Thursday. With no strong cold air
mass in place ahead of the front, there isn't much concern for
freezing rain in Bristol Bay, and this should primarily be a
rain/snow event. In the Kuskokwim valley/Delta, there should be
cold air in place, but given the uncertain trajectory of the warm
air mass, it's too soon to have a good idea on whether this will
be a freezing rain event. Be sure to monitor the forecast over the
next few cycles.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
gales have been observed over much of the eastern Bering and
south of the akpen as cold air wraps around the back side of a
vertically stacked low centered near Nunivak Island. In the
Pribilof Islands, snow with gusty winds has brought visibility
down to 1 to 2 miles over the last several hours. As the parent
low fills and shifts eastward, precipitation rates and winds will
diminish. A complex low near Kamchatka will push an embedded low
center towards the western Aleutians, Wednesday, and areas of
gales are possible south of the chain into Thursday. Otherwise,
expect high end small craft to low end gales in north flow over
the central Bering Wednesday into Thursday.
Marine (days 3 through 5)...
a vertically stacked area of low pressure in the eastern Bering
responsible for gales and storm force winds will be the main event
for early Friday morning. The 957 mb low will just be moving
ashore near 60n Friday morning with gales and a few storm winds in
the central Bering and gales across the western half of the Gulf
to include the coastal waters of southern Alaska and the akpen.
The surface low remains fairly stationary, drifting slightly
north, and fills to 970 mb by late Friday night with the gales in
the Gulf diminishing to a small area in the southern Gulf, while
northerly gales will persist across much of the central Bering and
into the eastern Aleutians until late Saturday night. While the
upper level flow remains rather active with zonal flow through
Sunday, an area of ridging builds into the central Bering on
Sunday before another low moves in from the Kamchatka peninsula.
The expected low will deepen to 953 mb by Monday morning bringing
gales and storms into the western Bering and Aleutians. Early
guidance suggests this low will track north of 60n again, with
some ridging in the Gulf of Alaska, but model discrepancies in
this active pattern leave some uncertainty 5 days and beyond.
Long term forecast (days 3 through 7)...
by Friday, a deep vertically stacked low will be centered over
the Kuskokwim Delta with the upper level portion of the low
extending to cover southwest Alaska as well as the central and
eastern Bering. To the west of the low, cold north to
northwesterly flow from Siberia will continue to spread south
across the Bering, Aleutians and Alaska Peninsula. Supported by a
strong west to southwesterly jet stream south of the Aleutians and
shortwave troughs rotating around the south side the upper level
low will continue to swing a series of fast moving frontal waves
along the Alaska Peninsula and into the Gulf.
The low over the Kuskokwim Delta will lift north over the Yukon
Delta Friday evening to be replaced by a second surface low
developing along the northern Kuskokwim Delta coast Friday night
and moving inland Saturday. A surface trough associated with this
second low will swing from north to south across the eastern
Bering Friday night through Saturday morning. A surface trough
will push east across the Alaska Peninsula and into southwest
Alaska Saturday afternoon through Saturday night as the upper low
opens to a trough. The upper level trough will then rotate away
to the northeast Sunday with a colder air mass pushing into
southwest Alaska behind it.
A transient upper level ridge will build into the western Bering
Saturday night followed quickly by an initial frontal system
swinging of the Kamchatka peninsula and reaching the western
Aleutians Sunday morning and central Aleutians Sunday night. A
stronger frontal system supported by a rapidly developing and
rather deep surface low will follow quickly behind, reaching the
western Aleutians and western Bering late Sunday night through
Monday morning and central Aleutians and Bering by Monday
afternoon. The front will reach the eastern Bering Monday night
and push onshore into southwest Alaska Tuesday morning. The front
will continue east into the Gulf and southcentral Alaska Tuesday
afternoon and evening. A new surface low will spin up just
offshore of the Kenai Peninsula Tuesday evening and strengthen
rapidly as it rotates trough the northern Gulf Tuesday night
through Wednesday morning.
Marine...Gale Warning 125 132 138 150 155 165 173 175>177 181 185.