Scientific Forecaster Discussion
fxak68 pafc 180217
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
517 PM akst sun Nov 17 2019
Analysis and upper levels...
the 500 mb low that was a large factor in saturday's snowfall in
southcentral has now moved northeast to near the alcan border in
the interior of the state. The next upper level low is digging
southward this afternoon over the Kuskokwim valley into the
Bristol Bay region. The 850 mb temperatures with this low are
quite a bit colder than the previous system as well. This is the
trough that will move rapidly eastward overnight and make it to
the alcan border by Monday morning.
The large low that is that start of a very complex and active
weather pattern this week can be seen on satellite moving into
the central Aleutians this afternoon. Farther southwest, another
smaller low can be seen. The 250 mb jet is running just south of
these two lows and digging southward under the second, currently
weaker one. This will cause the stronger low to the east to weaken
and while the jet strengthens the smaller one to the southwest and
make it the main low tomorrow.
models are becoming better aligned with the track of the low
that is rapidly intensifying near the Alaska Peninsula Monday
night. Before the 12z runs, we could not even pin it down to that
location at that time. All models are now trending toward the
solution the NAM had yesterday with slowing the progression of the
low inland after it moves onshore in the vicinity of Cape
Newenham Tuesday night. Previous model runs had some support for a
fast push inland, but this is looking much less likely which is
allowing for forecast decisions to be made with higher confidence
for both southwest and southcentral Alaska.
panc...the biggest question for the taf this evening is whether
or not there will be snow showers at the Airport due to an upper
level wave moving up from the southwest. Even if there are some,
conditions are not expected to drop below MVFR conditions for
ceilings or visibility in the snow showers. After these pass by
midnight, there is the potential for MVFR ceilings to develop and
persist into Monday morning due.
Short term forecast southcentral Alaska (days 1 and 2)...
For tonight, another dusting of light snow (1"-2") is possible
across the region as an upper level disturbance moves across from
west to east, however accumulations will mainly be over the mat-
su valleys. On Monday, the next wave of energy ejects from low
pressure lifting over the Bering. Precipitation will be largely
confined to the Gulf and surrounding coasts as the upper wave
shears apart quickly. For inland locations, Monday will be the
break between systems.
The next system organizing late Monday will impact the region on
Tuesday and Wednesday, bringing precipitation challenges and gusty
winds. The front with this storm system pushes across Kodiak
Island late Monday, spreading rain and gusty gale force winds to
marine areas from Kodiak to the north coastal areas by early
Tuesday morning. The surface front will stall along the coast as
the associated surface low lifts across the Alaska Peninsula mid
Tuesday morning. Gusty south to southeasterly winds will increase
along the front and then persist through Wednesday. Winds will
also increase for typical channeled terrain such as the Turnagain
Arm and hillside, Portage Valley, Matanuska Valley and Copper
Most of the inland locations will see a few light showers on
Tuesday as snow in the morning mixing with or changing over to rain
during the afternoon. The coastal locations and the Kenai should
warm steadily with temperatures climbing over freezing by the
afternoon. This warming trend is also expected for inland
areas...although to a lesser extent. The exception to this will be
along the western Alaska Range where a cold air mass at the base
of the mountains will support all snow. We are currently looking
at heavy snow falling across the Susitna Valley on tuesday;
although, there is some uncertainty on how far north warm air
intrudes from the Gulf. There is better certainty that the cold
pool holds north and west of Talkeetna. Another challenge presents
across the Copper River basin...the cold pool across this region
will be over-run initially by deep southerly flow from the
subtropical air mass. With better upper level dynamic support, the
warmer air may mix down to the surface more efficiently.
Precipitation will start as snow with freezing rain and rain
likely from Tonsina south. Heavy snow may also bring another
significant round of accumulations Tuesday into
Wednesday...especially along the southern slopes of the Alaska
Short term forecast southwest Alaska (days 1 through 3)...
(tonight through Tuesday evening)
The proverbial "calm before the storm" best sums up the weather
through Monday afternoon. A rapidly deepening area of low pressure
will move from the North Pacific into the Bristol Bay region.
There remains some questions as to what exact track the low will
take, and how strong is its central pressure. That said, the
general consensus is for a strong storm to move into the region.
This system will usher a very warm and moist air into the region,
as it has a strong tropical feed.
Temperatures at 850 mb (roughly4400 feet) increase to as much as
+6 degrees celsius in some locations. Meanwhile, colder air
entrenched at the surface will be reinforced by a northeast wind.
The question remains how far north does the surface warm front,
and 850 mb elevated warm front, make it north. The elevated front
will delineate which areas immediately along and to its south see
a period of freezing rain or sleet, and for what duration.
Locations south of the surface front will warm above freezing,
with all precipitation switching over to rain.
To add to the mix, the numerical models are inherently too fast
in dislodging cold air masses like this, which affects the
precipitation type and duration. Given that there's still a solid
24 hours or more for these details to get worked out, felt it was
more of a prudent move to hoist a Winter Storm Watch for now,
versus going straight to advisories or warnings. In general, the
Kuskokwim Delta will see a potential for heavy snow for the entire
Delta, with blowing snow and possible blizzard conditions a
possibility west of the Kilbuck Mountains. Further south and
generally west of a King Salmon to Port Alsworth line, freezing
rain and ice accumulations will be more of a concern, as the depth
of warm air here will be greater.
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
(tonight through Tuesday evening)
A rapidly strengthening storm, of sub 960 mb strength, moving
from the North Pacific into the Bristol Bay region will bring
storm force winds to the waters south of the Alaska Peninsula,
where seas will push or exceed 30 feet. North of the peninsula and
across much of the Bering and surrounding areas, widespread gale
force winds are expected. The storm will also bring precipitation
to much of the area, some of which will be heavy at time.
Marine (days 3 through 5)...
Beginning the extended period marine forecast on Wednesday, there
will be a weakening gale force low moving inland across southwest
Alaska as it exits the Bering Sea, with the associated near gale
force front moving across the Gulf of Alaska. Attention turns to a
second low which will move into the Aleutians Wednesday night and
Thursday. There still is lower confidence with the strength and
track of this low, but the likelihood is increasing that this low
will reach at least gale force, and possibly low end storm force,
as it rapidly tracks east into the eastern Bering Sea, and then
the coast of southwest Alaska by Thursday. Expect the highest wind
and seas across Bristol Bay and along/south of the eastern
Aleutians and the Alaska Peninsula. The associated gale force
front will move into the Gulf of Alaska Thursday with southwest
flow lasting over the Gulf through Friday. Expect a period of
quieter weather heading into the weekend as the storm track
briefly subsides over the Bering Sea.
Long term forecast (days 4 through 7)...
Beginning the extended forecast period Wednesday evening, the
North Pacific storm track will be unleashing its fury across
southern Alaska as another strong low and warm front will be
plowing through the southern Mainland. Expect warmer than average
temps to be in place along with inland rain/snow and coastal rain,
although the heaviest rain will be shifting east into the Yukon.
Attention very quickly turns to the very next storm which will
quickly amplify along the npac jet stream and move across the
Aleutian chain into southern Alaska. While uncertainty still
exists, this low will likely bottom out sub-960 hpa, and will make
landfall across southwest Alaska Thursday morning, then quickly
move into southcentral later in the afternoon. This low looks
rather warm, but given the challenges of local microclimates,
expect another Rainbow (myriad) of precipitation types ranging
from rain, to rain/snow mix, to possible freezing precipitation
for inland areas. However, most inland locales near the coast will
be rather warm, so most areas will likely just be rain during the
The upper level trough gradually moves east Friday and Saturday,
so temperatures will gradually cool aloft again, although showers
and clouds will still be in place, so a gradual changeover back to
snow or rain/snow showers will occur. Thereafter, a lot of
uncertainty develops, but the latest European model (ecmwf)/GFS are depicting the
chance for subtle basal shortwave troughs to undercut the larger
trough Sat/sun, which would be a favorable pattern for a return to
accumulating snow threats across parts of the southern Mainland,
but at this juncture, that is effectively fairy-tale land in the
world of atmospheric modeling.
Marine...storm 155. Gales 130 138 139 150 160 170-181 185 352 411-414.
Heavy freezing spray 181.