NWS Anchorage, AK wrote its last area forecast discussion before the first event at May 4, 4:18 PM AKST, 7 hours 42 minutes earlier.
Area forecast discussion, May 4, 4:18 PM AKST, 7 hours 42 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 PM AKDT Fri May 4 2018
.ANALYSIS AND UPPER LEVELS...
Satellite depicts a well-defined cyclone moving over Bristol Bay
with divergence stretching from the Alaska Range into the Gulf of
Alaska. A subtropical moisture connection strengthened through
the day.
With this Bering Sea low on the move, rain and wind are now focused
on Southcentral and Prince William Sound while cold air is
spilling into Southwest following the frontal passage.
A warm front lies just south of Prince William Sound while the
triple-point is moving north from Kachemak Bay and into the
southern Kenai Peninsula. The cold front has passed Kodiak city.
Gusty winds were blowing through the Turnagain Arm, Chugach, and
MatSu Valleys since early this morning. While blustery conditions
have been noticeable, all have remained below warning thresholds.
&&
.MODEL DISCUSSION...
Models initialized well and are tracking together through 84
hours over Southcentral. We see minor discrepancies after Sunday
in the west where the NAM takes an early lead out of the gate with
the next Aleutian low.
Pressure difference forecasts have been on target with today's
Turnagain Arm wind shift and continue to show the strongest
pressure differences through 00z Saturday / 5p AKDT. While the
strongest gusts subside, it's also clear that blustery conditions
will continue through the weekend for Southcentral.
High resolution models were the choice for Southcentral and Prince
William Sound again today for detailed wind and precipitation
analysis. GFS remained the pick for Southwest and the Aleutians.
&&
.AVIATION...
PANC...Southerly winds and gusts will remain along Turnagain Arm
through the period, but diminish slowly this evening. Showers and
short periods of rain are expect, however VFR is forecast to
prevail.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The front associated with the low out west continues to push
inland from the Gulf this evening, with the typical gap-driven
winds (Turnagain Arm, higher elevations, and the Knik River
Valley) beginning to slowly diminish through the tonight period.
The bulk of precipitation currently spread across much of the
northern Gulf coast area will shift further eastward tonight,
still covering the eastern portions of the Prince William Sound
area and Copper River Basin through tomorrow afternoon. Areas to
the west should become more showery tonight, and remain both
showery and breezy through Sunday morning.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The well-advertised low is currently pushing through SW AK
bringing very unsettled weather. The low center will drift from
the Kilbuck Mountains north through the YK-Delta overnight. This
will keep enough instability and lift across the area to make for
widespread rain and snow showers, some of them heavy at times.
Gusty northeast to southeast winds will persist until a trailing
trough lifts through the area tonight. This will allow the winds
to shift to westerly and allow some cold air to advect into the
area. This cold air and gusty onshore flow will bring widespread
snow showers to the area Sat. Accumulations will be near zero for
most population centers as the afternoon temperatures should
still rebound to near 40 for most locations.
Sun will bring continued unsettled weather as the cold trough
remains over the area. Some snow bands could set up across the
area coming off Bristol Bay and the Eastern Bering, but these
showers should favor the mountains. West to southwest winds will
slowly diminish through the day, though there will still be some
gusts with the unstable airmass in the afternoon.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
Sun)...
The main story for the weekend will be one low exiting the Eastern
Bering while another pushes into the Western Aleutians. As the
first low exits, it will leave a healthy amount of cold air
spilling down from the north. This should produce plenty of snow
showers, especially for the Pribilof Islands and the north side of
the Aleutians/Alaska Peninsula. Late Sat into Sun will bring a
brief break to the Central-Eastern Aleutians with a shortwave
ridge. However, the Western Aleutians will see yet another
developing low-pressure center push in Sat evening with another
round of winds, warm air, and rain. We increased winds to max-gale
force with the leading front with this system as it looks healthy
than it did on the past few model runs. The low will deepen to
near ~970 mb by Sun afternoon as it moves towards the Central
Aleutians. It will sling its warm occlusion out ahead of it from
west to east into Cold Bay by Sun night. This system will become
the dominant low across the entire state by the end of the weekend
stretching nearly 1100 miles from west to east. As it does so, it
will pull down some cold air on its backside and possibly bring
some storm-force north winds to the Western Aleutians.
&&
.LONG TERM FORECAST (Days 3 through 7: Monday through Friday)...
On Monday, surface high pressure will build across the Gulf,
extending northward across much of Southcentral. This will be the
cause of a rare wind-free, precipitation-free, partly cloudy day
for most of Southcentral. Outdoor plans should definitely be made
for Monday. Meanwhile, for Kodiak and most of Southwest Alaska,
the next front will move in on Monday, though not reaching Kodiak
until late in the day. The front is associated with a sprawling
low, the center of which is expected to be about halfway between
Adak and the Pribilofs Monday afternoon.
On Tuesday, the Bering low remains in roughly the same area as on
Monday as it stalls. However, the front will continue drifting
slowly eastward, thus, the coast from Prince William Sound west
will see rain developing through the day, but the front will start
up the gap winds, so areas downwind of the Kenai Mountains and
Anchorage will stay dry, albeit cloudier than Monday. Meanwhile
points north and east of Anchorage will remain dry as well, but
that will be simply because the front doesn't make it that far.
Rain, heavy at times, will be likely on Tuesday from Kodiak
northward to about Seward.
From Wednesday onward, the models diverge significantly as to how
the front behaves. The GFS and EC keep the front largely
stationary, holding the heaviest rain in about the same areas as
on Tuesday, with relatively little rain for northeastern Prince
William Sound and points north and east. Downsloping continues to
keep the Anchorage area mainly dry regardless. The Canadian model
advances the front across the area, so all of the Gulf Coast gets
hit with rain on Wednesday. The GFS/EC solution brings the
remnants of the front into the rest of the Gulf Coast on Thursday,
which in its highly weakened state would diminish the downsloping
enough for the Anchorage area to potentially get in on the
rainfall action.
Meanwhile the low over the Bering dissipates in place on Thursday
with its remnants absorbed into a new low moving north out of the
Pacific towards the Alaska Peninsula. With the easterly flow
continuing, western Prince William Sound will remain wet
throughout Friday, while the rest of the area north of Kodiak
starts out the day dry. The next round of precipitation with the
associated front reaches the north Gulf Coast by Friday evening.
The models in surprisingly good agreement on the behavior of the
front on Friday considering the previous days (Wednesday and
Thursday), there is far more disagreement.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 152.
MARINE...Gales 180 414.
&&
SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JW