Flood in Alaska, May 18 to 20, 2024

Northern Alaska Forecast Discussion...CORRECTED
National Weather Service Fairbanks AK
318 PM AKDT Sat May 18 2024

.SYNOPSIS...
Scattered showers and isolated afternoon thunderstorms are
expected  to develop along the thermal trough through the
weekend. Moisture and upper level energy moving across the eastern
Alaska Range and the southeastern Interior Sunday night into
Monday will bring the potential for heavy showers; at higher
elevations accumulating snow is possible. A weather front moves to
the West Coast and Western Interior Monday into Tuesday, bringing
showers and increased southeasterly winds.

&&

.DISCUSSION...
Synoptic Analysis and Forecast...
Aloft, at 500 mb, broad upper level troughing is in place over
much  of mainland Alaska. A 526 dam low at the base of the trough
near Perryville will move across Kodiak tonight and into the
northern Gulf of Alaska Sunday and to near Yakutat by Monday
morning continuing east into northern British Columbia by Monday
afternoon. A 534 dam low near Wrangel Island moves south to the
Gulf of Anadyr by Sunday afternoon, merging Monday with a 525 dam
low moving east away from the Kamchatka Peninsula, becoming
centered between the Pribilofs and St Matthew Island by Tuesday
morning. Between the low in the Bering and the low in British
Columbia, ridging develops across mainland Alaska

At the surface, a 1028 mph high 400 NM north of Nuiqsut will
deepen to 1031 mb Sunday afternoon as it moves north into the
high Arctic. A thermal trough stretches from McGrath to Nenana to
Northway this afternoon. The thermal trough shifts north on Sunday
to stretch from near Huslia to Fairbanks to between Eagle and
Northway. A 996 mb low moves into the far western Bering Sea late
Sunday and tracks east to be 350 NM northwest of the Pribilofs
Monday morning to 150 NM northwest of the Pribilofs Tuesday
morning. As this low tracks east northeast it will push a front
towards the West Coast Monday afternoon, moving onshore the Y-K
Delta and St Lawrence Island during the evening and overnight
hours and pushing northeast across the Seward Peninsula and
Western Interior Tuesday.

Models...
The 18/12Z models initialized well against the 12Z RAOBS and are
in good agreement with the overall synoptic pattern through
Monday. There are some minor differences in the timing of weak
impulse moving through the trough, but the impacts of the
differences are very minimal. One of the biggest differences seen
in model solutions compared to the last few days is the diminished
instability expected on Sunday and thus thunderstorm potential.
Given the agreement among the ECMWF, NAM, and GFS, opted to trim
back thunderstorm area on Sunday to Upper Tanana Valley, Fortymile
Country, and White Mountains. Models have not handled the
increased winds during the afternoon hours across the Interior
with the thermal trough in place, opted to blend ADJMAV and NAM to
capture the higher winds. Leaned toward the NAM for winds with
the front moving to the West Coast Monday into Tuesday.

North Slope and Brooks Range...
Fairly quiet across the North Slope and Brooks Range over the next
several days. High pressure over the Arctic will maintain areas
of stratus and periods of fog, as well east winds of 10 to 20 mph
range. Persistence expected temperature wise, with not much change
in high and low temperatures

West Coast and Western Interior...
Drier conditions anticipated through Monday afternoon, with a few
spotty showers. Precip chances increase late Monday as a weather
front pushes over the Y-K Delta and St Lawerence Island and then
moves northwest across the southwestern Interior to Seward
Peninsula Monday night. With the approach and passage of the
weather front Monday into Tuesday, southeast winds increase into
the 15 to 30 mph range across the Y-K Delta and St Lawrence Island
Monday afternoon, spreading north across Norton Sound and to the
Seward Peninsula by Tuesday morning. High temperatures Sunday in
the mid 50s to lower 60s across the Western Interior, in the 30s
over St Lawrence Island and Bering Strait Coast, and the mid to
upper 40s elsewhere along the coast. Slightly cooler temperatures
expected Monday for much of the area.

Central and Eastern Interior...
Scattered afternoon showers and isolated thunderstorms expected
across the Interior today and Sunday along the thermal trough. In
addition to the thermal trough, upper level energy moving across
the Interior will support isolated thunderstorm today over an area
stretching from near Manley Hot Spring north to Gobblers Knob and
east to Northway and Eagle. Confidence in thunderstorm potential
for Sunday is lower than today. Thunderstorm potential on Sunday
is largely over the Fortymile Country, but the White Mountains
can’t be ruled out. Upper level low moving out of the Gulf of
Alaska and into northern British Columbia Sunday night into Monday
will push a slug of moisture across the Eastern Alaska Range and
the southeast Interior, bringing increased chances for heavy
showers, especially over the eastern Alaska Range. Rainfall
amounts of half an inch to as much as an inch are possible in the
eastern Alaska Range Sunday night through Monday night. From Delta
Junction to Dot Lake up to half an inch of rain is possible. A
mix of rain and snow is possible Sunday night into early Monday
morning in locations such as Tok and Robertson River. Accumulating
snow is possible in the higher elevations, especially in upslope
areas of the Alaska Range where 4 to 6 inches are possible. Weak
southerly gap flow is developing in the Alaska Range today,
peaking tonight in the 20 to 30 mph range, and then diminishing on
Sunday morning. Sunday afternoon into Sunday evening winds become
west to southwest and increase to 10 to 20 mph range Sunday
night, diminishing Monday morning. High temperatures largely in
the 60s Sunday. Lows in the mid 30s and lower 40s. Slightly cooler
temperatures Sunday, especially over the southeastern Interior
due to shower activity.

Extended Forecast Days 4-7...Deterministic and ensemble means
continue to indicate a series of upper level lows in the Bering
that squash the eastern Pacific Ridge. This will likely maintain
continued chances for showers across the Interior and West coast
through the week. Temperatures cool slightly for the latter half
of the week.

Coastal Hazard Potential Days 3 and 4...None.

&&

.FIRE WEATHER...
Scattered showers are expected into early next week across the
Interior. Thunderstorm potential continues through Sunday along
the thermal trough, with the best chances occurring over the White
Mountains, Fortymile Country and Upper Tanana Valley. Thunderstorm
potential diminishes on Sunday. Weak southerly gap flow is
developing in the Alaska Range today, peaking tonight in the 20 to
30 mph range, with south winds of 15 to 20 mph extending north to
Delta Junction. The gap flow diminishes on Sunday morning. Northeast
winds increase to the 10 to 20 mph range and persist into Monday
from the White Mountains north across the Dalton Highway summits
and the Yukon Flats. High temperatures largely in the 60s Sunday
and Monday, with highs in the 50s for the far upper Tanana Valley
and Fortymile Country on Monday. Minimum RH values will be in the
20 to 25 percent range Sunday from Nenana north to the Yukon Flats
and near Eagle. Critical fire weather conditions are not expected
to occur. Wetting rains develop Sunday into Monday over the
southeastern Interior and eastern Alaska Range.

&&

.HYDROLOGY...
The Yukon River at Pilot Station broke up on Thursday with water
levels continuing to rise and starting to flow overbank into lower
portions of the community. Ice was reported as running bank to
bank as of Saturday morning at Pilot Station with the run of ice
slowing down with a possible ice jam downstream between Pilot
Station and Mountain Village. River levels will continue to rise
until this ice jam clears or moves further downstream. Water
levels have been steady at St. Mary's and Mountain Village. Water
levels upstream at Marshall have been steady over the last day.
Flooding will continue for low lying areas at Pilot Station until
the downstream ice jam releases. Once this jam releases or moves
downriver water levels will rise significantly at St. Mary's,
Pilot Point and Mountain Village.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
AK...Flood Watch for AKZ826.
PK...None.
&&