Flood Watch 1, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Wed May 2 2018

.ANALYSIS AND UPPER LEVELS...
A shift in the pattern is underway across the state this morning
as a departing trough continues eastward into the Yukon Territory.
A low in the central Bering is rapidly moving eastward toward
Bristol Bay and deepening as it approaches the Alaska Peninsula.
Primarily upper level zonal flow today will eventually break down
and transition to southerly flow as the leading front moves over
Southcentral. Light areas of rain showers are seen on radar this
morning in Southwestern Alaska and across the Susitna Valley. Most
of Southcentral is fairly clear this morning except for
considerable cloudiness across the Susitna Valley and Copper
River Basin.

&&

.MODEL DISCUSSION...
The numerical models are doing well with the large scale synoptic
pattern through the weekend, but they are still struggling and
differing with regard to key (and important) details as the flow
pattern dramatically amplifies the next three days.

The details are rather complex, but a leading shortwave currently
moving east through the central Bering Sea will strengthen and
stall with the associated front continuing east toward the North
Gulf Coast Thursday. Here, the NAM is the most significant outlier
with both features and was ignored. The ECMWF and GFS were
suitable for this forecast and were favored for the first two
days.

The next low, which will be a diabatic low level warm anomaly
phasing with the Bering parent upper trough, is expected to move
north toward the Alaska Peninsula Thursday night and Friday then
rapidly strengthen as it moves into SW Alaska. Here the models are
still flip-flopping back and forth among runs, but the overall
trend has been for the low to shift farther and farther west, with
the once outlier ECMWF solution now also shifting west. This will
have impacts to the forecast, but it does seem this storm will be
rather intense, quite intense actually, for May. A blend of the
GFS and ECMWF was also favored for this storm which will bring a
laundry list of impacts to Southern Alaska (winds to SW AK,
blustery conditions to most of Southcentral, heavy coastal rain,
etc.). Stay tuned...

&&

.AVIATION...
PANC...VFR conditions will persist. Continued breezy W-SW winds at
the terminal will end this morning. The next burst of more normal
(for this time of year) SE winds will develop off Turnagain Arm
this evening and continue through the overnight before diminishing
late Thursday morning as the down Cook Inlet gradient
strengthens.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Conditions will be fairly dry across Southcentral today as weak
high pressure builds in. The next system will approach from the
southwest tonight. The front is expected to push into Kodiak
this evening. This front quickly spreads to the northern Gulf
overnight. Gusty east to southeast flow increases along the front
with rain across the Gulf spreading inland. Most of the
precipitation will fall along the coast and coastal mountains, but
some could sneak inland with the flow aloft initially from the
southwest. The next system will rapidly move in from the southwest
Thursday night. This is rather strong system for this time of
year, with Gale force winds over the Gulf and considerable
precipitation moving into the coast by late Thursday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frontal system in the Bering will begin spreading precipitation
across the Alaska Peninsula today and then to the Southwest
Alaska coast this afternoon and evening. The front will then lift
north and inland over Southwest Alaska tonight through Thursday
before a significantly strong low pressure system tracks in from
the south Thursday night and Friday. Breezy easterly gap winds are
expected to develop out of passes in the Aleutian Range and the
Alaska Range tonight with substantially stronger offshore northeasterly
winds building Thursday night through Friday morning in advance of
the second system.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
North to northwesterly flow will increase through the cold
advection in the western and central Bering over the next several
days. Initially the strongest winds will be associated with the
wrap around frontal band to the west of the surface low near the
Pribilofs. As a rapidly deepening North Pacific surface low
tracks up towards the Alaska Peninsula, the low in the Bering will
be pulled south as the new and rather strong low slingshots north
up towards Bristol Bay Thursday night. As the new low becomes the
dominant feature and tracks north, northwesterly Gales will shift
east from the central Bering into the eastern Bering.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...
The very wet weather pattern ongoing at the start of the long-term
forecast will continue through Friday as a strong low, located
either in Bristol Bay or between Kodiak Island and the Alaska
Peninsula, brings a renewed surge of heavy rain to the north Gulf
Coast and snow at the higher elevations of the coastal mountains.
The associated front will also restart the barrier jet along the
Gulf Coast, along with the Turnagain Arm and Knik River Valley
winds. In a change from the past few days, the latest trend in the
models is to keep the surface low further south near Bristol Bay
until Friday night, which in turn will allow it to weaken further
before tracking north into the state. This will prevent winds
through the gaps from getting near warning levels. The winds will
continue into Saturday as the low lifts to near Norton Sound.
However, since by that point the low will be in a much weakened
state, the winds should not increase appreciably. The
precipitation behind the low over southern Alaska should become
more scattered, showery, and favoring the mountains.

The showery weather pattern continues into Sunday with the
potential for an upper level disturbance moving through. Generally
Sunday looks to be the dry day of the group for Southwest Alaska
as it gets in between weather systems. By this point the
disagreements in the models are great enough that the timing can
drastically change. Where the models all agree is that the
weather will be unsettled for most of Southcentral for both
weekend days. The disagreements become even greater moving into
early next week, but for now it appears Monday will be the dry
day in Southcentral. A front and a new low moving into the eastern
Bering looks to return at least some of the area to unsettled
conditions on Tuesday, especially Southwest Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Ice Jam Flood Advisory for 152 for Kuskokwim River near Aniak.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...JA/KO
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
LONG TERM...JW

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1108 AM AKDT WED MAY 02 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0815 AM     SNOW             19 ESE BUTTE            61.42N 148.52W
05/02/2018  M1.0 INCH                           AK   TRAINED SPOTTER

1000 AM     SNOW             25 SSW MATANUSKA GLACIE 61.47N 148.12W
05/02/2018  M3.0 INCH                           AK   TRAINED SPOTTER

            SNOW STILL FALLING

&&

KUTZ