Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
443 PM akdt Monday Apr 20 2015
Analysis and upper levels...
a look at satellite imagery shows a large closed circulation
dominating the Alaska region. A vertically stacked low is centered
just north of Nunivak Island with a continuous jet stream around
the periphery of the circulation resulting in a pinwheel of short-
waves. One of the key characteristics of this airmass is the very
cold air aloft...which is resulting in widespread convection with
isolated thunderstorms in the areas of greatest vertical motion.
The most significant of the upper short-waves is one moving
onshore of southwest Alaska today and another lifting northward
across southcentral...with locally heavy showers in both cases.
Temperatures were still warm enough for rain over much of
southcentral this morning but a cold front is progressing north
and eastward gradually changing precipitation over to snow. Precipitation over
southwest Alaska...the Bering Sea and Aleutians is all in the form
A separate branch of the upper level jet extends well south into
the North Pacific with an amplifying trough and developing surface
low noted in satellite imagery. A short-wave digging southward
along the Aleutians will be timed just perfectly to interact with
the southern stream and lift the low northward toward Alaska.
there are still some minor timing differences with troughs lifting
northward across southcentral today and tonight. Based on latest
radar trends the slower solutions are best. The 18z NAM seems to
really be catching on to this slower progression. With the track
of the Pacific low south of the Alaska Peninsula and Kodiak Island
Tuesday into Wednesday all solutions are now grouped into a tight
core...so confidence has greatly increased. More significant
differences continue with handling of low near the Kamchatka
peninsula the middle of this week...but solutions are fairly close
with the track of the frontal system across the Bering and
Aleutians. Overall forecast confidence is average to slightly
above average. Uncertainty largely lies with the difficulty in
forecasting exact location of convection with the typical large
variation in quantitative precipitation forecast over short distances.
Short term forecast southcentral Alaska (tonight through Wednesday afternoon)...
The much anticipated cold front expected to bring snowfall to
Anchorage and the mat-su valley has progressed much slower up the
inlet than originally expected. This has led to scattered snow
showers throughout the day with no accumulation. However...the
front is now on the doorstep of Anchorage with the snowfall
intensity beginning to increase. Significant snowfall is not
expected...with up to 2 inches expected in town and 2 to 4 inches
possible along the hillside as precipitation up slopes along the
chugach. The mat-su valley will have a much harder time
accumulating snowfall as temperatures have been in the upper 30s
to middle 40s as the cold air advection has not made it that far
north just yet. Snowfall will begin to taper off across the Kenai
Peninsula as the front lifts northward but with decent lapse rates
associated with the whole layer cooling... scattered snow showers
are not out of the realm of possibility through Tuesday afternoon.
Following this showery regime...an area of low pressure from the
North Pacific will move into the western Gulf of Alaska late
Tuesday. This will bring higher rain and snow chances to Kodiak
Island as well as the northern Gulf Coast. The low will shift
eastward across the southern Gulf of Alaska which will bring some
gale force winds to the offshore zones and keep any front/barrier
jet from impacting the northern Gulf Coast. There will be some
weak outflow winds with this event but with the low being so far
south...they should be relatively light.
Short term forecast southwest Alaska (days 1 through 3)...
snow developing ahead of low pressure just north of Nunivak
Island will bring accumulations across much of the southwest. A
brief mixing of rain will be possible across the lower elevations of
Bristol Bay and the Alaska Peninsula and then change over to all
snow as cooler air advects from the Bering. A tight pressure
gradient ahead of the surface low will keep winds gusting through
midnight and then begin to relax as the low lifts north and
diminishes. Blowing snow will reduce visibilities to one mile or
less at times through midnight...however there will be frequent
breaks in the current snow pattern keeping visibility criteria
above winter advisory criteria.
Rain and snow showers taper off Tuesday afternoon with scattered
activity remaining through Wednesday as weak upper level disturbances
move through the flow. The next stronger system will be developing
south of the eastern Alaska Peninsula on Tuesday night into
Wednesday with an eastward track. Low impact easterly flow will
increase across Bristol Bay on Wednesday with precipitation mainly
along the Alaska and Aleutian ranges.
Short term forecast Bering Sea/Aleutians (days 1 through 3)... a
Gusty winds and snow across the central Bering will bring reduced
visibilities across the pribilofs through midnight. Winds begin to
taper off after midnight as the low north of Nunivak Island lifts
and diminishes. Continued snow showers and gusty west to northwest
flow across the western Bering will begin to change as a stronger
front pushes into the western Aleutians Tuesday night. Gusty
southeast winds and mixed precipitation accompany this front with
stronger northerly flow and cooler temperatures behind this
Long term forecast (wednesday night through monday)...
after the gale force low over the Gulf dissipates and departs the
area Wednesday evening...a weakening Bering Sea front will enter
the southwest Mainland...bringing rain and snow to the area for
Thursday. This front will enter the Gulf as it continues to weaken
on Friday and bring rain mainly to the Gulf Coast...while
southcentral will remain cloudy but dry.
Starting this weekend there is very good model agreement that the
upper low associated with the aforementioned Bering front will
drag the jet well south of the state...allowing for ridging to
build in from the central Bering into the interior. This looks to
bring a much less amplified pattern to most of the state that
should persist into early next week. This pattern will limit
precipitation in the area mostly to diurnal showers as
temperatures return to above normal.
Marine...gale 131 132 136 150 175 176 177 178.
Synopsis and model discussion...sb
southwest Alaska/Bering Sea/Aleutians...kh