Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Northern Alaska forecast discussion 
National Weather Service Fairbanks Alaska
647 PM akdt Monday Sep 1 2014

Discussion...

A large variety of significant weather is occurring or expected
over the forecast area. Current model runs are similar on the
major features out through 4 am Thursday.

A winter depth upper low...about 502 dam at 500 mb...is centered
about 250 nm north northwest of Barrow. The upper low is prognosticated
to move to just north of Barrow by 10 PM Tuesday and to about 150 nm
north northeast of Demarcation Point by 10 PM Wednesday. West to
northwest flow aloft will continue over northern Alaska through
Thursday.

A strong short wave trough...not easily recognizable in the 500 mb
height field...is currently moving across the Alaska West Coast
and northwest Alaska and will sweep eastward to the alcan border
by 4 PM Wednesday. A cold front extending from Demarcation Point to
eastern Norton Sound and St Lawrence Island. The front will move
eastward overnight. By 10 am Tuesday the front will extend from near
Eagle to the western Alaska Range and southwestward.

There are very strong upper level dynamics and strong winds aloft
with this weather pattern which is very rare this early in the
season and much more representative of an active winter storm
pattern. A low level jet streak with southwest winds as high as
60-65 knots at 850 mb...will move eastward across the interior
overnight. A 995 mb surface low has developed about halfway
between Bettles and Ambler and the 6 hour forecast from the 18z GFS
run is verifying well on the low position and central pressure.
The fast flow aloft will carry this low center rapidly northeastward
across the northern Alaska interior tonight. By 4am Tuesday the low
center will be centered over the eastern Brooks range near the
alcan border...and moving to 250 miles east northeast of Barter
Island by 10 am Tuesday...maintaining a 996 mb central pressure.

The short wave trough and surface front are producing a widespread
area of significant rainfall. Rainfall amounts of up to 1.8
inches have occurred in the upper Kobuk valley...up to 1.3 inches
in the Nulato Hills...and about an inch over the upper Kobuk and
Noatak valleys. Significant rainfall is now spreading across the
central and eastern interior. Rainfall amounts of up to a 0.5 inch
have already occurred at some sites in the middle Tanana Valley.
The Pedro Dome WSR-88D is indicating widespread estimated
instantaneous rainfall rates ranging from 0.2 to as much as
0.6-0.8 inches per hour in the heavy rain area. While these
are only unverified estimates...it is good indication that
widespread heavy stratiform rain is occurring.

Heavy rainfall is expected overnight in the upper Kuskokwim
valley...central interior...middle Tanana Valley...Denali... and
nearby areas in zones 223 and 224. Current GFS and NAM indicating
additional amounts of 1 to 2 inches in middle Tanana Valley and
eastern parts of the central interior zone. Up to an inch is
expected in Denali and western parts of zone 223 during the same
time period. With the very strong west to southwest winds
aloft...there will be considerable orographic enhancement in the
chena and Chatanika basins...resulting in local rainfall amounts
higher than the model forecasts. The heavy rain will likely
produce flooding on the upper chena river... little chena
river...and the Chatanika river. Flood watches have been issued
for these rivers. High water levels may also occur on the smaller
streams and tributaries feeding into these rivers. Due to the low
freezing levels...no flood issues are expected in the Alaska
Range. However... significant and rare heavy snowfall this early
in the season is expected in Denali park...where amounts of 6 to
14 inches are expected at elevations above 2500 feet mean sea level. Several
inches of snow have already fallen on higher elevations of the
Denali park Road today.

The rapidly moving surface low will produce a period of strong
pressure gradients and rapid pressure rises over much of the
eastern interior tonight and Tuesday morning. This combined
with stronger winds aloft mixing down to the surface...will
produce strong and gusty west to southwest surface winds over
parts of the central and eastern interior tonight and Tuesday
morning...and strong southwest winds over the summits on
highways in zones 220-221 and 222 tonight. Wind advisories
have been posted for zones 220-221 and 222.

Snow will continue overnight in the Brooks range and taper off
Tuesday morning. Several inches of additional accumulation are
expected.

On the West Coast...winds over Norton Sound are decreasing
and sea levels along the eastern Norton Sound shoreline
will be falling this evening.

Over the Arctic region...a 985 mb low centered about 300 nm
north northwest of Barrow will move to about 50 nm north of
Barrow by 4 PM Tuesday...becoming a strong low pressure trough over
the eastern Beaufort Sea by 4 am Wednesday. This low will generate west
to northwest winds 25-30 knots over the West Coast and adjacent
marine areas beginning late Monday night and continuing through Wednesday
morning. The winds will produce high seas and surf along the
coast from Barrow westward...along with high surface and the
likelihood of localized minor beach erosion. West to southwest
winds 25-30 knots will spread eastward along the remainder of the
Arctic coast and adjoining marine areas Tuesday night and Wednesday morning.

Beyond the short term...another short wave trough rotating
around the upper low will arrive on the West Coast and
northwest Alaska Tuesday afternoon...moving east to the alcan border
by Wednesday afternoon. Precipitation with this next short wave
is expected to be considerably less than that expected
in the current ongoing event.



Coastal hazard potential days 3 and 4...none.

&&

Hydrology...
significant rainfall amounts have occurred in the Nulato
Hills...and a Flood Advisory has been issued rivers and streams
in that area. Heavy rainfall expected overnight in the chena and
Chatanika river basins will likely bring the upper chena
river...Chatanika river...and the little chena above flood stage.
A Flood Watch has been issued for these rivers.



&&

Afg watches/warnings/advisories...
Wind Advisory...akz220-akz221-akz222.
High surf advisory...akz201-akz202-akz212.
Winter Storm Warning...akz206-akz225.
Flood Watch...akz222-akz224.
Flood Advisory...akz212-akz216.
Winter Weather Advisory...akz218.
Small Craft Advisory....pkz200-pkz225-pkz230-pkz235.

&&

$$

Rf Sep 14

National Weather Service Glossary of Abbreviations