Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
723 PM EDT Mon Sep 09 2024

Valid 12Z Fri Sep 13 2024 - 12Z Tue Sep 17 2024

...Overview...

As the medium range period begins Friday, upper and surface low
pressure will provide support for lingering moderate to heavy
precipitation to Southeast Alaska before moving southeast over the
weekend. Upper ridging is forecast to push through behind this
trough and in response to additional energy and troughing coming
into the Bering Sea late week. These upper and surface lows in the
Bering gradually moving east will promote increasing precipitation
chances across the Aleutians late week, spreading into western and
central parts of the Mainland into the weekend, and affecting the
Panhandle into early next week.


...Guidance Evaluation/Preferences...

Recent model guidance continues to show general agreement with the
large scale features of note. A stacked upper/surface low is
pretty agreeable offshore of Yakutat Friday with a southeastward
shift into Saturday, so a multi-model blend worked well. As this
upper trough exits, overall ridging looks to press east from the
eastern Bering Sea/western Mainland Friday and across the Mainland
Saturday. Possible shortwaves moving around the ridge are
uncertain with their timing and strength, but without much
clustering around any particular solution yet, which is typical
for these smaller scale features.

Meanwhile, broad energy and troughing are likely across the Bering
Sea, gradually tracking east as the period progresses. The
non-NCEP models including the AIFS indicate one relatively strong
upper low within the trough pattern moving from east of Kamchatka
toward the Bering Strait over the weekend. This supports a deep
surface low, with the EC and UKMET in the 950s mb west of the
state. However, the 06Z and 12Z GFS runs show a split upper low
solution where the first (east of the non-NCEP consensus position)
weakens in favor of the secondary one (west of the other models'
position), and the surface reflection is similar with double lows.
The 18Z GFS trended in the direction of one stronger upper low
though. At least the deterministic models become agreeable with a
trough axis over the western Mainland by Monday, though with
differences in the energy distribution within the trough. Guidance
shows increasing differences in how the trough moves east by next
Tuesday and potentially forms a closed upper low in the Gulf per
GFS runs, with the pattern affected by a possible additional
upstream low as well, so there is considerable uncertainty by Day
8.

The WPC forecast was based on a blend favoring the 12Z
deterministic models early, with increasing proportions of the
GEFS and EC ensemble means as the period progressed due to
increasing spread, reaching over half means in the blend by the
end of the period.


...Weather/Hazards Highlights...

The first upper trough and surface low near the Yakutat region
into late week could continue to spread notable precipitation into
northern parts of the Panhandle into Friday, though gradually
trending down compared to the short range period. The highest
totals should be over the typically favored upslope regions like
Mt. Fairweather/Glacier Bay National Park. Lingering snow is
possible on the backside of the low into the Alaska Range, with
lighter precipitation to the north in central/eastern parts of the
Mainland. Then as the broad upper trough and surface low pressure
system get established in the Bering Sea, precipitation is likely
to affect much of the state, moving from west to east with time.
Precipitation is forecast for the Aleutians on Friday, moving into
the Alaska Peninsula/western coast into Saturday, and continuing
east to focus in Southcentral Alaska for the weekend and then into
Southeast Alaska early next week. The current forecast is for
moderate to locally heavy rain amounts for coastal/lower elevation
areas and some snow in the higher elevations. Today's forecast
trended up for inland precipitation totals, including the Alaska
Range to western/central parts of the Interior to the Brooks
Range. With low pressure in the northern Bering, there is some
chance for high winds and potentially some minor coastal flooding
along the western coast, but model guidance waffling on the
strength of the surface low as it approaches the state creates
uncertainty in the level of impacts. WPC will continue to keep an
eye on those potential threats.

High temperatures should mostly be in the 50s across lower
elevations of the state through the forecast period, so within a
few degrees of normal in most locations. However, the North Slope
with highs in the 40s and even low 50s will be above normal this
weekend into early next week. Prevalent clouds and precipitation
will favor mostly above normal morning lows, aside from a round of
slightly below average lows tracking across the Interior
Friday-Saturday. Southeast Alaska may see a mix of below and above
average lows depending on the day but with mostly below normal
highs.


Tate


Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html