Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EST Mon Dec 16 2024
Valid 12Z Fri Dec 20 2024 - 12Z Tue Dec 24 2024
***Multiple surface lows over the Gulf and North Pacific to
produce hazardous maritime conditions late in the week and into
the weekend***
...Overview/Guidance Evaluation/Preferences...
A nearly stationary and active weather pattern will remain across
all of Alaska through early next week. A longwave trough will be
parked over the Bering and Aleutians. This will keep the Pacific
subtropical jet both well south of Alaska over the North Pacific,
as well as in a hyperactive phase due to the contrast of the warm
equatorial air mass south of the trough and the cold air within
the trough. Thus, rapid-fire lows will race along the jet, then as
they round the southeastern periphery of the low, make a left turn
and move north into the Gulf of Alaska, and eventually curve back
to the west and into the low itself. This process will repeat
itself multiple times through early next week. With all the
weather across the state originating from the Pacific and not the
Arctic, expect a continuation of the warming trend almost
state-wide through the weekend. The North Slope will be the one
area of the state where cold air hangs on, though temperatures
even there will be around average.
Due to the aforementioned pattern remaining locked in place for
the foreseeable future, the models are certainly handling the
larger scale pattern changes quite well. The largest points of
uncertainty will focus on the track of the numerous lows that
track north out of the data-sparse North Pacific and into the
Gulf. On these details, as is normal, the models disagree,
especially from Day 6/Sunday and beyond. A GFS/EC/CMC blend was
used for Days 4 and 5 where there was better agreement, then
increasingly blended in the GFS and EC ensemble means for Days 7
and 8. The deterministic GFS became an outlier from the ensemble
means by Day 7/Monday, so it was removed from the blend at that
point.
...Weather/Hazards Highlights...
The primary hazard through this week will be periodic high winds,
especially along the Gulf Coast. A series of strong lows will move
into the Gulf, very slowly shifting east in track with time. The
high wind threat will be as the strong lows enter the Gulf and
activate the gap winds and barrier jets as their wraparound
occluded fronts approach and move into the coast. For the low on
the Day 4/Friday, the highest winds will be through the Barren
Islands, Shelikof Strait, and Kodiak and Afognak Islands. There
remains considerable uncertainty with the track of the next low
around Day 6/Sunday, but it's likely due to its strength and track
more towards the central Gulf, that the barrier jets will be the
larger wind threat. High-end gales to storm force winds are likely
through these aforementioned gaps and barrier jets.
The QPF has also been raised through the week due to increased
certainty on multiple lows moving into the Gulf, including another
one by Day 8/Tuesday. The Kenai Mountains look to be the greatest
beneficiaries of this onslaught of moisture, with the potential
for 12-18 inches of liquid equivalent expected in the Friday
through Tuesday period, and 6-12 inches for the other coastal
mountain ranges. At higher elevations this is likely to mainly
fall as extremely heavy snow, with rain at the lower elevations
along the coast. With the lows all expected to both weaken and
turn westward, it's unlikely there will be much precipitation of
any significance during this forecast period for inland locations.
Wegman
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at: The WPC Hazards Outline
depicts these threats.
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