Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
711 PM EDT Fri Mar 20 2020
Valid 12Z Tue Mar 24 2020 - 12Z Sat Mar 28 2020
...Significant snow possible for portions of the Interior next
week...
...Overview and Guidance/Predictability Assessment...
Upper ridging will continue to meander over the northeast Pacific
next week, punctuated by a weakening upper low across the
Aleutians and then back across the AKPEN. To the north, an upper
low is forecast to slide eastward near 80N from the New Siberian
Islands through the Arctic Sea toward Banks Island by midweek,
with a piece of that perhaps sinking southward through the
mainland next Fri/Sat. Upper jet (near 140kts) will move across
the Interior Tuesday and into Canada Wednesday with another jet
streak for Thursday along with a pair of surface cold fronts. With
a southward push of lower heights next Fri/Sat, much colder air
could spread across most of the mainland though ensemble spread
remains modestly high for this lead time.
A blend of the 12Z deterministic models (GFS/ECMWF/UKMET/Canadian)
offered a reasonable starting point for the Tue-Thu forecast with
the system over the Aleutians/Bristol Bay and into the Gulf. To
the north, the models have come into a bit better agreement with a
frontal system through the North Slope that may stall for a day or
so across the Interior. Ensembles were a bit split on how strong
to dig heights through the mainland as the GFS/GEFS generally
allowed more height falls out of the Bering to erode the ridge and
bump and digging heights to the east. The ECMWF and its members
generally allowed the ridge to hold back Bering height falls and
thus allow more northerly flow and colder temperatures. Favor a
tempered version of the 12Z ECMWF via its ensemble mean later in
the period since this will be contingent on smaller scale features
across the Arctic/Beaufort Sea that are not too predictable at
this time range. Weak system next Friday in the Bering may just
shear eastward as the ridge opens up over the northwest Bering.
...Weather Highlights/Threats...
Well above average temperatures across the North Slope on Tuesday
will give way to near average or below average temperatures
through the week along and north of the Brooks Range, but turning
much colder over the eastern Interior. Frontal system sinking
southward into central Alaska will be the focus for at least light
to modest snow accumulation but this could be heavy in an arc from
near Denali ENE across Fairbanks to the Yukon. QPF from the 00Z
ECMWF ensemble mean shows 2-day totals as high as is typically
forecast for this time of year (relative to its own model
climatology). In-house QPF guidance shows near 1" liquid
equivalent (12Z Tue - 12Z Thu) with higher amounts in favored
upslope areas east of Fairbanks which can be aggressive, but does
at least signal an unusual event may be in the offing.
Much colder air will push through most of the interior, especially
eastern sections, but the veracity of the cold is still uncertain.
Temperatures 10-20 deg F below average may be common. NW flow
should trend most of the mainland drier by the end of the week.
Fracasso
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy snow across portions of the Alaska Range, Mon-Tue, Mar
23-Mar 24.
- Much above normal temperatures for much of northwestern and
northern Alaska, Mon-Wed, Mar 23-Mar 25.
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