Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
745 PM EDT Sun Mar 08 2020
Valid 12Z Thu Mar 12 2020 - 12Z Mon Mar 16 2020
...Overview and Guidance/Predictability Assessment...
As a whole latest guidance continues to be more assertive with
upper ridging forecast to build over the northeastern Pacific and
at times extend into the mainland/parts of the Bering Sea, with
strong shortwaves riding around the northern periphery of the
ridge. The corresponding storm track would have low pressure
systems passing through Siberia and then into the Arctic. Some
elements of guidance are more agreeable than yesterday but there
is still significant spread and run-to-run variability with the
amplitude of shortwaves rounding the upper ridge.
The forecast is fairly consistent with the leading strong storm
tracking through Siberia and into the Arctic late this week though
some timing divergence arises after early Fri. To the southwest
of this system guidance develops a split regarding the amplitude
of shortwave energy crossing the Bering Sea and mainland--between
the faster/stronger GFS-UKMET and weaker/slower ECMWF-CMC.
Current preference is two-thirds toward the ECMWF-CMC idea given
the recent trends toward stronger ridging but tempered a bit by
the 12Z ECMWF backing away from its 00Z run a bit for heights over
the mainland. However note that by early Sat the 12Z ECMWF is
stronger than its previous run with the core of the upper high to
the south. It has turned out that the earlier 12Z/07 CMC provided
some leadership in the evolution toward a strong upper ridge.
Upstream the guidance is starting to develop some clustering for
the next progressive system tracking near Kamchatka around Fri-Sat
but with increasing spread thereafter. The ECMWF shows the
biggest reversal as the 12Z run is more amplified with the
associated shortwave versus most other solutions while the prior
run was extremely strong with the upper ridge, deflecting the
surface system much farther northward. An intermediate solution
with a decent operational model component (more total GFS/CMC
input versus the ECMWF) appears best in light of operational
guidance suggesting the system should not lose definition as
quickly as seen in most of the ensemble means.
Another question mark lurking in the forecast from Fri onward
involves to what extent shortwaves within the western
Pacific-Bering stream may interact with energy/moisture flowing
around the northern periphery of a broad upper low over the
central Pacific. A composite of 12Z model runs would suggest
decent potential for some moisture to reach the Aleutians, with
specifics of the mean ridge aloft ultimately determining how much
of this moisture could ultimately extend into the western/northern
mainland.
Early in the period the initial blend emphasized nearly half
weight of the 12Z ECMWF and the other half a combination of
GFS/CMC with a small remaining component reserved for the UKMET.
By the latter half of the period aforementioned issues with the
12Z ECMWF led to reducing its weight some while increasing weight
of the 00Z ECMWF mean. The forecast maintained inclusion of the
GFS/CMC.
...Weather Highlights/Threats...
The strong storm tracking through Siberia and the Arctic during
the latter half of the week will bring a period of strong winds
from the Bering Sea through the northwestern mainland/Arctic with
an episode of significant terrain-enhanced precipitation over the
northwestern mainland as well. Another system may also track
through Siberia and the Arctic during the weekend into Mon. This
system is likely to be weaker than the first one, so accompanying
winds and precipitation should be less extreme than with the
late-week event. Interaction of western Pacific/Bering shortwaves
and moisture around the northern side of a large system over the
central mid-latitude Pacific should bring some light-moderate
precipitation into the Aleutians Fri onward. Some of this
moisture could eventually reach the western/northern mainland.
Cold high pressure supporting below normal temperatures over about
the southeastern two-thirds of the mainland on Thu will quickly
give way to incoming warmer flow ahead of the strong late-week
Aleutians/Arctic storm. Then expect the pattern into early next
week to maintain above to well above normal readings over most
areas with highest anomalies over the northwest third of the
mainland. Some localized pockets of below normal temperatures may
persist over the southeastern mainland and Panhandle.
Rausch
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
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