Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EDT Fri Jun 19 2020
Valid 12Z Tue Jun 23 2020 - 12Z Sat Jun 27 2020
...Overview and Guidance/Predictability Assessment...
At higher latitudes an Arctic upper low and trough extending to
its south should be a persistent feature for most of the period.
The trough will extend into portions of the northern/western
mainland. Over the past day the ensemble means and some models
have trended deeper with the trough, leading to a farther
southeastward extent of the leading surface front at least early
in the period. The trough should still weaken somewhat later in
the week and allow the front to return northward. Thus far the
ensemble means have provided the most stable and agreeable
forecast in terms of the Arctic upper low and shape/axis of the
upper trough while operational model runs continue to vary among
each other and in consecutive runs. This recommends a
blended/mean approach for this part of the forecast.
Farther south the most prominent system will be North Pacific low
pressure that should gradually weaken as it reaches the vicinity
of Bristol Bay or the Alaska Peninsula late in the week. There is
fairly good clustering into day 5 Wed but then solutions display
increasing timing/longitude differences. The past couple ECMWF
runs are on the slow side of the full spread (including the
ensemble means, with the 00Z and new 12Z ECMWF means actually a
tad faster than the 12Z GEFS mean) while the 12Z CMC strays to the
fast side. The 12Z GFS is the closest operational model solution
to the guidance/ensemble mean average though it may be somewhat on
the deep side.
Ahead of this system there is a majority cluster suggesting that a
modest upper low will track just south of the Alaska Peninsula and
into the Gulf Tue-Wed, along with a weak surface wave at least
into early Wed. This majority maintains greater definition than
the 12Z GFS. Then late in the period guidance spread/variability
lead to low confidence for forecast specifics over the central
Pacific/Aleutians. There is some consensus for an upper trough to
evolve over or reach the central North Pacific by Fri-Sat.
However models have varied widely on what proportion of energy
comes from high latitude flow versus the northwestern Pacific,
with corresponding differences at the surface. Overall trends
from the past couple days have been toward slower and/or farther
south low pressure. Preference is for a conservative starting
point until better agreement develops.
Days 4-5 Tue-Wed used an operational blend followed by a rapid
increase of 12Z GEFS/00Z ECMWF means, with the means reaching 60
percent total weight by days 7-8 Fri-Sat. As for operational
inputs, the forecast excluded the 12Z CMC after Wed due to its
progressive North Pacific system while using both 00Z and 12Z
ECMWF runs through the period due to some very different high
latitude trough/upper low details at times along with central
Pacific divergence late. The blend included the 12Z GFS through
the period and the UKMET through the end of its run.
...Weather/Threats Highlights...
The mainland will see episodes of convection/rainfall of varying
intensity during the period. Shortwaves within the upper trough
extending south from the Arctic low will provide occasional focus,
as will the leading surface front that reaches the
northern/western mainland and eventually drifts northward. The
North Pacific system expected to weaken as it reaches the vicinity
of Bristol Bay late this week should spread some moisture to areas
from the Aleutians through Alaska Peninsula/southern coast and
possibly to the Panhandle. Track and speed are still in doubt so
confidence is not yet very high regarding intensity and duration
of precipitation. The most likely track may lead to some
enhancement over favored windward terrain. The pattern continues
to favor below normal highs over a large portion of the state
during the period, with greatest negative anomalies over the
northwest as well as south/southeast and Panhandle. Anomalies for
lows will likely be more moderate and some above normal lows will
be possible away from northwestern/southeastern areas.
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
Hazards: No significant hazards are expected over Alaska during
this forecast period.
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