Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Fri Apr 10 2020
Valid 12Z Tue Apr 14 2020 - 12Z Sat Apr 18 2020
...Overview and Guidance Evaluation...
Today's guidance agrees fairly well for the large scale forecast
aloft, with fairly strong mean ridging over the Northeast Pacific
and Alaska mainland/western Canada while mean troughing prevails
farther to the west. Flow between the upper ridge and trough will
carry along multiple smaller scale (thus having lower
predictability) surface waves over the Aleutians/Bering Sea and
North Pacific in the days 4-6 Tue-Thu period. By mid-late period
most guidance shows shortwave energy passing around/through the
initial ridge, followed by rebuilding of the ridge as low pressure
and supporting dynamics track into the Aleutians/Bering Sea.
Guidance shows the typically increasing divergence for details
aloft and the surface low track by late next week into the
weekend. During next week the pattern will favor a wet period
especially over the extreme southern mainland.
The initial forecast blend started with a 12Z operational model
blend to reflect the most common ideas of guidance early in the
period. The one detail of note is that the GFS seemed to differ
somewhat from other solutions for the fast-moving wave between the
Aleutians and northern Bering Sea Tue into Wed, with the blend
giving that run sufficiently low weight for the consensus to
prevail. The forecast was able to hold onto nearly exclusive
operational model weight into early day 6 Thu.
By the latter half of the period the ensemble means and
operational models differ for what may be separate upper features
over the western Bering and emerging from the Northwest Pacific,
with their specifics (and potential interaction) affecting the
strength and track of Aleutians/Bering Sea low pressure. There
are also some differences for the degree to which upper ridging
rebuilds over the mainland. With time the 00Z ECMWF mean strayed
to the southeast side of the model/mean envelope with the low
track. The 12Z GEFS mean was significantly slower and about the
same latitude as the 00Z ECMWF mean, though the GEFS/CMC means
were not as strong with the upper ridge late in the period.
Operational models tracked low pressure farther north than the
means and downstream favored the stronger ridge aloft of the ECMWF
mean late in the period. Trends of guidance as a whole over the
past day have been toward a stronger upper ridge. Transitioning
the early-mid period operational model blend toward a mix of
models and means (with operational ECMWF input accounting for both
12Z and 00Z runs given increasing detail uncertainty) reflected
the best intersection of traits in the guidance--a low pressure
system northwest of the ECMWF mean and upper ridge stronger than
the GEFS mean. The new 12Z ECMWF mean has trended a bit north and
then northwest of its 00Z run with the surface low and is as
strong or stronger with the downstream upper ridge, providing
added support for the favored approach.
...Weather/Threats Highlights...
The pattern will support a fairly wet period across a large
portion of the state, with models and ensembles showing one or two
sizable areas of anomalously high precipitable water values (at
least 3-4 standard deviations above normal). Expect highest
precipitation totals to focus across the southern mainland, most
likely from the extreme eastern Alaska Peninsula across the Kenai
Peninsula to the southeastern coast. Lower-predictability wave
details temper confidence in specifics across this area though.
There may be some relative maxima over parts of the western
mainland as well.
Expect a majority of the state to see above normal temperatures
during the period with lows tending to be more extreme versus
normal than highs. Northern parts of the mainland in particular
will likely see much above normal readings through Wed or Thu. A
wavy front pushing southward late week into the weekend should
lead to a moderating trend. Exceptions to the warmth will be the
Panhandle which will tend to be somewhat below normal and parts of
the southern mainland where there could be localized pockets of
modestly below normal highs.
Rausch
Hazards:
- Heavy precipitation across portions of mainland Alaska, Mon-Thu,
Apr 13-Apr 16.
- High winds across portions of mainland Alaska and the Aleutians,
Wed-Fri, Apr 15-Apr 17.
- High winds across portions of mainland Alaska, Mon, Apr 13.
- Much above normal temperatures across portions of mainland
Alaska, Mon-Wed, Apr 13-Apr 15.
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