Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
648 PM EDT Wed Apr 29 2020
Valid 12Z Sun May 03 2020 - 12Z Thu May 07 2020
...Guidance/Predictability Assessment...
The best majority of guidance offers good continuity in principle
from the Northeast Pacific into the Arctic. A broad mean low will
prevail well south of the mainland, with multiple embedded
circulations at the surface and aloft during the period. Flow
around these embedded systems may bring one or more periods of
precipitation to areas between the Panhandle and Alaska Peninsula.
Meanwhile an upper ridge initially over eastern Siberia and the
mainland should drift east or northeast (albeit with some
uncertainty over the details) and eventually merge with another
ridge over western Canada. Consensus continues to show
progressive Arctic flow bringing a cold front into the extreme
northern mainland around the start of the week. This front should
linger in place through Wed-Thu. Meanwhile guidance is somewhat
more variable and diverse between the western Pacific and the
Bering Sea. There is decent agreement that a low/frontal system
should reach the western Bering/Aleutians by around Tue but then
differences with upstream flow playing at least a partial role in
what ultimately happens with this feature, and then what the
pattern will look like over the western Aleutians by the end of
the period.
In an approach similar to yesterday, the initial blend emphasized
the 12Z operational model guidance during about the first half of
the period and then trended toward an even model (12Z/00Z ECMWF
and 12Z GFS) and ensemble mean (12Z GEFS/00Z ECMWF mean) blend as
solutions diverged over some areas. The forecast had to exclude
the 12Z CMC after day 4 Sun due to being more amplified than other
guidance for two separate Arctic troughs aloft.
Once again clustering for initial low pressure over the Northeast
Pacific held on fairly well into early day 6 (now Tue) with the
ECMWF/GFS/CMC the most similar aside from some noticeable strength
differences (GFS and questionably far westward UKMET deepest).
Ensemble means were close to this cluster for track/timing through
day 5 Mon before starting to lose definition somewhat as the low
center tracks south of the Alaska Peninsula. During the latter
half of the period confidence rapidly decreases for exact details
of trailing waves to the south/east of the first wave, favoring
the model/ensemble mean blend to capture the agreeable large scale
pattern. By day 8 Thu the ensemble means suggested low pressure
would be near 50N 150W. The 12Z GFS was closest to this position
while the 12Z ECMWF was farther east/00Z ECMWF to the south.
During the latter half of the period the guidance average has
trended weaker over the past day with any energy aloft and low
pressure/frontal system reaching the Bering Sea. The 12Z ECMWF
was one solution that ran counter to this trend. How quickly this
feature weakens will depend in part on progression of an upper
trough emerging over the western Pacific. So far guidance has
been mixed with the 00Z ECMWF/12Z GFS and last couple ECMWF means
somewhat faster than the 12Z GEFS mean/12Z ECMWF, favoring an
intermediate solution. The faster part of this spread would have
the western Pacific system extending its influence into the
Aleutians on day 8 Thu.
...Weather/Threats Highlights...
Expect flow around the northern side of Northeast Pacific systems
to bring some moisture across the Panhandle and westward along the
southern coast to the Kenai Peninsula/Kodiak Island and Alaska
Peninsula. However ongoing guidance spread leads to low
confidence for precipitation intensity and five-day totals.
Localized areas in the Panhandle and eastern Kenai
Peninsula/Kodiak Island/northeast Alaska Peninsula could see
enhanced activity depending on the exact evolution and track of
low pressure centers. The western half of the Aleutians may see
one or two episodes of precipitation, with the first system that
likely weakens and second one whose influence may or may not reach
the area by next Thu. Above normal temperatures should be most
prevalent over the southwestern mainland while below normal
readings are more likely over northern and eastern 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