Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
334 PM EDT Thu Oct 17 2019
Valid 12Z Mon Oct 21 2019 - 12Z Fri Oct 25 2019
The most significant system of interest during the period
continues to be the potentially deep storm that may track toward
or to either side of the Alaska Peninsula in the Tue-Thu time
frame. Most individual models have been offering a general signal
for this development but with considerable timing and track spread
due to differences in how northern stream/Bering Sea energy may
interact with a wave that emerges into the western Pacific during
the weekend. Ensemble members have been sufficiently diverse that
the means (especially the ECMWF mean) are much weaker than a
number of operational runs. However the ensemble means have been
more stable with an intermediate track/timing so preferences
continue to lean toward their overall scenario (more to the 06Z
GEFS mean due to its deeper surface low) but with some operational
input where possible and manual adjustment to ensure the central
pressure reaches at least into the 980's mb while awaiting better
agreement. With the remaining 12Z models having come in, the
UKMET has introduced a deep storm (versus none in the 00Z run)
while the CMC/GFS are fastest with northward progression and the
ECMWF is still a bit in the eastern half of the spread (though
similar to the 06Z GFS). The 12Z GEFS mean has trended a bit
westward. This storm will bring a potential for a period of
strong winds/high waves. Best confidence for highest
precipitation totals is from the Kenai Peninsula to the Panhandle,
with areas westward into the Alaska Peninsula also seeing
meaningful potential for enhanced precip.
Elsewhere... Farther west most operational model runs are siding
with the ECMWF mean with respect to the general idea of a system
affecting the Aleutians by the latter half of the week. Specifics
vary a lot though, so the manual forecast is fairly conservative
in the depiction of the surface low. Early in the period guidance
is still in the process of resolving multiple surface low
centers/frontal systems across the Bering and
northern-northeastern Pacific, favoring a model/ensemble mean
blend to capture the most agreeable aspects of guidance. Across
the higher latitudes, guidance has so far been showing a variety
of possible evolutions for an Arctic upper ridge and an upper low
that may retrograde to the south of the ridge. Overall the
average of guidance has shifted somewhat westward with the upper
low versus 24 hours ago highlighted by the 00Z CMC and 12Z
GFS/GEFS mean. This type of evolution tends to have below average
predictability so again a model/mean blend looks best for
producing a single deterministic forecast.
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
Alaska Hazards...
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Wed-Thu, Oct 23-Oct 24.
- High winds across portions of mainland Alaska and the Aleutians,
Wed-Thu, Oct 23-Oct 24.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Wed-Thu, Oct 23-Oct 24.
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