Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
309 PM EDT Wed Oct 02 2019
Valid 12Z Sun Oct 06 2019 - 12Z Thu Oct 10 2019
Guidance continues to show a fairly amplified but progressive
pattern during the period. There should be a tendency toward
amplification of troughs reaching the northeastern Pacific,
consistent with teleconnections relative to a strong positive
height anomaly center which multi-day means have been showing near
the central Aleutians.
Within this pattern the first significant feature is a
mainland/northeastern Pacific upper trough that may close off a
low over the Gulf of Alaska around day 5 Mon and support
associated surface low pressure. Guidance differences have been
fairly persistent with the ECMWF/ECMWF mean slower than the
GFS/GEFS mean, while other guidance has not fully committed to one
side of the envelope through the 00Z/06Z cycles. The 00Z CMC/CMC
mean highlight this idea, as the operational run trended closer to
the ECMWF versus its prior run but the CMC mean sides with the GFS
cluster. The overall progressive pattern does not particularly
favor the slowest side of the spread, plus by early Mon the 00Z
ECMWF is on the strong/northwest fringe of the ensemble envelope
with the Gulf surface low. However with teleconnections
suggesting mean troughing over the Northeast Pacific it is
possible the GFS cluster could be a bit fast. Thus prefer an
intermediate solution which keeps the forecast fairly close to
continuity. It remains to be seen if the trend holds in
successive runs, but most new 12Z guidance has generally adjusted
to the faster scenario.
Upstream most guidance has been fairly consistent in principle for
the system which is the extratropical reflection of current
Tropical Storm Mitag. The 06Z-12Z GFS/00Z ECMWF and corresponding
ensemble means (plus CMC mean) cluster quite well for this system,
with the only adjustment from continuity being a slightly farther
south track across the northern Bering Sea. The CMC strays
increasingly northward of consensus and thus could not be used in
the forecast after the start of the week. Also consistent with
yesterday, expect low pressure to develop over the Gulf by next
Wed-Thu as the weakening parent low reaches the western mainland.
Based on the above considerations today's forecast started with a
compromise of 00Z/06Z operational model guidance (and a little 12Z
GFS) early and then trended to a model/mean blend consisting of
the 06Z-12Z GFS/00Z ECMWF and the 06Z GEFS/00Z ECMWF means. The
initial Gulf system will promote a period of enhanced
precipitation near the southeastern coast and Panhandle with the
supporting upper trough bringing a broader area of lighter
activity to locations farther west. The second system will spread
moisture across the southwestern half to two-thirds of the
mainland and eventually to the southeastern coast/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
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sat-Sun, Oct 5-Oct 6.
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