Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Sat Jul 11 2020
Valid 12Z Wed Jul 15 2020 - 12Z Sun Jul 19 2020
...Overview and Weather/Hazard Highlights...
A relatively progressive flow pattern looks to persist across the
North Pacific through the first half of the extended forecast
period, with Alaska on the northern periphery of this area.
Ensemble guidance suggests a gradual amplification of the flow
over time, with troughing becoming more prominent across the
Aleutians, and ridging building north into the eastern Gulf of
Alaska and Southeast Alaska by late next week. Showers will be
winding down on Wed across portions of Southeast Alaska as a
weakening low pressure system pushes inland. Farther west, a low
pressure system is expected to cross the Aleutians through the mid
to latter part of next week, bringing an increase in showers and
perhaps some gusty winds. As this system moves east into the Gulf
of Alaska by next weekend, it should spread a round of showers
into southern portions of mainland Alaska, and eventually
Southeast Alaska as well. The setup farther north across the North
Slope an portions of the Interior will be a rather chaotic one,
with an arctic upper low initially north of eastern Siberia
retreating westward through time, and a rather murky picture of
what happens in its wake, but with some potential for additional
shortwave energy to affect northern Alaska, keeping some risk for
scattered showers and below average temperatures in place.
...Guidance/Predictability Assessment...
Model/ensemble guidance showed relatively good consensus early in
the extended forecast period, but this degraded quickly at the
smaller scales over time, despite some consensus on the described
amplifying large scale pattern across the North Pacific next week.
Similar to yesterday, 12Z ECMWF/CMC showed perhaps the closest fit
with the overall consensus early on, and a blend of these
solutions served as a basis for the WPC forecast during days 4-5
(Wed-Thu). After that, weight placed on ensemble means (ECENS and
NAEFS) was increased through time. This ensemble-heavy blend later
in the forecast period served to smooth differences among the
deterministic guidance when it comes to the timing/intensity of
individual, smaller-scale systems with low predictability at those
tie scales.
Ryan
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