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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1902Z Oct 15, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Oct 15 2018

Valid 12Z Fri Oct 19 2018 - 12Z Tue Oct 23 2018

...Multi-day heavy precipitation possible across
southern/southeastern Alaska by the end of the week and into next
week...

The large scale flow pattern across Alaska and the north Pacific
will remain relatively stable and unchanged during the medium
range period (Fri - Tues). This features a mean trough centered
near/south of the Aleutians sandwiched in between
persistent/blocking upper ridging. A number of shortwaves/low
pressure systems will rotate through this mean trough, with a
nearly constant plume of Pacific moisture directed into southern
and southeastern Alaska. While models show agreement on the larger
scale, there continues to be disagreement on the specific details
of individual systems. Two significant surface lows approach the
Alaska Peninsula during the medium range period. With the first
low, located in the northern Gulf by day 4 (Friday), the only
outlier appears to be the 00z ECMWF which has a low position well
south by this time than the deterministic GFS/CMC, the ensemble
means, and previous shift continuity. The second low right on its
heels, should begin approaching the western Gulf western Alaska by
Day 6-7 (Sunday - Monday). There is good agreement in the models
on the existence of this low, but differences arise with respect
to track and timing/placement, especially as we get farther out in
time. Some ensemble members are quite intense with the central
pressure of this second low, as indicated by ensemble mean SLP
values around low 980 hPa. This cycle of the WPC Alaska progs used
a majority deterministic blend (lesser weighting of the ECMWF,
especially on day 4) early in the period, with increasing
weighting of the GEFS/ECENS by day 8 to help mitigate the
small-scale noise/differences. This provided a forecast very close
to previous shift continuity as well.

The stable upper-level flow will result in a sustained period of
southerly mid-level flow into Alaska from the subtropics, with
each low pressure system enhancing precipitation potential. Models
show general agreement on a multi-day heavy precipitation event
for portions of southern and southeastern Alaska with several
inches (liquid equivalent) of precipitation looking likely,
especially at windward locations near the coast. The flow regime
will prevent any arctic air from making its way into Alaska, thus
expect temperatures to be near or slightly above climatological
norms through the extended period.

Santorelli


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html