Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
249 PM EDT Fri Jul 20 2018

Valid 12Z Tue Jul 24 2018 - 12Z Sat Jul 28 2018

At least for the moment (as of the 00Z-06Z cycles) guidance shows
better than average agreement for the overall pattern evolution
during the period.

An upper low with associated surface system will likely persist
for multiple days just south of the Alaska Peninsula.  Consensus
has trended a few degrees longitude west with this feature--and
the ridges aloft to the west and east--compared to yesterday.  The
deep-layer low will tend to focus precipitation along/near
portions of the Alaska Peninsula and southern coast of the
mainland.  Specifics are very uncertain but models/ensembles in
principle are suggesting that height falls from eastern Asia and
perhaps the extreme western Pacific should gradually migrate into
the Bering Sea toward the late week/weekend time frame.  This
energy may merge with the aforementioned feature to the east.  At
the very least expect surface pressures to trend lower over the
Bering in response to these height falls but model/ensemble spread
and variability thus far yield low confidence in the surface
details. 

Across the higher latitudes, initially progressive Arctic flow
should lead to a front lingering over the North Slope region into
day 4 Tue, followed by another front that arrives Wed-Thu.  Latest
consensus shows an upstream ridge aloft building eastward and
eventually connecting with the northwest part of the ridge
covering extreme western Canada into Mainland Alaska.  The ECMWF
ensemble mean has trended stronger with the late-period ridge
aloft over recent runs but in principle has suggested this
evolution with greater lead time than the GFS/GEFS
mean/operational ECMWF.  Confidence in the Arctic forecast is
still moderate at best given the guidance spread/variability
observed over preceding runs.  The 12Z GFS highlights the forecast
uncertainty as it has adjusted faster than consensus with Arctic
features early-mid period and then is flatter than the 00Z/06Z
runs with the trailing ridge aloft.

The forecast blend started with input from the 00Z-06Z GFS/00Z
ECMWF and lesser weight of the 00Z CMC/UKMET for days 4-5 Tue-Wed
given good agreement in principle (with detail differences typical
for an early-extended forecast) and support from the means. 
Question marks with Arctic flow precluded using the 12Z GFS in the
forecast though the run seemed more reasonable farther south. 
After Wed the forecast used increasing amounts of the 06Z GEFS/00Z
ECMWF means so that there was even model/ensemble weight by day 8
Sat.  This solution brought some degree of low pressure into the
western Bering late in the period with a surface trough/frontal
axis extending southeastward.  Potential exists for a system to
exist within a fairly broad area along and either side of the axis
of lower surface pressures.

Rausch

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