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Prognostic Meteorological Discussion (PMD)

Issued by National Weather Service (NWS)

                            
000
FXAK02 KWNH 181854
PMDAK 

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Sun Aug 18 2019

Valid 12Z Thu Aug 22 2019 - 12Z Mon Aug 26 2019

...A wet week coming up for the dry AK Panhandle...

The models indicate a could of cyclones are poised to bring
significant rainfall to the AK panhandle.  One occurs on day 3 and
another, within this forecast period, occurs days 4-6.

A blend of the 00Z EC ensemble mean and the 06Z GEFS mean,
together with a smaller portions of the deterministic 00z
ECMWF/00z Canadian/06z GFS were used to generate the 500 mb height
forecasts and sea level pressures/fronts and gridded probability
of precip and wind fields for days 4 to 8.

Following the Wed low moving onshore into southeastern Alaska
Wednesday, the next system upstream develops in the north Pacific
south of the Aleutians. The 12z Thu 22 Aug location is south of
Kodiak Island, with the low starting to weaken as it approaches
the AK panhandle Fri. The 00z ECMWF is on the south edge of the
cluster envelope with the 00z ECMWF Ensemble Mean further north
and close to the 00-12z GFS track, which is also supported by the
00z Canadian global and Ensemble Mean.

The occluded portion of the low curls back to the AK peninsula Fri
night to early Sat, keeping cyclonic flow across the Gulf of AK.
This low drifts east over the weekend. The 12z run of the GFS
slowed this system down a few hours.

Over the Arctic Ocean/Beaufort Sea, persistent low heights and a
few waves of low pressure are forecast this period. Several models
and ensembles indicate a wave on the front developing on days 4-5
as the system drifts east.  The 06z GFS
had a wave moving down the back side of the trough into northwest
AK next Sun 25 Aug, but the 12z run backed off on the amplitude of
the wave.

With both the GFS and ECMWF making big run run changes in the
timing/phasing of the trough and embedded shortwaves.
significantly more weighting was given to the 06z GEFS Mean and
00z ECMWF to avoid committing to any one run, given the large
variability run to run, and lack of agreement among the
ECMWF/GFS/Canadian global models.

Petersen

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

$$