Prognostic Meteorological Discussion
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FXAK02 KWNH 252308
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
708 PM EDT Sun Jul 25 2021

Valid 12Z Thu Jul 29 2021 - 12Z Mon Aug 02 2021

...Guidance/Predictability Evaluation...

The latest ensemble means from the ECMWF, GFS, and CMC remain
reasonably compatible with each other through Day 8--basically
keeping the Bering Sea low and the ridge across southern and
eastern Alaska in place.  Some erosion of the ridge is probable as
there is general ensemble consensus for an upper trough to edge
closer from the Gulf of Alaska next weekend and into Monday.  The
GFS has been the fastest with respect to bringing a developing
cyclone into the Gulf of Alaska next weekend while the ECMWF and
CMC have been slower.  Meanwhile, the 12Z GFS appears to develop a
spurious low pressure system south of the Aleutians.  The 06Z GFS
appears more reasonable in this regard.  Therefore, a general
compromise of the ensemble means together with the 12Z ECMWF, 06Z
GFS, and 12Z CMC should work well as a starting point.  The WPC
Alaskan grids were based on an even blend of these
models/ensembles with a smaller contribution from the CMC starting
on Day 4.  By Day 7, only 25% of the deterministic GFS/EC/CMC was
used with the reminder portion from the ensemble means.

...Weather/Threats Highlights...

The main areas of precipitation and unsettled weather will be
focused across the western and northwestern mainland throughout
the period as southerly flow ahead of the Bering Sea cyclone
interacts with stalled stationary boundaries under persistence
deep-layer southerly flow.  A reinforcing trough from eastern
Siberia could ingest moisture from T.S. Nepartak and is forecast
to reinvigorate the Bering Sea low next weekend into early next
week to sustain the general rainfall across western Alaska.  Heavy
rain can also be expected especially across the wind-facing
upslope terrains.  Temperatures should remain below normal for
western and northern parts of Alaska but they should trend warmer
over eastern parts of the state as upper level ridging builds in
aloft.

Kong


Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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


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