Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
706 PM EDT Sun Sep 05 2021
Valid 12Z Thu Sep 09 2021 - 12Z Mon Sep 13 2021
...Pattern Overview and Model Guidance...
The latest suite of model and ensemble guidance continues to show
very good agreement on the overall large scale synoptic pattern
through much of the Alaska extended range period which features a
broad upper level low which should drift southward across the
Bering Sea. Several shortwaves look to rotate through the base of
the upper low sending a couple of low pressure systems near the
Aleutians and into the Gulf/Mainland region. The first low, which
could become a fairly deep cyclone (several solutions suggesting a
central pressure near 980mb) should slide south of the Aleutians
and into the Gulf Thursday and Friday. Guidance shows enough
agreement to warrant an equal blend between the 12z GFS, ECMWF,
and CMC, but even the deterministic solutions still struggle
somewhat on exact location. After day 5, the guidance becomes
increasingly chaotic with the details of additional shortwave
perturbations and surface low pressure systems. Thus, the WPC
blend trended quickly towards the ensemble means which helped
mitigate these smaller scale uncertainties, while maintaining
modest amounts of the 12z GFS/ECMWF just for added system
definition. This drives a second cold front through the Alaska
Peninsula/southern coast on Sunday, reaching the Panhandle by next
Monday.
...Sensible Weather...
Moderate to locally heavy rainfall will accompany the first
surface low as it drifts through the Gulf from the AK Pen/Southern
Coast into the Panhandle Thursday-Saturday. Expect marine hazards
and gusty winds to also impact parts of the coast, the strength of
which highly dependent on exact location of the low in the Gulf. A
cold front moving through western Alaska will also bring
precipitation as well, especially for terrain areas, with some
precipitation in the form of snow. As this interacts with the next
shortwave south of the Aleutians next Sunday, another round of
moderate to locally heavy rain is likely for southern Alaska and
the Panhandle, with precipitation particularly enhanced over the
terrain. Thursday may be particularly warm for parts of northern
and eastern Alaska, but should trend back towards or below normal
by early next week. Across the Panhandle and southern half of
Alaska, general cloudiness and unsettled weather should keep
temperatures near or below normal through the entire period.
Morning lows however should remain above normal through next
weekend for most of the state.
Santorelli
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