Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
644 PM EST Fri Mar 06 2020
Valid 12Z Tue Mar 10 2020 - 12Z Sat Mar 14 2020
...Overview and Guidance/Predictability Assessment...
From day 4 Tue into day 6 Thu the model and ensemble guidance
shows remarkably good agreement and consistency for the overall
pattern evolution. There are some typical detail differences and
consensus has refined some specifics compared to yesterday. Over
and near the Gulf of Alaska the operational models are suggesting
that a short range wave tracking south of the Alaska Peninsula
should reach the Panhandle around the start of the extended
forecast early Tue, with a trailing northern Gulf wave supported
by an upper trough that extends from the mainland into the Gulf.
Expect surface pressures to rise with time as the upper trough
progresses eastward. Farther west models/means still show a
weakening system over the Bering Sea/Aleutians Tue-Wed though with
some variation in specifics. Meanwhile guidance is well-clustered
for a strong storm forecast to track near Japan and Kamchatka into
Siberia, then into the Arctic later in the period. Westward GFS
trend, especially in the 12Z cycle, has brought that model in line
with the majority while the GEFS mean is still a bit to the east.
The agreement seen in the first half of the extended period
favored a 12Z operational model blend (GFS/ECMWF/UKMET/CMC) to
represent important features while accounting for lingering detail
uncertainty.
From late Thu into the weekend guidance diverges with the
evolution and potential interaction of two streams of flow
emerging from eastern Asia. This leads to increasing
model/ensemble spread and variability for the character of flow
aloft reaching the Bering Sea/Aleutians and eventually into the
mainland. The past two ECMWF runs provide the most simple
depiction of this spread by day 8 Sat as they become nearly 180
degrees out of phase over the Bering. The past couple ECMWF means
and 12Z GEFS/CMC means hint at a flat ridge aloft over the Bering
by next Sat, suggesting that the 12Z ECMWF/GFS may be more correct
in principle versus the cyclonic flow in the 00Z ECMWF. However
the means are not yet too enthusiastic about the 12Z ECMWF/GFS
specifics upstream so preference would be to trend the forecast
more toward ensemble guidance late in the period. Thus the
initial forecast blend adjusted to 30 percent means by early Fri
and 70 percent for Sat. The ensemble component used the 00Z ECMWF
mean and 12Z NAEFS mean, the latter used to lessen GEFS influence
for the storm tracking through Siberia and into the Arctic. With
time there will be increased sensitivity of the storm track to
specifics of the upstream flow.
...Weather Highlights/Threats...
Enhanced precipitation associated with one or more lows near the
Panhandle/southeastern coast of the mainland into midweek will
trend lighter with time as supporting dynamics move off to the
east. The forecast has been consistent in showing cold high
pressure bringing a period of much below normal temperatures
across the state from west to east. Flow ahead of the strong
storm tracking across Siberia around Thu and then continuing into
the Arctic should bring a significant warming trend with western
and northern parts of the state being the first to see a rebound
to above normal readings. The greatest warm anomalies will likely
be over far northern locations. Any lingering below normal
temperatures should be confined to the southeastern
mainland/Panhandle by next Sat. Expect the aforementioned storm
to bring a broad area of strong winds across the Bering Sea and
northwestern mainland. Also moisture spreading into the mainland
late this week combined with terrain enhancement will produce some
areas of moderate to heavy precipitation.
Rausch
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
coastal southern Alaska, Mon-Tue, Mar 9-Mar 10.
- Heavy precipitation across portions of interior southwestern
Alaska, Thu-Fri, Mar 12-Mar 13.
- Heavy snow across portions of western Alaska, Thu, Mar 12.
- Heavy snow across portions of southern Alaska, Fri, Mar 13.
- High winds across portions of northwestern Alaska, Thu-Fri, Mar
12-Mar 13.
- Much below normal temperatures for much of western Alaska,
Mon-Tue, Mar 9-Mar 10.
- Much below normal temperatures across central to eastern Alaska,
Wed-Thu, Mar 11-Mar 12.
- Much above normal temperatures for much of the Arctic Coastal
Plain of Alaska, Fri, Mar 13.
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