Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
748 PM EDT Tue Mar 16 2021
Valid 12Z Sat Mar 20 2021 - 12Z Wed Mar 24 2021
...Overview and Guidance/Uncertainty Assessment...
Guidance shows a progressive and active pattern across the Bering
Sea/North Pacific into the Gulf of Alaska. A leading eastern
Bering system should track into the Gulf of Alaska during the
weekend and then a North Pacific storm will likely turn
northeastward into the Gulf during the first half of next week,
followed by another system that tracks into the Bering Sea. These
systems are likely to focus the highest precipitation totals over
the Panhandle and perhaps to parts of the southeastern coast, with
somewhat lesser totals from the Aleutians through the rest of the
southern coast. An upper trough initially over Siberia and
Kamchatka should play a role in the evolution of the second Gulf
system and support reinforcement of below normal temperatures as
it settles over the mainland next week.
Latest models and ensemble means display better than average
agreement for significant features through the period. Thus a 12Z
operational model blend provides a good representation of the most
likely scenario through the first half of the period, and in spite
of some detail or timing differences that develop later in the
period the models/means are sufficiently comparable to allow
keeping two-thirds operational model weight into day 8 Wednesday
with the 12Z GEFS/00Z ECMWF means providing the rest of the input.
Even with the decent clustering in today's guidance there are some
trends of note. At the start of the period early Saturday the
leading Bering Sea into Gulf system is farther north than
yesterday's consensus. The combination of stronger low pressure
on the trailing edge of this system and somewhat slower
progression of the upper trough approaching from Siberia/Kamchatka
ultimately leads to a better defined surface wave across the
western/northern mainland plus trailing front from the weekend
into next week. The slower adjustment of the upper trough also
leads to a slightly farther westward track of the storm that
initially moves south of the Aleutians and then continues toward
the Gulf as the trough phases with the Pacific dynamics.
Uncertainty over the specifics of this phasing process will likely
provide some lingering detail questions for a while. The primary
differences with the late period Bering storm and associated front
involve timing, with the GFS characteristically on the faster edge
of the envelope. The favored blend provides a good intermediate
starting point with operational model variability over the past
couple days making it difficult to rule out any individual
solution at this point.
...Weather/Hazards Highlights...
This weekend's system will generally produce light to moderate
precipitation from the Alaska Peninsula/southwestern mainland
through the southern coast and Panhandle. Early next week the
Aleutians and Peninsula may see a period of precipitation and
enhanced winds on the northwest side of the trailing North Pacific
system but this will be sensitive to the exact track of the
surface low. Then this storm will quickly bring increasing
moisture to the southern coast and especially Panhandle with
Tuesday-Wednesday being the most likely time frame for potentially
heavy precipitation. Models and ensembles currently disagree over
the magnitude of precipitable water anomalies (12Z ECMWF being the
highest, reaching at least plus 2-3 standard deviations for a
time) but this event merits monitoring over the coming days given
the lower-latitude origin of the moisture. The system forecast to
track across the central latitudes of the Bering Sea late in the
period would bring a front across the southern Bering and
Aleutians into midweek, accompanied by precipitation and brisk to
strong winds.
Expect a majority of the state to see below normal temperatures
during the period with anomalies tending to be colder for daytime
highs relative to morning lows (with some pockets of slightly
above normal anomalies possible for the latter). Extreme northern
areas may see moderately above normal readings. The upper trough
arriving into the mainland during the first half of next week
should push temperatures somewhat farther below normal compared to
the weekend.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Tue, Mar 23.
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