Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Mon Mar 11 2019
Valid 12Z Fri Mar 15 2019 - 12Z Tue Mar 19 2019
Confidence is relatively good with respect to the large scale flow
pattern across Alaska and the North Pacific during the extended
period, with persistent mean upper-level troughing near the
Aleutians/Alaska Peninsula, and an upper ridge axis rebuilding
across western/northwestern Canada into portions of mainland and
Southeast Alaska. ECENS and GEFS D+8 standardized anomalies
provide strong support for the persistence of this large scale
pattern, with maximum positive 500 hPa height anomalies centered
over west central Canada, and minimum negative anomalies centered
between the Aleutians and Hawaii. This setup will keep the
preferred storm track generally south of the Aleutians and into
the Gulf of Alaska, with persistent southerly flow into Alaska.
Models/ensemble members showed relatively good clustering for a
low pressure system moving northward across the Gulf toward Kodiak
Island late Fri-Sat, which should then quickly weaken as it moves
inland late Sat into Sun. Thus, the forecast during days 4-5
(Fri-Sat) was based heavily on a blend of the ECMWF/GFS solutions,
which showed relatively good consensus during this early portion
of the extended period. While spread is a bit higher given the
time frame, models show general agreement that another low
pressure system should follow a similar track north across the
Gulf on Mon, with yet another following quickly on its heels just
beyond the forecast period. Despite increased spread on the
timing, intensity, and structure of these systems later in the
extended period, ensemble scatter plots reveal some semblance of
clustering even through the end of the period. Thus, ensemble
means may perhaps provide some realistic semblance of the
evolution of these systems and their tracks. Thus, the forecast
was shifted gradually toward heavier weighting of ECENS and GEFS
ensemble means starting on day 6 (Sun), with deterministic
solutions completely absent from the forecast by day 8 (Tue).
The continuous track of storm systems across the Gulf into
southern Alaska, and the persistent deep southerly flow will make
for a period of widespread and occasionally heavy precipitation
for many areas from southern mainland Alaska to Southeast Alaska.
The air mass will quickly lost its moisture content farther
inland, however, with relatively dry conditions expected to
persist across the Interior and the North Slope. Additionally, the
Fri-Sat system may bring the potential for high wind gusts to
southern mainland Alaska, particularly for coastal areas.
Ryan
Bullet points from the WPC Days 3-7 Hazards chart:
https://www.wpc.ncep.noaa.gov/threats/threats.php
- Heavy precipitation across portions of mainland Alaska, Thu-Mon,
Mar 14-Mar 18.
- High winds across portions of mainland Alaska, Thu, Mar 14.
- High winds across portions of mainland Alaska, Fri-Sat, Mar
15-Mar 16.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can 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