Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Sun Oct 20 2019
Valid 12Z Thu Oct 24 2019 - 12Z Mon Oct 28 2019
The deep storm forecast to be near the Alaska Peninsula late in
the short range period (midweek) will weaken and progress eastward
later in the week, leading to a gradual lighter trend for
initially enhanced precipitation along the southern
coast/Panhandle. Meanwhile the very deep extratropical reflection
of Typhoon Neoguri will likely track near the Aleutians late week
and begin a pattern evolution toward greater amplitude and slower
progression. From Sat onward the details become hazy on specifics
of the parent surface low, a possible frontal wave farther north
over the eastern Bering Sea, and one or more waves that may track
northward along/ahead of the Pacific front associated with the
parent low. Regardless of the finer details there is a decent
signal for a strong and persistent flow of significant
moisture/precipitation aimed at the Alaska Peninsula and parts of
the southern coast of the mainland by the end of the week/next
weekend. Some of this moisture will overspread other portions of
the mainland. Extratropical Neoguri will produce areas of strong
winds and high waves as well.
Guidance clustering has improved significantly for the system
expected to weaken over the southwest corner of the mainland by
Thu. An operational model blend reasonably represents consensus
while accounting for lingering detail uncertainties, with timing a
little slower than the latest ensemble means. This seems
reasonable as recent trends have shown solutions gravitating
toward the middle to slower half of the prior guidance spread.
For the most part the models and ensembles cluster fairly well for
extratropical Neoguri near the Aleutians into day 5 Fri. After
that time solutions diverge (more north-south than east-west) for
the parent low. There are also indications of a possible frontal
wave to the north over the eastern Bering Sea, plus various ways
of depicting one or more waves that may lift northward along/ahead
of the front associated with the initial parent low. Trending the
operational model blend early in the period to an even model
(00-06Z GFS and 00Z ECMWF/CMC) and ensemble mean (06Z GEFS/00Z
ECMWF mean) blend provides a good balance of reflecting the most
agreeable aspects of the large scale pattern and providing a hint
of individual wave details.
Toward the end of the period day 8 Mon the guidance is very widely
dispersed regarding the extratropical evolution and track of
current Typhoon Bualoi. In addition models and even ensemble
means have been inconsistent for timing. The model/mean blend
preferred above provides an appropriately conservative forecast,
minimizing effects on the Aleutians until confidence can improve.
Across higher latitudes the main trend over the past day is toward
more troughing near the northwest corner of the mainland during
the first half of the period. The initial blend provides a good
account for this trend while not committing to a specific
scenario--given the fairly new nature of this trend and current
guidance spread. Also of note, by day 8 Mon the overwhelming
majority of models and ensemble means would suggest the 12Z GFS
becomes too quick with its eastward progression of eastern Bering
upper low/trough energy.
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
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