Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Mon Oct 21 2019
Valid 12Z Fri Oct 25 2019 - 12Z Tue Oct 29 2019
The dominant focus of the forecast will be the expected
amplification of the pattern as the deep extratropical evolution
of Neoguri reaches and lingers near the Aleutians. The
strengthening deep-layer gradient between the upper low/trough and
downstream ridge that builds into the northeastern Pacific and
mainland Alaska/northwest Canada will bring a significant amount
of moisture into the Alaska Peninsula and a many portions of the
mainland. The GEFS/ECMWF means reflect the overall guidance
agreement for this evolution, with both showing precipitable water
values reaching 3-4 standard deviations above normal in the late
Sat-Sun time frame. Expect highest precipitation totals along
southward/southeastward-facing terrain along the Alaska Peninsula
and southern coast of the mainland. Somewhat less extreme but
likely still very significant precipitation should extend into
other portions of the southern and western mainland.
Extratropical Neoguri will bring areas of strong winds and high
waves as well.
As for the guidance evaluation, solutions cluster fairly well for
extratropical Neoguri late this week with the main adjustment from
yesterday being a slightly southward trend by Sat--though not as
far southward as the 00Z CMC (12Z run has returned northward). By
the weekend into next week there are persistent uncertainties over
details of one or more separate surface lows along the leading
front and into the Bering Sea. Based on guidance available
through arrival of the 12Z GFS, an operational model blend
highlighting the 06-12Z GFS and 00Z ECMWF represents consensus
details for extratropical Neoguri during the first half of the
period. Transitioning the forecast toward an even weight between
the models and 06Z GEFS/00Z ECMWF means later in the period
accounts for the most common details in the guidance and also the
agreeable larger scale pattern.
Farther upstream the models and ensembles have so far been quite
varied and inconsistent with the ultimate extratropical evolution
and track of Typhoon Bualoi. Thus far a higher percentage of
guidance indicates a southern track that would miss the Aleutians.
Currently GFS/GEFS-based guidance is generally on the faster side
of the spread, and given historical biases the preference is to
lean the aforementioned blend partially toward the ECMWF/ECMWF
mean by days 7-8 Mon-Tue.
Guidance has not been very consistent with some aspects of higher
latitude flow as well. Latest consensus brings a shortwave
initially just west of the northern mainland eastward with time,
becoming flatter as the upper ridge builds to the south. The 00Z
CMC was questionably fast/amplified on Fri while the array of new
12Z guidance suggests the GFS could be a tad amplified with the
shortwave by late Fri-early Sat.
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
Alaska Hazards...
- Heavy precipitation across portions of mainland Alaska, Sat-Mon,
Oct 26-Oct 28.
- Heavy rain across portions of mainland Alaska and the Aleutians,
Fri, Oct 25.
- Heavy snow across portions of the Alaska Panhandle and
southeastern mainland Alaska, Thu, Oct 24.
- High winds across portions of the Aleutians, Thu-Fri, Oct 24-Oct
25.
- High significant wave heights for coastal portions of the
northeast Gulf of Alaska and the Alaska Panhandle, Thu, Oct 24.
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