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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1958Z Oct 13, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 PM EDT Sat Oct 13 2018

Valid 12Z Wed Oct 17 2018 - 12Z Sun Oct 21 2018

...Multi-day heavy precipitation possible across
southern/southeastern Alaska next week...

...Guidance and Predictability Assessment...

A nebulous forecast looms across the state and its periphery as
vast model spread plagues the Day 5-8 period. Initially, global
models are in agreement with a weakening surface wave spinning
over far southwestern Alaska. Amplifying flow underneath this
closed low is where the guidance quickly begins to break down. A
very impressive low is likely to materialize and lift toward the
northeastern Pacific during the Thursday/Friday timeframe. Recent
runs of the ECMWF have tended to be on the slower side of the
pack. Operational models over the past day or so agree on
developing such a system but very little continuity exists with
timing and placement. Given the lack of consistency amongst
individual models coupled with spatial differences between the
global models themselves, confidence is below average until some
semblance of clustering is evident. With the potential for a
surface low reaching pressures below 970-mb, wind fields will
likely be much stronger than advertised in the latest forecast.
What ever does materialize should lift sharply toward the north
given the potent upper ridge sitting over British Columbia and the
Yukon Territory. To complicate the forecast further, another deep
cyclone is possible over the far western Aleutians. Ensemble low
plots literally show a wave train of cyclones spanning the entire
northern Pacific so it may be a while until the guidance resolves
these complexities. As expected, the preference was heavily
ensemble based from Day 5 onward utilizing a combination of the
12Z GEFS mean and 00Z ECMWF/NAEFS ensemble means. Before then,
kept small proportions of the 12Z GFS/00Z ECMWF in the mix to
capture the southwestern Alaska low on Day 4/Wednesday.


...Weather Highlights/Hazards...

Vast differences in the solutions make some of the impacts more
uncertain than usual. Assuming these cyclones emerge as shown, a
fairly lengthy period of moderate to locally heavy precipitation
would be in store along the south-central Alaskan coast into the
panhandle region. Based on the high-resolution 12Z GFS model
output, 96-hour precipitation totals would be in the 3 to 6 inch
range across the former region while numbers nearly doubled would
occur toward southeastern Alaska. Additionally, gusty winds are
possible at times depending on the eventual pressure gradients,
potentially in the 40 to 60 knot range. At this juncture, it is
too difficult to pinpoint where this would even occur along the
coast. Regarding temperatures, a cool down is anticipated over the
northern section of the state with readings dropping into the
low/mid 20s during the day and teens at night next weekend.


Rubin-Oster

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html