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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1937Z Jun 13, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Wed Jun 13 2018

Valid 12Z Sun Jun 17 2018 - 12Z Thu Jun 21 2018

...Heavy rainfall expected along the southern Alaskan coast into
the panhandle...

...Guidance Assessment and Weather Highlights...


Reasonable model agreement exists on the more global-scale pattern
featuring an impressive mid-level ridge across western Canada.
While differing in precise location, the general idea depicted in
the GFS/ECMWF solutions is to build an extensive ridge with likely
anomalies in the 2 to 3 sigma above average range by the middle of
next week. This would support a persistent negative height anomaly
across Alaska and its periphery while details are a bit more
nebulous farther upstream. Such a pattern supports significant
poleward moisture fluxes aimed at the southern Alaskan coast
stretching into the panhandle region. This will lead to an
atmospheric river setup as precipitable water values reach the
97.5 to 99th percentile based on the NAEFS Ensemble Situational
Awareness Table. Comparisons of the 27-km GFS and ECMWF suggest
very hefty 5-day rainfall totals, generally in the 3 to 5 inch
range along the mentioned region. Recent GEFS probabilities showed
scattered locations with over 60 percent chances of exceeding 5
inches of rainfall during the period. Of course, multiple embedded
maxima well above that range are possible given the degree of
moisture transport coupled with persistent southwesterly mid-level
flow. Details will be ironed out while this multi-day event moves
into the short range but this signal has been noted for quite a
while now.

Considering further details of this pattern, decent model
agreement is in place through Day 5/Monday as the initial deep
cyclone (extratropical remnants of Maliksa) crosses the eastern
Aleutians over the weekend. While this system weakens, an
additional shortwave in the chain takes aim a little farther down
the Alaskan coast with forecast spread less than previous days.
The associated deep cyclone eventually moves into south-central
Alaska by roughly 18/1800Z. At this point, the upstream pattern
exhibits considerable uncertainties as suggested by ensemble
spaghetti plots and poor run-to-run model continuity. By early
next week, the 12Z/06Z GFS depict a surface ridge while the 00Z
ECMWF suggests an area of low pressure will be located upstream
across the north-central Pacific. This has remained a contentious
region of the map when doing multi-run comparisons. As such, felt
it was prudent to shift toward a more ensemble-based approach
beyond Day 5/June 18 with a three-way split of the 12Z GEFS/00Z
ECMWF/00Z NAEFS ensemble means utilized. Before then, was able to
maintain a fair amount of the 12Z/06Z GFS and 00Z ECMWF to capture
more details of the cyclonic flow affecting southern Alaska. 


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