Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Mon Apr 30 2018

Valid 12Z Fri May 04 2018 - 12Z Tue May 08 2018

...Guidance and Predictability Assessment...

Models have improved slightly compared to yesterday with some
signs of a consensus finally emerging for late this week into the
weekend, although any consensus remains rather tenuous due to some
continued run-to-run variability. Later in the forecast period,
uncertainty continues to be relatively high, with some significant
model differences remaining. The 00Z ECMWF/CMC along seemed to
best represent a consensus solution through the weekend. Based on
ensemble consensus, a solution including a surface low initially
over the southeastern Bering Sea at 12Z Fri was favored, with the
development of a secondary wave of low pressure along the surface
front farther south, reaching the Gulf of Alaska on Sat. The great
variability continues to surround the timing and intensity of this
secondary wave, developing ahead of a relatively vigorous
shortwave and upper jet streak.

Models show general agreement that the large scale pattern will
become increasingly amplified by the weekend into early next week,
with ridging building northward along the North American west
coast into Southeast/East Central Alaska. There has been a uniform
trend across most guidance in a somewhat slower evolution of this
flow amplification and the building ridge, although the consensus
remains that it will occur. Meanwhile, additional shortwave energy
reinforces the large scale persistent trough south of Alaska. The
result will be an elongated area of deep southerly flow ahead of
the trough, directed into southern Alaska and the Gulf. The
amplified flow regime will also increase the potential for
interaction/absorption of Arctic shortwave energy into the
westerlies, enhancing the uncertainty surrounding the forecast
specifics late in the period. Nonetheless, models/ensembles show
some consensus that another trough/upper low may amplify across
the western Bering Sea by late in the weekend, with another
potential surface cyclone moving eastward across the Aleutians as
well. Confidence in the placement or strength of this system
remains, given the aforementioned factors. Given the increased
uncertainty during the Mon-Tue time frame, ensemble mean (00Z
ECENS/NAEFS) weighting in the forecast was increased substantially
during that time period.


...Pattern Highlights/Threats...

An active pattern is anticipated for Alaska and vicinity late this
week into early next week. The southern Bering Sea low by late in
the week may threaten maritime interests with heavy seas/gusty
winds, as well as increasing rain/snow coverage and intensity
across the Alaska Peninsula and southern/southeastern Alaska. The
second surface low crossing the Gulf and moving into southern
Alaska on Sat may bring additional maritime hazards as well as
rain/snow and gusty winds across portions of southern/southeastern
Alaska. Meanwhile interior areas of Alaska will see some
occasional rounds of lighter precipitation, the first as the
Bering low and occluded surface front move inland/decay. As flow
trends more southerly by later in the period, this should result
in increasingly dry conditions for interior portions of Alaska.
Warming temperatures are also expected for much of Alaska by early
next week as heights rise in association with the building ridge.

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

Ryan