Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Wed May 09 2018

Valid 12Z Sun May 13 2018 - 12Z Thu May 17 2018

...Guidance/Predictability Assessment and Preferences...

On Sunday morning, a mean upper ridge along the western coast of
British Columbia will be in the process of eroding in response to
upstream height falls lifting through the Gulf of Alaska. To its
north, a slowly retrograding upper low should drift back toward
the far eastern Bering Sea although there are some hints at
absorption given close proximity to neighboring shortwaves.
Gradually the focus will shift toward amplification taking place
over the north-central Pacific. Relative to yesterday, there is a
bit more operational spread which is noted in recent ensemble
spaghetti plots. By Day 8/May 17, recent runs of the GFS favor a
quicker progression of the parent upper low which significantly
suppresses downstream heights along the west coast of North
America. This remains in stark contrast from the 00Z ECMWF/CMC
solutions which are slower which maintains a pronounced downstream
ridge. Ensemble means seem to favor this scenario which suggests
the 12Z GFS and preceding runs may be outlying in nature.
Regarding the preference, initially took a consensus of the 12Z
GFS and 00Z ECMWF/CMC with some contributions from the ensemble
means through Day 5/May 14. Thereafter, removed the 12Z GFS in
favor of a more ensemble based approach led by the 00Z ECMWF
ensemble mean. Confidence remains a bit below average late in the
period given how much the guidance has shifted during the past day.


...Weather/Threat Highlights...

A warm up will be in store across the Alaskan interior,
particularly over central portions of the state. Once the upper
low evacuates the region, building heights will support
temperatures pushing into the mid 60s while the coldest
temperatures remain along the northern coast bordering the Arctic
Ocean. Afternoon highs will likely struggle to get above freezing
in such locations. Considering precipitation, scattered showers
are likely over much of western Alaska in response to the drifting
upper low while a more organized threat is likely over
southeastern portions of the state. Eventually the focus is
forecast to shift to the upstream amplification which will enhance
the potential for widespread cloud cover and rainfall over the
Aleutians and into south-central Alaska from Tuesday onward.



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

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