Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Sun Apr 28 2019
Valid 12Z Thu May 02 2019 - 12Z Mon May 06 2019
There are still multiple uncertainties in the forecast but
compared to previous days there is improved clustering among
models/means for some features. One such improvement is for the
upper high expected to drift over the Arctic during the period.
Not surprisingly the operational models have been more
variable/diverse than the means but even the means have waffled on
the latitude of the feature. The current model/mean cluster is
close to and with time perhaps just a bit north/west of the
multi-day average of the means. The other system whose clustering
looks much better is potentially strong low pressure whose track
spread currently centers along the Aleutians but includes the
North Pacific and Bering Sea. Latest guidance is faster with this
storm versus solutions that had at least a hint of it yesterday.
Meanwhile confidence remains quite low for details within a
mainland upper weakness which may incorporate some flow from
northwestern Canada. There are still many different ideas on
depth/timing of retrograding shortwave energy. Prefer to hold
onto a moderate solution given the tendency for deepest/fastest
retrograding solutions south of a closed high not to work out the
best. Trends over the past few days do seem to be hedging away
from prior solutions that had been on the deeper side of the
spread with retrograding energy. Another low-confidence aspect of
the forecast will be upper dynamics/surface low pressure over and
southwest of the southwestern mainland early in the period.
Predictability is low due to the relatively small scale of energy
aloft. Some of this flow may also feed into the aforementioned
weakness to the south of the Arctic upper high. New 12Z guidance
has changed enough with the combination of these uncertain upper
features that the front initially near the Panhandle/southeastern
mainland does not progress southwest into the Gulf as seen in
prior established consensus (and today's manual forecast). This
change will have to be watched for confirmation in future runs.
The preferred forecast blend started with a 70/30 model/mean
weight for days 4-6 Thu-Sat, incorporating the 00Z ECMWF and
00-06Z GFS (more 00Z run due to its less aggressive retrograding
height falls over the mainland) along with the 06Z GEFS/00Z ECMWF
means. Relative agreement in the larger scale evolution held up
well enough to allow for nearly even model/mean input by day 8 Mon.
Rausch
Bullet points from the WPC Days 3-7 Hazards chart can be found at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
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