Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Sun Jun 09 2024

Valid 12Z Thu Jun 13 2024 - 12Z Mon Jun 17 2024

...Overview...

Today's guidance continues to show a blocky regime from the latter
half of this week into early next week, with upper ridging
initially over the Arctic settling over the mainland.  However
there are various uncertainties regarding the
shape/extent/strength of the ridge as well as for a mean
deep-layer low forecast to be over the North Pacific/Aleutians. 
An upper low expected to track northward from the northern Bering
Sea could ultimately have some influence on the northwestern
mainland but with low confidence in that idea so far.  The warm
upper ridge will increase the potential for fire weather concerns
over portions of the mainland while the mean low near the
Aleutians should support unsettled weather over that region.

...Guidance Evaluation and Preferences...

Dynamical and ECMWF-initialized machine learning (ML) guidance
agrees fairly well with the Arctic upper ridge dropping down into
the mainland later this week but spread steadily increases for the
shape, extent, and center of the feature with time.  There are
also meaningful differences for the latitude of the mean low over
the North Pacific/Aleutians, with resulting effects on the
Aleutians.  Most of the ML solutions eventually offer potential
for somewhat greater westward elongation of the ridge across the
central/north-central Bering Sea versus the average of dynamical
guidance (12Z ECMWF/ECens means closest to the ML theme), along
with potential for the upper high center over the mainland to be
displaced more southwest or west (at least as far as the 12Z
CMCens mean).  At the same time the ML models depict the mean low
near the Aleutians--composed of an initial feature and then
another one that moves in from the west--over or within 2-4
degrees latitude just south of the Aleutians for most of the
period.  Predictability appears fairly low regarding how these two
individual features may interact.  Depending on the valid time,
various averages of the ensemble means and 12Z GFS/ECMWF/CMC are
not too far from the overall ML model themes for the mean low.

One other lurking forecast issue is with the upper low forecast to
be over the northern Bering Sea as of early Thursday.  Recent
consensus had been to lift it steadily northward with minimal
influence on the mainland or other aspects of the forecast. 
However the 00Z ECMWF had it drifting over the Bering Sea most of
the period (not supported by any other guidance), and sporadic
model runs such as the 00Z CMC and 12Z ECMWF/UKMET have brought it
close to or into the northwestern mainland.  ML models show very
low potential for that scenario, favoring higher heights aloft
over the northern mainland.

With minimal effect on the Panhandle aside from a period of
easterly flow aloft, a northeastern Pacific upper low should track
eastward just south of Haida Gwaii.  There is still some timing
spread by late week but a slower trend in the 12Z ECMWF has helped
clustering somewhat.  Today's guidance is showing some potential
for trailing energy to form another upper low over the
northeastern Pacific late in the period.

The start of the forecast period employed a nearly even blend of
the 12Z GFS/ECMWF/CMC, with the UKMET not comparing was well to
the majority cluster for various aspects of the forecast.  With
time, some aspects of the ensemble means become preferable whether
to downplay uncertain dynamical model details or based on general
themes of the ML models.  Thus the blend gradually increased 00Z
ECens/12Z GEFS-CMCens input during days 5-8, reaching 70 percent
total weight at the end of the period.

...Weather/Hazards Highlights...

Expect the upper ridge settling over the mainland to bring a
warming trend toward multiple days of above normal temperatures,
especially over much of the Interior.  This will increase fire
weather concerns with specifics depending on winds and moisture,
as well as any scattered convection.  Some scattered areas of
rainfall will be possible over southern areas and the North Slope,
with these regions plus parts of the Panhandle seeing pockets of
below normal high temperatures.  There should be broader coverage
of above normal min temperatures.  The mean low near the Aleutians
(composed of one or more separate systems) will likely support
unsettled weather over the region, with coverage and amounts of
rainfall at any particular time depending on the exact shape of
surface low pressure and upper dynamics.

Rausch


Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart 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