Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
745 PM EDT Sun May 01 2022
Valid 12Z Thu May 05 2022 - 12Z Mon May 09 2022
...Overview...
The primary focus of the forecast will initially be on a compact
but deep upper low over the eastern Bering Sea/eastern Aleutians
late this week and a weaker feature over the Gulf of Alaska.
There is uncertainty over how far east the Bering Sea low will
progress after Friday but regardless, a moderate majority of
guidance suggests that northern stream flow should interact with a
mid-latitude Pacific system, ultimately bringing the latter on a
track to the south of the Alaska Peninsula and perhaps near/into
the Gulf of Alaska. Details of the system and any effects to its
north are still fairly uncertain though. Mainland Alaska should
see an ill-defined upper pattern, tending to favor weak ridging
Thursday-Saturday and then possibly weak southwesterly flow over
northern areas as a modest trough axis develops near the western
coast. Arctic high pressure should gradually make its way into
the Canadian Archipelago during the period.
...Guidance/Predictability Assessment...
Latest guidance provides some common themes but there is enough
spread to temper confidence in various details after about Friday.
The first notable divergence in guidance involves the eastern
Bering Sea upper low, with latest GFS runs already a bit west of
the ECMWF/UKMET/CMC to start the period early Thursday. With time
this difference leads to the GFS holding the feature over the
eastern Aleutians to interact with the approaching Pacific system,
in contrast to the others that progress it steadily eastward
(albeit with variations in latitude depending on any interaction
with a leading weaker low). Ensemble means generally lean toward
their parent models. Interestingly the ECMWF/ECMWF mean still
show enough lingering mean troughing/northern stream digging to
the west to influence the Pacific system, leading to the ECMWF/GFS
and their means depicting a similar overall pattern by late in the
period. Recent GFS/GEFS trends have been toward the ECMWF/ECMWF
mean idea. On the other hand the 12Z CMC/CMC mean suggest less
stream interaction by mid-late period, leaving the Pacific system
well to the southwest.
For the purposes of a single deterministic forecast, preference
favored the majority scenario for both the Bering Sea upper low
and then the evolution that brings low pressure to the south of
the Alaska Peninsula. This was achieved by way of a 12Z
operational model composite early, followed by phasing out the CMC
and trending the forecast toward an even blend of means (12Z
GEFS/00Z ECens) and models (12Z/00Z ECMWF and 12Z GFS) by day 8
Monday. The new 12Z ECMWF mean and 18Z GFS offered no significant
changes in continuity from their previous runs.
...Weather and Hazard Highlights...
Upper lows over the eastern Bering Sea/Aleutians and farther east
into or near the Gulf of Alaska, along with their surface
reflections, may produce periods of precipitation from the
Aleutians to the Panhandle late this week with most amounts on the
lighter side. The Alaska Peninsula and vicinity could see some
increase of moisture around late weekend/early next week with low
pressure that may track to the south. Moisture could eventually
extend eastward to the Panhandle. Track and timing of this
late-period system and path of its moisture are still uncertain so
confidence in specifics is fairly low. Mostly scattered and light
precipitation is possible at times primarily over the southern
half of the mainland.
Temperatures should maintain a consistent pattern for most of the
period, with below normal highs across the southern mainland and
Panhandle as well as the North Slope, with moderately above normal
highs over Interior areas. Morning lows will tend to have warmer
anomalies, leading to more coverage of above normal readings.
Sufficiently warm temperatures combined with the remaining snow
pack could present flooding concerns across portions of the Yukon
River Basin in eastern Alaska.
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