Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
700 PM EDT Sun Apr 12 2020
Valid 12Z Thu Apr 16 2020 - 12Z Mon Apr 20 2020
...Overview and Guidance Evaluation...
Guidance is consistent in showing a strong mean ridge aloft
prevailing over the northeastern Pacific into the Alaska mainland
and western Canada through late this week. Then most solutions
advertise some flattening/progression of the ridge in response to
weakening shortwave energy associated with the system reaching the
Aleutians Thu-Fri. Detail uncertainties with this energy and
nearby Siberia/Arctic flow continue to keep confidence fairly low
for wave details along a front likely to extend from the Bering
Sea across/near the northern mainland for most of the period.
Relative to typical late-period forecasts there is better than
average clustering for the next system which consensus shows
tracking across the North Pacific, though precise effects of this
storm on the Aleutians are still dependent on details not easily
resolved several days out in time. By next Mon this system may
promote a downstream ridge from the Northeast Pacific across the
southwestern mainland into the Bering Sea.
Broadly speaking today's guidance offered better agreement today
versus 24 hours ago. In addition operational models have been
consistent for the general evolution of the North Pacific storm
during the latter half of the period. Thus the initial model
blend started with an average of the 12Z operational models and
then maintained about two-thirds operational input (with the rest
consisting of the 12Z GEFS/00Z ECMWF means) through day 8 Mon.
Continuity/consensus are good for the system reaching the
Aleutians Thu-Fri. It is after Fri when detail uncertainties
arise downstream. There has been some trending of guidance toward
the GFS idea of Aleutians/Bering Sea low pressure (or
redevelopment) tracking into the northwestern mainland and beyond
by next weekend. However the 12Z GFS is again on the
deep/northward side of the spread for this surface low with the
06Z run closer to the consensus pattern by mid-late period. The
06Z GFS also looks better for eastern Asia through Arctic flow
aloft in this time frame. Therefore GFS input used the 06Z run
instead of the 12Z version after day 5 Fri. Other models
including the ECMWF exhibit meaningful run-to-run changes with the
specifics of weakening shortwave energy, favoring splitting ECMWF
input among the last two runs after Fri. Guidance has been
inconsistent with the southern periphery of cyclonic flow over the
Arctic as well, with the combination leading to variability in
position of the wavy front over the northern mainland. For the
second day in a row the consensus forecast has backed away
somewhat on southward progression of the front during the latter
half of the period.
Thus far there has been enough consistency/clustering in the
operational models to support an operational tilt to the forecast
of the Sun-Mon system south of the Aleutians. Latest ECMWF means
are closest to the operational scenario. The GEFS mean shows more
potential for interaction with lingering Bering Sea energy to draw
the surface low somewhat farther north. There is now a stronger
signal that a compact but deep bundle of energy, possibly an upper
low for a time, may emerge from eastern Asia (tracking over/near
Kamchatka during the weekend). Models increasingly diverge on the
speed of this feature, with some fast enough to merge with the
North Pacific system by next Mon and others considerably slower
with less direct influence. Issues with this upstream flow
suggest guidance could experience a period of greater spread than
what is currently the case.
...Weather/Threats Highlights...
One area of widespread precipitation through the late short-range
period (Wed) should progress eastward with some activity possibly
lingering into Thu over the extreme southeast mainland and
Panhandle. Expect another episode of precip across much of the
state from late week into the weekend with moist flow ahead of the
late week Aleutians/Bering Sea system and then waviness along a
Bering Sea into northern mainland front plus remaining shortwave
energy aloft progressing eastward. Best potential for significant
totals will be over southern and western portions of the mainland.
Rain/snow should become lighter and more scattered by late
weekend into early next week. Precipitation coverage/intensity
along the Aleutians and Alaska will be sensitive to exact details
of the storm forecast to track over the North Pacific by next
Sun-Mon.
Continue to expect most of the state to see above normal
temperatures during the period, with the exception of some
modestly below normal highs over the Panhandle and localized
pockets of the southern mainland. In most cases morning lows
should be more anomalously warm than the highs, while the northern
mainland will likely see the most extreme departures from normal.
Recent trends toward delayed southward progression of the wavy
front over the northern mainland would allow for greater
persistence of the much above normal temperatures.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards 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