Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
146 PM akdt Friday Jul 25 2014
Analysis and upper levels...
there is an upper low over central Alaska and another over the
northern Gulf of Alaska. There is a large area of high pressure
over the Bering Sea and Aleutian Islands. A trough/front/deformation
band is currently draped across south central Alaska between the
two upper lows.
the numerical models are in good agreement on the over-all
pattern through 84 hours. However...there is still quite a bit of
difference on how they handle the transition of the central Alaska
low into the Gulf low on Saturday night through Sunday
night...then still yet more differences on how they handle
easterly waves associated with the consolidated (by monday) Gulf
low...and with a system associated with the re-developing central
Alaska low late Sunday into Monday. Therefore forecast confidence
is below normal.
Short term forecast...
For south central Alaska...periods of rain will continue
tonight...tapering off towards morning as the deformation band
weakens and slowly slides east. By later Saturday the central
alaskan low will begin to dive south. As mentioned above...the
models are not in good agreement on the timing. The NAM is by far
the fastest with this system...and without support from other
models can probably be safely disregarded. The European model (ecmwf) and global
Gem are fairly close to each other bringing the low through south
central late Saturday night and Sunday morning...and fairly strong
with a closed low. The GFS is dancing to its own tune and takes
very little energy southward...basically slowly dissipating the
central alaskan low through Sunday evening. The "truth" is likely
closer to the European model (ecmwf)/Gem solution...but with a nod towards the GFS
due to the forecast uncertainty. This results in a forecast that
keeps some showers over the area...but generally east of
Anchorage. The European model (ecmwf)/Gem solution is also stronger with an
easterly wave on Sunday...bringing stronger winds and more
precipitation to the Gulf Coast than the other models.
There is another forecast challenge for Monday with the arrival of
a system from the northwest as the central Alaska low strengthens
again. The GFS/NAM suggest precipitation will arrive as early as
Monday afternoon to the western portions of Southeast Alaska while
the European model (ecmwf)/Gem are slower...with the system still back in southwest
For southwest Alaska...dry conditions are expected for the most
part through Sunday afternoon as the Bering ridge protects the
area. The exception is a weak system sliding south through the
eastern Bering tonight into Saturday morning that may bring some
light precipitation to coastal areas. On Sunday night and Monday a
system will dive in from the northwest. With the large range of
model solutions at this point...probability of precipitation will be "broad brushed" until
there is better clarity between the models.
Long term forecast...
High amplitude ridging centered over the Bering Sea will be
dominant beginning the extended period on Tuesday...with a classic
Omega block pattern across Alaska region. The dominant feature
across southern Alaska Tuesday will be a sprawling upper low in
the Gulf of Alaska...with a northerly shortwave trough moving
across southwest Alaska. By midweek...embedded vorticity maxima
rotating around the Gulf low will begin to impinge upon southern
Alaska...with the threat for strong easterly waves tracking west
across the southern part of the state. Timing differences among
the various global models are rather large...but they all hint at
a similar synoptic pattern and evolution. Stay tuned...
By late week...the pattern will not change dramatically...with
the European model (ecmwf) and GFS ensemble depicting a continued high latitude
ridging pattern over the Bering and a persistent low in the Gulf.
This pattern tends to favor temperatures near climatology...with
individual days strongly modulated by individual shortwave length
Ludwig/ja Jul 14