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Area Forecast Discussion (AFD)

Issued by Juneau, AK (AJK)

                            
897 
FXAK67 PAJK 212357 CCA
AFDAJK

Southeast Alaska Forecast Discussion...CORRECTED
National Weather Service Juneau AK
257 PM AKST Sun Jan 21 2018

.SHORT TERM...Our focus continues to be on widespread snow showers
and accumulations on Monday but attention was also given to
another system bringing more showers along the coast and for the 
south going into Tuesday. 

Our stable longwave pattern with two ridges, one over the Bering
and another over Western Canada, and a deeply amplified trough
extending south from Mainland Alaska well into the Gulf of Alaska
has been and will be dictating our weather for some time. Cold 
air from the Bering regions of Alaska long-ago dove south to round
the base of the trough and currently rests in the outer gulf 
poised to enter through our southern reaches on Monday. With 
plenty of lift due to steep lapse rates close to 8C/km and 
positive vorticity advection, we should see potentially intense 
showers spread across Southeast following an occluded front on
Monday. Showers will begin in the far south late Sunday night and
through Monday spreading north across the rest of our CWA. 
Temperatures will remain marginal for snow in the south and along
the eastern and southeast coast, and with some building 
thicknesses tonight ahead of the main low- level trough, we feel 
increasingly confident that some areas in the far south may only 
see rain showers tonight. However, as the front lifts north, 
colder air will rush in to support a mix, even as temperatures 
will remain in the middle to upper 30s. North of Petersburg and
inland of Sitka temperatures should support all snow. 

In the north and interior Panhandle we have concerns on snow 
accumulations and potential impacts. We have raised snow amounts 
to ranges just shy of advisory, perhaps 1 to 4 inches Monday. With
more southerly flow in the low to mid levels than easterly, 
orographic lifting will be enhanced in many areas including 
Juneau. Steep lapse rates will migrate inland as well. Given the 
convective nature of the snow, we do foresee some areas 
experiencing some higher snowfall accumulations and potential 
short-term white-outs. But this polar, more marine, air mass, 
should also result in a wetter snow, one that compacts a little 
more easily. Thus, we have kept ratios around 10:1. And in 
addition, to all this, the southerly gradient behind the front has
tended to eat well- prepared higher snow forecasts for lunch. So 
the rain-snow line could potentially make its way farther north 
than forecast. The band of enhanced precipitation looks to be
around 6 hours, but we may see a bit longer duration of 9 hours 
for Juneau, which is one reason we felt confident enough to raise 
amounts, but somewhat concerned as not enough.

Looking farther in time for Tuesday, the wave lifting Monday's
system stalls moisture across the northern Panhandle and the
eastern gulf. A more southern originating wave lifts northward to
contribute to more snow shower enhancement with some rain mixed 
in along the immediate coast. Models are mainly holding back 
precipitation expanse for the central interior and northern Inner 
Channels, in part due to increasing outflow, so impacts may be 
most felt along the coast and the south. But we do look for more 
light snow accumulations for portions of the western and southern
interior communities. 

Outflow strong winds are still afflicting Lynn Canal and Cross
Sound with 30 kt and gusts to 40 mph in Skagway. These are 
forecast to steadily decline through tonight with some enhanced 
southerlies appearing in the far south around daybreak Monday 
morning. These southerlies will lift northward following the 
frontal passage. But there remains some uncertainty as to weather 
they reach Lynn Canal and Skagway. Currently, we are skeptical and
have held back. Leading into Tuesday, a new low develops well to 
our south that changes the gradient quite rapidly back to 
northerly and increases through the time period. Thus, we have 
stronger outflow winds redeveloping into Tuesday. 

We relied on ECMWF and NAM for any updates to pop/qpf, but the gfs
and ecmwf with some wind edits later on Tuesday. Forecast 
confidence is good for snow in the north/central regions tomorrow 
with a mix to the south. However, while we give general ranges for
amounts, Mother Nature will likely gift us a few surprises in 
snow accumulations for a few locations Monday evening. 

.LONG TERM.../Tuesday through Sunday/...Long range period begins
with a large upper level low dominating the Gulf of Alaska and
another located over the northern interior. Upstream, a building
upper ridge will be situated over the Bering Sea. The gulf low
exhibits a slight positive tilt, but becomes negatively tilted by
Wednesday morning as the northern interior low drops south. By
Thursday, the original low shifts east to Hecate Strait and 
weakens as the northern interior low moves south into the 
northwest gulf and deepens. By this time. the upstream ridge will 
have strengthened into an Omega block, with another upper low near
Hokkaido and Sakhalin island. By the end of next weekend, the 
Omega block remains strong and in place and a large upper low once
again dominates the Gulf of Alaska. 

The combined effect of the upper low(s) over the gulf and the
building upper ridge to the west will be to advect in colder low 
level air over the panhandle. Some modification of this airmass 
will occur as it crosses a considerable distance over the open 
ocean, but the ridge will prevent any warmer air from the west 
from mixing in. 850H temperatures marginally supportive of 
snowfall on Tuesday and then again from Thursday onward. Similar 
story for 1000-500 thicknesses. 1000-850 thicknesses remain 
marginal to favorable for the entire week. Daytime highs over the 
southern half of the panhandle from Frederick Sound south 
indicating that there is a good chance that at least some of the 
precipitation for this coming week will be in the form of rain. 
Otherwise, what precip we do see may be mostly in the form of 
snow. No well defined fronts currently being depicted in medium to
long range models, so convective precip (showers) is anticipated 
through at least Tuesday night. 24 hour snow totals at this time 
indicating possibly advisory level snowfall at most. Major caveat 
there is that a snowfall forecast for 3-5 days into the future in 
a convective environment in Southeast Alaska is a stretch at best,
so keeping track of how this forecast changes is in everybody's 
best interest. 

No significant changes to pressure or winds. PoP and QPF from GFS
and the Canadian NH with some help from ECMWF through Thursday
night and the latest WPC thereafter. Daytime highs and overnight 
lows generally adjusted down slightly for the second half of the 
week based on current model spectrum plots. Overall forecast 
confidence is average, but below average for snow totals.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 9 PM AKST this evening for AKZ018.
MARINE...Gale Warning for PKZ012. 
     Small Craft Advisory for PKZ013-022-031-033-035-036-041>043-051-
     052. 

&&

$$

JWA/Fritsch

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