Southeast Alaska forecast discussion...corrected
National Weather Service Juneau Alaska
356 PM akdt Friday may 24 2013
Short term...weather is still rather quiet across the area but
there is a bit more to talk about today. The sunny weather of the
past few days has given way to a mostly cloudy to overcast middle to
high level deck and a few scattered rain showers. The showers
have mainly been confined to the southern Panhandle and along the
Coast Mountains as Ketchikan, Juneau, and Stewart, British
Columbia have all reported some very light rain showers today.
This is the result of a low in British Columbia that is sending
some weak upper level waves over the Coast Mountains. However low
level offshore flow is drying out the lower layers of the
atmosphere creating a rather substantial warm and dry layer in
upper air soundings particularly in the north, and is making it
hard for the rain that does make it over the mountains to reach
the surface.
This high to middle level cloud layer and the showers will continue
into tomorrow as more weak upper level waves move through and try
to overcome the low level offshore flow. Chance to slight chance
probability of precipitation and mostly cloudy to cloudy skies remain in the forecast for
most areas of the Panhandle into tomorrow. With the highest probability of precipitation
concentrated near the Coast Mountains or the southern Panhandle.
The exception is Yakutat since it will remain mostly dry as it is
too far west for the weak waves to reach it.
Meanwhile the outer coast will have drier conditions but will
have to deal with the marine layer clouds and fog again tonight.
Last night saw the marine layer creep into Yakutat, Cross Sound,
Pelican, Sitka, and Klawock with reduced visibilities observed at
the former three. I expect a repeat performance tonight with the
low clouds possibly getting to Gustavus and Hydaburg late tonight
before they retreat again tomorrow morning.
With all the clouds around temperatures today and tomorrow will
not be as warm as they were yesterday. Most places will most
likely only make the upper 50 to low 60s tomorrow. However this
will also result in warmer overnight lows as the high overcast
prevents a rapid cooling effect from radiational cooling. As a
secondary result, sea breeze circulations will not be as strong
over the next 24 hours either.
Fire weather. Fuel conditions remain dry over the northern
Panhandle today. While humidity values moistened up overnight the
Skagway and Haines areas already have humidity values in the
25 to 35 percent range. Skagway also had a brief burst of wind
this morning so for a short time some very high fire danger did
exist in Skagway this morning. Those winds have since slackened
off a bit and will continue to decline this afternoon. Plus, with
the high clouds moving in the air temperature is not expected to
reach much higher than it already is. So extreme fire weather
conditions are not expected for the rest of the day but will be
watched closely. Dew point temperatures are expected to rise
tomorrow as low level flow turns to the south and with continued
cloud cover temperatures will remain low so no extreme fire
danger is expected for tomorrow at this point.
Short range models were in good agreement on overall features.
Although there were some consistency issues on how the models
handled the precipitation once it got over the mountains. GFS in a
previous run was a bit overaggressive bringing the precipitation across
the southern Panhandle today. Decided on using the NAM with a bit
of the European model (ecmwf) or GFS here and there. Forecast confidence is above
average.
&&
Long term...models remain in general agreement with a weakening
ridge of high pressure through Monday across the eastern Gulf and
the Panhandle followed by a weak frontal passage associated with a
low in the Gulf through the middle of next week. The low will
expand its area to become a broad across Southeast Alaska. Blended
the pressure field with the GFS and European model (ecmwf) for their good consensus
through late Wednesday then used the new wpc outputs for the
remaining periods. Used 50/50 of the GFS and European model (ecmwf) for probability of precipitation and
quantitative precipitation forecast values for entire periods.
A developing low in the western Gulf early next week will
continue to expand over the Gulf and weaken the ridge of high
pressure to the east. As the ridge weakens and dissipates the
expanding low will become a broad area of low pressure across
Southeast Alaska by the middle of next week. The weak frontal
passage will bring likely probability of precipitation to the central and southern
Panhandle coastal areas Wednesday and Wednesday night before
dissipating by late Wednesday night. As the low becomes a broad
area across Southeast Alaska...the extended forecast GOES mostly
cloudy skies with a chance pop and no significant winds/seas.
&&
Ajk watches/warnings/advisories...
public...none.
Marine...none.
&&
$$
Eal/ahn
