Scientific Forecaster Discussion
fxus65 kboi 130311 cca
Area forecast discussion...corrected
National Weather Service Boise Idaho
811 PM MST Tue Dec 12 2017
Discussion...dense fog continues in and near the western Magic
Valley and eastern portions of the upper Treasure valley, and
therefore the dense fog advisory will continue as issued. We have
made minor adjustments to the location of fog and stratus across
the area based on the latest satellite information and
expectations for changes overnight. Minor adjustments were also
made to the temp forecast overnight, as a few sites have already
gotten down to the predicted low. However, with relative
humidities nearing 100% in those same areas, the temps won't be
going much lower. Update out shortly.
Aviation...widespread low stratus and fog in the lower valleys,
with MVFR/IFR/LIFR conditions, will persist through Wednesday.
Surface winds will be light and variable. Winds aloft at 10 kft mean sea level
will be northwest 15-25 kts increasing to 20-35 kts on Wednesday.
Air stagnation...inversion remains with little change expected
tomorrow. Change possible with a system Friday night into
short term...persistence, meaning another round of dense fog
tonight and Wednesday morning in and near the western Magic Valley
and a continuation of low overcast and patchy fog in all other
areas below 4000 feet msl, and also in the Long Valley near
McCall. These conditions may repeat Wednesday night and Thursday.
Temperatures under the inversion will stay nearly constant through
Thursday with diurnal ranges of 5 degrees or less. Above the
inversion (about 5200 feet msl) skies will remain mostly clear
tonight, with slowly increasing high clouds Wednesday through
Thursday. Much larger diurnal temperature ranges in these areas
with lows zero to 15 and highs 35 to 45. Light winds.
Long term...Thursday night through Tuesday...patchy valley fog
and stratus will continue under inversion conditions Thursday
night through Friday morning as a ridge remains over the Pacific
northwest. Temperatures will be below normal once again along the
lower valleys. Looking for the inversion to finally mix out by
late Friday evening as the ridge erodes and a trough and its
associated front push through the region. This will bring windier
conditions for the interim. Precipitation is expected to fall as
snow across the mountains and as a wintry mix along the valleys.
There is potential for freezing rain in sheltered valleys that
erode slowly, but generally looking for the surface to warm above
freezing ahead of any falling precipitation. A west-northwest flow
will set up behind the trough Saturday evening, keeping a slight
chance of precipitation across the east-central Oregon and west-
central Idaho mountains, as temperatures warm to about normal
across the region for the remainder of the extended period.
Aviation...low stratus and fog in the lower valleys continuing to
cause MVFR/IFR conditions this afternoon. Seeing some clearing near
kbke and kmyl, but not expecting big changes at this point for the
remainder of the day. Looking for a repeat of low stratus/fog
beginning around 13/06z through the morning. Surface winds: light
and variable. Winds aloft at 10 kft msl: variable 5-15 kts.
Idaho...dense fog advisory until 11 am MST Wednesday idz014-016-030.