Area forecast discussion...corrected
National Weather Service Boise Idaho
925 PM MDT Friday may 17 2013
Discussion...a negatively tilted upper level trough axis from
around Portland Oregon to Elko Nevada at 8 PM brought large scale
lift across southeast Oregon and SW Idaho this afternoon. Isolated
thunderstorms extended from north Malheur County to central Owyhee
County and were lifting towards the northeast at 10-15 miles per hour. This
trough axis will spread through SW Idaho tonight and likely keep
isolated thunderstorms going with it...until around midnight
including for the Treasure valley. Evening weather balloon
sounding showed instability of -1.4c Li and 341 j/kg cape along
with light shear of less than 20 M/S...so storms look to remain
pulse-like with local gusts of 30 miles per hour but up to 45 miles per hour...such as
occurred at Grassy Mountain RAWS in the late afternoon. Small hail
was reported with another stronger looking cell near Buhl and a
menacing looking shelf cloud and rain or rain and hail shaft was
shared via facebook from the tcastleford area. Forecast is on
track with no updates expected.
&&
Aviation...scattered showers and thunderstorms mainly south and
west of the Snake River Plain at 02z will push east-northeast this
evening. Expect brief MVFR ceilings...heavy rain and gusty winds
with these storms. Otherwise...scattered to broken clouds deck
generally at 5k-7k feet above sea level. West-northwest surface winds at 10-15
kts. West winds aloft up to 20 kts at 10k feet mean sea level.
Weekend outlook...scattered MVFR showers and a slight chance of
thunderstorms Saturday afternoon - mainly across southeast Oregon and
higher terrain of SW Idaho. Sunday...predominantly VFR with showers
confined to the central Idaho mountains and eastward.
&&
Previous discussion...
short term...tonight through Sunday...heating of the day combined
with moisture associated with an upper trough was resulting in
scattered showers and a few thunderstorms. Instability was marginal
to support thunderstorms...although enough to keep isolated coverage
in the forecast through this evening. Drier stable air will
temporarily move into southeast Oregon late tonight and Saturday morning
with a short wave upper ridge. Another trough will then move into
the pacnw Saturday afternoon...then slowly work eastward across the
area through Sunday. The second trough will bring more moisture and
a chance of mountain/slight chance of valley showers...although the
system will be weaker and with less moisture. Snow levels will lower
to around 7500 feet mean sea level with light snow accumulations possible on
the peaks. Temperatures will remain slightly below normal through
the weekend. Breezy northwest winds are expected Saturday afternoon
with a little stronger winds expected Sunday afternoon...especially
in the upper Treasure valley and western Magic Valley.
Long term...Sunday night through Friday...a shortwave ridge will
build into the area from the west early Monday...allowing skies to
clear and temperatures to rise to near normal values Monday afternoon. As a
deep low pressure system slides southward into the Pacific northwest
on Tuesday...southwest flow will pick up over the intermountain west
which will drive the afternoon high temperatures to around 5 degrees
above normal. The low will continue to move southward...bringing a
strong surface cold front through southeast Oregon and SW Idaho late Tuesday
night. Expect high temperatures on Wednesday to be nearly 20 degrees cooler
than Tuesday. The low pressure system will stall out over the Pacific northwest
through the end of the week...keeping conditions cool and unsettled.
Most of the available moisture looks to be on the northern half of
the low circulation...with the southern portion of the low affecting
our area. Thus the best chance for showers will be across our
northern and Western Mountain zones.
&&
Boi watches/warnings/advisories...
Idaho...none.
Or...none.
&&
$$
Weather.Gov/Boise
Discussion...vm
aviation.....Js
previous short term...bw
previous long term....cb
