Area Forecast Discussion
Issued by NWS Boise, ID
FXUS65 KBOI 200358 CCA
Area Forecast Discussion
National Weather Service Boise ID
857 PM MST Sun Feb 19 2017
.DISCUSSION...Pacific upper level trough brought showers and
isolated thunderstorms to our CWA today. Trough was exiting east-
northeast and showers have decreased this evening, but skies will
remain mostly cloudy through the night. Next Pacific moisture
surge, currently entering central Calif, will turn northeast into
our area by noon Monday, more into Idaho than Oregon. More
energetic Pacific storm upstream will overtake that moisture and
increase rain in our CWA Monday night and Tuesday in both Oregon and
Idaho. Southwest winds will increase in southeast Oregon Monday
night and Tuesday under very strong west-southwesterly flow aloft.
Wind advisory or high wind watch is under consideration for Tuesday
in southern Harney and Malheur Counties. Tuesday will be mild
temperature-wise but temps will drop significantly Wednesday through
the rest of the week as a long wave trough develops over western
North America. Currently forecast temps, PoPs, and winds look good
through the short term so no updates for now.
.AVIATION...VFR tonight except for local MVFR/IFR conditions in
heavier showers. Precipitation becoming widespread by Monday morning
with mountain obscuration and areas of MVFR and local IFR conditions.
Snow level near 5k ft msl. Conditions improving Monday afternoon
becoming mainly VFR in the Snake River Valley and lower elevations
of southeast Oregon. Surface winds southeast to south 5-15 kts
tonight increasing to 15-25 kts by 18z on Monday. Winds aloft near
10k ft msl southwest 25-40 kts.
SHORT TERM...Tonight through Tuesday...The upper wave that`s
supporting shower and thunderstorm development over the area this
afternoon will lift northeast overnight. Any thunderstorm threat
will dissipate by sunset and precipitation chances will decrease
across southern portions, to include the Snake River Plain, into
Monday morning. However, showers will continue across the east-
central Oregon and west-central Idaho mountains overnight, but to
a lesser coverage and intensity. Warm advection on southwest flow
ramps up on Monday bringing a resurgence of rain/snow to the area.
Given the more stable air mass mountains will be focus of heavier
precipitation, with shadowing possible in downwind valleys. Snow
levels will rise to between 6-7k feet for Monday afternoon into
Tuesday morning, dropping back to around 5k feet by Tuesday
afternoon. Expect breezy conditions across higher terrain on
Monday, but the windiest day will be Tuesday, as mid-level winds
(700 mb) push 50 kts over much of the area. With the stronger
winds and mixing, Tuesday will see the warmest temperatures,
especially at lower elevation sites.
LONG TERM...Tuesday night through Sunday...Southwest flow aloft
Tuesday night with a deepening trough offshore. Snow levels start
Wednesday around 4000 feet MSL, but quickly drop to valley floors by
Wednesday afternoon. The majority of the precipitation and moisture
stays out of the valley during this time, but chances still remain
for snow showers. With temperatures climbing above freezing during
the daytime, impactful accumulations are not expected in the lower
elevations. After Thursday a cold northerly flow is in place aloft
keeping the snow levels down at the valley floors, however models
continue to disagree on any feature coming through the CWA. Kept
precipitation chances in through the extended period, but still not
expecting any major accumulations in the lower elevations.
Temperatures stay below normal during the period.
.HYDROLOGY...Flood watches are now in effect for both the
Weiser River near Weiser, ID, and the Malheur River near Vale, OR,
Tuesday and Wednesday, based on current QPF and snowmelt forecasts.
These inputs may change before then, but confidence is growing that
the rivers will reach or surpass flood stage. A hydrologic outlook
(ESFBOI) also remains in effect, highlighting the continued threat
of lowland and small stream flooding at low-elevations sites still
holding a snowpack. Rivers should recede rapidly after Wednesday as
colder and drier air comes in.
PREV SHORT TERM...DG
PREV LONG TERM....JC