Area forecast discussion
National Weather Service Boise Idaho
831 PM MDT Friday may 24 2013
Discussion...asymmetric upper low over Washington was shifting north
after rotating a lobe cyclonically across our northern zones earlier
today. A new lobe was moving eastward across Oregon this evening
and it also is prognosticated across our northern zones overnight and
Saturday morning with another chance of showers. Our southern and
eastern zones will stay dry. A much stronger lobe will come into our
Oregon zones Sunday and then into our northern Idaho zones Sunday
night/Monday with more showers...and general troughing and
chance of showers will continue into at least the middle of
next week. In the short term...current forecast has highest
probability of precipitation Sunday afternoon but latest models from 18z say Sunday
night will be at least as wet. We await 00z models for new information
beyond Sunday. No updates for now.
&&
Aviation...VFR with areas of high and middle level clouds. Surface
winds...west to northwest around 10 kts. Winds aloft...southwest up
to 20 kts at 10k feet mean sea level.
Weekend outlook...VFR Saturday. Scattered showers and isolated
thunderstorms developing Saturday afternoon over the higher terrain
of eastern Oregon and the west central Idaho mountains...spreading
over most of southeast Oregon and southwest Idaho by Sunday
afternoon. Snow levels 7500-8500 feet mean sea level rising to 8000-9000 feet
mean sea level Sunday afternoon.
&&
Previous discussion...
short term...tonight through Sunday...the Memorial Day weekend will
start off dry as the upper low that has been parked over Washington
state lifts into Canada. Showers and isolated thunderstorms over
east-central Oregon and SW Idaho this afternoon will dissipate by
sunset. Air mass modifies into Saturday on strengthening SW flow.
Only a slight chance of showers or thunderstorms over mountains on
Saturday as large scale forcing is absent so development is
dependent on afternoon instability which is marginal. Clouds
increase Sunday ahead of a short wave trough moving into the Pacific northwest.
There are some differences on timing the feature. It becomes
negatively tilted as it moves inland which will likely slow it down
which would follow the GFS and European model (ecmwf) solutions. Height falls and
increased dynamical forcing will favor shower and thunderstorm
development over southeast Oregon and much of SW Idaho by late Sunday
afternoon. Cloud cover could cap development in the Snake River
Valley of Idaho so have only slight chance there. Raised the chance
of precipitation elsewhere across southeast Oregon and mountains of SW Idaho for
Sunday afternoon where confidence is higher. More clouds and some
cooling aloft will take 3-7f degrees off of high temperatures from
Saturday to Sunday.
Long term...Sunday night through Friday...the Pacific northwest
remains under a broad upper trough through the extended period.
Monday looks to be the driest and warmest day next week as region
will be between low pressure systems. Increased moisture associated
with the next upper trough on Tuesday will bring precipitation
across the entire area. At the moment...this looks like the best day
for widespread precipitation. Latest model guidance is trending this
system further north across the forecast area Tuesday. Models are
not in total agreement with this idea as a few GFS ensemble members
and the 12z European model (ecmwf) continue to show the trough closing over central
Nevada as it moves east. However...more solutions are trending
towards the GFS open wave across the forecast area and thus have
trended the forecast in this direction. This system lingers over the
region through Wednesday and is followed quickly by another system
Thursday/Friday. These systems will continue the chance of showers
and thunderstorms through Friday. Temperatures will remain below
normal throughout the extended period.
&&
Boi watches/warnings/advisories...
Idaho...none.
Or...none.
&&
$$
Weather.Gov/Boise
Discussion...lc
aviation.....Jt
previous short term...dg
previous long term....js/wh
