Wind Advisory 23, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
258 AM MDT Sat Aug 19 2023

.SHORT TERM...Today through Sunday night. Satellite imagery shows
deep low off California coast continuing to pump moist southerly
flow through Great Basin into Idaho. Further south, Hurricane
Hilary rotating north off southern tip of Baja peninsula. Will see
another day of isolated to scattered thunderstorms with locally
heavy rainfall before impacts from Hilary begin to be felt in East
Idaho. PWATs today increase again to just above 1" across most of
the Snake Plain and southern highlands. By late Sunday, the entire
forecast area is blanketed by PWATs of at least an inch, and over
1.5" across portions of the Snake Plain. Hilary plume trajectory
targets the western portions of the forecast area Sunday, with
deep southerly flow enhancing precip amounts into the Central
Mountains, and slightly downsloping the interstate portions of the
southern Snake Plain regions. 24hr probabilistic means ending
Sunday night at midnight support 0.50-1.0" of rainfall, with 90th
percentile amounts reaching over 1.5", the bulk of which falls
during daylight hours. The remainder of the area looks to be less
impacted, though isolated/locally heavy rainfall will still be
possible given the deep moisture air mass. If the plume shifts
further east under the influence of approaching northern stream
trough off PacNW coast, then so will the heavy rain threat. Winds
starting to become an additional concern as mid-level flow starts
to ramp up significantly during the day Sunday into Sunday night.
The 40-45kt 700mb flow likely to help mitigate some of the
precipitation potential in favored downslope regions, especially
I-84/I-86 corridors from Burley to Pocatello. Again, a slight
shift east in this regard will have potentially significant impact
on developing winds Sunday night and beyond, and precipitation.
For now, have gone with a Flash Flood Watch for the central
mountain zones given current expected plume trajectory and
orographic enhancement of rainfall into the region. Watch extends
all day Sunday through Monday. DMH

.LONG TERM...Monday through Saturday. The first part of next week
will be pretty active across central and eastern Idaho. Two big
stories will be unfolding on Monday. The one we have been advertising
the most is the rainfall remaining across the central mountains. We
are in the SLIGHT RISK FOR EXCESSIVE RAINFALL in collaboration with
WPC. We currently are forecasting for 24 hour rainfall 0.50-1.0" for
the Wood River Valley and surrounding mountains, Sawtooths and
Stanley Basin. Similar amounts are possible for the Lost River
Range, while the rest of the central mountains are looking 0.20-
0.50". Looking at probability forecasts, there is a 30-70% chance
of EXCEEDING 1" for the Sawtooths, Stanley Basin and surrounding
mountains...with 20-40% of over 1" for other areas mentioned before.
The other story which has been getting less attention is the very
highs across the area...with the strongest winds across the central
mountains and Montana border and along and south of the Snake Plain.
(The latter looks to be a very strong downslope event from Burley to
Blackfoot.) The current forecast calls for 30-45mph gusts common
across central and eastern Idaho. For the corridor from the Wood
River Valley to Island Park, we have gusts over 50 mph especially at
mid and upper elevations. Probability forecasts show 50%+ of seeing
over 50 mph and a wider range of 60 mph winds, 10-50% depending on
elevation. From Burley to Blackfoot and southward, 50-60 mph gusts
are likely. There is a 40% chance or higher of 60-70 mph winds along
and just south of 84 and 86...with a small area of 70 mph winds (up
to 60%) across the Albion Mountains. We will likely see changes in
the wind forecast as we get a better idea of where Hilary will
track. Rain and wind quickly wind down into Tuesday, but we still
expect pockets of showers and thunderstorms as weak "wave" sweeps
northeast around the main low to our northwest. 48 hour totals are
in the 1-2" range for the central mountains, with a bullseye of 2-3"
across the highest elevations. We will issue a FLOOD WATCH for those
areas Sunday and Monday. Beyond that, we will off and on showers and
thunderstorms as multiple storms swing across the state. Keyes

&&

.AVIATION...VFR conditions give way to at least a VCTS threat for
MVFR thunderstorms all terminals this afternoon. Southerly flow
already increasing with gusty winds across the southern Snake
Plain mainly impacting KPIH this morning, expanding this afternoon
to include KIDA and KBYI. Showers continue overnight, increasing
in intensity for KSUN early Sunday as first push of deep tropical
moisture arrives. Southerly flow strengthens, for locally breezy
winds enhancing early in the day. DMH

&&

.FIRE WEATHER...The weather across central and eastern Idaho will
continue to be pretty active into next week. For today, another
surge of moisture spells showers and thunderstorms...with the best
chances for thunder across the central mountains (422/475/476),
eastern highlands (411) and southeast highlands (413). We will see
a mix of wet and dry storms today, with the best potential for
wetting rains across the central mountains, 411 and 413. For
Sunday and Monday, the focus turns to very heavy rainfall across
the central mountains thanks to the remnants of Hilary rapidly
moving into the northwest U.S., which is quite the rare occurrence
this far north. We currently have widespread 1-2" amounts over 48
hours across that area, with the potential of 2-3" across higher
elevations of 422 and adjacent areas of 475 and 476. There is
definite concern for debris flows with these amounts, especially
if we get higher rainfall rates due to embedded thunderstorms
across Ross Fork, Boundary and Hayden Burn areas. There is a FLOOD
WATCH in effect for portions of the central mountains, and
potentially be expanded to all of the central mountains within the
next 24 hours. There is a small strip of 0.40"+ along the Lava
Beds in 425, the Craters/Arco Desert portion of 410 and into the
northern part of 411. Otherwise, rainfall amounts are lighter and
more sporadic in nature. Besides the rain, Monday looks to be
windy, especially across the central mountains and along the
Montana border, as well as from Burley to Blackfoot south. We
currently calling for 30-45mph gusts common across central and
eastern Idaho. For the corridor from the Wood River Valley to
Island Park, we have gusts over 50 mph especially at mid and upper
elevations. Probability forecasts show 50%+ of seeing over 50 mph
and a wider range of 60 mph winds, 10-50% depending on elevation.
From Burley to Blackfoot and southward, 50-60 mph gusts are
likely. There is a 40% chance or higher of 60-70 mph winds along
and just south of 84 and 86...with a small area of over 70 mph
winds (up to 60%) across the Albion Mountains. Outside of the
central mountains, even with higher humidity, that winds of this
magnitude could still promote fire spread. We will be closely
monitoring the potential over the next 24 hours. Beyond that
period, we will see multiple rounds of showers and thunderstorms
as more than one area of low pressure impacts the state. Keyes

&&

.HYDROLOGY...Remnants of Hurricane Hilary impact East Idaho Sunday
through at least Monday with periods of moderate to heavy
rainfall. Main impacts for now appear to be targeted across the
Central Idaho Mountains where deep moisture coincides with deep
strengthening southerly flow. Probabilistic means show at least
0.5-1.0" of rainfall each day Sunday and Monday. 90th percentile
supports 1-1.5" Sunday and 1.5-2.0" Monday. A Flash Flood Watch
has been issued for 6am Sunday-Midnight Monday night. DMH

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Sunday morning through Monday evening for
IDZ068>075.

&&