Flash Flood Warning 29, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
938 AM MDT Mon Jun 12 2023

.UPDATE...We are pushing out a quick update mainly for rainfall
amounts and chance of rain through tonight. The highest amounts
are expected across the central mountains, Snake Plain/Magic
Valley and the bench areas from Burley to the Big Holes.
Probability forecasts have 30-60% of over 0.50" in those areas
this afternoon through late evening. The threat of flash flooding
and river/stream spikes is there across the entire area today,
along with small hail. Gusts or outflow boundaries with winds over
35 mph are possible, with a bullseye across the Snake
Plain and Magic Valley. Showers and isolated storms are possible
overnight, with a much lower risk of heavier rainfall.  Keyes

&&

.PREV DISCUSSION... /issued 329 AM MDT Mon Jun 12 2023/

SHORT TERM...Today through Wed night. The next two days will
feature thunderstorms, again, and some potentially heavy rainfall
with them. For today, the probability of exceeding 0.50 of an inch
is very high on the southeast side of the central Idaho mountains
and in the Bear Lake basin area; for Tue, this focal point
continues. Increased cloud cover for Tue will cool temperatures
somewhat for afternoon highs, but keep overnight lows mild. On
Wed, the pattern shifts eastward and the heaviest precipitation
will be east of Interstate 15. Also, the thunderstorm coverage
will trend to isolated or low scattered conditions. A cooler air
mass will settle in, continuing the cooling trend everywhere. Wind
should pick up in the mid- morning hours with a Lake Wind
Advisory likely for the American Falls Reservoir as wind in the
Snake River plain exceeds sustained 20-plus mph wind at that time.

The main forecast issue is how the precipitation will affect
rivers. The convective nature of this event has shown a large
deviation for precipitation, as much as 0.70 of an inch of rain in
6 hours to less than 0.10 of an inch for the same location. Would
not be surprised to see Flood Watches for flash flooding as the
afternoon heating spikes up thunderstorm activity. Messick

LONG TERM...Thursday through next Monday.
Thursday marks the start of a brief break from the rainy/stormy
pattern we've been stuck in for quite some time now. That doesn't
mean we will completely dry out, but more of the area will see some
dry time. For Thursday, PoPs are highest through the Eastern
Highlands, but staying generally at or below 30 percent. We will
finally be out of the WPC's Excessive Rainfall Outlook on Thursday,
too. This comes as we are in between two lows. One is to our south,
over southern California, and the other is up to our north in
Canada. We will primarily be influenced by west/northwest flow from
the Canadian low. PoPs are even lighter on Friday, less than 20
percent through the Eastern Highlands, as the Canadian low lifts
northward and the low to our south begins to weaken. Models begin to
diverge more by Saturday. The ECMWF shows a mostly dry day while the
GFS brings another round of showers and storms back to the area in
the second half of the day. This is mainly due to how both models
are handling the speed of the next low coming into the PNW with the
ECMWF bringing it in a bit slower than the GFS. Either way, both
models agree on the return of rain and storms next Sunday and
continuing into early next week. Still, the 8 to 14 day outlook from
the CPC shows a 40-60% chance of above average precipitation from
the 19th-25th, so don't expect to totally dry out any time soon.

It looks like we'll be breezy during the extended forecast period as
these systems roll through the area. Look for wind gusts 20-30 mph
each afternoon, though we likely get stronger winds early next week.
Saturday will be the warmest day of the week with highs in the 70s
to low 80s. Otherwise, expect afternoon highs generally in the 70s
and then 60s next Monday.
AMM

AVIATION...12Z Monday to 12Z Tuesday.
Generally VFR conditions are expected throughout the day once again,
until we get the afternoon/evening convection going. Any patchy fog
early this morning looks to stay outside of any terminals, though it
may get close to PIH with the NBM 4.1 showing a 30% chance of CIGS
less than or equal to 3000ft around 13Z/7AM. Rain and storms will
get going again today with the HREF 24 hour thunder showing an 80 to
90 percent chance of storms at SUN, BYI, and PIH, and a 90 to 100
percent chance at IDA and DIJ. With storms, look for wind gusts up
to 40 kts, heavy rainfall, and VIS/CIGs down to MVFR/IFR conditions.
More storms will be on tap again for Tuesday afternoon/evening.
AMM

FIRE WEATHER...Low pressure will dominate that pattern, with the
low center actually entering from the south during the day
Tuesday. The cooling aloft will continue the instability, and with
moisture in the air increasing as the low approaches, expect
thunderstorms with light to very heavy precipitation. Flooding
will be the primary concern, especially in the Caribou NF, the
northern Sawtooth NF, and the eastern Salmon Challis NF along with
the adjacent BLM lands. The showers and thunderstorms shift
eastward on Wed, affecting the Caribou-Targhee NF mainly, along
with nearby BLM lands. Wind will increase, but humidity should
remain high in the windiest parts in the Snake River plain and
southern highlands of Idaho. The hazard lessens from there into
the weekend, with another low approaching and bringing
precipitation back by Sun. Messick

HYDROLOGY...Some minor flooding in Cassia and Minidoka Counties
in the late evening and early morning appears to have subsided,
and the Flood Advisory for this area was allowed to expire.

Flooding continues on several rivers including the Big Wood at
Hailey, the Big Lost at Howell Ranch, Antelope Creek north of
Arco, the Big Lost below the Mackay Reservoir, the Big Lost near
Arco, the Portneuf at Pocatello and Topaz and the Bear River at
the Wyoming border. The Big Lost River below Mackay Reservoir is
currently under a flood warning as well. The Big Lost near Arco is
under a flood advisory.

All these sites are running above normal due to high mountain snow
melt and recent moderate to heavy rainfall. Additional rainfall will
push all these rivers (and others) higher than current flows. All or
portions of central and eastern Idaho are under a MARGINAL RISK of
excessive rainfall through midweek. The forecasts are trending
toward lower chances of rain, especially heavy rainfall, for the
latter part of the week and into next weekend. Keyes

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&