Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 170900

Area Forecast Discussion
National Weather Service Pocatello ID
300 AM MDT Wed May 17 2017

.SHORT TERM...Today through Thursday night. Challenging forecast
through the period as a cold, closed area of low pressure tracks
right across SE Idaho today and into the Great Basin on Thursday.
The amount of cold air associated with this low is historic, as
heights and associated temperatures aloft exceed the coldest on
record over the last 30 years for the date. Expect highs today to
be the coldest observed for May 17th across much of SE Idaho.

This morning, the heaviest snowfall is expected to be falling over
the Eastern Highlands and Pashimeroi and Lemhi ranges. An upper-
level dry slot is forecast to penetrate into far southern and
Eastern Idaho, signaling a reduction in snowfall for these areas.
Meanwhile, snow showers are expected to continue through the day
across the Continental Divide region and Pashimeroi and Lemhi
ranges. This afternoon, the combination of the proximity of the
low pressure and high mid-May sun angle will result in a re-
development/expansion of showers across the region, with some
isolated thunder possible this afternoon. By this afternoon, the
heaviest showers are expected over the Pashimeroi/Lemhi/Sawtooth
ranges southward across the Arco Desert and Snake Plain. Can`t
rule out an occasional changeover to snow (with the potential for
heavier bursts) late this afternoon/early evening across the Snake
Plain. On Thursday, the low slides southeast into Utah, with the
heaviest precipitation shifting southward across the Southern and
Eastern Highlands. Snowfall is expected to linger across the
Central Mountains and Continental Divide region beyond the
expiration of the current Winter Weather Headlines, which may
require these to be extended. Will allow day shift to monitor
trends. Slushy and snow-covered roads possible, particularly at
pass level. Backcountry recreation will become dangerous with
snow and cold conditions.

Speaking of cold, temperatures will plummet to around freezing by
sunrise this morning for many areas. For tonight, statistical
guidance suggests the potential for low temperatures across the
Snake Plain to plummet well into the 20s. However, ensemble
guidance is much warmer, suggesting temperatures falling to near
or just above freezing. Current forecast reflects these warmer
values due to the expectation for extensive cloud cover to linger
for much of the night, combined with abundant low-level moisture.
Will hold off on a Freeze Warning at this time due to the low
confidence in forecast low temperatures tonight into Thursday
morning. However, only a short time under clearing skies could
allow temperatures to plummet, so those with sensitive vegetation
should still take proper steps to protect this vegetation from
freezing temperatures.

Finally, have downgraded the previous High Wind Warning to a Wind
Advisory for the Snake Plain and adjacent Southern Highlands.
Stability profiles and winds aloft have become unfavorable for the
development of winds strong enough for a High Wind Warning.
Nonetheless, winds will still be strong and gusty this morning,
creating difficult travel for drivers of high-profile vehicles,
and in particular on north/south routes. Strongest winds with
gusts meeting or exceeding 50 mph expected at times over favored
areas of Cassia County, including I-84 from Burley to the UT
border, and for the Craters of the Moon area. Winds will slacken
this afternoon for all areas except the Eastern Magic Valley and
adjacent Southern Highlands before diminishing this evening
across these areas. AD/Valle

.LONG TERM...Friday through Monday...Much of the extended forecast
remains unsettled as somewhat moist northwest flow is set up across
the region. Looks like there will be a weak front that will move
through Sunday which may produce a better chance for showers across
the region. Temps are expected to gradually warm during the
extended. By midweek, we may be slightly above normal on temps.



.AVIATION...Tough forecast today as a low pressure area will move
across the region. This will bring showers along with a variety of
ceilings to the region, along with gusty conditions. Low confidence
with the ceiling forecast for today, but vfr along with mvfr and
possibly ifr ceilings are all possible depending upon where/when
showers pop up.



.HYDROLOGY...A strong low pressure system will sweep through the
region today and gradually sink SSE through the eastern Great
Basin through Thursday. Snow levels are forecast to fall sharply
to valley floors by this morning behind a strong passing cold
front. Additional precipitation through Thursday should fall
in the form of snow in the mountains with mainly snow in valleys.
An exception is during the afternoon hours when temperatures may
warm enough for a rain/snow mix. We should see a brief bump
upwards on area rivers Wednesday and then a receding leg with the
much colder temperatures Thursday into Friday. Warming
temperatures are expected into and through the weekend as a ridge
builds along the coast. The Big Wood at Hailey is expected to
remain below action stage through the weekend rising to bankful
Monday through Wednesday and possibly reaching flood stage once
again next Wednesday night as a result of warming temps. The
Portneuf at Pocatello is expected to make another run into
moderate flood stage Wednesday afternoon/night before gradually
falling next week. The Bear River at the Wyoming Border will
remain in flood stage through the weekend. The Henry`s Fork near
Rexburg is expected to rise to bankfull Thursday and Friday before
tapering off through the weekend. Huston/AD


Wind Advisory until noon MDT today for IDZ017-020>022.

Winter Weather Advisory until 6 PM MDT this evening for IDZ019-

Winter Storm Warning until 6 PM MDT this evening for IDZ018-031-


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