Frost Advisory 1, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
213 PM MDT Thu Apr 30 2020

.SHORT TERM...Tonight through Saturday night. Most active period
is through this evening with strong thunderstorm development
ongoing. As suspected, cells developing in destabilized airmass
where clearing was realized mid to late morning. Convective models
painting a clearer picture this afternoon, with ongoing activity
likely to help development later this afternoon further east and
north, as outflows work with terrain interaction in more recently
destabilized air to feed additional cells. Of additional concern
is the activity working into western Idaho from Oregon, on
trajectory to reach central mountains later this afternoon into
early evening. Overall threat remains the same with wind gusts and
hail approaching or exceeding severe limits within SPC MRGL risk
area, depending on the instability pockets. While thunderstorms
are expected to continue past sunset, intensity should diminish as
instability wanes with loss of heating. Showers linger across the
northeast corner past midnight as remnants of the central
mountains activity, but otherwise drier and more stable airmass in
place behind shortwave. Will see second weak shortwave drive
through the region Friday, but significantly drier, though a few
lingering showers and storms are possible along higher elevation
areas. Another shortwave with associated moisture pushes across
East Idaho on Saturday, with next round of showers and
thunderstorms favoring higher elevations. DMH

.LONG TERM...Sunday through next Thursday. The three main potential
impacts in the long term period include: 1) potential for wind/dust
issues Monday 2) Thunderstorm potential Monday 3) Potential for
frost/freeze Tuesday morning. Forecast models are in decent
agreement regarding the passage of a mid-level disturbance and
attendant surface cold frontal passage on Monday. At this time,
there is the potential for westerly winds across the eastern Magic
Valley, southern Snake Plain and south hills to reach the 20-30 mph
range with gusts potentially approaching 40 mph. This would be close
to Wind Advisory. Severe weather parameters for Monday look marginal
at this point, but we'll continue to monitor this potential as w e
get closer. The airmass filtering in behind the cold front may be
cold enough to allow for the development of frost and possibly
freezing low temperatures across most of east Idaho Tuesday morning.
Drier conditions and a modest warming trend is then in the cards for
the remainder of the week, with the best potential for the re-
introduction of precipitation holding off until Friday, despite the
GFS's bullish precipitation presentation on Wednesday. Forecast is
heavily based on the NBM with only modest tweaks to winds and cloud
cover. AD

&&

.AVIATION...Morning showers and high-based stratus
continues to lift off towards Wyoming early this afternoon.
Upstream, clearing skies continue to work eastward into east Idaho,
and this will warm the surface and lead to the gradual development
of instability, making thunderstorm development more likely as the
afternoon progresses. Activity is expected to develop farther west
towards SUN and BYI between 20-21Z, then spread north and east
towards PIH, IDA and DIJ as the afternoon progresses. Inherited TAFs
include a TEMPO group detailing TS potential and associated impacts
and have left this intact. Given uncertainty on exactly where
thunderstorms will develop, have left these TEMPO groups in tact for
now and will amend as the picture becomes a bit clearer. Outside of
thunderstorms, south winds in the 10-20k range have developed this
afternoon and should continue. A cold front will reach SUN and BYI
around 06Z this evening, PIH around 07Z, IDA around 08Z and DIJ
around 09Z. Wind shear is likely with this boundary, with models
indicating westerly winds of 30-45kt between 2-6kft, respectively.
Model cross-sections have indicated the potential for MVFR/IFR
stratus across the Snake Plain Friday morning, especially at IDA
between 12Z-18Z. Winds will gradually be on the decrease through the
day Friday with VFR conditions expected outside of areas of
potential stratus. AD

&&

.HYDROLOGY...Let the Willow Creek warning go earlier this morning
as yesterdays diurnal bump stayed below flood stage, and the trend
continues to be a lower crest. Further north, dam releases
producing downstream rises along the Henrys Fork. Along with
continued snow melt and precipitation through tonight, these rises
are expected to continue downstream. Will be watching the gages
and expectations for impacts as the release-wave reaches Rexburg.
DMH

&&

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