Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 222026

Area Forecast Discussion
National Weather Service Pocatello ID
226 PM MDT Sat Jul 22 2017

.SHORT TERM...Tonight through Tue evening. Upper level high over
the desert southwest is shifting to a position that allows the
subtropical unstable air from the Gulf of California to start a
northward trek into the Utah-Idaho border region by Sun
afternoon/evening. Precipitable water is still fairly low and
expect dry thunderstorms both Sun and Mon afternoon. By Mon
afternoon, this air mass will continue to moisten up and continue
unstable, so the dry thunderstorm threat spreads farther north. By
Tue morning, the thunderstorms should transition to wet. The slow
moving nature under the upper level high could mean a flash
flooding threat on Tue, and this needs to be monitored, especially
the southern half of the forecast area.

There are large discrepancies between the GFS and NAM
precipitation accumulations, both in position/location and general
amounts. The GFS is much much wetter, which is not normally the
case--perhaps new convective parameterization schemes in this
model? Have tried to split the difference, but not rule out any
location that could receive thunderstorms or precipitation. This
includes a new area in the central Idaho mountains on Mon.

Winds are not expected to be a factor during this time period, and
temperatures peak still on Mon, with strong cooling to highs and
warming of lows beginning Sun night.

.LONG TERM...Late Tue night through next Sat night. Wet
thunderstorms are pretty certain to continue Wed/Wed night. The
GFS continues to be solid here, but the ECMWF has backed off for
Wed/Wed night. This episode of heavy rains should end Wed night,
with drier but still unstable air replacing the subtropical
airmass as the upper level flow turns more westerly for a short
period. The thunderstorm threat continues, just a lower
likelihood, and tempertures start a warming trend.


High pressure remains firmly in control across SE Idaho, yielding
high confidence in VFR conditions at all terminals with clear skies,
light winds <12 knots, and no forecast concerns through early Sun
afternoon. Sun afternoon, there continue to be signs in the model
guidance that monsoonal moisture wrapping around the high over SW
Utah will start to work north toward far southern Idaho. Very
marginal VCTS chance may get close to KBYI, but dry elsewhere.
Shower/t-storm potential increases slightly Mon and more Tues, with
VFR conditions expected through Tues AM. - KSmith/Messick


Dry westerly flow will remain in place through the weekend. Some
increase in winds on Sunday as an area of low pressure skirts
northern Idaho Sunday into Monday. Forecasting wind gusts in the 20-
25 mph range for zones 410, 425, 422 and 476, particularly at mid
and upper slopes. These winds combined with minimum RH below 15% in
lower elevation areas and elevated Haines values will create an
elevated fire risk. At this time winds are forecast to remain below
Red Flag criteria so have decided against Fire Weather Headlines.
Flow is forecast to gradually become southwesterly by Monday ahead
of an emerging Pacific trough. This will open the door for an
increase in moisture as an expanding plume of Monsoonal moisture
over the Great Basin expands into SE Idaho. By Monday afternoon, SE
Idaho will reside on the northern fringes of this activity. Isolated
showers and thunderstorms are expected by afternoon continuing
through the night. Enough dry air will be initially in place Monday
afternoon and evening to support dry thunderstorms. At this time
coverage is expected to be isolated but will need to watch this
closely. Monday night into Tuesday the atmosphere becomes quite
moist, with precipitable water values of 200-250% of normal expected
to move into the region. Widespread showers with isolated
thunderstorms are expected Tuesday, the majority of which
transitioning to the `wet` variety. Locally heavy rain is possible.
Certainty is low on amounts and placement of the heaviest activity.
However, locations that experience the heaviest activity may see
rainfall amounts approach 1 inch. Isolated to scattered showers with
isolated thunderstorms are expected to persist through the week,
although becoming less numerous each day as moisture decreases in
response to a building ridge of high pressure nudging back into the
area by late-week. AD/Messick



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