Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 182017
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
217 PM MDT Fri Aug 18 2017

.DISCUSSION...Forecast remains quite stable today, both in terms of
current conditions and changes during the 7 days. Shortwave
dropping out of PacNW overnight and into Saturday expected to
bring some high cloud cover mainly to central mountains and areas
along the Divide during the day. In addition, moisture prevalent
over NV/UT expected to creep north. Could see more of a convective
influence to cloud cover along the UT border, dependent on how
far north the development occurs. Have kept precip chances dry
through the day. Afternoon winds remain breezy as well with fire
concerns; see fire discussion below. Trough over the PacNW deepens
some into Sunday, and increasing south flow should advect some of
the mid level moisture north toward the southern highlands
Saturday night into Sunday. There should be more convective
influence Sunday afternoon and evening mainly along the UT border
and Bear Lake region so carrying isolated threat for
thunderstorms. Focus is really on the eclipse time period Monday.
Shortwave drops through PacNW trough toward East Idaho Sunday
night. Models showing agreement on some moisture crossing the
region, mainly at higher levels where cirrus cloud cover would
impact viewing of the event. However, timing and amount remains
slightly off between models and runs. Current expectations would
bring thin band of cirrus across the region Monday morning,
clearing west to east mid-day. Thicker cloud cover along the UT
border could linger from NV/UT convection Sunday night (and the
possibility for early non- precipitating build ups). Have kept the
chance for cloud cover in the 30-40% range most areas, but cannot
stress enough that the forecast remains highly variable depending
on timing of this feature. Regardless, shortwave does shift east
into WY late in the day (after totality), and could see convective
build-ups later in the day especially over the central mountains.
Have made few changes to the remainder of the forecast period.
Upper ridge strengthens again over the region, allowing moisture
to curl north into OR/ID Tuesday and beyond. Daily threat of
afternoon/evening thunderstorms continues mainly higher elevations
for each day. DMH


&&

.AVIATION...VFR conditions expected through the period. Gusty winds
through about sunset, particularly at BYI, IDA and PIH with gusts in
the 20-25 kt range from the west-southwest. With a trough
approaching for Saturday and its associated winds, expect a similar
situation with winds again for Saturday, although perhaps a couple
of knots higher. AD/DMH


&&

.FIRE WEATHER...Westerly flow at mid and upper levels of the
atmosphere will be firmly in place thorough the weekend. This will
open the door to allow for weak waves of low pressure to swing
thorugh the area, which will do two things 1) Increase winds 2)
Allow for increased vertical mixing of winds through the atmosphere,
which will allow for winder and drier conditions at the surface.
Have thus issued a Red Flag Warning for Fire Weather Zones 422, 475
and 476 from Noon-9PM Saturday for the combination of critical
minimum relative humidity values of about 10-15% and valley wind
gusts around 25 mph, with gusts to 35 mph at mid and upper slopes.
On Sunday, have increased the wind speeds in response to a second
weak wave, which brings near-critical wind and relative humidity
conditions, particularly for southern portions of zones 476 422
southward across the Arco Desert. Less wind expected for Monday.
An upper level low pressure system forecast to develop off of the
California coast is expected to weaken while approaching SE Idaho
by mid-week, allowing for an increase in shower and thunderstorm
activity beginning Tuesday, along with an increase in RH.
Otherwise dry conditions expected until then. AD/DMH


&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon to 9 PM MDT Saturday for IDZ422-475-
476.

&&

$$



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