Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 230502
Area Forecast Discussion
National Weather Service Riverton WY
1102 PM MDT MON AUG 22 2016
.UPDATE...06Z Aviation discussion added.
.SHORT TERM...Tonight through Thursday
...Critical fire weather tomorrow...
Imagery shows broad ridging across the central CONUS with an
upstream trof moving through the northwest CONUS and an embedded dry
shortwave spinning out ahead of the trof over northwest WY. The
surface has a trof over eastern WY with higher pressure off to the
The rest of today, southwest flow aloft and quite unstable with dry
adiabatic lapse rates 10 to 11.5 up to at least h5, winds of 25 kts
to 35kts, and increasing stable through the vertical giving wind
gusts at the surface from 25 to 35 kts. So, warming and drying with
RH values dropping back into the single digits and teens at lower
elevations and the foothill to near 20 percent across some mountain
locations combining with the winds are giving enhanced to critical
fire danger everywhere today and across portions of central,
southern and southwest Wyoming on Tuesday. Red Flag Warning
continues today for all areas west of the Divide and for portions of
central and northern Wyoming east of the Divide into the evening. A
Red Flag warn has also been posted for Tuesday for all of the
southwestern and southern fire zones in addition to small portions
of central Wyoming. Only a few locations near the southern and
southwest border areas may see an isolated shower/storm or two this
afternoon and evening.
Otherwise, a front sweeps through the northern/eastern FA tonight.
Tuesday will then return to similar conditions to that of today, but
with perhaps slightly increased RH values across the northern
forecast zones. The gustier portion of the wind field will shift
further south and perhaps be slightly less aggressive than today.
However, winds and low RH values will be at or near critical for
another day south of the frontal passage across southern and
southwest WY...where the above mentioned RFW resides. Only very low
chance for a couple of isolated showers/storms across the far
south once again.
Tuesday night another front moves through the FA east of the Divide
with Wednesday seeing a change in the weather...at least east of the
Divide and over portion of northwest WY...as the area goes post
frontal as the PAC NW trof begins to influence the CWA with some
increase in moisture and yet another front moving into the northern
zones Wednesday afternoon. There will be increased chances for
precipitation across the north and east of the Divide...but with
only modest increase in moisture and most of the showers/storms
providing little rainfall...at least until Thursday morning. Cooler
everywhere Wednesday but southwest WY will again remain very dry.
Winds, however, look to behave under the trof`s influence and
frontal passages...so no fire weather headlines expected ATTM.
Wednesday night through Thursday, hope abounds with much cooler
temperatures and even some light snow expected across the higher
terrain of central and northern WY overnight through Thursday
morning...especially in the Beartooth Mountains and northern
Bighorns with perhaps and inch or two on the ground above 9500 feet
by noon. Otherwise, most precipitation for the day Thursday is
expected to fall across northern WY and to a lesser extend over
portions of central WY. A few storms may become briefly strong over
northwest/northern WY with brief heavy rain, small hail and gusty
.LONG TERM...THURSDAY NIGHT THROUGH MONDAY
Medium range period shows overall retrogression with strong mean
ridge centered off the B.C. coast retrograding out into the Central
Pacific while the mean trough sets up over Western Canada. We are
still downwind of the trough axis into Friday (if not Saturday) with
expected shortwave energy overrunning front that should remain east
of the divide. Chance of showers/isold storms will continue through
Thursday night and Friday (and possibly longer). The 12z GEM and
Euro are much slower (by a day) and do not swing the trough axis
through until Saturday. So it could end up taking until Sunday now
before we finally get rid of this trough and its associated
shower/tstm chances. Monday looks warmer in the developing sw flow
with potential for embedded shortwaves creating isold showers/storms.
Overall looks dry but probably at least isold storm potential in and
near the mtns. Generally near 70 to 70s east of the divide Friday
with 60s to around 70 west. Gradually warming up over the weekend
into Monday with mainly 80s by the end of the period east of the
divide and 70s west.
VFR conditions to occur through Tuesday night. Smoke from wildfires
will impact visibility overnight and Tuesday in parts of northern WY.
Breezy to windy by 19z over much of the far west and south with gusts
around 25 knots expected until 01z...then decreasing wind speeds.
...Critical fire weather conditions Tuesday across southwest and
southern Wyoming in addition to small portions of central Wyoming...
Fire weather conditions elevated to critical across the forecast
area through Tuesday. With a warm and very dry transitional period
continuing until the next fall-like system arrives Tuesday night, RH
values will drop quickly, ranging from the single digits to lower
20s (percent) for most locations across the forecast area. Winds
gusting upwards of 25 mph will combine with these very dry
conditions and critically dry fuels to give critical to extreme fire
weather conditions over all areas west of the divide in addition to
portions of the foothills and basins in northern and central Wyoming
on today; and across much of southwest Wyoming on Tuesday. A
reprieve from these fire weather conditions looks to arrive
Wednesday as cooler weather with some modest chances for
precipitation invades the region...especially across northern and
Red Flag Warning from noon to 9 PM MDT Tuesday for WYZ277>279-