Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200826

Area Forecast Discussion
National Weather Service Riverton WY
226 AM MDT WED JUL 20 2016


Prominent monsoon flow is streaming north from Mexico north to
AZ/NM/Utah/Co/SE WY as this moisture rides north along the back side
of the high centered over the southern Plains. The back edge of
this moisture plume currently runs from RKS to CPR. What sparse
convection our CWA has experienced this evening has fired off along
the back edge of this moisture boundary, namely over Flaming Gorge
Reservoir, where quite a bit of lightning was observed. This
moisture will be shunted a bit farther east today as a vort lobe
tracks northeast across Idaho, Montana, and northwest Wyoming and
veers the flow slightly to more of a west southwest trajectory. As
for the fire weather watch already in place for today valid from 12
noon Wednesday afternoon through 9 pm Wednesday evening, it appears
that the winds will not be quite strong enough to justify a red flag
warning over Natrona County and especially Johnson County. Will
leave the southern Big Horn Mountains in the warning as Split Rock
did blow 20 knots Tuesday and will probably push criteria today. The
rest of the watch area will be upgraded to a warning including the
Wind River Basin, the west slope of the Wind River Mountains and the
far west mountains where the gradient will be a little tighter
closer to the track of the vort lobe. The RH will certainly be
there. North winds will also increase in response to a southbound
surface boundary tonight that will only affect Johnson and northeast
Natrona counties tonight but by the time the winds increase in
response to this surface feature, so will the RH behind the weak
boundary. We could see some isold convection over southeast
Sweetwater County this afternoon along the exiting back edge of the
monsoon flow. Temperatures will reach close to 100 in the Big Horn
Basin today.

Thursday appears to be similar to today but with less wind. Not much
in the way of convection is expected Thursday except for isold
activity to the southeast and mountains. Some areas in the Big Horn
basin will come close to the century mark once again Thursday

Friday appears to be a more solid critical fire weather day as a
prominent shortwave tracks east across the area stirring the winds
up along with continued hot and dry conditions accompanied by more
of a dry westerly flow.


Beginning of forecast period - Flat flow across the CONUS with low
amplitude ridging over the ern part of the country and minor
troffing across the west. WY will lay under the influences of a
minor SW passing through the northern portion of the state.
However, WY will be post frontal and dry east of the Divide, with
low level westerlies west of the Divide and very dry.

Sunday will see modest ridging aloft across the FA with a surge of
monsoonal moisture through the southern and central Rockies into
eastern WY. a few showers/storms may catch the far eastern CWA as
moisture is somewhat wrapped back into the least across
portions of Johnson county and to a lesser extent eastern Natrona.

Through the rest of the forecast, relatively flat flow aloft with
surface high pressure set up across the eastern half of the CONUS
and low pressure across the west will keep the moist monsoonal flow
through the southern and central Rockies and into the central and
northern plains where a fair number of MCSs will likely form each
night...but leave central and western WY relatively untouched and
unable to produce any precip at the surface except for an occasional
spurious shower/storm over a higher peak or two...or across far
southern WY near the border, closer to the monsoonal moisture.

With some significance to the current fires burning, the flat flow
pattern aloft and pressure fields at the surface will allow winds to
turn and blow more out of the northwest across the western zones
instead of the southwest...especially Tuesday and Wednesday...which
could push fires in a new direction from time to time. However, that
said, winds do not look to be too brutal under this pattern.



The main aviation concerns:

1. Very isolated high based convection mainly along and southeast of
a KRKS-KCPR line. Gusty/Dry microburst winds will be the main

2. Smoke from ongoing wildfires across western Wyoming will cause
MVFR restrictions at times especially on routes between KJAC-KRIW
and KJAC-KBPI/KPNA. KJAC terminal could see MVFR restrictions from
smoke this morning, and will have 6SM FU in KJAC TAF for now.



Rather dry southwest flow aloft will continue over the area through
the weekend.  Isolated to widely scattered showers and thunderstorms
will be possible over and near the mountains and across the south.
Today will see brisk southwest winds over the western mountains and
across Fremont County and the southern Big Horn Mountains. Red flag
warnings are in effect for zones 414 415 416 283 and 285 this
afternoon and evening. Temperatures will be hot especially across
the north with highs at or around 100. Little change in the weather
pattern is anticipated other than a gradual shift to more westerly
flow by the weekend. Friday appears to be another critical fire
weather day.


.RIW Watches/Warnings/Advisories...
Red Flag Warning from noon today to 9 PM MDT this evening for



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