Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180821

Area Forecast Discussion
National Weather Service Riverton WY
221 AM MDT Tue Jul 18 2017

.SHORT TERM...Today through Thursday night
Issued at 220 AM MDT Tue Jul 18 2017

As expected, we still have a couple of storms out there this morning
but they appear to be weakening and should be gone by morning. Zonal
flow will dominate today with a vast majority of the deeper moisture
getting shunted to the south of the area. As a result, a vast
majority of the area will be dry today. We agree with the day shift
that a thunderstorm or two may still fire across portions of Johnson
and Natrona Counties but even here storms would be few and far
between. Like previous days, the NAM appears to be initializing the
convection the best and will be the model of choice. In addition,
expect more sunshine today and as a result temperatures will be
warmer than yesterday. Many of the lower elevations will see high
temperatures return to the 90s with 80s across the lower elevations
West of the Divide.

On Wednesday, we will begin to see a bit of monsoonal moisture move
into the area. The question is, how far north will it extend? The
GFS moves it further north than the NAM at this point. Again, we
are favoring the NAM. For one, it has been more reliable. And two,
we also share the point of the day shift that more numerous
thunderstorms across Colorado and Utah may rob most of the
moisture from moving this far north. Again, continuity looked
reasonable with keeping any thunderstorms across southern Wyoming
and possibly as far north as Casper. Temperatures will also
continue to warm, with some of the warmer spots in the Bighorn
Basin approaching the century mark once again.

There is more uncertainty for Thursday however. Previous runs of the
GFS showed a good push of moisture moving into the state. However,
both the GFS and now the NAM have come in much drier and keeping
most of the moisture across southern Wyoming. The European is
still wetter but even it has the shortwave a bit further south. For
now, given the uncertainty and that this is the first run to show
this, we made few changes. However, this may have to be changed
depending on if the trend continues. Temperatures are also a bit
uncertain, as a lot depends on how much cloud cover from the
moisture moves into the area.

.LONG TERM...Friday through Tuesday
Issued at 220 AM MDT Tue Jul 18 2017

The weather pattern turns more quiet over the region Friday
through Sunday with not much thunderstorm activity expected.
A few storms in far southern WY Friday afternoon and evening.
Very isolated activity is possible Sunday afternoon and evening
over and near the mountains. The GFS and ECMWF models have a weak
disturbance in the flow for Monday. It may not even exist by the
time Mon gets here and have only very isolated thunderstorms in
the mountains Monday afternoon and evening. Next Tuesday looks dry
for now. Very warm to hot high temps Friday. The models wish to
drop 700MB temps a few degrees on Saturday as a weather system
passes well north of WY. Not sure what to make of that cooling and
only dropped highs a few degrees up north and central with no
change in the south. Very warm to hot temps prevail Sunday through
Tuesday with Tue being the hottest day at this point. Again, this
prolonged heat and dryness will increase fire danger with time.


.AVIATION...12Z Issuance
Issued at 220 AM MDT Tue Jul 18 2017

VFR conditions will continue through the forecast period. Have adjusted
winds across the taf sites to account for expected late morning/early
afternoon winds. Winds should become more southwest toward the latter
portion of the forecast and have accounted for this involving the TAFs.


Issued at 220 AM MDT Tue Jul 18 2017

Mainly dry and warm to hot weather will dominate the weather today.
The only chance for a shower or thunderstorm would be along the
Wyoming-Colorado border and possibly eastern Johnson and Natrona
Counties. Although relative humidity may approach and possibly fall
under 15 percent at times this afternoon, wind should remain below
critical levels. Expect, of course, in and near any isolated shower
or thunderstorm. Mixing and smoke dispersal will range from fair to
good in the Eastern Basins to very good to excellent across the
western Mountains and Sweetwater County.





SHORT TERM...Hattings
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