FXUS65 KRIW 152006

Area Forecast Discussion
National Weather Service Riverton WY
206 PM MDT Sat Jun 15 2019

.SHORT TERM...(This afternoon through Sunday)
Issued at 151 PM MDT Sat Jun 15 2019

Diurnal heating combined with weak instability has allowed
isolated convection to develop over the higher terrain mainly
along and east of the Divide as well as in Johnson County early
this afternoon. A weak right-entrance region of an upper jet will
pull across N. Wyoming late this afternoon through the predawn
hours Sunday. A weak mid-level shortwave looks to push into
western Wyoming late this afternoon, and slowly push east across
the forecast area tonight. Both of these factors will enhance the
thunderstorm coverage during the rest of the afternoon and early
evening along and east of the Divide as well as northwest Wyoming
from Jackson northward. Cannot rule out isolated convection over
the southwest. Convection will likely persist well into the night
for portions east of the Divide from the mechanisms discussed
earlier with convection ending from west to east. Much of the
convection should exit the area between 09Z and 12Z.

The primarily hazard looks to be very heavy rainfall as storms
will be relatively slow moving (10-20 mph), precipitable water
values around 0.75 inch, and some locations could be impacted a
couple of times. However, not enough confidence on any given
location or amounts to warrant a flash flood watch. Model
soundings indicate hail occurring with the stronger storms,
possibly marginally severe if updrafts can maintain themselves
long enough.

Isolated showers could be ongoing into Sunday morning. Another
round of diurnal showers and thunderstorms are expected again
Sunday with initiation a bit earlier than what occurred today. The
focus looks to remain along and east of the Divide as models
continue to show weak convergence from surface to 700mb along the
Continental Divide with storms moving east into an unstable and
moist atmosphere. Could see some convection over the far
west/southwest, but overall moisture and instability fields will
be weaker in that area.

.LONG TERM...(Sunday night through Saturday)
Issued at 151 PM MDT Sat Jun 15 2019

Sunday night will see a weak shortwave trough moving through the WNW-
ESE mid-level flow with a surface low over southern WY moving
eastward. This pattern will continue showers and thunderstorms over
the northern half of the state along and north of a surface
boundary, while southwest WY will see windy drier weather. Both the
NAM and the GFS are continuing showers across north-central WY
overnight into Monday morning. Another shortwave trough moves
through the flow on Monday, and with the surface low moving well to
the east and higher pressure moving southward from Montana, more
widespread showers and thunderstorms are expected. Wont be
surprised if this pattern on Monday results in some areas getting
over half an inch of precipitation. Stronger zonal flow pushes into
the Pacific Northwest on Tuesday and this flow will start drying the
forecast area out from the northwest. Tuesday though will keep some
instability around for mountain showers and thunderstorms and places
downwind (southeast), though the GFS is more aggressive with.
precipitation than the NAM. Breezy to windy northwest surface flow
can be expected in the afternoon Tuesday.

Zonal flow increases on Wednesday with a weak boundary along the
WY/MT border for isolated afternoon showers and thunderstorms.
Models are showing a surface low developing over central WY
Wednesday afternoon then moving eastward. This pattern will
increase the WNW wind during the afternoon and evening and bring the
warmest temperatures of the week to the area. Similar weather is
expected again on Thursday though temperatures will decrease a bit
due to 700mb temps dropping by 2-3C. Medium range models are
showing a more significant trough moving from southern British
Columbia into Montana Friday into Saturday. Mid-level flow should
become more westerly and stronger Friday afternoon with some
thunderstorms across NW and northern WY. The low movement on
Saturday will determine the impact heading into the weekend. Overall
looks to be a breezy and windy day across the area, with a good
chance of showers and thunderstorms over the northern half of the
state. Temperatures at this time might even suggest snow in the
northern mountains on Saturday, so temps will be dropping below
normal for sure.


.AVIATION...(For the 18Z TAFS through 18Z Sunday)
Issued at 1130M MDT Sat Jun 15 2019

VFR conditions generally expected for the next 24 hours, though
localized MVFR is likely under and around showers and thunderstorms.
Also, an area of showers from 06Z/Sun to 12Z/Sun over central WY
could generate widespread MVFR with lowered ceilings. Convective
activity starting off later today compared to Thursday, with most
activity today expected along and northeast of the continental
divide. Again, brief heavy rain, small hail, and wind gusts around
30 mph are likely with the storms as they generally move in a SE
direction. Scattered showers and thunderstorms can be expected this
afternoon until early evening with isolated coverage from 03Z/Sun to
06Z/Sun with possibility of showers after 06Z/Sun as mentioned
before. Models are indicating a similar setup for Sunday, with
convective activity starting earlier by a couple of hours, heavier
showers likely across northern WY later in the day, and breezier
west wind across SW WY.


Issued AT 151 PM MDT Sat Jun 15 2019

Isolated to scattered thunderstorms are expected to impact areas
roughly along and north of a Jackson to Lander to Casper line this
afternoon into tonight. Veru isolated thunderstorms are expected
south of this area this afternoon and early evening. Showers and
thunderstorms will linger past midnight for northern and central
Wyoming east of the Divide. Another round of showers and
thunderstorms are again expected Sunday afternoon and evening with
the best chance over the north.

Although winds will generally be light overall, winds in the
afternoon and evening today and Sunday over central and northern
areas will likely become dominated by breezy to windy conditions
caused by thunderstorms. Smoke dispersal looks to be fair to low
good east of the Divide, to good to very good along and west of
the Divide.





SHORT TERM...Murrell
LONG TERM...McDonald
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