Weather Service Report


989 
FXUS65 KRIW 282140
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
340 PM MDT Wed Jun 28 2017

.SHORT TERM...(This evening through Thursday)
Issued at 126 PM MDT Wed Jun 28 2017

First line of convection moving into the nw early this afternoon 
ahead of upper circulation moving into wcntrl MT with fairly strong 
thermal trough extending swd in the mid levels. Also approaching 
next jet streak moving into Oregon attm. Nose of this jet may add
some upper support later this afternoon into this evening as it 
starts moving ewd later. Strong to possibly severe storms are 
still expected the this afternoon and early evening across the nw.
As this first line gets farther east, it will hit some better low
level moisture and lower lcls so idea of some strong/svr storms 
in the ern Bighorn Basin over to Johnson is still good. Other 
areas across the west and north half could see strong to 
marginally severe due to both hail and wind (spreads increasing to
40-45 already at noon) and with front pushing s/se across the 
area. Storms will last well into the evening and even after 
midnight, especially east of the divide. On Thursday, cold trough 
axis moves across the area with much cooler air and considerable 
showers and thunderstorms for the west and north half. Cold enough
for snow in the nw mtns and even down to the floor in Yellowstone
National Park during convective showers (snow pellets). Quite 
chilly in the park with much of the day only in the 40s above 7K 
feet. Even the lower elevations east of the divide will mostly see
60s behind the next front. Even though the instability is not as 
great as today, the cool mid levels and decent sfc dp will still 
create a favorable environment for more tstms in this nw flow 
(and next cold front). Quite cool/chilly behind this trough 
Thursday night with mainly 40s east of the divide and upper 20s to
30s west. Mostly dry Friday with the potential for some building 
showers/isold storms off the nrn mtns. Shortwave ridge moves ovhd 
Saturday for a warmer day with an approaching shortwave late in 
the day or early evening. Enough heating to fire some isold storms
across the nw and north with gusty wind and maybe some small mtn 
hail. Spreads increase so strong wind gust threat will be there 
with any storm, especially across the lower elevations. 

.LONG TERM...(Thursday night through Wednesday)
Issued at 126 PM MDT Wed Jun 28 2017

The middle of the weekend will see a departing weak ridge to the 
east, while a shortwave trough moves into western WY from the Pacific 
Northwest.  The disturbance should bring scattered showers and 
isolated thunderstorms to western and northern WY from Saturday 
night through Sunday night, as it moves slowly eastward. 
Temperatures will warm slightly on Sunday, but moreso on Monday and 
Tuesday as the upper level high pressure over the southwest US 
strengthens.  Still forecasting some isolated thunderstorms mainly 
over the mountains Monday and Tuesday, with a better chance across 
northern WY Monday evening as a weak wave moves across MT.  Tuesday 
evening on the Fourth of July may see more typical diurnal 
thunderstorms for natural fireworks.  Wednesday looks to be a 
continuation with less convective activity, although the NE areas 
may see a weak boundary move south.  Overall, the strongest winds 
during the period should occur from thunderstorm outflow as the 
surface pressure gradient weakens from the weekend into the early 
part of the week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday afternoon) 
Issued at 340 PM MDT Wed Jun 28 2017

Shortwave trough moving through the area early this evening in 
westerly flow, with scattered showers and thunderstorms.  Primary 
threats this evening are gusty winds over 40 kts along with small 
hail and brief heavy rain.  VFR conditions generally, with local 
MVFR possible under and around storms.  The convective activity is 
likely to continue through 03z-06z/Thu tonight core of the 
disturbance moves through from 00z-03z/Thu.  Lingering showers 
likely as well after 06z/Thu across northern WY. After a few hours 
of some relative calm, another shortwave trough will approach the 
area from the north-northwest, mainly along and east of the divide. 
Showers likely around KCOD starting at 15Z/Thu. By 18Z/Thu, 
instability and cooling in the mid levels should allow scattered to 
widespread thunderstorms as well.  General flow Thursday will be W-
NW across SW WY, and west becoming northerly east of the divide. 
Some local areas may see heavier precipitation on Thursday,. 
such as KCOD and KLND.  Showers to linger until about 06Z/Fri.

&&

.FIRE WEATHER...
Issued AT 126 PM MDT Wed Jun 28 2017

All fuels are currently in green-up and below critical levels for 
all locations. Fire danger remains low across all mountain 
areas...but will continue somewhat elevated in the afternoons across 
the lower elevations east of the Divide over portions of southern 
and central WY. These areas with elevated fire concerns will 
generally coincide with low afternoon RH values and/or gusty 
winds...through Friday. Winds west to northwest 15 to 25 mph into 
this evening...north to northwest 10 to 20 on Thursday...and north 
to west 5 to 15 on Friday...today and Thursday with higher gusts in 
the afternoons and especially near stronger showers/storms. This 
afternoon and evening, we will see isolated to scattered storms 
producing brief heavy rainfall, hail to 1 inch and strong gusty 
outflow winds of 40 to 55 mph...especially across portions of the 
northern half of Wyoming. This unstable pattern will continue 
through Thursday night. Smoke dispersion will be good to excellent 
each afternoon and evening. 

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE. 

&&

$$

SHORT TERM...Skrbac
LONG TERM...McDonald
AVIATION...McDonald
FIRE WEATHER...Skrbac

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Western Regional Climate Center, wrcc@dri.edu