905 
FXUS65 KRIW 061721
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1121 AM MDT Wed Sep 6 2017

.SHORT TERM...(Today through Friday night) Issued at 1120 AM MDT 
Wed Sep 6 2017

Imagery shows large, broad amplified ridge centered over the western 
CONUS (nrn UT) over a slowly deepening low off the srn California 
coast. Embedded shortwave in the flow is now located well to the 
north within the ridge over swrn/srn Canada...due to mildly affect 
the FA by Friday. The SFC has strengthening heat induced low 
pressure over the Desert SW with seasonally strong high pressure 
across the nrn/cntrl Rockies and High Plains with a front between 
these two areas through the Great Plains. No precipitation in or 
around WY this morning. 

Today and Thursday, fire Weather conditions will begin to increase 
once again and return to Elevated for much of the FA due to very low 
minimum RH values this afternoon/evening...especially for lower 
elevations east of the Divide and across southern Wyoming...as both 
warming and drying take place under the influence of a very dry 
airmass accompanying the strong upper level ridge/SFC High combo. 
Short and medium range mdls showing some minor moisture return 
getting into the FA on Thursday from the e/se around developing low 
pressure over srn WY/nrn CO. However, there is severe doubt there 
will be enough to produce much in the way of precipitation (maybe 
some isolated virga over the central mountains)...especially after 
the atmosphere has been so dry up to that point...so basically 
keeping POPs at or below mins everywhere Thu/Thu night. 

Friday, the upper ridge axis shifts ewd with the axis near 
through/ovr WY while weak sub-tropical monsoonal moisture...squeezed 
towards the region between the offshore upper low and the upper high 
then centered over TX/NM/MX area. Additionally, the embedded 
shortwave currently located ovr swrn/srn CA (mentioned above) will 
move south and close enough to WY to aid in triggering isolated 
showers/thunder mainly across the w/cntrl mountains Friday 
afternoon/evening. Little rain and gusty winds most likely
scenario. 

.LONG TERM...(Saturday through Tuesday) Issued at 1120 AM MDT Wed
Sep 6 2017

Overview...Above normal temperatures, high temperatures near 10 
degrees above seasonal normals, will prevail through the period. 
Some late season monsoonal moisture will bring chances of showers 
and thunderstorms across the west on Saturday.  Mainly dry 
conditions will prevail Sunday through Wednesday.  

Discussion...Upper air pattern on Saturday features upper high over 
eastern New Mexico, upper low off the southern California coast, and 
hurricane Irma near the Bahamas in the southern stream; northern 
stream features upper ridge across central Canada and upper Midwest 
with shortwave trough moving eastward across British Columbia into 
Alberta.  

Some monsoonal moisture is expected to rotate northward on backside 
of southern Rockies high, into Wyoming on Saturday.  Slight 
instability (LIs -1c to -3c) is expected across western Wyoming 
Saturday afternoon with a more stable airmass east of the Divide; so 
expecting some some high-based showers/virga, isolated 
thunderstorms with gusty wind along and west of the Divide.  

Northern stream trough will move across central Canada on Sunday and 
drive a weak, mainly dry cold front south into northern/central 
Wyoming.  Upper ridge will flatten over the Rockies with this 
system's passage, suppressing 'monsoonal' moisture further south, 
giving way to a drier, more stable westerly flow over Wyoming. 
Temperatures on Sunday are only expected to be a few degrees cooler 
across the north and central, most high still in the mid to upper 
80s across the basins.  Upper ridge will build back up over the 
Rockies Monday and Tuesday, keeping northern subsequent northern 
stream shortwaves across Canada, upper low in the southern stream 
off the California coast through Wednesday.  There are some 
indications by both GFS and ECMWF just beyond the extended period of 
the merging of northern and southern streams over the area (Thur-
Fri) to bring cooler temperatures and better precipitation chances.  

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday morning) 
Issued at 1120 AM MDT Wed Sep 6 2017

The main hazard for aviation will be persistent smoke that will 
continue through Thursday. Local MVFR visibilities are possible west 
of the divide, but most areas will remain VFR with local mountain 
obscuration. It will remain dry across the region through Thursday. 
A few cumulus buildups are expected over the mountains Thursday 
afternoon, mainly west of the divide.

&&

.FIRE WEATHER...
Issued AT 1120 AM MDT Wed Sep 6 2017

...Elevated Fire Weather Conditions today and Thursday during the 
afternoons and early evenings east of the Divide and across
Southern Wyoming...

Fuels at critical levels for most of the low elevation forecast 
zones in addition to many mountain zones below 8500 feet. (Please 
see the Fire Weather Forecast product for more information). 

Today will be post frontal with generally warmer high temperatures 
compared to those on Tuesday. RH values will range from around 10 to 
20 percent across the lower elevations and foothills...with 15 to 30 
percent covering most mountain areas. Winds, however, will remain 
light for the most part with only a few afternoon mixed out gusts 15 
to 20 mph. Thursday will be warmer with similar to slightly higher 
RH values compared to those of today. Winds will remain quite light. 
Smoke dispersion in the afternoons will be fair to good today...good 
to very good on Thursday...and good to excellent on Friday. 

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...PS
FIRE WEATHER...Braun