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