893 FXUS65 KRIW 200426 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1026 PM MDT Thu Sep 19 2019 .SHORT TERM...(Tonight through Saturday) Issued at 255 PM MDT Thu Sep 19 2019 A trough over the Great Basin will slowly pull across the Rockies Friday and Saturday. Models are in good agreement with a better defined mid-level circulation developing across eastern Idaho tonight. Have lowered PoPs and precipitation amounts tonight as models are showing relatively weak forcing ahead of the system. However this is expected to change rapidly towards sunrise Friday. This circulation is then expected to push across northwest Wyoming Friday into southeast Montana Friday night. The heaviest and most widespread precipitation is expected to occur ahead and near the circulation (Friday into Friday evening) across western and northwest portions of the forecast area. Some significant mountain snowfall is expected. Farther southeast, models are showing a dry slot with possibly very little precipitation in the Rock Springs to Casper corridor. Cannot rule out a few robust thunderstorms over the north/far east Friday afternoon/early evening. Instability will be weak, but shear decent enough with forcing for possibly a strong thunderstorm or two. 700mb temperatures and soundings would indicated snow levels by 12Z Friday around 9500 feet over the far west to 11,000+ feet over the east. A cold front will push across the forecast area Friday, with snow levels by 00Z Saturday around 8500 ft west, and 9500 to 10000 ft east. There is enough cold air to have snow levels drop to around 7000 feet late Friday night. On Saturday, a moist northwest flow will occur with snow likely over the northwest mountains. This looks to be high PoPs, but low precipitation amounts. Issued a winter weather advisory for the Absaroka Mts. and Yellowstone despite not expecting the widespread 6 to 12 inches of snow as this is an early season snowfall with some mountain pass impacts. .LONG TERM...(Saturday night through Thursday) Issued at 255 PM MDT Thu Sep 19 2019 Showers will linger across the northwest Saturday evening and overnight as the trough will be slow in departing. Enough moisture in cyclonic northerly flow to necessitate at least low precipitation chances in this region. A shortwave ridge will quickly replace this trough for Sunday with a sunny and seasonal day in store. Ridge axis will traverse the forecast area Sunday afternoon with gusty southwest wind expected from eastern Sweetwater County northeast through Natrona County. Progressive zonal flow will bring the next trough across the Intermountain West and toward Wyoming on Monday. Models have differing solutions for Monday regarding this trough. GFS brings an open wave directly across the forecast area Monday afternoon and evening, while both the ECMWF and Canadian drive this trough toward the Desert Southwest. Tend to favor the idea of the energy dropping more southwest with just general troughiness across the forecast area Monday. Thus, have scattered precipitation chances across the northwest mountains and isolated chances in the valleys. Conditions elsewhere Monday will be generally dry, especially east of the Continental Divide, with seasonal temperatures expected. The flow aloft will be west-northwest Tuesday and Wednesday in the wake of the open wave trough. Any embedded ripple within this flow could lead to light showers across the far northern mountains. Otherwise, it will be dry and seasonal Tuesday and Wednesday. Wind speeds across southwest Wyoming will begin to ramp up Wednesday as the next trough takes shape across the Pacific Northwest. These winds will further increase on Thursday and likely more so Friday as the flow backs more to the southwest, the pressure gradient tightens, and the mid-level flow strengthens. The southwest flow and warmer mid-level temperatures could lead to Thursday being warmer than normal and possibly set the stage for elevated fire weather conditions. && .AVIATION...(For the 06Z TAFS through 06Z Saturday) Issued at 1020 PM MDT Thu Sep 19 2019 Trough over the Intermountain West will be accompanied by jet energy which will help to increase areal coverage of showers west of the Continental Divide around 12Z/Friday. Rain showers will increase at KJAC, KBPI, and KPNA between 11Z-15Z/Friday. Conditions at KJAC will be MVFR most of the daytime hours Friday with the potential for IFR. Low-end VFR likely at KBPI and KPNA with heavier showers possibly creating short periods of MVFR. Mountain tops will be obscured by 12Z/Friday with mountain obscurations becoming more frequent along and west of the Continental Divide by 18Z/Friday. Central terminals and KRKS will see VFR conditions with increasing west to southwest wind between 16Z and 19Z/Friday. KCOD will have upslope flow behind a weak frontal boundary draped across the northern Bighorn Basin. Low- end VFR and light rain showers should increase there after sunrise Friday. Would not be surprised to see MVFR at KCOD Friday afternoon. KWRL will be VFR throughout the period with generally a northwest surface wind and an increasing chance of showers Friday afternoon. Wet weather will continue over north and northwest WY through 06Z Saturday with MVFR conditions prevailing. && .FIRE WEATHER... Issued AT 255 PM MDT Thu Sep 19 2019 Elevated to critical fire weather conditions (strong gusty winds, low RH) will continue through early evening. Otherwise the main focus will be a fall storm system that will bring widespread valley rain/mountain snowfall for western and northern Wyoming Friday into Saturday. The heaviest precipitation will occur Friday into Friday evening. Areas between Rock Springs and Casper might end up not seeing much rainfall. Accumulating snowfall is expected generally above 8000 feet with this storm system but the bulk of the accumulating snowfall will be above 9000-9500 feet. Warmer and drier conditions return Sunday with elevated fire weather conditions possible between Rock Springs and Casper. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 6 AM Friday to noon MDT Saturday for WYZ001-002. Red Flag Warning until 7 PM MDT this evening for WYZ280. && $$ SHORT TERM...Murrell LONG TERM...Jones AVIATION...CNJ FIRE WEATHER...Murrell