437 FXUS65 KRIW 312001 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 201 PM MDT Sun Mar 31 2019 .SHORT TERM...This evening through Monday Clear skies across the forecast area this afternoon, as temperatures continue to slowly warm above freezing. Southwest portions will remain the coldest, rising into the middle 30s, while the rest of the forecast area rise into the 40s and 50s. Clouds will begin to increase over the far north late this afternoon, as a shortwave approaches N WY from W MT. There will be a chance for snow showers over the NW and northern portions of the Bighorn Basin by early evening. These chances will spread across the rest of the west and the Bighorns through the overnight hours tonight. The forecast area will start to become split along the Divide Monday. Cooler, NW flow will occur east of the Divide, especially over NE portions. NW flow will also occur west of the Divide, however, 700mb temperatures will be warming slightly through the day. This will be a precursor to increasing isentropic lift that will become prevalent by Monday night. More on this will be discussed below. All in all, isolated to scattered showers will continue over northern and northwestern portions through the day Monday. Rain will be expected in the lower elevations, with snow levels near 6000 ft or lower. This will result in steep lapse rates and showers with cloud bases near 10kft. However, rain and snow accumulations will be little to none. The best chances for wetting rain and/or accumulating snow will occur over northern portions of the Bighorns and central portions of Johnson County near Mayoworth. .LONG TERM...Monday night through Sunday Northwest flow aloft will slowly back to the west Monday night in response to backing flow ahead of trough along the West Coast. Pacific moisture ahead of this trough deepens and overspreads western Wyoming Monday evening where specific humidity values rise to between 3-4g/kg on 300K isentropic surface. Jet dynamics and instability are lacking, so the western snowfall will be primarily driven by warm advection. As such, temperatures support the potential for rain Monday evening in the western valleys before dynamic and evaporative cooling help make the changeover to snow. Tuesday morning will find a cold front pushing south across areas east of the Continental Divide. This front will reach a Lander to Casper line Tuesday morning and become a focus for precipitation through the day Tuesday. Pacific moisture will overrun this colder air in southwest flow aloft and consequently generate more widespread precipitation between 15Z-18Z/Tue over lower elevations of the east. The heaviest precipitation will occur in upslope favored areas in Fremont and Natrona counties where the front will provide additional lift. Temperatures and timing of precipitation after sunrise may allow for rain initially. Suspect that as the precipitation increases we will see more areas changing over to snow Tuesday afternoon. The rain/snow will become more widespread over central Wyoming Tuesday afternoon. Have substantially cooled Tuesday high temperatures and they still may be too warm. If the precipitation is steady, it may be very difficult for central areas to warm to 40-42F. Again, dynamics are not favorable as jet streak and energy are headed well to our south over Colorado. Still, depending upon temperatures, we will need to monitor central Wyoming for potential heavier and more widespread snowfall Tuesday and Tuesday night. Temperatures Tuesday will be the coldest of the period and well below normal. A mid-level disturbance crosses the forecast area Tuesday night and early Wednesday potentially enhancing precipitation east of the Divide. The moisture will continue to feed into the west Wednesday while eastern areas begin to dry out. Wednesday accumulations will be highest across the northwest mountains which are favored as the upper flow veers to the northwest. Transient ridge crosses the forecast area Thursday with drier and warmer conditions expected. Moisture and weak disturbances embedded in progressive zonal flow will keep the weather unsettled across the west Friday and Saturday. Eastern areas look to be generally dry with a warming trend Thursday through Saturday. Sunday looks drier and a little cooler across the entire forecast area. && .AVIATION...For the 00Z TAFS through 00Z Tuesday West of the Divide...KBPI/KJAC/KPNA/KRKS Routes Fog could again form around KBPI and KPNA early Monday morning. VCFG likely at both terminals and we may add prevailing group before transmitting before 00Z/Mon. Otherwise, all terminals will be VFR through 00Z/Tue. Mid and high clouds will spread across the region from the west and increase through Monday afternoon. There could be a few light snow showers over the mountains late in the period with tops occasionally obscured. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes VFR conditions will prevail through 00Z/Tue. Clipper system still on- track to move southeast across northern Wyoming late tonight and Monday morning. Mid and high clouds will increase at KCOD and across the north. Light snow showers will generally be confined to the northern mountains with tops occasionally obscured after 03Z/Mon. Northwest wind 10-15kts will blow in favored locations between 17Z-23Z/Mon. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... A weather disturbance will sweep quickly southeast across northern Wyoming late tonight and Monday morning. Only light mountain snow showers are expected. Gusty northwest wind of 15 to 30 mph will blow across northern Johnson County and the northern Bighorn Basin Monday in the wake of this disturbance. Other locations will be dry through Monday, before moisture returns to the west Monday evening. Precipitation will overspread western Wyoming Monday evening and overnight. The western valleys may see rain Monday evening, before becoming all snow by midnight. Snow intensity and coverage will increase west of the Continental Divide through sunrise Tuesday and continue through the day. To the east, a cold front will push south to zone 289 Tuesday morning. This front will become the focus for enhanced precipitation Tuesday afternoon, as moisture runs over the top of it. Eastern areas could see rain, a rain/snow mix, or snow depending upon elevation and precipitation intensity. Precipitation should be of less intensity across northern areas, such as Johnson County and the Bighorn Basin Tuesday. The precipitation will become all snow across the entire forecast area Tuesday night. The snow will decrease east of the Divide Wednesday, while the west can expect the snow to continue. Drier and warmer conditions are expected Thursday as a ridge passes overhead. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...LaVoie LONG TERM...CNJ AVIATION...CNJ FIRE WEATHER...CNJ