Area forecast discussion National Weather Service Riverton Wyoming 354 am MDT Sat may 18 2013 Short term...today through Monday night A very unsettled few days expected across central and Wyoming. A few showers linger over western Wyoming this morning...with a weak boundary moving west through Fremont County. Webcams have been indicating the presence of fog across the Big Horn basin and Johnson County. Patchy fog will remain in the forecast through the middle morning hours...with shower activity remaining for much of western and central Wyoming. A circulation embedded in the trough over northeastern Utah continues to lift towards southwestern Wyoming with the progression of the trough. This circulation is anticipated to move through much of the state today. Showers are expected through the day...with thunderstorms developing in the late morning and early afternoon hours. Instability to move northeast through the day...with the Big Horn basin and Johnson County again expected to see the greatest amount of instability today. Any thunderstorms may contain gusty wind today. In addition...storms across northeastern Wyoming may contain small hail. Storms may contain period of heavy rain...which may result in localized flooding. National guidance indicates a slight chance for strong wind and large hail to occur...primarily across eastern Johnson County. As mentioned before....flooding will be possible...with models indicating the highest precipitable water values across this area. Special attention will continue to be paid to the burn scar regions...especially the Gilead burn scar should significant rain fall near the area. The Big Horn basin and Johnson County are again noted to have the greatest cape and lowest lifted index values through the afternoon and early evening hours. Consequently...it is certainly not out of the question for a storm or two to turn severe across northeastern Wyoming today. Notable cape and lifted index values sneak into Natrona County during the late afternoon and early evening hours. Daylight will be fading with this arrival...but some stronger storms may occur. Thunderstorm activity is expected to wane with the loss of daylight this evening...with shower activity continuing through the overnight hours. Patchy fog development will again be possible across northern Wyoming in response to anticipated shower activity. Under the continued influence of the trough of low pressure...thunderstorms will again be possible on Sunday. A northwest flow will begin to build over the area early Monday morning...though some instability may keep isolated thunderstorm activity across the southern two thirds of the state on Monday afternoon. Long term...Tuesday through Saturday A Rex block across southern Canada and central U.S. Will gradually weaken by late in the week...with the general flow across the much of the Continental U.S. Remaining rather stagnate through the extended. There could be some lingering light precipitation across the east Tuesday associated with an upper low near Sioux Falls South Dakota. However...models are showing some weak ridging moving across the forecast area Tuesday/Tuesday night which would suggest a drying trend and the beginning of a warming trend. For the rest of the extended...attention turns to a strong upper low that moves into the Pacific northwest Tuesday/Tuesday night. Global models keep this strong upper low around the Pacific northwest area through Saturday. This will result in increasing south to southwesterly flow over the region and above average temperatures...and possibly some strong convection at times. On Wednesday...medium range models are showing a decent difluent flow over the forecast area ahead of a significant shortwave rotating around the upper low. In addition...at the surface...models are progging a moist east/southeast flow developing east of The Divide. Models are showing the bulk of instability/steep lapse rates on Wednesday across the west/northwest. Thus could see some strong convection across the west/northwest Wednesday...and possibly farther east if more instability occurs than currently forecast. This system could also surge a cold front eastward into western Wyoming late Wednesday/Wednesday night...but quickly retreat during the day back west/northwest Thursday. Models then show a dry slot moving into southwest Wyoming Thursday...but a continued moist east/southeast surface flow across the extreme east...with a quickly veering wind profile just above the surface. Global models are showing the bulk of the instability Thursday across the east...with the west fairly stable. Thus the bulk of the convection should be concentrated across the north and east Thursday...with some strong convection possible especially along and north and east of the pseudo dry line/front...which should be across Johnson and Natrona County as well as the Big Horn basin. The basic set up will remain the same Friday and Saturday...with models showing the drier southwest flow pushing ever so slightly farther east. && Aviation.../12z issuance/ Showers continue across the west terminal area as another wave moves overhead in southwest flow...kjac kbpi and kpna will continue to be in and out of MVFR to IFR conditions through the morning with widespread mountain obscurations ongoing. MVFR to IFR conditions are also possible across the Bighorn Basin and at kbyg through the early morning hours as fog forms in locations where heavy rain fell with the storms on Friday afternoon. Showers and some storms will increase in coverage across the entire forecast area after 18z...so will keep thunderstorms in the vicinity and vcsh in forecast for all terminals. The highest based storms will be vicinity kcpr between 20z Sat and 02z sun...the threat for downburst winds will be greatest at this time. Showers and thunderstorms will clear from southwest to northeast after 03z sun...with some showers continuing across the northeast and in the kcpr area through the end of the taf period. && Fire weather... Patchy morning fog will be possible across the Big Horn basin and Johnson County today in response to previous rain showers. Another round of thunderstorms will occur this afternoon...with the prime spot again being found over the Big Horn basin and Johnson County. Storms in these areas may produce gusty wind and small hail...with a few storms becoming strong. Isolated thunderstorms expected elsewhere...with gusty wind possible. This unsettled pattern continues through the weekend. Northwest flow will begin to move over western Wyoming Monday morning...though thunderstorms may again be found across central and southern Wyoming. && Riw watches/warnings/advisories...none. && $$ Short term...branham long term...wm aviation...Allen fire weather...branham