Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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545 FXUS65 KRIW 031140 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 540 AM MDT Fri May 3 2024 .KEY MESSAGES... - Snow and rain showers today, ending from West to East. - Saturday looks dry and mild. Snow returns to the west Sunday, with windy and warm conditions East of the Divide. - Cool, blustery and unsettled much of next week. The heaviest precipitation is expected in northern Wyoming. Details past Monday remain uncertain. && .DISCUSSION... Issued at 334 AM MDT Fri May 3 2024 The forecast today will have two prominent words beginning with W. The first is the state we are in Wyoming. The second is the season that seems to last longer than any other here, winter. Winter is reminding us that it is not done yet. Snow is currently ongoing across much of the west, courtesy of a trough and cold front moving into western Wyoming, with a circulation noted over eastern Idaho. As I write this around 2:30 am, areas East of the Divide are mainly dry. There is rather dry air at the surface, this evening`s sounding showed it and the tell tale donut hole in the imagery over the radar site is showing the snow is having trouble reaching the ground. Dew point depressions are still rather large, but it should be snowing by the time most people wake up. If you don`t like snow, this will be good since the longer it takes to start, the less that can accumulate before the sun comes up. After around 8 or 9 am, the strong May sun should keep roads mainly wet and limit accumulation. Even now, only some of the passes are slick. Reasoning on the timing of the precipitation remains the same, with the steadiest in the morning, then gradually tapering off in the afternoon as the system moves to the east. All showers should end by around 9 pm tonight. Many locations will see some accumulation of the grass, but this will be elevation dependent. For a rough guide, the 1 in 2 chance of an inch or more of snow lies mainly above or below 6000 feet in areas of central Wyoming.Far northern areas will see the least chance of snow. With the clouds and precipitation, temperatures will average below normal. Much of the weekend looks dry and warm, especially East of the Divide. Saturday looks to be the nicest day with high pressure over the area, bringing sunshine, near to somewhat above normal temperatures and mainly light wind. As for Sunday, the approaching, more potent Pacific system appears a bit slower, so POPs and QPF were reduced across the west during Saturday night and Saturday morning. We did maintain a small chance of convection East of the divide late, but this is looking less likely with less than a 1 in 5 chance in any location. Most locations East of the Divide should be dry through sunset with downsloping, southwest flow continuing. This will bring in additional concerns though. One, the potential for strong to potentially high wind. I would put the chance at 1 out of 3 at this point. This will especially be case in areas prone in southwest flow, like Casper, the Green and Rattlesnake Range and possibly Lander. The strongest winds may end up being across southern Wyoming, closer to the jet energy, courtesy of the right front quadrant of an 100 knot jet streak. Another clue is 700 millibar winds, with some 50 to even 60 knot barbs showing up in southern Wyoming. There could also be elevated fire weather, although increasing dew points may mitigate this. Temperatures will peak Sunday East of the Divide, with mainly locations rising into the 70s. Heavier precipitation will move into the west in the afternoon and especially in the evening as the low moves toward and area and the best lift from the left front quadrant of the aforementioned jet streak approaches the area. Snow levels look to remain above 7000 feet through the day as 700 millibar temperatures hold at minus 3 or warmer. Colder air will move in at night though and drop snow levels to the floor across the west. Favorable, moist westerly flow will keep snow going in the mountains into Monday as well. There is greater than a 3 in 4 chance of over a foot of snow across the Tetons and over 6 inches across the remainder of the western mountains. Some highlights will likely be needed over this period. Meanwhile, in areas East of the Divide, rain and snow will overspread the area. With the expected track of the low to be across central Wyoming, the main impacts would be in northern Wyoming. This would especially be the case in Johnson County and the Bighorns, where upslope will be favorable and wrap around precipitation may continue as the low moves slowly to the east. There is more uncertainty for amounts here with more model differences. It also looks warmer here, with 700 millibar temperatures falling to around minus 5 or so. This would keep snow levels around 6000 feet or higher, keeping the populated areas mainly rain. Several inches of snow will be possible in the Bighorns though, with a greater than 1 in 2 chance of advisory level amounts. Reasoning remains the same through much of the week, as a blocking ridge sets up across the Great Lakes and the upper level low can only move slowly away and long wave troughing hangs over the Rockies. No major storms are expected. The main concern will be wrap around precipitation and if it can move into northern Wyoming. Uncertainty is high on details here through. A tight pressure gradient will also stay across the area, keeping gusty to strong wind going across many areas possibly through Wednesday before finally weakening. So, to sum up the extended, we have high confidence (greater than 4 in 5) of a cool, blustery and damp pattern through much of the next workweek. Confidence on the details past Monday remains very low, however. && .AVIATION /12Z TAFS THROUGH 12Z SATURDAY/... Issued at 540 AM MDT Fri May 3 2024 Widespread snow showers will continue and associated IFR to occasional LIFR conditions will persist across most terminals this morning through at least 16Z. Snow showers will end from west to east through the morning, so expect to see KBPI/KJAC/KPNA terminals see improvements between 14Z and 16Z with mostly clear skies prevailing by mid afternoon. Hi Res models continue to indicate that a snow band or series of bands will pass through KRKS between 12Z and 16Z this morning, but lack of radar coverage near the terminal has made it difficult to pinpoint exact timing of that band. East of the Divide, snow may changeover to rain late in the morning before ending west to east between 16Z and 20Z. By 20Z, all terminals are expected to see prevailing VFR conditions. Winds will be gusty with many locations seeing northerly or northeasterly winds of 10 to 15kts with gusts 20 to 25kts at times. Winds will diminish quickly towards sunset. Light winds and mostly clear skies will prevail across the region after 03Z. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 3 PM MDT this afternoon for WYZ015. && $$ DISCUSSION...Hattings AVIATION...Hensley