Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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028 FXUS65 KRIW 231128 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 528 AM MDT Thu May 23 2024 .KEY MESSAGES... - Snow continues for the mountains today, with the highest amounts in the Bighorns. - Strong wind is likely this afternoon, with a potential for a frost or freeze in some areas tonight. - Saturday and Sunday still look unsettled and cool, although details remain in flux. && .DISCUSSION... Issued at 319 AM MDT Thu May 23 2024 Winter has returned to portions of the area this morning, but this is not an unusual occurrence in western and central Wyoming in May. Decent snow has already fallen in the western mountains, with Grand Targhee already showing over 6 inches of snow around 2 am. Snow has also occurred in the lower elevations. Webcams show accumulation is only on the grass though, the roads look largely wet or maybe having some slush. The low pressure area that is causing this is now seen rotating over eastern Idaho. It will move across northern Wyoming today and shift the heaviest precipitation east of the Divide today. Reasoning today remains similar to yesterday, with the heaviest snow falling in the Bighorns range and more specifically from Granite Pass northward. The question is the difference in accumulation vs impacts. Areas north of Granite Pass do have a greater than 1 in 2 chance of over a foot of snow, which is warning criteria. However, areas south of the Pass have a much lower chance, with only a 1 in 10 chance at Powder River Pass and no chance further south. In addition, the heaviest snow is expected between 9 am and 3 pm. In other words, during the time of peak solar insolation and we are now less than one month from the summer solstice. As a result, a lot of the snow may have trouble sticking to roads during this time unless it is very heavy. As a result, impacts will be less. This, plus the fact that over 85 percent of the zone has less than a 1 in 2 chance of warning amounts, we will stick with the advisory. Some moisture will also wrap back into western Wyoming. However, here as well, accumulation on paved surfaces will be limited. We will maintain the winter highlights and Special Weather Statements as is for now. Elsewhere, precipitation will be mainly rain. There could some brief mixing with snow at times, but no impacts. Another concern will be wind. This will occur in favored areas, mainly from Rock Springs to Casper this morning into the afternoon, and then across Johnson County in the afternoon after the low passes and the wind turns northwest. There are a few 50 knot wind barbs at 700 millibars in this area, however these occur rather briefly late this afternoon into the evening. For now, we will issue Special Weather Statements to cover for this. There could be a stray gust to 60 mph in the favored areas, like Wyoming Boulevard or the Buffalo airport, but it does not look widespread. There is a greater than 1 in 2 chance of wind gusts to 50 mph though. All precipitation should end this evening as the low moves over the Black Hills and into the Dakotas. Then we have one more concern for tonight. That is for the potential of a frost or a freeze. This looks to be mainly the Bighorn Basin at this time. It is borderline, but some locations here there is a greater than 1 in 2 chance of temperatures falling to 32 or below. It does not look like a hard freeze though. Clearing looks to be fairly certain. The question is if wind can decrease fast enough to allow for temperatures to plummet to freezing or below. Still questions on this. We will issue a Freeze Watch for now and let the day shift decide if they want to upgrade or not. Friday at this point looks quiet with transitory ridging across the area. The quiet weather won`t last though. The third and last in the series of lows will move onshore in the Pacific Northwest for Friday night. A weak shortwave ahead of it could bring a few showers later Friday night across southern Wyoming. The models continue to struggle with the details of this system. Saturday looks fairly unsettled with a scattering of showers and storms. Models then diverge starting Sunday, some guidance moves the low away quickly, drying things out. Others linger the moisture and unsettled weather into Sunday. For Sunday, we tapered chances from east to west. Uncertainty is still high though. And even on Saturday, details on placement and precipitation amounts are still rather uncertain. Ridging should then bring dry weather for Memorial Day and Tuesday. The next system may try to approach on Wednesday with the next chance of showers, but details this far out are always questionable. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 528 AM MDT Thu May 23 2024 The latest storm system has moved over northwestern portions of WY this morning. Precipitation extends from MT southward into the Bighorn and Wind River Basins. This rain is largely inconsequential for KCOD, KWRL, KRIW/KLND with conditions remaining VFR. This rain will largely end and become more focused over northwestern portions through 14Z, as the low continues its eastward trek. Rain and snow will spread southward through the morning and early afternoon, as the storm makes its way toward the Black Hills through the day. Snow will remain likely at KJAC, with KCOD and KWRL are expected to remain as rain (although a mix of snow is not out of the question). IFR/MVFR conditions are expected with the heaviest precipitation between 18Z and 00Z. Mountain obscurations will be common through 06Z. Wind gusts of 30 to 40 kt will be widespread today, as the gradient tightens as a result of the storm. These winds will increase across the area through 15Z, before decreasing after 00Z. Precipitation will also quickly end across the area, from west to east, from 00Z to 06Z. 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... Freeze Watch from late tonight through Friday morning for WYZ003>006. Winter Weather Advisory until 6 PM MDT this evening for WYZ008- 009. Winter Weather Advisory until 4 PM MDT this afternoon for WYZ012. && $$ DISCUSSION...Hattings AVIATION...LaVoie