Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
906 FXUS65 KRIW 230921 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 321 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 a 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, The heaviest snow will fall in the Bighorn range. The heaviest will be in the northern Bighorns, especially north of Granite Pass. 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 the chance is much less, 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 be the favored area. Mainly from Rock Springs to Casper earlier in the day and afternoon, and then across Johnson County in the afternoon after the low passes and wind turns northwest. There are a few 50 knot wind barbs at 700 millibars, However, these occur rather briefly. 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. And then we have one more concern. That is for the potential of a frost or a freeze in areas where that is a concern. This is mainly the Bighorn Basin at this time. It is borderline but in some locations 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 enough. 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 on Sunday 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 /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1028 PM MDT Wed May 22 2024 The next weather system continues to approach Wyoming tonight with moderate to heavy precipitation impacting the western part of the state. Rain has turned over to snow at KJAC and should remain IFR through 07-08Z as the area of precip moves northeast. A mid-level cold front will move west to east overnight, bringing snow and rain to KBPI and KPNA between 06-12Z/Thu. A low chance of showers is possible at KRKS around sunrise too, not enough to mention in the TAF though. Behind the cold front, surface winds will increase from the west and gust 30-40 kts at most sites through the day Thursday. As the storm center moves across the middle of the state, wrap- around precipitation with SW then NW winds will bring showers to KJAC during the middle of the day. And then as the storm center moves to eastern WY in the afternoon, a surface cold front will push southward into the Bighorn Basin to bring MVFR/IFR conditions to KCOD and KWRL with gusty north winds and rain turning to snow. The cold front will push into central WY around 00Z/Fri for 1-2 hours of gusty north winds at KRIW/KLND/KCPR and a 30 percent chance of light showers. Some east winds then push into KRKS just before 06Z/Fri. The mountains will be mostly obscured for the TAF period with moderate snow in the west-central mountains tonight, and moderate snow over the Bighorns from early morning through sunset Thu. 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...McDonald