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
173 FXUS65 KRIW 171745 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1145 AM MDT Mon Jun 17 2024 .KEY MESSAGES... - A cold front passes through today. As a result, gusty winds, shower and thunderstorm chances, mountain snow, and much cooler temperatures. - A few strong to severe storms are possible for Johnson County today, mainly between 2PM and 8PM local time. - Sub freezing temperatures occur for western valleys and basins tonight. Frost and freezing to outdoor vegetation is possible. - Much cooler tomorrow (Tuesday), with temperatures climbing back above normal by Thursday. && .DISCUSSION... Issued at 148 AM MDT Mon Jun 17 2024 Current water vapor imagery shows an approaching cold front, the associated upper-low currently over the Washington/Oregon/Idaho border. This low tracks eastward into Montana today. The trough swings down through Wyoming as a result, bringing the cold front with it. Models show the main push of the front moving through western Wyoming this afternoon, pushing east of the Continental Divide this evening and overnight. The consequences of this frontal push will be gusty winds, precipitation chances (including snow), and cooler temperatures. These three things will be elaborated on in the coming paragraphs. Starting with winds, gusty southwest winds ahead of the front pick up today late morning to early afternoon. Gusts 30 to 40 mph will be common across central and southern areas, with gusts in the 20 to 30 mph range elsewhere. Stronger winds are possible with the frontal passage. As the front passes through, winds shift more northwesterly to northerly. Cooler temperatures follow the front as well. Today`s highs west of the Divide will range from the mid 70s in Sweetwater County to the upper 50s and 60s for the western most basins and valleys. The cold front passage will halt the diurnal temperature climb, so western locations will be cooler today. East of the Divide, the front passes in the afternoon and evening, so these locations should be able to reach peak heating before the front passes. Highs here will be in the mid 70s to mid 80s, warmest towards the Casper area where the front passes latest. Low temperatures Monday night/Tuesday morning are on the cool side. Of note will be the western valleys and basins, where temperature below freezing (32 degrees) are looking likely (75% to 100% chances). The chance of less than 28 degrees (a hard freeze) are around a 25% to 30% chance for the lower elevations of southern Lincoln County, and for the upper Green River Basin, including Big Piney and Pinedale. Star and Jackson Valleys are less than 10% chance for less than 28 degrees. The one wrinkle in these temperature forecasts will be cloud cover. Models have post-frontal clouds. If clouds are more numerous than forecast, temperatures will be warmer than forecasted. In any case, it may be a good idea to prepare outdoor vegetation for sub-freezing temperatures. Lastly, is precipitation. Starting with snow, snow levels look to drop to around 6000 to 6500 feet at the lowest through sunrise Tuesday, behind the front. Based on precipitation chance locations, this puts snow over the western mountains (including Yellowstone National Park) and the northern Bighorn Mountains. Most of the these places are looking to receive somewhere on the order of 0.5 to 2 inches of snow. The higher elevations of the Tetons, Absarokas, and Bighorns could see up to 6 inches. Notably, the Tetons have about a 60% chance to get more snow at the highest peaks based on the latest model runs. Much of this snow falls Monday night, through about sunrise Tuesday. Expect little to no impacts for Yellowstone, where up to inch of snow is forecasted for the lower elevations. For liquid (rain) chances, those will mostly be across the northern half of Wyoming. Shower and thunderstorm chances occur this afternoon, mainly east of the Divide, ahead of the incoming front. Chances look isolated (15% to 30%) for most of these areas. Slightly better chances (30% to 60%) occur with frontal passage for the latter half of the afternoon and through the evening across the eastern Bighorn Basin and for Johnson and Natrona Counties. Model soundings are showing a favored environment for strong storms over Johnson County between 20Z/Mon and 02Z/Tues (2pm and 8pm local). CAPE values in the 1000 to 1500 J/kg range are possible based on a few models, and given decent shear as well (bulk shear 50-60kts based on latest RAP), some stronger storms may develop. Strong outflow winds and hail are the main threats. The Storm Prediction Center (SPC) has Johnson County in a marginal (category 1 of 5) risk today, to account for the wind and hail threat. Additional shortwaves behind the main trough keeps rain (and mountain snow) chances through the day Tuesday, mainly across the northern half of the area and mainly in the mountains. Dry slotting early Tuesday and through the day will limit precipitation chances across central areas. Highs Tuesday are noticeably cooler behind the front. Most places will see highs in the 60s, which are 10 to 20 degrees below average for mid-June. Temperatures begin to rebound Wednesday, with highs returning back to seasonal normals as southwest flow dominates the state. Drier air and lack of forcing will keep rain chances to near zero. The next feature of interest is a trough in the late Thursday to Friday timeframe. This looks to bring widespread rain chances, but confidence is low at this range. High pressure builds in for the latter half of next weekend, bringing warmer and drier conditions. The Climate Predication Center (CPC) has Wyoming favored for warmer than average temperatures in both the 6-10 and 8-14 day outlooks. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1145 AM MDT Mon Jun 17 2024 A cold front associated with a shortwave trough will move through the region today. This will bring increased chances for showers, thunderstorms, and even snow showers to portions of the region. The front will move northwest to southeast across the region during the afternoon, with first chances for associated showers expected at KJAC after 20Z. KCOD and KWRL will be the next terminals to see chances for showers, with isolated thunderstorms also possible. KCPR also is expected to see showers and thunderstorms associated with the cold frontal passage, with best chances after 06Z. MVFR conditions and mountain obscurations are expected with the shower and thunderstorm activity, most notably at KJAC and KCOD. KJAC may see occasional IFR conditions, and there is an outside chance (<20%) that some snow flakes may mix in with rain showers after 06Z. Otherwise, VFR will generally prevail through the period. Gusty winds will occur both ahead of and behind the front. West of the Divide, winds will generally remain westerly with a slight turn to northwesterly at KBPI and KPNA. East of the Divide, southwesterly winds early this afternoon will turn northwesterly behind the front. This shift will be most notable at KRIW and KCPR. Gusts could reach 30 to 35 kts during the afternoon and evening at most terminals ahead of and with the front, and winds will slowly decrease through the late evening and overnight behind the front, but remain breezy. KRKS has the best chance (40%) of occasional wind gusts reaching 40kts late this afternoon. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Wittmann AVIATION...Hensley