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
43
475 FXUS65 KRIW 171925 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 125 PM MDT Mon Jun 17 2024 .KEY MESSAGES... - An abrupt wind shift and the arrival of cooler air follow passage of a cold front sweeping east across the region through Monday evening. Light snow falls in northwest Wyoming later this evening and overnight. - The potential for strong to severe storms exists over mainly northern Johnson County between 3PM and 6PM MDT. - Tuesday morning is chilly across the western valleys and basins as overnight lows fall to 27-32F. - After an unseasonably cool Tuesday, temperatures begin to climb Wednesday through the end of the weekend. The best chance for widespread showers and storms is Friday. && .DISCUSSION... Issued at 125 PM MDT Mon Jun 17 2024 As of early Monday afternoon, satellite clearly shows an approaching shortwave near the ID/MT border along the Bitterroot Range. A jet streak has rounded the base of the associated trough and is making its way toward southwest Wyoming. This SW-NE oriented jet streak will aid precipitation production late this afternoon and evening as it traverses the forecast area. Additionally, a surface cold front moving through far west Wyoming continues east and works in concert with the jet streak to boost coverage of showers and thunderstorms east of the Continental Divide late Monday afternoon and evening. Gusty west to southwest wind veers to the northwest behind the cold front. This will led to persistent 20 to 35 mph northwest wind through Monday evening in favored locations such as northern portions of Johnson County and the Bighorn Basin, along with the Wind River Basin. A prevailing south-southeast surface wind has increased dew points into the mid 40s across Johnson and Natrona Counties as of 1PM Monday. CAPE is forecast to reach 500-800 J/kg between 2PM and 4PM and CAMS depict convection strengthening during this time as CIN slowly erodes. The strongest storms are expected over northern Johnson County ahead of the front, with the deepest convection moving east and coming to a close between 6PM and 7PM Monday. SPC has included a slight risk for this region. Large hail will be the primary hazard with the strongest storms. To the west, orographic northwest flow will favor a period of precipitation this evening and overnight across northwest Wyoming. The flow regime favors the heaviest snow in the higher elevations of the Teton Range, while lower elevations above 7000 ft MSL in Yellowstone could see snow accumulations up to around one inch. The best chance for snow accumulation is from 8PM Monday to 2AM Tuesday. As cooler air filters into the region, Tuesday morning lows will fall to 27-32F. Cloud cover may elude locations south of an Alpine to Bondurant to Pinedale line, where the coldest temperatures are most likely. It would not be surprising to see overnight lows in the mid 20s in a few locations, like Bondurant and south Lincoln County if the westerly wind diminishes under a clear sky by sunrise Tuesday. Tuesday keeps a moist westerly flow across the northern-third of the forecast area. Precipitation lingers across this region through the morning and becomes more widespread as instability increases Tuesday afternoon. Temperatures will be unseasonably cool Tuesday with most locations in the upper 50s to upper 60s, about 8 to 15 degrees below normal. The trough departs to the northeast Wednesday and temperatures begin to rebound and most areas return to dry conditions. The exception could be the I-25 corridor late in the day. A weak trough over California Thursday ejects eastward and crosses the forecast area Friday. The Friday convection provides the best chance for widespread showers and thunderstorms during the Thursday to Sunday period. Temperatures warm Thursday and are somewhat hindered by cloud cover and showers Friday. Nonetheless, seasonal readings are anticipated both days. Zonal flow in the wake of this wave allows for upward trending temperatures and drier weather Saturday. Ridging then builds over the Northern Rockies Sunday leading to above normal temperatures and dry conditions to end the weekend. && .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...CNJ AVIATION...Hensley