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
113 FXUS65 KRIW 281742 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1142 AM MDT Sun Apr 28 2024 .KEY MESSAGES... - Remaining unsettled today and Monday with a scattering of showers and thunderstorms. - Windy conditions likely Monday and especially Tuesday with elevated fire weather possible. - Remaining unsettled through much of the work week. Potential for some accumulating snow Wednesday night and Thursday morning. && .DISCUSSION... Issued at 227 AM MDT Sun Apr 28 2024 Steady precipitation has ended across the area as the low that was moving across Colorado has pulled into the Plains states, the trigger for the severe weather outbreak across the central and southern Plains states. The weather looks to remain unsettled through most of the period, but some details are still in flux, especially further out in the forecast period. To sum the pattern up, I will use a Sesame Street reference, as I heard the song Rubber Ducky earlier today, sparking some memories from my young childhood many years ago when big collars and bell bottoms were in style. It also got the creative juices flowing. So, this forecast is brought to you by the letters Z (Zonal flow) and U (Unsettled) and the number 4 (the number of waves that will likely impact the area through the week). We continue to have some showers across western Wyoming as a shortwave moves in from Idaho and slowly crosses the state, bringing a chance of showers and thunderstorms, anytime in the west and mainly afternoon East of the Divide. Many places won`t see one though in the lower elevations, the chance is generally 1 in 3 or less at any point and less than 1 in 5 for a thunderstorm. Gusty wind will be possible with any storm, and with low wet bulb zero levels, small hail will also be a possibility. This could also be a day of self-destructive sunshine, when a clear sky will lead to more clouds through the day with the unstable atmosphere (lifted indices fall to around minus 1). Temperatures will be warmer than yesterday, but still averaging around 5 degrees below normal. Showers should end around midnight as the trough moves away. After a brief break from late tonight through Monday morning, the next Pacific shortwave and weak cold front will swing across the state. With the later arrival, temperatures should be warmer than today, rising to near to slightly above normals levels. We have two concerns for Monday. The first is for the potential of elevated fire weather. The tightening pressure gradient ahead of the system will increase southwest flow ahead of the front. This will lead to some gusty wind. The 700 millibar wind does not look sufficient for high wind though (only around 40 knots), there is a greater than 1 in 2 chance of a few wind gusts past 40 mph. Relative humidity will be rather low as well, falling to around 20 percent. The main area of concern would be Johnson and Natrona Counties, which did not receive the soaking rain of further west. The second concern is convective wind gusts. There will be some dry air ahead of the front. This will likely lead to some high based convection. Model soundings are showing the tell tale inverted V signature that leads to dry downbursts. With dew point depressions approaching 50 degrees in the afternoon, there is a 1 in 2 chance of a wind gust of around 50 mph with any thunderstorms or shower. This could be our first "Little Green Blob" day of the season. Tuesday looks drier as we will be in between systems with zonal flow with somewhat cooler temperatures behind the front. Coverage of showers should be less than 20 percent. The main concern will again be fire weather and possibly strong to high wind. A rather potent 120 knot jet will be crossing the area, with the core coinciding with maximum heating and mixing. The main area of concern will be across the southern two thirds of the state, which has the best possibility to be in the right front quadrant of the jet, helping to enhance downward momentum. There is greater than a 1 in 2 chance of wind gusts past 45 mph in these areas. And with this, the air will be very dry, with dew points falling in the single digits, pushing relative humidity into the teens. This could, again, bring a day of elevated to near critical fire weather. Models continue to trend southward with the Wednesday system, bringing a better chance of precipitation to the area. There are still large differences in placement of the eventually developing upper level low though, so confidence in details remains low. Of particular interest is Wednesday night, as we could have a north to northeast upslope pattern developing that could bring some mountain snow to areas like the Absarokas, Wind Rivers and Casper Mountain during this timeframe. And, with low temperatures falling to around freezing and 700 millibar temperatures falling to around minus 8, snow levels could lower to around 4500 feet, bringing the chance of accumulating snow to the populated areas as well. This is still uncertain though, as it all depends on the eventual track of the low pressure area, but we will have to watch this carefully. Transitory ridging should bring mainly dry weather for Thursday before another wave possibly approaches for Friday with another chance of showers. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1137 AM MDT Sun Apr 28 2024 Scattered showers will continue to bubble across the state today, with the best focus on KJAC, KCOD, and KCPR. Any showers may bring a brief threat for thunder, but this will be very isolated in nature. Cigs with showers will be briefly MVFR, but should be staying above 2500 through the day. Northwest winds will gust 20 to 25kts west of the divide this afternoon, before decreasing after sunset. Next round of showers follows quickly Monday morning, with likely rain at KJAC and KCOD through the day. This precip is not expected to reach east of the divide before Monday evening. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Hattings AVIATION...Straub