Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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028 FXUS65 KRIW 201720 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1120 AM MDT Fri Sep 20 2024 .KEY MESSAGES... - Fire weather concerns this afternoon across central WY, with gusty wind and low RH. Red Flag Warnings have been issued from noon to 7 PM MDT. - A couple of cold fronts will bring cool temperatures to the area from Saturday through Tuesday. These cold fronts look quite dry, so precipitation chances/amounts do not look high. - A dirty ridge will bring back above normal temperatures by the middle part of next week. && .DISCUSSION... Issued at 159 AM MDT Fri Sep 20 2024 A loosely coupled upper-low currently located over central CA and a trough in western MT will slide eastward today into tomorrow. These two upper-level features will pass south and north of WY respectively, which means most of the area will miss out on any precip chances. As this tandem passes through Friday night into Saturday a cold front will sweep through the area bringing much cooler temperatures Saturday into Saturday night. The upper-low to the south looks to be a bit slower to pass by and may bring a bit of moisture to extreme southeastern Sweetwater County/Natrona County as it does so Saturday night. Rainfall amounts would be anywhere from a trace to a few hundredths of an inch in the aforementioned areas. Overnight lows Saturday night will be chilly in the mid to upper 30s for lower elevations east of the Continental Divide. Wouldn`t be surprised if there was frost in low spots Sunday morning. For those who still have a garden it doesn`t look like this will be a freeze event, so impacts should be minimal to vegetation. The most notable weather impacts from these weather systems will be the elevated to critical fire weather conditions this afternoon across central WY, which will be discussed in more detail in the Fire Weather Discussion below. A brief ridge moves overhead on Sunday in between weather systems, which will allow for temperatures to rebound a bit, back to about seasonal averages for this time of year. Another trough surges down from the northwest very quickly on Monday. Very light precip accumulations are possible (30%) with this weather system as it really won`t have enough time to drop more than a few hundredths of liquid for any location. By Tuesday there`s a bit of a disconnect in model guidance with how the pattern evolves. By that time ensemble model guidance starts to diverge greatly. The deterministic GFS wants to develop a cut-off upper-low from the Monday system and have it retrograde northwest through the middle part of next week, which does seem to align with a few ensemble members of the GEFS, but the majority do have a ridge developing behind Monday`s trough. For now the latter seems to be the more likely (70%) scenario, so expect a substantial warming trend by the middle part of the week as the ridge builds overhead. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1120 AM MDT Fri Sep 20 2024 VFR conditions at all terminals through midday Saturday. A shortwave sweeps east across Montana Friday afternoon and evening, nudging a cold front south through areas east of the Continental Divide. There is enough lift and mid-level moisture to generate isolated showers over far northern Wyoming late today, but no impacts beyond SCT-BKN mid-cloud deck at KCOD. Gusty westerly wind 10-20kts increases at almost all terminals by 21Z/Friday before slowing again by sunset. The cold front begins a southward trek Friday evening and generates gusty northerly wind 10-20kts overnight. The front does make it to KRKS around 08Z/Saturday, but fails to reach other western terminals. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .FIRE WEATHER... Issued at 159 AM MDT Fri Sep 20 2024 Today will be the most consequential day in terms of fire weather concerns in the next 7 days. Increased wind ahead of a strong cold front and good mixing leading to low RH will result in elevated to critical fire weather conditions across central WY. As such, have issued Red Flag Warnings for zones 280, 281, 289, and 300. Zone 281 is more marginal, especially across the northern half of the zone, but criteria is likely (60%) to be met across the southern half of the zone. This also aligns better with the Red Flag Warning in southern Campbell County. The Livingston Draw Wildfire is located in northeastern Johnson County where wind will be lower and RH higher, but conditions will still be favorable for fire growth and spread of that wildfire this afternoon. A strong cold front will bring an abrupt northerly wind shift Friday night, with wind gusts of 15 to 25 mph across the Bighorn Basin and Johnson County down into Natrona County. && .RIW WATCHES/WARNINGS/ADVISORIES... Red Flag Warning until 7 PM MDT this evening for WYZ280-281- 289-300. && $$ DISCUSSION...Rowe AVIATION...CNJ FIRE WEATHER...Rowe