Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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463 FXUS65 KRIW 200759 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 159 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 /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 1010 PM MDT Thu Sep 19 2024 Light winds and mostly clear skies prevail overnight into Friday morning. During the early afternoon hours on Friday, typical 15 to 25 kt breezes will develop for many TAF locations. A cold front will push in from the north during the evening and early overnight, causing winds to shift to northerly and become gusty (see TAFs for more precise timing). An hour or two of 20 to 30 kt gusts are likely (60% chance) at sites east of the Continental Divide as the front passes, then decreasing to lighter northerly flow overnight into Saturday morning. && .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 from noon today to 7 PM MDT this evening for WYZ280-281-289-300. && $$ DISCUSSION...Rowe AVIATION...VandenBoogart FIRE WEATHER...Rowe