Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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068 FXUS65 KRIW 211842 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1242 PM MDT Sat Sep 21 2024 .KEY MESSAGES... - Cooler today. Areas of frost are possible tonight, mainly for low-lying areas or cold prone locations. - A little warmer Sunday, with no precipitation chances. - A quick moving system Monday brings a north wind and cooler temperatures. Little to no precipitation chances. - A warming trend Tuesday through the end of the week, with no precipitation chances. && .DISCUSSION... Issued at 1241 PM MDT Sat Sep 21 2024 North to northeast flow dominates much of the area today in the wake of a cold front that moved through last evening and overnight. Temperatures are noticeably cooler as a result, with highs expected to only be in the 60s for much of the area. The main synoptic feature over the next 24 hours is a low south of Wyoming, tracking eastwards. Its main impacts will keep south and southeast of the CWA, however, a tighter pressure gradient brings some stronger northeast winds for the Wind Corridor (Sweetwater County up to around Casper). Afternoon gusts of 25 to 35 mph is forecast for this area. These winds decrease this evening. Precipitation chances have really trended down over the last couple days. Recent runs of the NBM, NAM, and hi-res models keep precipitation out of the area. Would not rule out a stray shower over far southeast Sweetwater County late this afternoon or evening, but little to no liquid is expected, even if something does form. Of interest is recent hi-res models producing some showers over the Wyoming and Salt Ranges between 6pm and 11pm MDT, perhaps in response to a little moisture advection from the southwest. Low confidence in this recent trend, but like with Sweetwater County, not expecting much, if any, measurable liquid. With a mostly clear sky and cooler temperatures in place, there is discussion on frost tonight/early Sunday morning. The lower elevations east of the Continental Divide, as well as most of Sweetwater County, have yet to have a growing season ending freeze. It does not look like this will occur tonight. That being said, widespread temperatures in the the mid-to-upper 30s occur by sunrise Sunday morning. For those in cold prone locations, such as low-lying areas or near rivers, localized freezing temperatures (and frost) are a real possibility. One location of note is Natrona County. Models have been consistent in showing there being a chance of slightly more widespread freezing temperatures (generally 28 to 32 degrees). Casper-town looks to remain just above freeze, but locations west of town, such as the airport and towards the town of Natrona, temperatures are looking near or just below freezing. A Special Weather Statement (SPS) has been issued for lower elevations east of the Divide and for Sweetwater County to cover these around freezing temperatures. Impact-wise, it may be a good idea to protect any gardens or vegetation tonight, even if temperatures are not quite forecasted to be freezing, as there is a chance (even if a small one) that frost forms. As the low tracks east Sunday, some slightly warmer air filters in. Highs climb a few degrees, to seasonal values. Winds should remain relatively light, with no precipitation expected. A very quick trough moves southward through Wyoming Monday. Models have trended drier with this weak system. The recent NAM run only produces precipitation in the Bighorn Mountains. Elsewhere, the moisture is likely to just produce some clouds. Will have to see how hi-res models resolve this system once we get into range. Otherwise, a weak cold front returns cooler temperatures and a north wind for locations east of the Divide. Past this quick system, the GFS and ECMWF build in high pressure, keeping the jet stream to our north. This should trend temperatures up, while keeping precipitation chances near zero. We could see the return of some 80 degree temperatures towards the end of next week. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1102 AM MDT Sat Sep 21 2024 With a cold front having passed from north to south east of the continental divide this morning, and a cutoff low over Arizona moving NE to the Four Corners area, the surface pressure gradient over much of WY will increase NE winds from northeast to central to south-central WY this afternoon. KRKS and eastern Sweetwater County could see gusts over 30 kts, with gusts 15-20 kts in the Bighorn Basin, Johnson County, and Wind River Basin. Partly cloudy skies over northern WY will decrease through the day, with clouds increasing over southern WY due to the cutoff low. VFR conditions are expected at all TAF sites through the next 24-30 hours. A couple of high resolution models are indicating some weak convection over SE Sweetwater County as well as the southern parts of the Wyoming Range late this afternoon and early evening, but chances are quite low. Primary impact from the low pressure system over UT/CO the next 36 hours will be the gusty NE winds and increased clouds over southern WY through early Sunday afternoon. Skies then become mostly clear Sunday afternoon from the west to east. Clouds will increase over northern WY Sunday night into Monday morning due to a weak system moving through MT. 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...McDonald