Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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426 FXUS65 KRIW 072326 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 526 PM MDT Tue May 7 2024 .KEY MESSAGES... - An upper low will continue to lead to cool, wet, and windy conditions across much of the region over the next few days. - Snow showers persist across the west and southwest this afternoon, then, as the low progresses westward, a cold front will bring rain and mountain snow to most areas east of the Divide Tonight through Wednesday. - Lingering upslope enhanced showers and snow showers will persist through the day Thursday. - The upper low weakens and moves out of our region by Friday, leading to warmer and drier weather through the weekend. Dirty ridging looks to keep the pattern warmer but active next week. && .DISCUSSION... Issued at 231 PM MDT Tue May 7 2024 The broad region of low pressure that has been impacting the region for the past several days will continue to do so. As of 1 PM, the low is circulating over the ND/SD state line. Over the next 24 hours, the low will slowly shift westward and back into WY. With it, a cold front will drop southward and bring rain and mountain snow to much of the region tonight through Wednesday night. In the meantime, northwest flow is keeping persistent show showers across the far west through the afternoon today and into tonight. Snowfall rates look to increase overnight across the northwest mountains and Yellowstone Park as the main precip band associated with the low rotates southward into the region. Throughout the evening and overnight, northwest flow will also persist, enhancing the orographics along the Tetons. Given these trends, in conjunction with webcams that show very snowy conditions across those areas, have made the decision to extend both the Winter Weather Advisory for Yellowstone Park and the Winter Storm Warning for the Tetons through midday Wednesday. In addition to the snow across the west, snow moves into the northern portion of the state east of the Divide tonight as well. The Bighorn Mountains are of particular note. QPF has trended upward significantly across the northern portion of the Range, as well as across portions of Sheridan County and points north across Montana. The latest QPF forecast seems high, however, hi-res models also have trended QPF higher compared to previous forecasts. Flow will also begin to turn northerly and then eventually gain a bit of an easterly component as well, which is favorable for a period of upslope along the northeast slopes of the Bighorns. This will mainly impact the Sheridan County and MT side of the mountains, however, given the forecast and latest trends, have gone with a Winter Storm Warning for the Bighorns, which will cover the threat through Thursday morning. Upslope showery snow may continue through much of the day Thursday, however, additional accumulations after about 5 to 6 AM Thursday should be minimal. The east slopes of the Wind River Mountains, as well as Casper Mountain also need to be monitored, as upslope snow could bring more than anticipated snowfall amounts to those locations should QPF continue to trend higher. Elsewhere east of the Divide, lower elevations will mostly be rain with this event. There is some cooler air that will filter down with the cold front this evening and tonight, however, given cloud cover, precipitation occurring, among other factors, temps across most lower elevation locations should remain warm enough for precipitation to remain rain, rather than snow (85% chance of temps >33F). QPF amounts will be generally in the 0.25 to 0.5" range across most lower elevations east of the Divide, with highest amounts across Johnson County (mainly along the east slopes of the Bighorns) and also across the Cody Foothills and Lander Foothills due mainly in part to upslope effects. Besides precipitation, you would have to have been living under a large rock to not notice that it has been especially windy over the past few days. Persistent northwest flow as a result of a tight pressure gradient and associated low pressure, has kept winds quite gusty across most areas for several days. The good news is that this will finally begin to diminish overnight. As the cold front turns winds northerly and eventually northeasterly, wind speeds will diminish. Gusty post-frontal winds will persist across the Bighorn Basin and favored Johnson County through much of the day Wednesday, but across other areas, we will see a significant decrease in wind speeds and gusts tonight through tomorrow morning. The low finally weakens and rotates out of our area (to the south) by late Thursday night. Precipitation will end and we will see a warmer and drier trend through the weekend. Also, as the low begins to weaken Thursday, and especially Friday, winds will become mostly light and variable. Winds will stay light through the weekend, and with temps returning to seasonal norms and even above normal temps by the end of the weekend, we are in for a very pleasant Spring weekend. Weak "dirty" ridging looks to set up for next week, so temps will remain warm, but there does look to be chances of showery activity returning Monday, continuing through much of next week. && .AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 525 PM MDT Tue May 7 2024 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals. Occasionally MVFR snow takes a brief lull in the vicinity of KJAC this evening with VFR returning for a short spell. Snow showers return in northerly flow aloft as a shortwave spins south across the state late tonight and Wednesday. MVFR conditions prevail at KJAC after 08Z with occasional IFR possible until 13Z/Wednesday. Snow lingers into the afternoon, but should be on the decrease. Other than scattered brief MVFR snow showers early this evening, the remaining terminals are VFR through 00Z/Thursday. KPNA could see some light snow late in the period. Breezy westerly wind persists overnight with an increase in speeds around 15Z-16Z/Wednesday. Mountain tops frequently obscured above 10K feet MSL through the period. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals. Gusty west-northwest surface wind of 25-40kts continues until around sunset, but 15-25kt winds continue for many locales through Tuesday evening. Shortwave energy within a broad trough pinwheels south across the region late tonight and Wednesday. VFR conditions gradually give way to lowering ceilings and a better chance of rain and snow showers over the Bighorn Basin after 08Z/Wednesday. MVFR or low-end VFR to develop at KCOD and KWRL between 12Z-15Z/Wednesday, while other terminals remain VFR through 16Z/Wednesday. Favorable northerly surface and mid-level flow leads to increasing precipitation chance and lowering ceiling across the central terminals after 18Z/Wednesday. Some MVFR possible, mainly at KCPR and KLND, Wednesday afternoon. Mountain tops obscured after 09Z/Wednesday. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until noon MDT Wednesday for WYZ001. Winter Storm Warning until 6 AM MDT Thursday for WYZ008-009. Winter Storm Warning until noon MDT Wednesday for WYZ012. && $$ DISCUSSION...Hensley AVIATION...Jones