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Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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429 FXUS65 KCYS 112329 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 529 PM MDT Sun Aug 11 2024 .KEY MESSAGES... - Near critical fire weather conditions return to Carbon county today along with areas of smoke and haze due to distant wildfires. - Much lower thunderstorm coverage is expected this afternoon, but storms that manage to form may quickly become strong to severe. - An active pattern continues through the week ahead with near normal temperatures and daily chances for afternoon and evening showers and thunderstorms. && .MESOSCALE... Issued at 1211 PM MDT Sun Aug 11 2024 A few cloud streets have begun to crop up in the past hour across portions of WY, showing signs of increased daytime instability in the atmosphere via GOES satellite imagery. A few towering cumulus have also started to appear on satellite in Laramie County. SPC Mesoanalysis depicts low-level lapse rates between H7-H5 at 7.5 to 9.5 degrees C/km as of 18Z. The upper level shortwave axis is presently situated across northern WY, and that is where the current majority of convection and thunderstorms reside. Additionally, MUCAPE of 500 to small pockets of 2000 J/kg have popped up via SPC mesoanalysis for portions of our cwa thanks to the morning sunshine. 0-6km effective bulk shear is sufficient for rotating thunderstorms this afternoon, as it is analyzed at at 25-50 kts, especially east of the Laramie Range. This is where SPC has updated their Day 1 SPC severe weather risk to a Slight/Marginal, instead of just a Marginal Risk east of I-25. DCAPE values exceeding 1000 J/kg also have been analyzed by the 18Z mesoanalysis page. All of this weather jargon being mentioned overall is suggesting isolated to potentially scattered severe thunderstorm initiation in the next 1-3 hours, particularly off the North Laramie Range and eastern Laramie County near the CO/WY/NE state line. Hi-res model guidance suggests that any development should be all but done near 0Z for our cwa. The main hazards will be brief, gusty winds up to 60mph, large hail, and heavy downpours. However, the coverage is expected to be isolated in nature as discrete thunderstorm activity may become right-moving supercells for what turns up to occur this afternoon. && .SHORT TERM /THROUGH MONDAY NIGHT/... Issued at 330 AM MDT Sun Aug 11 2024 Another round of nocturnal showers is moving across the area this morning, fueled by modest synoptic lift ahead of the next upper level shortwave trough. That features is moving across western Wyoming at this hour, with its axis expected to cross through the forecast area late this morning or early this afternoon. Once the axis passes through, we`ll see synoptic scale subsidence take over which will significantly limit today`s thunderstorm coverage. Meanwhile, plentiful moisture remains in place in the lower atmosphere with weak upslope flow supporting another round of low clouds and fog over the High Plains. This is less thick and widespread than the last few mornings, but travelers should still prepare for patchy low visibility in fog this morning. Any lingering fog will clear by midday. Today will be a dry outlier in the middle of a cool and wet weather pattern. A developing surface low pressure system over northeast Colorado by this afternoon is expected to pull in much drier air along the I-80 corridor. This dry region is visible on GOES satellite water vapor channels moving across southern Idaho at this hour. Breezy westerly winds and low RH will return near critical fire weather conditions to Carbon County, while forming a sharp dryline in the vicinity of I-25. The progressive dryline and a convergence zone along the WY/CO border will provide the best potential to produce thunderstorms this afternoon, but coverage will be much lower than Saturday. However, any isolated storms that manage to develop over the High Plains in the rich moisture environment will exist in a fairly potent environment with decent vertical wind shear, lapse rates, and instability. Thus, we are looking at a conditional threat for severe thunderstorms today, contingent on thunderstorm updrafts overcoming the synoptic subsidence in the wake of the shortwave trough aloft. Large hail, strong winds, and locally heavy rainfall will also be possible. Directional shear in the lowest 1-2km also means that an isolated landspout or tornado can`t be totally ruled out. On the bright side, the window for storms will be fairly short today with synoptic subsidence and dry air winning out by around 6PM. In addition to the drier air making intrusions today, this trough may bring another round of smoke and haze from distant wildfires. The dry air break of Sunday will be short lived though as the ridge axis shifts back to eastern Wyoming during the day Monday. Southwest flow across the Great Basin will allow for monsoon moisture to rapidly fill back into the area for Monday. With a continuous train of vorticity maximums also returning, we`ll kick off another round of scattered to numerous afternoon and evening showers and thunderstorms Monday afternoon, which may carry well into the evening hours once again. Monday`s storms will once again bring the chance for heavy rainfall and isolated strong winds and large hail. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Issued at 330 AM MDT Sun Aug 11 2024 An active extended forecast period for southeast Wyoming and the western Nebraska Panhandle w/ numerous mid and upper-level short wave disturbances embedded within quasi-zonal flow aloft. Expect daily chances for showers and thunderstorms as several pieces of energy continue to emanate from the slow moving parent trough in the Pacific Northwest. Multiple rounds of strong/severe thunder- storms appear possible given substantial low-level moisture with dew points in the 50s and 60s and occasional mid-level jet maxes contributing to adequate deep layer vertical shear for organized storms. Still too early to determine any specifics of which days may feature a more organized severe risk as this will be heavily dependent on the eventual timing of individual waves. Otherwise, expect temperatures to be near seasonal norms with daytime highs mainly in the 80s. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 529 PM MDT Sun Aug 11 2024 Quiet this evening as storms have moved out of the forecast area. Southeast flow will lead to the return of low stratus later tonight across western Nebraska terminals. Expect IFR CIGs through at least mid-morning at most of these terminals. Dense fog will likely also develop at KSNY, KAIA, and possibly KBFF, leading to significant visibility reductions overnight and into Monday morning. Scattered showers and thunderstorms are once again expected Monday afternoon. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ MESOSCALE...BW SHORT TERM...MN LONG TERM...CLH AVIATION...SF