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Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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908 FXUS65 KCYS 101931 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1200 PM MDT Mon Jun 10 2024 .KEY MESSAGES... - Dense fog with visibilities as low as 1/8 mile will impact the southern Laramie Range and adjacent foothills (including the I-80 Summit) through early this morning. A Dense Fog Advisory remains in effect until 9 AM MDT. - Numerous showers and thunderstorms are expected along and east of the Laramie Range after 18z today. Strong to severe storms are possible with the potential for hail, strong winds, and perhaps an isolated tornado with any quasi-discrete storms. - Mainly dry and rather warm conditions will prevail for Wednesday and Thursday, with increasing chances for showers and thunderstorms for Friday and Saturday, along with slightly cooler temperatures. Sunday will see another warming trend with decreasing precipitation chances. && .UPDATE... Issued at 1137 AM MDT Mon Jun 10 2024 Mesoscale trends are being closely monitored late this morning relevant to the severe weather threat in southeast Wyoming and western Nebraska this afternoon. As of 17:30z, surface obs from KCYS northward through KGUR and KDGW indicate mid to upper-50s dewpoints with general south-southeasterly surface flow between 5-15 kts. To the west, locations in the Laramie valley and Shirley basin have shown a dramatic lowering of surface dewpoints as the lingering shallow moist layer has mixed out and drifted eastward in dry westerly flow AoA 15kts. As such, a convergent boundary/dryline (enhanced by orographic upslope flow) has developed over the Laramie Range, stretching from the Colorado border near Pumpkin Vine northward through far eastern Albany County and bending back westward along the western slopes of the Laramie range toward Casper. Despite the radar data outage from KCYS WSR88D, this boundary is still visible over southern Wyoming from KFTG in Denver. SPC RAP mesoanalysis indicates over 3000 J/kg of SBCAPE as of 17z to the east of the Laramie Range, as well as 700-500mb Lapse Rates over 8 across all of southeast Wyoming. With a little bit of MLCIN to still yet to erode, but yet formidable instability in place once it does, the expectation is for thunderstorms to rapidly develop over the next hour in southeast Wyoming. Initial discrete thunderstorm development will be generally tied to the dryline/mountain convergence boundary and move east around 10-15 knots. It is in this stage that we will likely see the highest tornado/landspout threat, as any surface-based storm in the developing stage anchored to this boundary will get a little boost of low-level shear. However, given poor mid-level flow (h5 wind below 25 knots in the entire CYS CWA) and a lack of turning aloft (chaotic/short hodographs above 700mb), the expectation is for early discrete cells to quickly pool their cold, hail-laden outflows and transition to a linear wind threat as they head into the Nebraska Panhandle. The one exception to this afternoon linear mode transition may be across the far northern portions of Converse and Niobrara counties later today. As a shortwave trough evident in Water Vapor imagery slides eastward, wind profiles aloft will improve after around 21z in northeast Wyoming. If boundary layer moisture can remain untapped and avoid overturning from early convection, a better threat for a sustained discrete cell may evolve. We will monitor these trends closely this afternoon. && .SHORT TERM /THROUGH TUESDAY/... Issued at 420 AM MDT Mon Jun 10 2024 An area of extremely dense fog persists early this AM across the southern Laramie Range and adjacent foothills w/ numerous webcam and highway observations indicating visibilities only around 250 to 500 feet at several sensors along I-80 between CYS and LAR. A Dense Fog Advisory was issued earlier this AM, which will remain in effect until 9 AM MDT as T/Td spreads remain unusually low w/ dew points in the middle/upper 50s. Otherwise, light rain shower activity or perhaps just a few sprinkles can be expected for our western zones through mid-morning. The focus for the afternoon & evening hours shifts to the threat for strong to severe thunderstorms w/ much of the high plains in a SLGT risk from SPC. South/southeast low-level flow is expected to maintain robust boundary layer moisture profiles with surface dew points in the 50s (perhaps locally 60+ F) in the pre-frontal air mass along and east of the Laramie Range. CI should occur as early as 18z over the central/southern Laramie Range w/ enhanced low-level convergence associated w/the Chugwater Cyclone. Should see convection rapidly increase in coverage to the north along & east of I-25 between 18z-21z along the dryline & cold front. The overall evolution is expected to feature rapid upscale growth to a forward-propagating MCS due to merging cold pools as well as a favorable wind profile with strong low-level directional shear & deep unidirectional flow aloft. Would prefer to see stronger mid level flow for a more substantial severe weather threat, but 30+ knots of 0-6 km shear should support organized clusters. Threats should include both severe hail/wind given steep the robust low- level moisture and steep lapse rates supporting MLCAPEs of 1500- 2000 J/kg. CAMs are in excellent agreement showing a quite large linear complex impacting areas along/east of a line from Lusk to the Colorado state line and beyond, mainly between 18z & 00z. We may see several additional rounds of convection across far south east Wyoming into the southern Nebraska Panhandle w/ the passage of a train of H5 vorticity maximums through the evening and into the overnight hours. The intensity of this activity remains very uncertain at this time, and will most likely depend on how those earlier storms evolve. Aside from the convective threat, it will be quite warm today w/ most of the area seeing daytime highs in the 80s, near 90 across the Platte River Valley including EAN/TOR/BFF. Expect to see 700 millibar temperatures peak near 12-15 deg C this afternoon, then dropping 8-10 deg C during the evening and overnight w/ the cold frontal passage. Substantial WAA will occur early Tuesday, ahead of a building broad/flat upper-level ridge. That said, we should not see much of a change in daytime highs between Mon and Tue. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Issued at 420 AM MDT Mon Jun 10 2024 Tuesday night/Wednesday...Even warmer as the flow aloft turns westerly, and with 700 mb temperatures near 17 Celsius, maximum temperatures will be in the mid 80s to mid 90s. Warmer temperatures aloft will increase the convective inhibition, and with less low and mid level moisture around, it looks like it will be dry. Thursday...Similar synoptic pattern as on Wednesday, though low level winds will become east, and with an increase in low and mid level moisture, we expect to see isolated late day showers and thunderstorms. Should be a bit cooler than Wednesday with upslope winds and more cloud cover. Friday...Ridging aloft, over our forecast area, begins to break down as the next shortwave trough aloft approaches, and with surface lee trough convergence and an increase in atmospheric moisture, we expect scattered late day showers and thunderstorms. Slightly cooler with an increase in cloud cover and precipitation coverage. Saturday...Looks like we will likely see a shortwave trough aloft passage in the afternoon, helping to spark scattered late day showers and thunderstorms. Maximum temperatures look similar to those of Friday. Sunday...The flow aloft backs to southwest, inducing the development of another surface lee trough, and with less atmospheric moisture and 700 mb temperatures near 18 Celsius, maximum temperatures will increase into the 80s at most locations. With less moisture aloft, only isolated to widely scattered late day showers and thunderstorms are expected, mainly along and east of Interstate 25. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 1140 AM MDT Mon Jun 10 2024 A surface low across northern Wyoming and eastern Montana will move east today as a cold front to the south of the low moves across eastern Wyoming and into western nebraska later this afternoon through this evening. Thunderstorms will start to develop around 19z, and should form a solid squall line as they move south and east...impacting the western Nebraska terminals late this afternoon through this evening. HAZARDS/WEATHER TRENDS: VFR conditions will prevail over KRWL and KLAR as most of the activity will remain to the east of these terminals. However, can`t rule out an isolated shower/tstorm after 20z with gusty winds being the primary concern. KCYS will see thunderstorms develop here shortly (~19z) with a solid line of thunderstorms by 21z. Strong gusty winds, hail, and very heavy rain will be the main threats. There is a chance that this line will form just to the east of KCYS (similar to what happened on Saturday) and spare the terminal of the worst conditions. Confidence with IFR VIS and strong gusty winds is low to moderate (20% to 40%) For the western Nebraska terminals, there is high confidence that all terminals will see a squall line push through between 21z today to 01z this evening. Expect a period of IFR CIGS and VIS with frequent lightning, strong gusty winds, and possibly hail. Thunderstorm activity may linger through the late evening hours behind the main squall line. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ UPDATE...MAC SHORT TERM...CLH LONG TERM...RUBIN AVIATION...RUBIN