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Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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555 FXUS65 KCYS 182333 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 533 PM MDT Thu Jul 18 2024 .KEY MESSAGES... - Widely scattered thunderstorms will continue through the evening hours. A few isolated storms may become strong to severe. - Another round of scattered to numerous thunderstorms is expected on Friday late morning through the afternoon. A few isolated storms may become strong to severe. - Cooler weather expected this weekend with daily chances of storms through early next week. && .SHORT TERM /THROUGH SATURDAY NIGHT/... Issued at 300 PM MDT Thu Jul 18 2024 Our area remains on the northeast fringe of a potent upper level ridge centered nearly exactly over the Four Corners. Decent monsoon moisture is trapped underneath this ridge, while weakly increasing pressure to the east is keeping dewpoints in the upper 40s to 50s across the High Plains. Between these two components, above average precipitable water is present across the area today, ranging from around 0.6 inches over the Sierra Madre to just over 1 inch in the nebraska panhandle. The environment is fairly unstable today with 700 to 500-mb lapse rates exceeding 8 C/km for most of Wyoming. Shear is fairly weak, but present over the High Plains with weak low level southeast winds underneath northwest flow aloft. There is not much synoptic lift nearby today, so the higher terrain and a weak convergence boundary along the Laramie range will provide the main axis for storm development today. Scattered showers and storms have already developed over Carbon and Albany counties right now, but have remained weak so far. Modest MLCIN is still present east of the Laramie range, but this is expected to break down over the next few hours and allow for a few storms to tap into the better environment further east. No major changes to the outlook for this afternoon, but expect isolated strong to severe storms with the potential to produce hail and gusty winds. Locally heavy rain will also be a concern with weak steering flow continuing. Friday will have a similar overall setup, but it will be a bit more dynamic than today. A modest shortwave currently visible on water vapor satellite imagery along the Canada/Montana border will rotate over the top of the ridge and slide down the eastern side on Friday. A surface cold front will also add some modest frontogenesis over the plains. Additional lift from these elements will help to increase shower and thunderstorm coverage compared today. The shortwave will also nudge stronger flow aloft back into our area, which will lead to slightly higher shear values than today. Mid to upper level moisture will remain fairly plentiful Friday, so we`ll have another threat for isolated large hail, locally heavy rainfall, and gusty winds with thunderstorms through the afternoon and evening hours. Expect an early start to convection, possibly kicking off even before noon. Expect cooler temperatures on Saturday across the area as the ridge retrogrades westward and cooler air is allowed down into the plains. Look for another chance for showers and thunderstorms in the afternoon hours with the next in a series of vort-maxes moves over the top of the ridge. At least right now, the lapse rates look more modest on Saturday, which may reduce the severe weather chances, but cannot rule out some isolated severe. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Issued at 316 AM MDT Thu Jul 18 2024 Overall, looking at cooler weather this weekend with afternoon highs around 80 on Saturday and potentially dropping into the mid-70s for Sunday. This will be all while much of the western CONUS is expecting above average temperatures developing under the vast upper level ridge, however much of our CWA will be positioned well east of the ridge axis and more under the influence of the large scale trough across the central and eastern CONUS allowing for cooler weather. Daily chances for showers and thunderstorms continue with multiple disturbances passing over and along the eastern periphery of the ridge. Saturday and Sunday look to have the best chance for more widespread coverage as stronger upper level support will be present associated with the upper low over the central CONUS mixed with post-frontal east/northeasterly flow up against the Laramie Range. Looking ahead to the middle of next week, a large scale trough approaches the PacNW attempting to breakdown this stale upper level pattern. Latest cluster analysis shows growing uncertainty with the timing of the trough passage to the north. One cluster mainly comprised of GEFS members shows a quicker trough progression across the Canadian border that shifts the thermal ridge overhead while other clusters more comprised of EC members are on the slower side. Will need to continue to monitor model trends as this will impact how quickly we return to above normal temperatures for the later portions of next week. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 528 PM MDT Thu Jul 18 2024 Northwest flow aloft will continue. Wyoming TAFS...Scattered to broken clouds from 7000 to 12000 feet will prevail. Occasional thunderstorms will occur at Laramie and Cheyenne until 01Z, producing wind gusts to 35 knots and visibilities to 4 miles. Outside of thunderstorms, winds will gust to 22 knots at all terminals until 02Z, and to 23 knots at Rawlins and Cheyenne after 15Z Friday. Nebraska TAFS...Scattered to broken clouds from 8000 to 15000 feet will prevail. Winds will gust to 25 knots until 02Z, except gusting to 24 knots at Scottsbluff until 08Z. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MN LONG TERM...BW AVIATION...RUBIN