Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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812 FXUS65 KCYS 151744 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1144 AM MDT Sun Sep 15 2024 .KEY MESSAGES... - Scattered thunderstorms may develop late this afternoon and evening along and east of I-25. Isolated storms will have the potential to become strong to severe. - Chances for showers and thunderstorms continue through Tuesday, followed by another chance for precipitation and cooler temperatures late in the week. && .SHORT TERM /THROUGH MONDAY NIGHT/... Issued at 345 AM MDT Sun Sep 15 2024 Mostly clear skies are present over the area this morning. The sharp dryline has retreated westward overnight, now located roughly from Cheyenne to Van Tassel. Moist air with dewpoints in the 50s is located to the east of this boundary, while those to the west remain quite dry, with dewpoints mostly in the 20s. Another night of light winds is allowing temperatures in the dry sector to plummet once again, but we are not as cold as 24 hours ago thanks to the warmer airmass encroaching into our area. This dryline will be an important player in today`s weather, as afternoon thunderstorms return to the forecast. While a deep longwave trough digs southward along the West Coast today, a modest shortwave traversing around the leading edge of the trough will approach the area. This is currently visible on GOES water-vapor imagery just beginning to move into southwest Wyoming. Synoptic scale lift ahead of this trough axis will help support the development of thunderstorms this afternoon. Forecast soundings show a fairly potent environment for this time of year, with MLCAPE between 1000 and 1500 J/kg and lapse rates exceeding 8 C/km. However, soundings also show substantial CIN in place this afternoon. This, combined with the late arrival of the shortwave trough axis, should result in fairly slow development of storms. It may take until about 3-4PM before an updraft is able to overcome the cap. The latest run of the HRRR also presents a failure mode scenario today, in which storms are nearly totally unable to overcome the cap resulting in very few if any storms. This is plausible based on environmental parameters, but is currently not the favored scenario since this model may be progressing the dryline too far to the east and thus resulting in a too-dry boundary layer that wouldn`t support thunderstorms. Most of the convective activity (if it occurs) should be mostly between 5PM and 11PM, coming to an end by around midnight when the shortwave trough axis clears the area and we enter a synoptic scale substance region. Isolated strong to severe thunderstorms will be possible through the late afternoon and evening hours. While vertical wind shear will be present, it looks too weak to support supercellular storm modes. Looking ahead to Monday, the strong West Coast trough will dive into central California, supporting amplification of the downstream ridge to our east. Another subtle shortwave will push out of the south as this amplification occurs, bringing with it a fairly significant surge of lingering moisture out of the monsoon region. Integrated water vapor transport Monday evening is over the 90th percentile of climatology. This will be concurrent with notable isentropic lift associated with the amplification of the standing Rossby wave pattern. Thus, look for scattered showers and thunderstorms developing mainly along and west of the Laramie range Monday afternoon and evening, which may carry into the overnight hours as well. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Issued at 345 AM MDT Sun Sep 15 2024 Primary forecast concerns in the long range will primarily relate to the longwave pattern change expected by mid to late week, and the potential for some moderate to locally heavy rainfall amounts (snow accumulations in the higher mountains) next weekend as the Rex Block over the eastern United States results in very slow movement of Pacific storm systems moving east across the Front Range. It looks like the warm late summer temperatures will come to an end on Thursday, with a more fall- like pattern setting up across the region after September 20th. Early this week, the Front Range will still be under the influence of southwest to southerly flow aloft with 700mb temperatures near 12c Tuesday. This translates to high temperatures in the 80s to near 90 for the high plains and low to mid 70s west of the Laramie Range. Models show the first of a series of Pacific upper level troughs moving eastward across the Great Basin Region on Monday and across Wyoming on Tuesday. Most of southeast Wyoming and western Nebraska will be in the region of diffluent flow aloft ahead of the trough axis. With some low level instability, afternoon showers and thunderstorms are expected...especially once the trough axis lifts northeast and a surface cold front begins to move into central and eastern Wyoming. Not expecting severe weather at this time due to limited moisture, but a few strong storms are possible due to low level and midlevel forcing, decent 0-6 km shear, and moisture advection through the afternoon hours. Surface cold front will push through the area and into western Nebraska by Tuesday night, with daytime temperatures closer to normal for this time of the year Wednesday. Windy conditions are expected Tuesday and Tuesday night with the wind prone areas likely seeing gusts up to 45-55 MPH Tuesday and early Wednesday, with gusts up to 40 MPH outside of the wind prone areas. Temperatures will rebound slightly on Thursday as the next Pacific trough moves across the southern Great Basin region. This is when models start to diverge considerably as the speed of this trough is nearly 24 hours faster with the GFS compared to the less progressive ECMWF and Canadian. Thankfully, models come into better agreement for next weekend as the strong Rex Block over the eastern United States develops and remains in place through the extended. There is high confidence with a gradual cooling trend over southeast Wyoming and western Nebraska Friday through next weekend, but confidence is limited with precipitation amounts. This is pretty typical with blocking patterns, and forecast confidence may not change much over the next several days. Lower than average confidence is mainly influenced by the strength of the Rex Block and exact position when/where it develops. Gradually increased POP Friday and Saturday for widespread rainfall, with the possibility of considerable early season snowfall across the mountains above 9500 feet. With good ensemble support, kept the cooling trend going towards the end of the week and next weekend with highs in the upper 50s and 60s by next weekend. Can`t rule out the first freeze of the season Saturday morning, but Sunday morning looks more probable for areas west of the Laramie Range as another strong upper level trough, this time digging south out of western Canada, pushes across Montana and Wyoming with 700mb temperatures lowering towards -3c by late next weekend. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1138 AM MDT Sun Sep 15 2024 VFR conditions expected across area terminals through the forecast period after low clouds this morning cleared out of the NE panhandle per GOES imagery. Showers and thunderstorms will begin developing along the higher terrain in southeast WY this afternoon, propagating east towards NE panhandle terminals this evening. Gusty, erratic winds in the vicinity of storms are possible. Any heavier storms that develop could lead to brief MVFR conditions, possibly near KSNY and KCDR later into the evening. Storms should dissipate and clear out of the area by 06z with clearing skies through the night. However, will need to monitor for low cloud development across the NE sand hills possibly reaching KAIA early Monday morning. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MN LONG TERM...TJT AVIATION...MB