Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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509 FXUS65 KCYS 161146 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 550 AM MDT Mon Sep 16 2024 .KEY MESSAGES... - Isolated to scattered showers and thunderstorms are possible this afternoon and evening, mainly along and west of I-25. - A powerful storm system will move through the area Tuesday, bringing widespread showers and thunderstorms with the potential for strong to damaging winds. - Strong winds may continue in the wind prone areas through Wednesday morning. A High Wind Watch has been issued. && .SHORT TERM /THROUGH TUESDAY NIGHT/... Issued at 420 AM MDT Mon Sep 16 2024 After an active day of thunderstorms yesterday, fairly tranquil conditions have taken over this morning as last evening`s shortwave pushes northward across the Dakotas. The break will be short lived though, as active weather will continue through the next few days. The next player is visible on GOES water vapor channels as an area of higher mid to upper level moisture content over Colorado and New Mexico drifting northward this morning. Meanwhile, the next major weather maker is a powerful upper level low pressure system currently centered over northern California. As this digs southward into the western US, the shortwaves ejecting out ahead of it will help to amplify the downstream ridge today. Today`s wave will bring with it more abundant moisture pulled out of the lingering monsoonal moisture. Models show decent isentropic lift developing mid to late afternoon, mainly over the western part of the forecast area and continuing through the evening hours. This should support the development of scattered showers and a few thunderstorms thanks to some marginal convective instability present. A few of these may be capable of producing strong winds. While most of the lift supporting this activity will dry up around midnight, a potent moisture plume will continue streaming into southwest Carbon county, likely keeping showers around through Tuesday morning. Tuesday will be a very dynamic day across the area. The closed low over California today will start to race to the northeast on Tuesday, reaching northwest Wyoming by the afternoon or evening hours. Strong southerly flow ahead of the storms system will continue to pull in deep moisture. 700-mb meridional wind component and integrated water vapor transport will both be approaching climatological maximums Tuesday afternoon according to the NAEFS mean. As the upper level low approaches, it will start to run into the stubborn Rex Block to the east and gain a negative tilt. Meanwhile, warm temperatures and decent low level moisture will still be present over our area Tuesday morning and early afternoon ahead of the storms system. All of these pieces will come together for what is increasingly looking like a very active afternoon and evening. The cold front is expected to begin pushing into Carbon county around noon, leading to the development of an impressive pressure boundary. A meso-high behind this will run into lee- troughing ahead, likely supporting some strong surface winds along the front. In addition, extremely strong frontogenesis and modest positive theta-e advection will support widespread showers along the frontal boundary. Instability near the frontal boundary will also support thunderstorms embedded along. Current best estimates on timing are for this to move through Rawlins around noon, Laramie around 2-3PM, Cheyenne, Wheatland, and Douglas around 3-4PM, Scottsbluff, Torrington, and Lusk around 4-5PM, and Chadron, Alliance, and Sidney around 5-6PM, but this is certainly subject to change. Due to the impressive dynamics associated with this system, it is possible we could see strong to severe winds along nearly the entirety of a line of thunderstorms that stretches across the entire forecast area. Scattered additional thunderstorms may also linger behind the frontal boundary. Models are currently showing the mid- level frontal boundary (i.e. the end of the moisture) running about 3 hours behind the surface pressure trough that is likely to be roughly the front edge of thunderstorm activity. In addition to the rainfall and wind, expect a very rapid temperature drop of 20-30F within perhaps just a few minutes. While for most areas this will mean a drop to the upper 40s to low 60s behind the front, those in the high mountains should be prepared for a rapid drop into the 30s along with rain quickly changing to snow. While snow accumulation will will not be significant, those expecting a summer day in the mountains may be taken off guard by the front in the afternoon hours. Dry air advection will occur very quickly behind the secondary front, clearing out skies quickly late Tuesday evening. However, the upper level low will then eject out across northeast Wyoming, bringing strong westerly flow overhead as the main trough axis clears through the area. Strong surface pressure and low level height gradients along with WSW winds at 700-mb of 50 to 60 knots will support a second round of high winds in the typical gap wind areas. With the slightly southerly component still remaining to the flow, the areas most likely in play will be Arlington/Elk Mountain and the I-25 corridor between Bordeaux and Casper. The period of strongest winds look to between midnight and 9AM Wednesday. For now, decided to issue a High Wind Watch just for the Arlington/Elk Mountain area starting Tuesday at noon to capture the initial frontal boundary and the strong post-frontal winds. This area is the least likely to see a break, which motivated the single headline. Elsewhere, confidence is lower in winds remaining strong between the initial thunderstorm-associated strong winds and the second overnight round. The first round could be handled with short-fused Severe Thunderstorm Warnings, with a High Wind Watch starting much later in the evening to handle the second round overnight. Will let a few more model cycles come in before making that decision though. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Issued at 420 AM MDT Mon Sep 16 2024 Long range models are in reasonable agreement through Friday night, but are now showing significant discrepancies for the weekend regarding the potential for prolonged rainy and cool weather for the weekend. For the middle of this week, models in good agreement with the timing of a pretty strong cold front pushing across the area by early Wednesday morning. High temperatures on Wednesday will likely be 10 to 15 degrees cooler compared to Tuesday, but still should be around average for this time of the year. It will be pretty windy behind the cold front with 700mb winds over 55 knots for central and east central Wyoming, with some evidence of mountain wave activity and stronger winds on the lee-side of the Snowy Range and Laramie Range. Increased winds a bit more with in- house wind models showing the wind prone areas flirting with high wind criteria, including areas near the I-25 corridor. Temperatures will rebound slightly on Thursday as the next Pacific trough moves across the southern Great Basin region and moderate to strong southwesterly flow aloft develops along the Front Range. Models are in better agreement for Thursday and Thursday night as the GFS has backed off on its more progressive solution...and now is more in line with the Canadian and ECMWF solutions with the Pacific upper level trough slower and much further west compared to yesterday`s 00z run. Most of Friday is looking drier and warmer in comparison to what some models were showing a few days ago, mainly due to the Pacific trough taking its time moving eastward and still over the far eastern Great Basin region, so increased high temperatures into the 70s for most locations with POP generally below 30 percent through the day. For Friday night and the weekend, this is when models start to diverge considerably with the 00z ECMWF and 00z GFS showing completely different solutions compared to yesterday`s 00z cycle. This is to be expected with blocking patterns as models struggle to resolve them in the medium to extended forecast range. Regardless, it looks like unsettled and cool weather for next weekend. The potent upper level trough is forecast to cross into the central Front Range and slow down...potentially forming a closed low around the Colorado/Wyoming/Nebraska border on Saturday. The ECMWF is now showing a prolonged moderate to heavy rain event with embedded thunderstorms Saturday afternoon through Sunday evening...mainly because of low to midlevel convergence associated with strong frontogenesis and cyclogenesis across the region. The GFS on the other hand is now more progressive at the onset of the event, mainly because it delays the Rex Block formation across the eastern U.S. until later in the weekend. Therefore, not as cool and somewhat drier weather for southeast Wyoming. The Canadian maintains its aggressive solution, similar to the ECMWF, but is 18 to 24 hours earlier...showing moderate to heavy rainfall developing as early as Friday evening. Still expect a gradual cool-down this weekend with highs in the 60s and nighttime lows in the 30s and 40s. There is still lower than average confidence in POP and timing of rainfall and mountain snowfall. All models have trended warmer in the 700mb level, so heavy mountain snow accumulations look less likely at this time. Kept POP between 30 to 50 percent, but increased the length of time through Saturday night. Unsettled weather could linger well into Sunday, as the 00z ECMWF is suggesting. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 550 AM MDT Mon Sep 16 2024 Southwest flow aloft will continue today ahead of the next Pacific low pressure system pushing across California. Relatively quiet weather expected with low impacts to Aviation. HAZARDS/WEATHER TRENDS: VFR conditions will prevail with occasional south to southwesterly wind gusts between 20 to 25 knots. Widely scattered rain showers and some embedded thunder possible near KLAR and KRWL this afternoon. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...High Wind Watch from Tuesday afternoon through Wednesday morning for WYZ110. NE...None. && $$ SHORT TERM...MN LONG TERM...TJT AVIATION...TJT