Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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550 FXUS65 KCYS 161140 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 540 AM MDT Sun Jun 16 2024 .KEY MESSAGES... - Scattered thunderstorms possible this evening, mainly east of the Interstate-25 corridor. Storms could contain hail and strong winds. - A cold front will bring cooler temperatures for Tuesday and Wednesday, along with a chance for late day showers and thunderstorms. A warming trend will develop for Thursday through Saturday, along with daily chances for afternoon and evening showers and thunderstorms. && .SHORT TERM /THROUGH MONDAY/... Issued at 410 AM MDT Sun Jun 16 2024 Looking at a fairly quiet afternoon on this Father`s Day. A trough over the Pacific Northwest will gradually move southeastward over the next several days. This will push a cold front south into northern Colorado where it will likely stall out this afternoon, becoming more of a stationary front. With the passage of the cold front, high temperatures this afternoon will be noticeably cooler, especially east of the Laramie Range. Here, highs will be in the 70s and 80s, about 10 to 15 degrees cooler than Saturday. Because of the cooler temperatures, surface-based CAPE appears almost nonexistent, particularly across the far northern zones where highs in the 70s will lead to a more stable environment. There is a bit more surface instability further south, closer to the stationary boundary where temperatures will be warmer. As the front weakens, winds will turn more out of the southeast. This will lead to increasing dewpoints and increasing MUCAPE values late this afternoon and through the evening hours. All this to say that some scattered convection could be possible this evening. Hi-Res guidance continues to show limited convection across the CWA, keeping most of the storm activity in the northern-most zones where a weak convergence boundary is located. However, most models show a strongly sheared environment in this area with 0-6 km shear values around 60 kts. This strong of shear may work against the longevity of any storms that develop. Model sounding profiles actually look much more promising for convection further south in the CWA, and a bit later in the evening. MUCAPE values over 1500 J/kg, 0-6 km shear around 45 kts, and dewpoints in the 50s lead to a better environment for convection. However, there is a lack of forcing further south, especially with the decaying cold front so storms may not form at all, but they cannot be ruled out. Any storms that do develop during the evening, regardless of where they are, will have the potential to be severe. The primary concern with any elevated convection will be large hail with the higher MUCAPE values, but an isolated strong wind gust or two could also be possible. A slight warm-up is expected Monday as southwesterly flow advects warmer 700 mb temperatures into the CWA. Looking at mostly dry conditions throughout the day, but a few weak vorticity maxes ahead of the strong Pacific Northwest trough could spark an isolated storm or two across the area. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Issued at 410 AM MDT Sun Jun 16 2024 Pacific cold front is still on track to arrive late Monday night and early Tuesday morning, with the front pushing through the far southeast counties (Kimball, Cheyenne Co.) by mid morning Tuesday. Models continue to be in excellent agreement with this solution and have continued to trend slightly cooler with each run. 700mb temperatures are now down to -1c to -3c, mainly for Carbon, Converse, and Albany counties...which will likely translate to freezing temperatures over 9000 feet. The high valleys (including Rawlins, Laramie, Saratoga, Shirley Basin) have a chance to see some late-season near-freezing temperatures or some frost early Tuesday morning and early Wednesday morning once the colder air settled over the area. Models show little or no precipitation as this front moves through the area, so high elevation snow looks unlikely at this time. Tuesday will be the coolest day for areas west of the I-25 corridor and east central Wyoming as this is when the midlevel cold pool will lift northeast across the region. For the rest of the forecast area, Wednesday looks like the coolest day due to increasing cloud cover over the eastern plains and a better chance for scattered rain showers and isolated thunder. Highs on Tuesday will likely reach the low to mid 70s over the plains due to ample sunshine, especially earlier in the day, while areas west of I-25 and east central Wyoming struggle to reach the low to mid 60s. Wednesday will be similar, but most of the eastern high plains will likely be stuck in the mid to upper 60s or near 70 during the day while Rawlins, Laramie, and Douglas moderate into the low to mid 70s. Low temperatures are going to be tricky since deterministic models and ensemble forecasts indicate solid cloud cover, especially Tuesday night/Wednesday morning. Therefore, did not trend forecast towards the colder MOS guidance and kept lows roughly between the ensemble mean and the 25th percentile. Even the 10th percentile NBM and ENS/GEFS are still above freezing, but do show a low-end chance of frost for Rawlins, Laramie (and vicinity) and up towards Douglas and Lusk. Forecast temperatures are still 8 to 15 degrees below normal for this time of the year. Thankfully, probability for strong or severe thunderstorms appears very low both of these days due to a relatively stable boundary layer. For late this week, models show a slowly building upper level ridge across the central plains, which should lead to a general warming trend across southeast Wyoming and western Nebraska. Models show another weak Pacific trough moving onshore early Thursday, with weak disturbances ejecting northeast into Wyoming as the main trough weakens. The subtropical jet will become more active during this time. NAEFS mean PWAT are generally in the 90th to 98th percentile with values around 1.00 inch over the Wyoming eastern plains and around 1.25 to near 1.50 for western Nebraska. This will result in a greater chance for afternoon thunderstorms across most of the area, with a chance for some nocturnal convection across the eastern high plains depending on the timing of the shortwave activity/vort maxes. Increased POP towards 50 percent for high-end scattered coverage of showers and thunderstorms through Friday and into next weekend. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 540 AM MDT Sun Jun 16 2024 A surface cold front will push through the area early this morning and stall near the Colorado border and in the mountains. This front is forecast to slowly lift northeast this evening and tonight, with isolated showers and thunderstorms possible in addition to low stratus clouds and some fog by early Monday morning. HAZARDS/WEATHER TRENDS: Low confidence forecast for precipitation today as the cold front stall near the I-80 corridor. Limited coverage of precipitation, based on high resolution models, suggest mostly dry conditions for all terminals this afternoon and tonight with occasional breezy east to northeast winds. Kept the mention of precip out of the TAFs for now and only mentioned VCSH for Chadron tonight. Nocturnal thunderstorms are possible for KCDR, KAIA, and KBFF but coverage and confidence are too low to include in the TAF at this time. After 06z tonight, models in agreement with the onset of low CIGS and some patchy fog between 08z and sunrise. Added MVFR to near IFR CIGS for KCYS and all western Nebraska terminals. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...SF LONG TERM...TJT AVIATION...TJT