Area Forecast Discussion
Issued by NWS Grand Junction, CO
Issued by NWS Grand Junction, CO
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434 FXUS65 KGJT 250930 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 330 AM MDT Tue Jun 25 2024 .KEY MESSAGES... - Residual moisture will continue to fire isolated afternoon showers and thunderstorms this afternoon. - A push of deep subtropical moisture moves in by Wednesday, returning widespread showers and thunderstorms through Thursday. - Mid-week moisture content, exceeding 200% of normal returns the threat of flash flooding due to heavy rain showers and/or training storms. && .SHORT TERM /THROUGH WEDNESDAY/... Issued at 324 AM MDT Tue Jun 25 2024 Satellite imagery shows the models have a grasp on the general weather pattern with the polar jet along the Canadian Border, an upper level high extending from Texas west along the Mexican Border with a low off Baja, and these features extending down into the mid- levels with the high from Louisiana west to the Desert Southwest and a weak low off Baja with these features slightly north of the upper- level features. This is pretty much the limit of the models as going into the lower levels, there are many meso-scale features in the satellite imagery that the models can`t resolve. even the high resolution models miss these features likely due to the initial conditions and to a lesser extent, smoothing in the ensembles. imagery shows a low-level high over the southwestern Utah-Arizona Border with a disturbance rotating around the northwestern corner supporting a few showers in the Four Corners area, a second high circulation over the White Mountains of Arizona and a trough over central New Mexico. Though these low-level features seem minor, they are driving the current weather across much of the Southwest allowing tropical moisture to sneak north under the high pressure circulation aloft. The models want to dry out the region, but dewpoints continue to run in the 40`s to low 50`s across eastern Utah and Western Colorado which combined with recycled moisture is plenty to support isolated to scattered orographic showers and thunderstorms through the afternoon across the higher terrain. As the high pressure builds north over the region, subsidence will try to cap the convection, but diurnal heating will likely break the inversions. Temperatures today will run five to ten degrees above normal for late June under the high pressure with isolated to scattered showers and thunderstorms across the higher terrain much like yesterday, but there may be a slight down tick with the increased subsidence under the high. The primary threats from these storms will be gusty outflow winds to 50 mph, small hail and lightning. Deep moisture from the Desert Southwest starts to pour back into the region Wednesday with clouds increasing from the southwest. Temperatures will be a little cooler Wednesday due to the increased cloud cover and with the increased shower and thunderstorm coverage. With pwats topping one inch again through the afternoon, concerns for heavy rain from the thunderstorms leading to localized flash flooding return, especially in eastern Utah and the far western Colorado along the Utah Border. Confidence on the timing of the moist surge is too low to issue a flood watch at this time, so stay tuned for forecast updates over the next 24 hours. This threat increases Wednesday night into Thursday, but more on that below in the long term discussion. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Issued at 324 AM MDT Tue Jun 25 2024 Nocturnal showers and embedded thunderstorms can be expected on Wednesday night as a trough of low pressure begins to dig into the Pacific Northwest. This will see the ridge flatten with flow aloft shifting to the southwest into Thursday. Impressive moisture advection will bring precipitable water values up to at least 200 to 220 percent of normal across the Western Slope for late June. As a result of this juicy atmosphere (with surface dewpoints staying in the mid 50s), look for widespread shower and thunderstorm activity on Thursday. The Pacific trough will quickly push east into Idaho / Montana by Thursday afternoon, tightening the gradient overhead and bringing embedded shortwaves across the forecast area. Modest lift paired with the abundance of moisture will lead to some stronger thunderstorms throughout the day, those capable of producing hail and gusty winds in addition to periods of heavy rain. In fact, the Weather Prediction Center already has a widespread marginal risk for excessive rainfall on Thursday with almost the entirety of western Colorado under a slight risk. Therefore, flash flooding will be a concern with storms, especially depending on how much moisture falls with activity on Wednesday. Stay tuned to any potential flood highlights in the coming shifts. The trough takes on a positive tilt Thursday night before lifting overhead on Friday. Showers will gradually taper off once the trough passes through but it certainly won`t be a clean transition. PWATs will stay elevated and daytime heating and orographics will keep showers and thunderstorms in the forecast through the end of the week. Areas along the Divide in particular will see the best coverage of convection as low level moisture persists. On Saturday a transitory ridge of high pressure builds overhead, though as usual diurnally-driven showers and storms will develop over the higher terrain, favoring the Divide. Sunday will see another, broader trough of low pressure push onshore across the western CONUS. Flow aloft will shift to the southwest which will re-introduce subtropical moisture into the region as an elongated ridge extends across the southeast states. The trough will push farther inland on Monday which will further enhance the clockwise flow around the high. So, after a brief downturn in convection on Friday but more-so on Saturday, unsettled weather looks to return from Sunday onwards. Temperatures will be a bit of a grab-bag through the long term period given the fluctuating shower and thunderstorm coverage. Overall, Thursday will be the coolest day of the period due to the uptick in clouds and showers with Friday also being below normal for most areas behind the trough. Temperatures will rebound slightly on Saturday under the weak ridge before values likely trend back down with the next round of moisture. Overnight lows will remain on the mild side through the period. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1030 PM MDT Mon Jun 24 2024 VFR conditions will develop overnight and continue for most terminals through the period. Isolated showers and thunderstorms will remain a threat on the high terrain tomorrow afternoon, with less coverage than today. A few showers will drift off their terrain and decay over valleys, leading to gusty outflow winds and the potential for light hail. Winds will hold westerly, with a few terrain influenced gusts tomorrow afternoon. Cloud cover will remain scattered at the mid levels, with a few TEMPO ceilings in the vicinity of showers and storms. Shower activity will quickly diminish towards sunset. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...DB LONG TERM...MMS AVIATION...TGJT