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Area Forecast Discussion
Issued by NWS Grand Junction, CO
Issued by NWS Grand Junction, CO
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880 FXUS65 KGJT 121153 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 553 AM MDT Mon Aug 12 2024 .KEY MESSAGES... - Showers and thunderstorms continue each afternoon and evening until further notice with gusty outflow winds, small hail, and frequent lightning possible. - Heavy rain rates with storms will bring an enhanced threat of flash flooding and debris flows through Tuesday. Saturated soils and steep terrain will be most susceptible to flooding. - Temperatures will remain near to several degrees below normal thanks to the abundance of clouds and showers. - Drier air will result in a downturn in storm coverage mid to late week with a potential return of moisture by this weekend. && .SHORT TERM /THROUGH TUESDAY/... Issued at 359 AM MDT Mon Aug 12 2024 Storms are still firing on the southern edge of the cloud shield across southwest Colorado where brief moderate to heavy rain and frequent lightning is still occurring. Most of the showers have stratiformed out down south or come to an end for all areas north of the San Juans and Abajo Mountains. These storms across the southern foothills are moving towards the east so anticipate the duration to be short-lived with MRMS currently indicating 0.5 to 1.0 inch an hour rates. Hi-res CAMs that are picking up on this activity indicate a downward trend over the next couple hours with most activity coming to an end towards sunrise. We are still left with plenty of cloud cover across the area, much like early yesterday, which could result in a bit of a delay in convective initiation and later start. For today, we will kind of be in a transition phase between the westerly flow and switch to southerly flow for Tuesday. Hi-res CAMs and even deterministic models indicate a bit of a downturn in convection across western Colorado. Even though deep monsoonal moisture remains in place, what we are lacking is sufficient instability (probably due to more CIN and more cloud cover this morning) where CAPE values are maybe up to 200 J/kg. Also, we lose the jet support today as the jet lifts north of the region. However, better jet support due to the incoming PacNW trough is seen across Utah today ahead of a rather robust shortwave trough that is projected to move through on Tuesday. Higher CAPE values up to 1000 J/kg are also seen across northwest Utah today. So, we will still see some scattered storms today but mainly forming over the high terrain late this afternoon with better chances across eastern Utah, and especially northeast Utah due to better instability and forcing. Any storms will still be capable of moderate to heavy rain as well as gusty winds, small hail and frequent lightning...but the flooding threat appears more localized and isolated today due to less storm coverage, instability and forcing than seen in previous days. Therefore, opted not to issue a Flood Watch at this time. Storms actually look to increase this evening across eastern Utah and spread into western Colorado overnight due to the approaching shortwave trough and increasing moisture in the southerly flow. On Tuesday, a rather robust trough moves through, so we gain back the jet support as well as the instability as CAPE values climb higher up towards 500 to 1000 J/kg for Tuesday afternoon. Tuesday looks more widespread with storms and a better potential for heavy rain and flash flooding as training storms look possible, so stay tuned for updates on the latest forecast as Tuesday appears more active. As far as temperatures are concerned, we continue to remain a few degrees below normal today with the coolest day occurring Tuesday due to increased cloud cover and better chances and coverage of showers and storms. Highs on Tuesday look to drop to 6 to 12 degrees below normal as a result. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Issued at 359 AM MDT Mon Aug 12 2024 The trough passage on Tuesday will nudge high pressure into eastern Texas/Louisiana by Wednesday, which, in turn, shifts both sources of moisture east of the Divide. A new area of high pressure builds back in over the Desert Southwest in the wake of Tuesday`s wave. This hot and dry air mass will work its way into the CWA from the southwest, reducing storm coverage across the Four Corners and lower valleys on Wednesday. However, lingering moisture will still be sufficient for afternoon convection to flourish atop the high terrain. Ridge of high pressure shifting overhead on Thursday and Friday will continue to support dry air advection across the Western Slope, as such, we`ll see a downturn in terms of storm coverage and likely rainfall intensity for the end of the work week. Confidence in forecasting the evolution of the high decreases leading into the coming weekend. Half of the ensemble cluster analyses show above average moisture across the Western Slope by Days 6 and 7, which would allude to the monsoon redeveloping. However, subtle shifts in the high`s center may keep the monsoon plume to our west, and a dry air mass overhead. Temperatures during the long term start off approximately 3 to 8 degrees below normal, thanks to cloud cover and the abundance of moisture. We`ll see a rise mid week through Friday as the new area of high pressure builds overhead, with highs hovering right around normal as we end the work week. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 553 AM MDT Mon Aug 12 2024 Some showers will linger this morning across the southern valleys but should dissipate shortly after sunrise. Plenty of cloud cover this morning should result in a later start to convective initiation this afternoon. Expect scattered storms developing over the high terrain around 20Z with better potential across eastern Utah. Storms increase this evening after 00Z across lower elevations of eastern Utah and northwest Colorado. Storms may continue to spread into western Colorado after 06Z with nocturnal activity possible due to a disturbance. Kept VCTS at higher elevation TAF sites of KASE, KTEX and KGUC as well as eastern Utah sites of KVEL and KCNY where confidence was higher. Confidence in lower elevations of western Colorado was less certain so left VCTS out of the TAF for now. Expect primarily VFR conditions with MVFR and ILS conditions where storms occur due to moderate to heavy rain. Gusty winds are also possible with storms but overall winds should remain light and terrain driven outside of storm activity. && .HYDROLOGY... Issued at 359 AM MDT Mon Aug 12 2024 Monsoonal moisture will keep showers and thunderstorms over the region through at least Tuesday with rainfall rates of 1 to 2 inches per hour possible with storms. The flooding threat appears more localized today given less storm coverage expected but storm coverage and flooding potential increase Tuesday as a low pressure trough moves through the area. Given how saturated soils have become over the recent days, there is increased potential for flash flooding and debris flows across much of the area. Stay weather aware over the coming days, especially if you live in terrain vulnerable to flooding, or are planning to recreate outdoors. Stay tuned to the latest forecast updates for any additional flood watches, warnings, and / or advisories over the coming days. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...MDA LONG TERM...MDA/ERW AVIATION...MDA HYDROLOGY...TGJT