Area Forecast Discussion
Issued by NWS Grand Junction, CO

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458 FXUS65 KGJT 211921 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 121 PM MDT Fri Jun 21 2024 .KEY MESSAGES...
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- SPC has issued Severe Thunderstorm Watch #441 for hail possibly reaching 2 inches and wind gusts up to 70 mph through 7PM this evening. - A Flash Flood Watch remains in effect for much of eastern Utah and southwest Colorado for the potential of very heavy rain and flash flooding. - Drier conditions return for Saturday onward, with a slow warming trend into next week. Chances for daily afternoon showers and thunderstorms remain over the higher terrain.
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&& .UPDATE... Issued at 1211 PM MDT Fri Jun 21 2024 The Storm Prediction Center has issued Severe Thunderstorm Watch #441 that encompasses all of eastern Utah and much of western Colorado. Anomalous moisture remains across the forecast area this afternoon with forecast CAPE values reaching between 2500 to 3000 J/kg. Along with this exceptionally high CAPE, substantial speed shear is also present which may allow some supercells to form this afternoon. There is some concern that cloud cover from this morning`s showers and storms will limit destabilization this afternoon. Having said that, models have remained consistent with the severe weather threat. The Severe Thunderstorm Watch will remain in place until 7PM this evening. Main threats will be scattered large hail and isolated very large hail up to 2 inches in diameter. Wind gusts up to 70 mph will also be possible. && .SHORT TERM /THROUGH SATURDAY/...
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Issued at 120 PM MDT Fri Jun 21 2024 With deep tropical moisture in place today, the threat for heavy rain and flash flooding is maximized. The PWAT on this morning`s 12z GJT sounding was 0.96 inches, which is well above normal and is approaching the record value for the time and date, 0.98 inches. Projected CAPE values across the region today are in the 1000-1500 J/kg range, with some models even suggesting 2000 J/kg across SE Utah, which is unusually juicy for the Western Slope. The abundant moisture and high forecast instability will combine with jet divergence aloft and dynamic lift provided by a passing shortwave to keep widespread strong thunderstorms going through the afternoon and evening. In addition, there is substantial wind shear in place across much of eastern Utah and western Utah, again maximized near the Four Corners. A widespread area of 40 knots of effective bulk shear, with an embedded area of 50 knots of shear over southern San Juan County in southeast Utah. In addition to the deep layer shear, helicity levels are maximized in this same region, with nearly 300 m2/s2 of storm relative helicity. All that is to say that not only is this atmosphere unusually juicy, but it`s capable of producing tilted, rotating updrafts. This setup is extremely conducive to producing large hail, damaging winds, and even tornadoes, although the chance of that is very low. The Storm Prediction Center has taken a look at this highly potent and highly unusual environment and decided to issue a Severe Thunderstorm Watch for all of eastern Utah and almost all of Western Colorado, bar the farthest eastern counties along the Continental Divide. Per the wording of this watch, and what environmental parameters suggest, threats with these storms include large hail up to 2 inches, gusty winds of 55-70 mph, and an extremely low but present chance of tornadoes (2%). This environment will persist into the evening, with discrete supercells eventually merging into more linear convective systems. As this change in storm type occurs, look for a shift away from the larger hail toward stronger winds. One final shortwave will track through late this evening, dragging one final line of thunderstorms before things begin to settle down. Behind this wave, drier air and subsidence will build in, especially across northeast Utah and northwest Colorado, leading to clearing skies. Flow aloft will weaken through the day tomorrow as high pressure begins to build in, although moisture trapped at the surface will continue to feed chances for showers and thunderstorms over the higher terrain tomorrow afternoon. This will be a much more normal afternoon thunderstorm setup, with convection driven first by differential heating over the terrain, and then by propagating outflows through the late afternoon and evening. With lower levels of the atmosphere still quite moist, heavy rain remains a possibility, but the potential for flash flooding will be low tomorrow. Lightning, gusty outflow winds, and small hail will also be threats. The usual summer afternoon mix. As skies will be clearer and moisture will be on the decrease, temperatures will be able to climb up to near normal values. Quiet weather will be in control for tomorrow night, with mild lows near to a few degrees above normal.
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&& .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
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Issued at 120 PM MDT Fri Jun 21 2024 With the subtropical high to our south and a ridge of high pressure overhead, drying and warming are expected through the long term period. Rich tropical moisture trapped under the high will fuel afternoon showers and thunderstorms forming daily off the terrain, with threats slowly transitioning away from heavy rain and toward gusty outflow winds. Frequent lightning and small hail will also be possible. Passing waves rounding the high will lead to expansion in storm coverage for Wednesday and Thursday. A draw of subtropical moisture also looks to set up during the late week period, which may see a return of the heavy rain threat to the Western Slope. Temperatures will run around 5 degrees above normal to begin the long term period, and climb to around 10 degrees above normal by mid- next week. This means the potential return of triple digit heat to the lower desert valleys.
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&& .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1128 AM MDT Fri Jun 21 2024 Widespread showers and thunderstorms are once again expected this afternoon and evening. Large hail in excess of 2 inches is possible in the Four Corners Region, along with gusty winds of 40-50 knots, and torrential rain. There is a small (2%) chance of tornadoes in the Four Corners Region as well. Frequent lightning will also be likely. Low ceilings are likely through the day, and periodic reductions in visibility with passing storms. With all the passing convection, winds will be erratic until late this evening, when typical terrain patterns kick back in. VFR conditions will prevail outside of shower activity, within showers and storms, look for reductions below ILS breakpoints and MVFR to IFR conditions with heavy rain. && .HYDROLOGY... Issued at 435 AM MDT Fri Jun 21 2024 Deep subtropical moisture remains over eastern Utah and Western Colorado. This combined with an upper-level disturbance moving up from the southwest this morning brings the threat of training thunderstorms and/or thunderstorms with heavy rain producing possible flash flooding across the region throughout the day and into the evening. Therefore, a Flood Watch for flash flooding remains in effect from 8 AM this morning to midnight tonight across far western Colorado and southeast Utah, generally along and south of the I-70 corridor. Continue to monitor for forecast updates and be prepared to take action should Flash Flood Warnings be issued. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...Flood Watch until midnight MDT tonight for COZ003-006-007-011- 017>023. UT...Flood Watch until midnight MDT tonight for UTZ022-025-027>029. && $$ UPDATE...TGR SHORT TERM...BGB LONG TERM...BGB/KAA AVIATION...TGJT HYDROLOGY...DB