Area Forecast Discussion
Issued by NWS Grand Junction, CO
Issued by NWS Grand Junction, CO
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542 FXUS65 KGJT 211820 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 1220 PM MDT Fri Jun 21 2024 .KEY MESSAGES... - SPC has issued Severe Thunderstorm Watch #441 for hail possibly reaching 2 inches and wind gusts up to 70 mph through 7PM this evening. - Significant moisture supports the potential for heavy rain and flash flooding favoring west-central and southwest Colorado and southeast Utah. - Drier conditions with more general afternoon thunderstorms are expected daily beginning Saturday. && .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/... Issued at 435 AM MDT Fri Jun 21 2024 Showers and thunderstorms continued to sweep northeastward across the forecast area during the overnight hours along with with a few stronger storms. Cells were clipping right along at around 30 miles per hour moving to the northeast, so little threat of flooding at this point. Precipitable water levels are expected to remain high today, with HREF guidance indicating levels ranging from 1 to 1.2" inches with locally higher amounts in the Four Corners region. Meanwhile, a short wave trough ejected from the main trough over the West is expected to result in increased storms this morning with daytime heating bringing more widespread tstorm activity this afternoon. Yesterday`s activity raised the subcloud moisture level as evidenced by dew point readings in the upper 40s and lower 50s across the region early this morning. Consequently, developing storms will be efficient rain makers. Cape values between 1000 to 1500 J/Kg combined with jet divergence and moderate bulk shear of 40 to 45 knots raises the potential for some severe thunderstorms, especially over the Four Corners region. The main threat will be large hail with the potential for supercells along with a lesser chance for localized tornadic activity. Meanwhile, though average storm motion is projected to be out of the southwest at 20 to 25 mph, stronger cells will be capable of generating excessive rainfall bringing the potential for localized flash floods. Given forcing aloft, training storms are possible which only exacerbates flash flood potential. Given the previous, felt the current Flood Watch needed to be expanded, so in coordination with the ABQ and PUB forecast offices, added the remainder of southwest Colorado including the San Juan Mountains and the Uncompahgre River Basin to the watch. Upper level forcing wanes this evening while diurnal cooling helps stabilize the atmosphere. As a result, showers and thunderstorms will decrease during the latter part of the evening, largely dissipating by sunrise Saturday. Drier air begins to move into the area on Saturday in zonal flow beneath a shallow ridge of high pressure over the Great Basin. This will begin to erode the moisture over the area, but enough will remain for scattered activity, mainly over the higher terrain Saturday afternoon/evening, though the threat of flooding and severe weather becomes minimal. Increased clouds and showers today will cause afternoon highs to fall below normal today while overnight lows tonight should remain mild. Drying on Saturday will allow for more solar insolation during the morning and as a result, forecast highs are expected to be near normal. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Issued at 435 AM MDT Fri Jun 21 2024 High pressure overhead will generally lead to warmer and drier conditions through much of the extended forecast. However, the lack of strong forcing to fully evacuate this anomalously moist air mass allows residual moisture to hang around into the new week. Precipitable water values remain at least one standard deviation above normal for this time of year through the forecast period. This will likely result in diurnal showers and storms every afternoon in the high terrain. Lightning is the primary threat of any storms that form. Early guidance also points to the potential for higher coverage precipitation on Thursday afternoon as the crest of the ridge begins to move off to the east and ripples of shortwave energy move in. Temperatures warm to 5-10 degrees above normal by midweek. Valley locations look to flirt with 100+ degree temps, especially on Wednesday. && .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...NL LONG TERM...KAA AVIATION...TGJT HYDROLOGY...DB