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Area Forecast Discussion
Issued by NWS Grand Junction, CO
Issued by NWS Grand Junction, CO
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100 FXUS65 KGJT 190000 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 600 PM MDT Sun Aug 18 2024 .KEY MESSAGES... - Scattered to numerous showers and storms will continue today and tomorrow. Storms will develop each afternoon with showers likely to persist into the overnight hours. - Storms will be capable of producing gusty winds, frequent lightning, small hail, and periods of moderate to heavy rain. Localized flash flooding remains a concern. - Brief decrease in storm coverage on Tuesday before better monsoon moisture arrives from Wednesday onwards, resulting in widespread showers and storms through at least Friday. && .SHORT TERM /THROUGH MONDAY/... Issued at 118 PM MDT Sun Aug 18 2024 Moisture advection was in full force yesterday under southerly flow aloft. PWAT values this morning are 140-180 percent of normal with dew points in the 40s and 50s for most locations. Currently there is an MCV left over from convection in AZ last night. This MVC is near the UT/AZ state line now with clouds spreading over the Four Corners region. This cloud cover will hinder surface heating and therefore instability in that region. Not sure it will be able to recover. The northern two thirds of the forecast area has been clear all morning so instability has been allowed to build. Convection has already taken place over the mountains and that activity will track northeastward causing additional convection downstream. Storms are expected to form over the higher terrain through the afternoon. Given the abundant moisture and CAPE on the order of 500 J/kg heavy rainfall is possible and storm training could lead to flash flooding. This flooding potential is highest in the southern mountains based on precip amounts from the latest high-res, but a nonzero chance just about everywhere. The strongest cells may produce small hail, and wind gusts upwards of 45 mph are possible especially around storm clusters. The CAPE is expected to decrease overnight leaving mostly stratiform, but perhaps there is enough CAPE to support a few pockets of lighting. The left over MCV could track over portions of northeast Utah and northwest Colorado tonight and support persistent showers there. The MCV looks to strengthen into a weak shortwave tonight and lift out over the Northern Plains tomorrow. This will cause the flow to become more southwesterly tomorrow. Moisture does not appear to change much tomorrow with this flow shift. Convection will develop over the higher terrain around mid day before drifting over the valleys in the afternoon. Heavy rain and some flash flooding remains a concern once again. By the evening the storms will weaken and coverage becomes more isolated. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Issued at 118 PM MDT Sun Aug 18 2024 An incoming wave in the PACNW will push the blocking high around on Tuesday. This will shunt the monsoonal tap back southward, thereby quieting the afternoon shower/thunderstorm production of recent days. Ensemble forecasts have trended PWAT`s downward on recent runs, with moisture values 120 percent of normal across western Colorado and slightly higher in eastern Utah. Southwesterly flow ahead of the incoming PACNW wave will likely spark a few showers on the favored aspects of the terrain Tuesday afternoon. Drier surface conditions will likely spell out some gusty outflows for these storms. The blocking high never really gives up and a series of waves drop into the PACNW against it from Wednesday onward. The monsoonal plume fills back in Wednesday and storm coverage picks back up. Some of the richest moisture feeds in north of the Colorado River Wednesday. By Thursday, much of the CWA will see PWAT`s in excess of 170 percent of normal. This will arrive along a region of forcing near the jet streak stretching from the Great Basin into Wyoming. Thursday looks like a good bet for one of the wettest days in this extended period. Storms will be widespread under this regime, should all hold on as expected. Things get interesting Friday as models bring a closed low ashore in NORCAL and punch it into the Great Basin by Saturday. This will keep an unsettled pattern in place into next weekend. With the blocking high out of the way, the monsoonal tap effectively closes. Recycled moisture, along with anything entrained in the passing low will keep some showers and storms in the forecast Saturday and, perhaps, Sunday. With such a drastic pattern shift, models are displaying some inconsistencies. Temperatures don`t ever get a chance to surge upward much this week. Tuesday will likely turn out to be the warmest day of the week. Cloud cover and showers will take the bite out of highs the remainder of the week. Thursday looks like another day, where area highs struggle to reach mid 80`s in our warmest valleys. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 537 PM MDT Sun Aug 18 2024 Isolated showers and thunderstorms moving to the northeast generally northeast of the line KGUC-KVEL will diminish or move out of the region over the next few hours, but a few showers may linger into the overnight. Except for brief periods of MVFR conditions associated with the showers and thunderstorms, expect VFR conditions through the TAF period. Look for widely scattered showers and thunderstorms to fire up over the higher terrain again after 18Z. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...KJS LONG TERM...LTB AVIATION...DB