The first few days of July have started out much the same way June ended – with hot and dry conditions, and now smoky skies across much of the state due to numerous wildfires. Heading into the Fourth of July holiday, fire danger is very high state-wide, so it is prudent to be cautious with fireworks and campfires and to obey fire restrictions in place. Despite the dry start to the week, changes are on the way for the Fourth, with potentially active weather developing across the Front Range and Eastern Colorado.
A trough of low pressure swinging across the Northern Plains will send a backdoor cold front into Northeast Colorado during the day Wednesday. At the same time, a subtropical dome of high pressure over the Southeastern U.S. will be retreating westward, allowing for a deeper easterly flow to advect low level moisture into Eastern Colorado behind this cold front. Dewpoints in the 30s on Tuesday will rise into the mid 50s behind the front on Wednesday – plenty high enough to result in heavy rainfall potential with thunderstorms. The image below (courtesy of the College of DuPage) shows the projected moisture increase behind the cold front on Wednesday afternoon.
The increase in low level moisture along with cooler air aloft will also result in a significant increase in atmospheric instability, setting the stage for greater thunderstorm potential across Northeast Colorado. The biggest question marks will have to with how early in the afternoon the front arrives, and also the potential for a stable layer (known as a “cap”) to develop behind the front. These stable layers can often inhibit or at least delay thunderstorm development until the cap weakens to the point at which storms can quickly develop, or another outside trigger (such as outflow from distant thunderstorms or terrain forcing) allows the cap to break.
Areas of elevated terrain will have the best chances of seeing thunderstorms on Wednesday afternoon, such as along/near the Front Range foothills up to the Continental Divide, as well as along the Palmer Divide and around the Pikes Peak region. However, the Denver, Boulder, Colorado Springs, and Ft. Collins areas will all stand a good chance of seeing a round of thunderstorms Wednesday afternoon or evening, with the potential for a strong thunderstorm or two as well. Any rainfall will certainly be welcome, but you’ll want to keep your guard up if you have outdoor plans for the Fourth of July, especially with the “prime time” for thunderstorms likely to occur between about 4pm-sunset when most of the barbeques and festivites are happening, and there will be some potential for later evening storms as well.
Fourth of July Outlook for Western Colorado and Surrounding States
Across the Southern and Central Rocky Mountain Region, the Front Range really will be the prime spot for moisture and thunderstorms. Along the Continental Divide, there will be a good chance of thunderstorms as well, so keep that in mind if you plan to go hiking or summit a 14er on Independence Day, and get below treeline before the afternoon.
Farther west, the western slope of Colorado including some of the western mountain ranges (such as the San Juans), thunderstorm chances will be pretty minimal. However, wildfires and smoke will continue to be an issue with very dry conditions in place. Beyond Colorado, most of Utah and Arizona are likely to stay dry as well with similarly high fire danger. Up north in Wyoming and Montana, thunderstorm risk is looking minimal as well, and fortunately these areas are in good shape in terms of moisture with limited fire danger to worry about.