FTB 06-16-2021: An Uptick in Storm Coverage Expected Amidst The Smoky Heat Wave

Issue Date: Wednesday, June 16th, 2021
Issue Time: 10:15AM MDT

— A LOW flood threat has been posted for parts of the Northeast Plains, possibly persisting into the early overnight hours

As the summer solstice approaches, we are reminded that the devil is always in the details when it comes to summer forecasting. Today is a good example of that. As shown in the visible satellite image, below, this morning’s mostly cloud free conditions across Colorado are a testament to the strength of the strong upper-level ridge. The ridge is centered directly over the Four Corners this morning. Compared to yesterday, dewpoints have crept up slightly west of the Continental Divide, but even with that, the Grand Junction PW shows a meager 0.48 inches of water vapor with most of that held high above the boundary layer. In short, little if any rainfall is expected west of the Continental Divide today.

East of the Continental Divide, Denver’s sounding showed a PW of 0.49 inches, which is below normal for this time of year. However, higher PW, just under 1 inch, was observed at the North Platte (NE) and Dodge City (KS) soundings. Furthermore, model guidance appears to be biased low in all measures of moisture such as PW, which was underestimated by 10-20% along the CO/KS/NE borders. Even larger dry biases were noted in the high-resolution guidance today. As shown below, this morning’s 4AM (10Z) run of the HRRR underestimated the 8AM dewpoints across eastern Colorado by up to 6F. This is a very large bias for only a 4-hour forecast, which can easily make the difference between heavy rainfall and dry conditions. The bias is likely a testament to both how shallow the moisture is, along with the presence of isolated pockets of higher moisture due to wet soils remaining from the wet May.

Returning the focus back on today’s forecast, we see a few forcing features of note. First, a cool front (more appropriately called a wind shift boundary) was noted over central Wyoming, which will race south across eastern Colorado this afternoon. Second, another wind shift boundary was also noted in northeast Colorado, with higher dewpoints and more southerly winds to its south. We expect these features to help concentrate the stronger storm activity today along eastern Northeast Plains, near the Kansas border. With forecast soundings indicating the potential for training cells due to the combination of (i) deep southerly boundary winds, (ii) light northerly winds aloft but (iii) weak shear overall, a Low flood threat has been issued for a small part of the Northeast Plains for isolated flash flooding.

Elsewhere, isolated to scattered showers and thunderstorms are expected mainly over the higher terrain east of the Continental Divide. But, with only shallow boundary layer moisture, flooding is not expected for these areas although short-term moderate rainfall will be possible.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northeast Plains:

Continued very hot today with scattered showers and thunderstorms developing this afternoon and possibly lasting into the early overnight hours. Max 1-hour rainfall up to 2.3 inches will be possible with the strongest cells, leading to the risk of isolated flash flooding. A Low flood threat has been posted for parts of the region. Large hail will be possible with the strongest storms. Note that this will persist into the early overnight hours.

Primetime: 1PM through 2AM

Front Range, Urban Corridor, Palmer Ridge, Southeast Plains, Raton Ridge, Southeast Mountains:

Continued very hot with isolated to scattered showers and thunderstorms this afternoon and evening. Max 1-hour rainfall up to 0.6 inches above 6,000 feet and 0.8 inches for lower elevations. Flooding is NOT expected today.

Primetime: 12PM through 9PM

San Luis Valley, Grand Valley, Southwest Slope and Northwest Slope, Central Mountains, Northern Mountains and San Juan Mountains:

Mostly sunny and continued very hot today with a isolated to widely scattered showers and weak storms possible over the San Juan Mountains. Max 30-minute rainfall up to 0.4 inches. Flooding is NOT expected today.

FTB 06-15-2021: Intense June Heat Mixed with Smoke

Issue Date: Tuesday, June 15th, 2021
Issue Time: 8:55AM MDT

— Flooding is NOT expected today

It’s going to be a scorcher this afternoon under the anomalously strong June ridge. The High has moved into the four corners region and is helping to advect smoke into the state. The smoke this morning is pretty thick across the western border, which can be seen by the gray/brown hue below. It remains incredible dry over the majority of the state (clear skies below), and PW at Denver has dropped to 0.56 inches. The lack of moisture will help to keep precipitation chances and totals very low today. Water vapor imagery indicates some mild moisture trapped under the ridge across the southern border. So, as the diurnal flow develops, a couple high based storms are possible across the southern mountains. Perhaps even an isolated, high-based storm or two over the Palmer/Raton Ridges near the mountains. More wind than rainfall is forecast from storms that do develop today, so flooding is NOT expected.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Southeast Mountains, Front Range, Palmer Ridge & Raton Ridge:

Best chance for measurable rainfall this afternoon will be over the Southeast Mountains and Raton Ridge. A high-based storm or two may also pop over the southern Front Range and Palmer Ridge. Storms should track more south today with the location of the High. Brief, max 1-hour rain rates up to 0.40 inches will be possible across the ridges, while max rain rates will drop to 0.15 inches per hour over the mountains. Outflow winds will be the main threat from the storms that do develop, so flooding is NOT forecast.

Primetime: 2PM to 10PM

Grand Valley, Southwest Slope, Northwest Slope, San Juan Mountains, Central Mountains & Northern Mountains:

The excessive heat warning continues through Friday. Highs will reach 100F+ again for the Grand Valley and portions of the Southwest and Northwest Slope. Plenty of daily temperature records will be broken this afternoon, so be sure to take breaks from the heat and carry water with you. Afternoon cumulus will likely pop over the San Juan Mountains and may produce some very light rainfall/sprinkles (east). Smoke will continue to cause hazy conditions and poor air quality. This feels more like an August forecast than mid-June forecast. Flooding is NOT forecast.

Urban Corridor, San Luis Valley, Northeast Plains & Southeast Plains:

It’s going to be hot this afternoon, so be sure to take lots of breaks if you’re outside. Afternoon high temperatures will be in the upper 90Fs and likely reach 100F across a few areas. That means many daily temperature records have the potential to be broken. Rainfall is not forecast.

FTB 06-14-2021: The Anticipated Extreme Heat Wave Begins

Issue Date: Monday, June 14th, 2021
Issue Time: 10:00AM MDT

— Flooding is NOT expected today

Today marks the first day of a very hot temperature streak. While temperatures already reached 100F across the lower elevations of western Colorado yesterday, the building 594mb ridge today will raise afternoon high temperatures statewide. The wake of last night storms over the Nebraska panhandle are currently located eastern Nebraska (orange “X”). There’s some residual low level moisture from the feature near Colorado’s eastern border. Elsewhere, the dry air mass continues to dominate as shown by Grand Junctions morning sounding where PW was measured at 0.26 inches. That will translate to another dry day for the majority of Colorado (areas of orange below).

Looking back to the eastern adjacent plains where there is some residual moisture under the ridge, PW at Denver remains about the same as yesterday (0.70 inches). Similar to yesterday, this moisture gradient increases quite a bit as you move towards the eastern border. However, it looks like the dry westerly flow aloft will have a better chance of mixing out the surface moisture throughout the day when compared to yesterday, especially along the I-25 corridor. Outside of a couple weak showers over the elevated ridges, rainfall is not forecast for the immediate adjacent plains. There’s no identifiable shortwave moving through the flow, so that should help to keep the eastern plains capped as the storms and outflow boundaries move into the area late this afternoon and evening. There’s also a chance for a couple late night storms to cross over the northern border from Nebraska, but with lower instability, flooding is not expected.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northeast Plains & Southeast Plains:

Temperatures will reach into the upper 90Fs and may reach 100F over portions of these regions this afternoon. A cap will likely inhibit any convection over the area as storms and outflow boundaries move into the area this afternoon and evening. Low confidence storms will break the cap, but if they do, max 1-hour rain rates up to 1.75 inches will be possible along with severe thunderstorms (hail and damaging outflow winds). Again, chances of this occurring are low enough that no flood threat will be issued. Late night storms may cross the northern border, and if they can survive, rain rates up to 1.5 inches per hour will be possible along with damaging outflow winds and some hail. Flooding is NOT forecast.

Primetime (if storms break the cap): 5:30PM to 2AM

Urban Corridor, Southeast Mountains, Front Range, Palmer Ridge & Raton Ridge:

Temperatures this afternoon will reach into the upper 90Fs across the Urban Corridor with upper-80Fs/low 90Fs forecast for the mountain valleys. Rising heights and decreasing surface moisture throughout the day mean storm develop chances will be low. A couple isolated storms could form over the elevated ridges where the surface flow has a bit more of an easterly component to it. Isolated max 1-hour rain rates up to 0.40 inches will be possible with wind as the main threat. Flooding is NOT forecast.

Primetime: 3PM to 8PM

Grand Valley, San Luis Valley, Southwest Slope, Northwest Slope, San Juan Mountains, Central Mountains & Northern Mountains:

An excessive heat warning has been issued again for dangerously hot conditions across the lower elevations out west. Highs will reach past 100F again for the Grand Valley. The century mark will also not be out of reach for the lower elevations of the Southwest and Northwest Slopes. San Luis Valley will reach into the 90Fs. Rainfall is not forecast.

FTB 06-13-2021: Overnight Flood Threat Issued for the Northeast Plains

Issue Date: Sunday, June 13th, 2021
Issue Time: 9:50AM MDT

— A LOW flood threat has been issued for portions of the Urban Corridor, Northeast Plains and northern Southeast Plains. This flood threat is an overnight threat.

A decent plume of moisture has worked its way under the ridge thanks to clockwise rotation around a surface High located in Missouri/Iowa. This plume is outlined in the green below, and the high surface moisture is helping to produce fog and cloud cover over the northeast corner of Colorado this morning. Out over the west border, it’s quite the opposite story. It remains incredibly dry (no rainfall forecast), and excessive heat becomes an issue this afternoon with highs forecast to reach over 100F for the Grand Valley.

Over eastern Colorado, PW at Denver has nearly doubled from yesterday morning to 0.68 inches. Equally important is that as you move towards the eastern border and closer to the plume, PW increases to an inch or above. So, it is very humid this morning with dew points at or above 60F. South/southeast flow at the surface around the aforementioned High and from the diurnal flow pattern should help to keep the moisture somewhat intact throughout the day despite the dry westerly/southwesterly flow aloft continuing.

A couple storms are expected to fire over the eastern elevated ridges this afternoon, which will drift southeast towards the High to our south. Then, a separate set of storms is expected to develop to our north late this afternoon and evening along a convergence boundary (cool front). These PM storms to our north are likely to spread in coverage and intensity as they move south/southeast into the higher moisture and the left exit region of the low level jet (increased lift). As the cluster of storms cross over the border tonight, widespread convection in a high moisture environment could cause flooding issues, if instability can hold on. Thus, a Low flood threat has been issued overnight. In addition to heavy rainfall, the more severe storms could produce large hail and damaging outflow winds tonight.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northeast Plains, Southeast Plains & Urban Corridor:

Heavy rainfall may be possible this evening and overnight for these regions. Back to the west over the northern Urban Corridor, the first set of storms that develop over the Cheyenne Ridge are expected to drift across the border by early this evening. With elevated moisture in the area, instability still present, slow steering flow and southeast/east flow increasing, training storms may produce storm totals just over 1 inch.

Later tonight, a second set of storms are expected to cross the northern border further east. While some of the morning model runs have been backing off the convection (rapidly decreasing instability overnight), if instability can hang on, numerous outflow boundaries will likely trigger additional convection over the Northeast Plains. With elevated surface dew points present, heavy rainfall becomes a concern alongside large hail and damaging outflow winds under the more severe storms. Isolated storm totals up to 2.75 inches will be possible, so a Low flood threat has been issued. Flood threats would include road flooding, field ponding and local stream flash flooding. Should the flood threat drastically drop off in probability later this afternoon, a PM update will be issued.

Primetime: 8PM to 4AM

Southeast Mountains, Front Range, Palmer Ridge & Raton Ridge:

It’s a bit too dry for storm develop over the mountains this afternoon, though some cumulus fields may still be possible. Better chance for storm development exists today over the elevated ridges, especially the Palmer Ridge where surface moisture remains higher. Max 1-hour rain rates up to 1 inch may be possible if the moisture does not mix out. Stronger storms will also be capable of producing small hail and strong outflow winds. Flooding is NOT expected today.

Primetime: 1PM to 8PM

Grand Valley, San Luis Valley, Southwest Slope, Northwest Slope, San Juan Mountains, Central Mountains & Northern Mountains:

An excessive heat warning has been issued for the Grand Valley where afternoon highs are forecast to exceed 100F. Equally hot temperatures are possible for the lower elevations of the Southwest and Northwest Slopes. Hazy and smokey conditions will continue from fires near the area. It’s going to be too dry for any rainfall with PW near a record low value at 0.19 inches.