FTB 09-30-2021: A Few Rounds of Precipitation Forecast for Western Colorado

Issue Date: Thursday, September 30th, 2021
Issue Time: 10:20AM MDT

This is the last scheduled Flood Threat Bulletin post for the 2021 season. The next FTB season begins on May 1, 2022. It has been a pleasure to serve you!

— Flooding is NOT expected today

Unsettled weather continues with cooler temperatures and plenty of cloud cover forecast over Colorado for the last day of September. The upper-level pattern is expected to split today, and another Low will become cut off from the main flow (Utah). This should help to hold slightly higher PW values in place over the western portion of the state (0.68 inches at Grand Junction this morning). Mid-level lift will rotate around the cutoff Low (orange “X”) as it traverses southward throughout the day. This vorticity maximum and remaining moisture is expected to spark a few rounds of showers over western Colorado with better coverage of precipitation south of I-70. Not confident that a lot of instability will be able to build, which should assist in moderating the rainfall rates. Nonetheless, some embedded convection may be possible and 2 to 3 hour accumulations will likely bring a good soaking to the area. As far as the flood threat for the area, rainfall over the southwest corner has been spotty with accumulations on the lower end the last couple of days. That means that soils in the region likely aren’t saturated yet, and they should be able to absorb a large portion of the rainfall today. The one exception is over the southern/eastern San Juan Mountains where a second round of precipitation tonight, driven by the jet, may help to boost 24-hour accumulations. Rain turning to snow tonight should help to limit the rainfall threat over the area as the snow line falls just below 10K feet. Outside of some nuisance ponding both during the overnight hours and this afternoon, flooding is NOT expected today.

Over the rest of the state, outside of isolated showers over the eastern mountains, it should remain dry. Forecasting below average temperatures today and for the cloud cover to hang around. Drier air is expected move into the plains this afternoon, which should allow some blue skies to break through for a few hours. This will help to increase afternoon high temperatures a couple of degrees over the region. Glad to see that the FTB will be ending on a fall-like note with another round of precipitation for western Colorado.

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:

San Juan Mountains, Central Mountains & Northern Mountains:

A couple rounds of showers are anticipated today with the snow line sitting above 10K feet during the day and dropping to around 9.5K overnight. More widespread showers are anticipated over the San Juan & Central Mountains with more isolated PoPs forecast over the Northern Mountains. Max 1-hour rain rates up to 0.70 inches (south) and 0.45 inches (north) will be possible if some embedded convection can develop. Small hail could also accompany those stronger storms. Over the southern/eastern San Juan Mountains, 24-hour accumulations up to 1.60 inches may be possible with the overnight precipitation forecast. Outside of some possible nuisance road and field ponding, flooding is NOT expected today.
Primetime: 1PM to Ongoing

Grand Valley, San Luis Valley, Northwest Slope & Southwest Slope:

Best chance for rainfall in the valleys will be over the SLV and Southwest Slope. Spottier shower coverage is expected over the Grand Valley and only light showers/cloud cover are anticipated over the lower elevations of the Northwest Slope. Max 1-hour rain rates up to 0.50 inches (south) and 0.25 inches (central) will be possible. Storm totals by morning could reach up to 0.40 inches for the SLV, 0.90 inches for the Southwest Slope and 0.40 inches for the Grand Valley. Flooding is NOT expected today, although there could be increased runoff in the Grand Valley if storms track over an area with saturated soils.
Primetime: 1:30PM to Ongoing

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

Much cooler today without much rainfall forecast as drier air moves over most of the lower elevations. Weak upslope flow and some residual moisture may cause some isolated showers to develop over the eastern mountains this afternoon. The southern Southeast Mountains/Raton Ridge intersect may see some weak precipitation overnight as well. Totals over the next 24-hour period up to 0.60 inches (south) and 0.25 inches (north) may be possible. Cloud cover will likely remain present all day over the Urban Corridor and elevated ridges, but there could be some clearing over the eastern plains for a bit this afternoon. Flooding is NOT expected.
Primetime: 1PM to 3AM

FTB 09-29-2021: A Narrow Low Flood Threat Amidst Widespread, Cool Rain and Snow

Issue Date: Wednesday, September 29th, 2021
Issue Time: 10:30AM MDT

— A LOW flood threat has been posted for parts of the Urban Corridor, Front Range and Palmer Divide

When it comes to analyzing the heavy rainfall threat in Colorado, there are two factors that have significant predictive strength: moisture and dynamics. Very often, only one is present (today, it is dynamics), but in sufficient strength, one can overcome the other’s absence. Such is the case today. As shown in the visible satellite image below, a strong shortwave trough has begun to move into Colorado from the west. In response, a surface low pressure will be developing along the NM/CO border today, responsible for very breezy conditions over the lower elevations of eastern Colorado today. Precipitable Water (PW) at Grand Junction has increased to 0.89 inches this morning, while Denver is steady at 0.67 inches. However, PW is expected to slightly increase today, especially over eastern Colorado. Additionally, the moisture profile has changed to a moister boundary layer over the past 24 hours. This increases chances for moderate to heavy rainfall, especially east of the Continental Divide. The aforementioned surface low pressure will maintain a strong upslope flow east of I-25 over northeast Colorado. With plenty of morning breaks in the cloud cover, there should be enough sunshine to boost instability into the 800-1,000 J/kg range this afternoon. With the stronger dynamics, organization of storms will be likely today, which could lead to prolonged heavy rainfall at a given location. In all, there will be a ~3-5 hour window this afternoon and early evening where heavy rainfall could occur over the Palmer Ridge and surrounding area. A LOW flood threat has been posted for this area.

To the west of the Continental Divide, we expect more light to moderate intensity showers and weaker thunderstorms. Outside of fire burn areas, we do not expect flooding for western Colorado today. The rain/snow line will progress from the 10-11k feet level to 7.5-8.8k feet as colder temperatures aloft move into the state.

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, Palmer Ridge and Raton Ridge:

Increasing clouds with scattered to numerous showers and thunderstorms developing early this afternoon and persisting into the late evening. Over the Palmer Ridge, max 30-minute rainfall up to 0.8 inches with max 1-hour rainfall up to 1.2 inches possible, and a LOW flood threat has been posted. For other regions, max 1-hour rainfall up to 0.8 inches possible. Hail up to 0.75 inches will be possible with the strongest cells this afternoon

Primetime: 2PM through 10PM

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

Mostly cloudy with scattered to numerous showers and weaker storms ongoing throughout the day, then diminishing during the late evening. Max 1-hour rainfall up to 0.8 inches (Front Range) and 0.6 inches (elsewhere). With multiple rounds of precipitation, up to 2.0 inch of total rainfall will be possible especially over the Front Range. Notable accumulating snow will be likely above 10,000 feet, so keep an eye on NWS products and CDOT road conditions.

Primetime: now through 9PM

FTB 09-28-2021: Widespread Precipitation Forecast for the Mountains and Western Colorado

Issue Date: Tuesday, September 28th, 2021
Issue Time: 10:25AM MDT

— Flooding is NOT expected today

Currently, there’s a hearty plume of late season moisture over the state associated with the cutoff Low (blue and white shades in the water vapor imagery below). This moisture and lift from the Low are helping to produce some morning, scattered cloud cover as well as some isolated, weak showers over the elevated regions of western Colorado. This cutoff Low is expected to track NNE throughout the day, so moisture and lift are expected to remain over the state. The Low may also generate some weak showers over the plains this afternoon, but only light rainfall accumulation is expected. With the plume overhead, PW at Grand Junction has risen to whopping 0.83 inches, which is close to the 95th percentile for this date. Over Denver, PW has risen more modestly to 0.68 inches, but that’s still in the 90th percentile for this time of year. That means there is more than enough moisture for widespread precipitation to develop over the mountains and western Colorado this afternoon and evening. An incoming trough from our west (orange dashed line) will also help increase lift and perhaps add a little more moisture later this evening over western Colorado. So, expect a second round of precipitation to arrive to the western border later this evening and continue through tomorrow morning as the trough axis moves east.

 

There are a few things that should limit the flood threat today. The first is that both soundings still show a relatively dry surface layer. That means that the initial storms that develop this afternoon will likely produce some brief windy conditions, and higher cloud bases should help temper rainfall rates. The second is that CAPE values are expected to be on the lower end, which will limit the convective/downpour potential of storms. More stationary storms, as well as the possibility for multiple storm cores tracking over the same area, are expected to increase local totals; however, dry soils should be able to absorb most of the precipitation and limit the runoff. With instability dropping off quite a bit after sundown, overnight precipitation is expected to only produce moderate hourly rainfall rates. Additionally, colder overnight temperatures are expected, which should help turn rainfall to snow over the highest elevations. For these reasons, flooding is NOT expected today. This event should be a much needed, widespread soaking for the mountains and western Colorado.

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:

San Juan Mountains, Front Range, Central Mountains & Northern Mountains:

Widespread showers should develop by early afternoon. Minimal CAPE and more pop like storms should limit the flood threat, although some decent, but isolated, storm totals will be possible by morning. The reason for the higher storm totals is that a second round of overnight precipitation is forecast as the trough axis moves overhead. Some snow will likely fall at the top of the highest peaks. Max 1-hour rain rates up to 0.65 inches will be possible this afternoon, and isolated 2-3 hour totals up to 1.75 inches are forecast by morning (Central/San Juan Mountains). Flooding is NOT expected today.
Primetime: 11:30AM to Ongoing

Grand Valley, Northwest Slope & Southwest Slope:

Best chance for some afternoon storms will be over the elevated plateaus/mesas and Southwest Slope. A second round of precipitation is expected overnight, which will likely help bring measurable precipitation to the lower elevations of the Grand Valley and Northwest Slope. Max 1-hour rain rates up to 0.50 inches and isolated 2-3 hour totals up to 1 inch will be possible. Flooding is NOT expected.
Primetime: 11:30AM to Ongoing

Southeast Mountains, San Luis Valley, Urban Corridor, Northeast Plains, Southeast Plains, Raton Ridge & Palmer Ridge:

It will be a bit drier over the Southeast Mountains/Raton Ridge and San Luis Valley today; however, isolated storms are still likely to develop. Max 1-hour rain rates up to 0.25 inches will be possible over the Southeast Mountains/Raton Ridge and storm totals up to 0.15 inches can be expected over the SLV. Light showers may also be generated over the eastern plains as the Low moves overhead. Rainfall accumulation over the plains should be light, with totals remain at or under 0.15 inches.
Primetime: 1PM to 9PM

FTB 09-27-2021: Isolated Heavy Rainfall Threat Returns To Southwest Colorado

Issue Date: Monday, September 27th, 2021
Issue Time: 10:18AM MDT

— A LOW flood threat has been posted for parts of the San Juan Mountains

Although cut-off lows usually require meteorologists to carry extra ibuprofen, we still secretly like them. This morning’s visible satellite image, below, shows a distinct cut-off circulation over western New Mexico responsible for ongoing showers and weak thunderstorms over a large swath of the Four Corners region. The low has moved slowly eastward since yesterday morning, has deepened a bit and will continue to do so as it begins to feel the synoptic scale lift from a strong incoming trough further to the northwest. Notably colder temperatures aloft have now moved into New Mexico and will enter southwest Colorado today. Yesterday’s QPE up to 0.50-0.75 inches over the San Juan Mountains carried max hourly rain rates of up to a rather impressive 0.5 inches with very limited instability and moisture. In addition, isolated heavy rainfall, near 1.5 inch/hour, was observed over central AZ higher terrain yesterday. Thus, we expect that today’s rainfall intensity will very likely be higher over southwest Colorado. This morning’s Precipitable Water (PW) was 0.61 inches at Grand Junction and 0.48 at Denver. A PW of 0.78 inches was measured at both Albuquerque and Flagstaff, and boundary layer moisture advection will be from the southwest today. Thus, we expect PW up to 0.7-0.85 in the far southwestern portion of the state. With the aforementioned cooling aloft, instability up to 1,000 J/kg will be possible this afternoon especially over areas that see some breaks in cloud cover as is expected. Steering winds will be modest, in the 20-25mph range and will be pushing storms towards the west.

Translating everything into sensible weather, with the strengthening cut-off low, we expect several rounds of showers and thunderstorms over southwest Colorado today. During the afternoon and early evening, there will be a chance for heavy rainfall over the San Juan Mountains roughly south of a South Fork-Silverton line. A LOW flood threat has been posted for this region.

Elsewhere, expect one more day of significantly above normal temperatures before cooling overtakes the state starting tomorrow and especially Wednesday.

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:

San Juan Mountains, Southwest Slope, Southeast Mountains and San Luis Valley:

Mostly cloudy with numerous showers and thunderstorms developing later this morning and persisting into the late evening hours. Max 1-hour rainfall up to 1.1 inches, with max 3-hour rainfall up to 1.5 inches, warranting a LOW flood threat for parts of the region. Isolated flash flooding, mud flows and debris slides will be possible especially over steeper terrain. Gusty winds and hail up to 1 inch could also accompany the very strongest storms.

Primetime: 2PM through 11PM

Front Range, Urban Corridor, Northeast Plains, Southeast Plains, Palmer Ridge and Raton Ridge:

A few clouds this afternoon, along with continued very warm temperatures up to 12F above normal. An isolated shower cannot be ruled out over southern areas later this evening and into the overnight hours. However, flooding is NOT expected today.

Northwest Slope, Grand Valley, Central Mountains and Northern Mountains:

Partly cloudy with temperature slightly above seasonal normal. An isolated shower or weak storm cannot be ruled out this afternoon and evening over far southern areas, but flooding is NOT expected today.