Issue Date: Thursday, September 22^nd, 2022
Issue Time: 10:10AM MDT

— A LOW flood threat has been posted for parts of the San Juan Mountains and Southeast Mountains
— Fire-Burn Forecast Summary: 4 burn areas under LOW threat; click HERE for more info

A very impressive, but rapidly shrinking plume of monsoonal moisture continues to hang overhead Colorado today. Morning PW at Grand Junction came in steady at 0.93 inches, with though some mid and upper-level drying noted. Socked under a thick cloud deck, Platteville’s PW came in at 0.90 inches, also steady since yesterday. Model analysis suggests a pocket of very high PW of up to 1.2 inches located over southwest Colorado this morning. If true, this would be near record values for late September. In addition to impressive total column moisture measured by PW, boundary layer moisture is equally, if not more impressive. For example, the current dewpoint at Cortez of 57F is close to a record for late September.

There are a couple of significant changes in today’s atmosphere, in relation to the heavy rainfall threat. First, notable drying was observed upstream in AZ/UT. The drying will be generated from the upper-levels downward, and may take a while to get into Colorado. But, by late afternoon, we do expect significant boundary layer drying especially north of I-70. This will limit heavy rainfall potential over the northern third of our state. Second, the quick steering flows have only increased since yesterday with storm motion expected to be in the 30-40 mph range today! This is due to the approach of a very strong upper-level trough, currently centered over the northern Great Basin (see water vapor image, above). Although instability will be higher today compared to yesterday (perhaps reaching 700-1,000 J/kg CAPE especially over the southern/central high terrain), there will simply be too much wind shear to sustain heavy rainfall beyond 15-30 minutes. The one exception to this will be over the far southern high terrain. There, the combination of higher instability, weaker steering flow and retention of higher moisture could allow for some terrain anchoring effects. Collectively, this will allow for heavy rainfall potential this afternoon for primarily the 1-2 hour duration. Thus, a LOW flood threat has been posted for this limited area, especially given the steeper terrain.

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 Plains, San Juan Mountains, Southeast Mountains & Raton Ridge:

Mostly cloudy and a bit warmer with scattered to numerous showers and storms re-developing this afternoon. Most locations will see only brief moderate to heavy rainfall of up to 0.5 inches in 30-minutes. However, isolated locations of the San Juan Mountains and Southeast Mountains (along the NM border) could see max 1-hour/2-hr rain rates up to 1.2/1.8 inches. Thus, a LOW flood threat has been posted for these areas. Gusty winds could accompany most storms, along with small hail.

Primetime: 12PM to 8PM, with more isolated activity possible into the early overnight hours

Northern Mountains, Front Range, Palmer Ridge, Central Mountains, Grand Valley, Southwest Slope, San Luis Valley & Northwest Slope:

Partly to mostly cloudy and breezy with scattered to numerous showers and weaker storms redeveloping this afternoon. Max 30-min rain rates up to 0.5 inches possible, along with very gusty winds and small hail. Flooding is NOT expected today.

Primetime: 10AM through 7PM

Northeast Plains & Urban Corridor:

Overcast early with some breaks in the clouds possible by later this afternoon. Continued cool with the chance of isolated to widely scattered showers and perhaps a weak embedded storm (closer to the foothills). Max 30-min rain rates up to 0.4 inches possible. Flooding is NOT expected today.

Primetime: 12PM through 7PM

Issue Date: Wednesday, September 21^st, 2022
Issue Time: 10:10AM MDT

— A LOW flood threat has been posted for parts of the Central Mountains, Southeast Mountains, San Juan Mountains and Southwest Slope
— Fire-Burn Forecast Summary: 3 burn areas under MODERATE threat; 4 areas under LOW threat click HERE for more info

The well anticipated plume of late season monsoonal moisture begins its foray into Colorado beginning today, lasting through tomorrow. In addition, plenty of dynamical support provided by a cut-off low over northern California (see visible satellite image below) will keep rain in the forecast nearly statewide. PW at Grand Junction has increased from 0.73 to 0.92 inches over the past 24-hours. Moisture has also increased sharply over eastern Colorado with Platteville PW climbing from 0.43 to 0.93 inches! Moreover, moisture is well distributed through the entire atmosphere, including a very moist boundary layer especially by late September standards. In short, moisture will not be the limiting factor to the heavy rainfall threat.

There are two limiting factors to the flood threat today: (increasingly) quick steering flow and limited instability. Steering flow this morning is already at 20+ mph and will continue to increase to 25-30 mph by afternoon. Regarding instability, as seen in the visible satellite image, below, an expansive, thick cloud deck will limit sunshine for most of central and western Colorado today. In turn, maximum instability will be a meager 500-700 J/kg CAPE, and only for a period of several hours. On a day like today, with a tropical-style “moist adiabatic” sounding, instability does not tell the entire heavy rainfall threat. Cloud warm layer depths of up to 1km will overcome limited instability to produce relatively efficient rainfall rates, but still constrained overall by quick steering flow.

To sum everything up, we expect a quick increase in shower and storm activity mainly over the central and southern higher terrain. Over primarily southern portions of the higher terrain, the combination of highest instability, warmest boundary layer and relatively slower steering flow (especially through mid-afternoon) warrants a LOW flood threat for primarily the 2-3 hour duration through this afternoon. A lull in activity is expected later this evening, ahead of another round of showers and weaker, embedded storms that are expected to accompany the approaching upper-level trough overnight.

Elsewhere, low to briefly moderate intensity rainfall is expected with significant total accumulation likely by Thursday afternoon, but flooding issues are 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:

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

Overcast with showers and storms increasing in coverage by late morning. Max 1-hour rain rates up to 0.9 inches with max 3-hour rain rates up to 1.8 inches. A LOW flood threat has been posted for southern and central parts of the higher terrain for this afternoon through early evening, where multiple rounds of moderate rainfall could trigger isolated flash flooding, debris slides and mud flows over steeper terrain.

Activity should subside temporarily by late evening, but another round of showers and storms is expected overnight. Max 1-hour rain rates up to 0.7 inches with this activity.

By Thursday morning, total rainfall up to 2.5 inches is expected over parts of the San Juan Mountains and Central Mountains.

Primetime: 1PM through 6AM Thursday (continuing into Thursday afternoon)

Northwest Slope, Northern Mountains, Front Range, Urban Corridor, Palmer Ridge & Northeast Plains:

Mostly cloudy and cool with scattered to widespread showers and weak storms expected this afternoon and evening. Max 30-min rain rate up to 0.5 inches possible, with max 3-hour rainfall up to 1.2 inches. Some ponding of rainfall may occur in low lying areas but flooding is NOT expected today.

Showers and embedded storms are expected to continue into the overnight hours, with max 1-hour rain rates up to 0.6 inches.

By Thursday morning, total rainfall up to 1.5 inches is expected over parts of the Front Range.

Primetime: 2PM through 6AM Thursday (continuing into Thursday afternoon)

Raton Ridge and Southeast Plains:

Mostly cloudy and much cooler today with widely scattered showers and storms developing this afternoon. Max 30-min rain rates up to 0.5 inches (west) and 0.8 inches (east, right along KS border) are possible, but flooding is NOT expected today.

Primetime: 2PM through 6AM Thursday

Issue Date: Tuesday, September 20th, 2022
Issue Time: 9:20AM MDT

— Flooding is NOT expected today

The aforementioned subtropical moisture plume has arrived to Colorado. Southwest flow from the Low off the coast of California and High over the southern Great Plains will continue to syphon in more moisture over western Colorado throughout the day causing widely scattered storms to develop over the high terrain by later this afternoon. PW at Grand Junction has risen to 0.73 inches already with slightly higher values estimated across the southern border. This morning’s sounding shows most of this moisture located in the mid and upper levels of the atmosphere, meaning the surface (boundary) layer continues to be quite dry. This should translate to high bases on storms that develop and less efficient rain rates today, which will help to temper the flood threat. After a couple rounds of rainfall this afternoon, dew points will likely increase, so light accumulations may be possible across the valleys by this evening and overnight. One other limiting factor to the heavy rainfall threat today will be fast steering flows. With strong upper and mid-level winds over the Great Basin, storms should be moving rather quickly to the northeast. So, with only light to moderate rainfall rates forecast today, flooding is NOT expected.

Over eastern Colorado, it remains quite a bit drier with PW at Platteville measured at 0.42 inches. While some mountain storms may be possible late this afternoon and evening, storms should be very isolated in nature and produce only light rainfall. Some overnight rainfall may also be possible for the mountains and heading into tomorrow morning, but 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:

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

There will be an increase in high terrain storms by this afternoon. Isolated max 30 to 45-minute rainfall rates up to 0.5 inches (central/south) and 0.2 inches (north) will be possible. Flooding is NOT expected, but stronger storms may produce some brief windy conditions and lightning. Isolated, light overnight rainfall is likely for the mountain zones.

Primetime: 2PM to Ongoing

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

There’s one more hot day forecast in this late season heat streak before a cold front and increase in moisture make it feel more like fall by tomorrow morning. Isolated PM storms are possible over the mountains, but it should remain dry elsewhere. With totals forecast to remain under 0.1 inches, flooding is NOT expected.

Primetime: 3PM to 9PM

Issue Date: Monday, September 19th, 2022
Issue Time: 9:05AM MDT

— Flooding is NOT expected today

With a Low off the coast of California and a High centered over the southern Great Plains, southwesterly flow continues across the state. It’s still quite a dry air mass over the northwest portion of the state as shown by the orange shade in the water vapor imagery below, and PW at Grand Junction coming in at 0.49 inches this morning. Outside of a couple possible sprinkles over the high terrain near the Continental Divide, the drier air should help to keep precipitation chances close to zero over the northern and central portions of the state.

Also circulating around the High are a couple disturbances and a bit better moisture (orange X’s and blue shades). This moisture and lift are combining to produce cloud cover and fog over eastern Colorado this morning with perhaps a couple isolated, light showers across the southern border. To a much lesser extent, there is also some morning cloud cover over the San Juan Mountains, which indicates some mid-level moisture in the area. PW at Albuquerque was measured just under 1 inch, so as clockwise flow around the High continues today, it’s likely this weak influx of mid-level moisture across the southern border will continue. This may help to produce isolated storms over the southern mountains and adjacent Raton Ridge. However, surface moisture over the area should remain quite limited, which means that the storms that develop will likely have high bases causing more virga and wind than meaningful rainfall accumulation. Therefore, 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:

Central Mountains, San Juan Mountains, San Luis Valley, Southeast Mountains, Raton Ridge, Southeast Plains & Southwest Slope:

Best chance for isolated storms today will be over the southern mountains and adjacent Raton Ridge area with the highest accumulations anticipated east. Max 30-minute rain rates up to 0.1 inches (west) and 0.5 inches (east) will be possible. Many storms are likely to only produce virga or sprinkles, and a stronger storm or two may produce brief outflow winds. Flooding is NOT expected. A much stronger moisture plume is expected to make its way northward early tomorrow morning into Tuesday, so expect a more active rainfall day tomorrow.

Primetime: 1:30PM to 8:30PM

Northwest Slope, Grand Valley, Northern Mountains, Front Range, Urban Corridor, Northeast Plains & Palmer Ridge:

Some stronger upper level winds are likely to mix down to the surface this afternoon causing breezy conditions over the northwest quadrant of the state. Gusts up to 40mph may be possible over the high peaks. Outside of the possibility for isolated sprinkles along the Continental Divide, it should remain dry. High temperatures are expected to continue to run above normal with another 90°F day anticipated for elevations under 5,500 feet.