FTB 09-22-2020: Widespread Scattered Storms Forecast for the Mountains

Issue Date: Tuesday, September 22nd, 2020
Issue Time: 9:15AM MDT

— Flooding is NOT expected today

A well-defined area of lift can be seen over Utah in the mid-level water vapor imagery below (orange “X”). As this feature moves eastward throughout the morning, it will help trigger more widespread scattered storms over the mountains this afternoon. Precipitation is expected kick off a little sooner today with plenty of moisture over the area, and storms should begin to pop over the mountains by midday. Steering flow remains light, but is slightly stronger than yesterday, so storms will slowly move to the east this afternoon. This will cause rainfall to spill into the adjacent eastern plains favoring the elevated ridges for accumulations once again.

PW has increased from yesterday and was measured at 0.60 inches in Grand Junction and 0.65 inches in Denver this morning. A slight increase in mid-level moisture is also forecast as the upper level wave arrives. With the majority of the moisture in the mid-levels again, high cloud bases will limit the amount of rainfall reaching the surface. However, totals are still expected to slightly rise from yesterday, but still remain well below flood threat criteria. Therefore, flooding is not forecast. Best chance for a rumble of thunder will be over the southern San Juan Mountains and Palmer/Raton Ridges where more daytime heating can build instability. The main threat from the weak thunderstorms that develop will be gusting outflow winds and lightning.

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

The majority of the rainfall this afternoon should occur south of I-70 once again, but an isolated storm or two may develop over the northern Front Range. Higher accumulations are expected over the Southeast Mountains and elevated ridges where isolated totals up to 0.50 inches (west) and 0.75 inches (east) are possible. Elsewhere, rainfall totals from the larger storms will generally be between 0.20 and 0.30 inches. Storms aren’t expected make it too far off the mountains, so the eastern plains should remain dry. The main threat from stronger storms that occur today will be gusting outflow winds, especially where virga is present, and lightning. Gusts up to 45 mph may be possible. Flooding is not forecast, and near surface smoke should clear out a bit this afternoon over the northern Urban Corridor.

Primetime: 12PM to 11PM

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

Best chance for measurable rainfall today will be over the high terrains south of I-70, and the most accumulation is again expected further south. Over the Central Mountains, rain totals up to 0.35 inches will be possible, and over the southern San Juan Mountains, isolated totals up to 0.65 inches will be possible. Best chance for a thunderstorm or two will be over the San Juan Mountains, and the main threats will be strong outflow winds and lightning. Light rainfall may fall over the valleys as storms move east, but plenty of virga is forecast as well. Over the western high terrains, totals should be between 0.10 and 0.15 inches. A second wave of energy may keep storms lingering over the mountains this evening, but the majority of activity should end just after midnight. Flooding is not forecast, and near surface smoke should improve throughout the day across the northern portion of these zones.

Primetime: 11:30PM to 1AM

FTB 09-21-2020: Scattered Showers Return to the Forecast

Issue Date: Monday, September 21st, 2020
Issue Time: 9AM MDT

— Flooding is NOT expected today

The departing trough can be seen over the eastern plains in the visible satellite imagery below, and radar is showing some light showers occurring along this line of convergence. Behind the trough, another ridge will begin to build northward and slide east throughout the day. Typically, this would begin to dry out the atmosphere and produce subsidence. However, today there are a couple things happening. One, over eastern Colorado, the trough and residual moisture will linger. That means as upslope flow develops this afternoon, there should be enough moisture to help produce another round high-based storms over the eastern mountains and Palmer/Raton Ridges. Secondly, there is some mid-level energy and a slight increase in moisture over southwestern Colorado. As both of these ingredients rotate around a strengthening High to our south, additional storms are forecast over the southern high terrains. With PW values reaching up to 0.50 inches and slow steering winds, rainfall totals should increase a bit when compared to the last couple of days. However, flooding is not forecast. A couple weak thunderstorms are also possible with the main threats being brief, strong outflow winds and lightning.

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

Expect scattered cloud cover to increase over the high terrains this afternoon as instability builds. Best chance for precipitation and scattered storms will be over the San Juan and Central Mountains along and near the Continental Divide. The main threats from the stronger storms that develop will be outflow winds and lightning. Isolated rain totals up to 0.30 inches (south) will be possible, but most storms will produce under 0.20 inches. So, flooding is not forecast. Near surface smoke is forecast to intensify along and just south of the northern border.

Primetime: 12:30PM to 8PM

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

Scattered storms are forecast over the high terrains this afternoon with activity staying mostly south of Clear Creek/Gilpin County. Additional storms are forecast to fire over the Palmer and Raton Ridges where there is the best chance for a weak thunderstorm. Isolated rainfall totals up to 0.50 inches will be possible with most storms producing totals between 0.15 and 0.25 inches. Storms may also produce some brief, but strong outflow winds with the higher cloud bases. Flooding is not forecast. Smoke is forecast to remain over these regions, and near surface smoke looks to remain heavy over the northern Urban Corridor.

Primetime: 1PM to 11PM

FTB 09-20-2020: Light Showers Possible

Issue Date: Sunday, September 20th, 2020
Issue Time: 09:45AM MDT

— Flooding is NOT expected today

The trough that impacted Colorado yesterday will continue to impact Colorado weather today as it skims the northern edge of the state. The image below shows this morning’s surface analysis with the infrared satellite imagery, which shows a large Low pressure system associated with the advancing trough now centered in Saskatchewan, Canada. A frontal boundary extends south from this Low through central Colorado, which has stalled out over the mountains. This frontal boundary will be the focus of some light precipitation today as the main storm energy stays well north of Colorado. The frontal boundary that worked its way into Colorado did not seem to bring significant moisture with it, but surface dew points have risen into the low 40Fs for the Northwest Slope region. Dew points are also in the 40Fs across much of the plains of eastern Colorado as well, with 30Fs dew points over the Palmer Ridge. The Denver sounding shows a slight increase in PW compared to yesterday, up to 0.61 inches. This limited moisture will be enough to allow some showers to develop across the central and northern part of the state, but strong upper-level winds will keep these showers moving quickly to the east. No flooding is expected today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. 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, Urban Corridor, Palmer Ridge, Raton Ridge, & Southeast Plains:

It will be warm again today with high temperatures increasing from the low 80Fs along the northern border to the low 90Fs in the Southeast Plains. Showers will likely develop over the Palmer Ridge, Northeast Plains, and Urban Corridor this afternoon near a stalled frontal boundary. These showers will not produce significant rainfall as there is limited moisture and strong winds pushing them east quickly. Total rain accumulations should stay under 0.1 inches from these showers. The Southeast Plains will likely stay dry, but western portions of the region may get a few sprinkles from showers coming off the higher terrain. No flooding is expected today.

Some smoke an active wildfire in southern Wyoming is affecting air quality over the northern plains regions. An Air Quality Alert has been issued for some of the northern counties. See your local NWS office for more information. Elsewhere, the trough has pushed out some of the thicker smoke to our east, so skies should be somewhat clearer today than it has been over the last week.

Primetime: 1PM to 10PM

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

Most valleys will stay dry and warm again today, but some showers are possible over the mountains. The highest chances for seeing rainfall is over the northern San Juan and Southeast Mountains where the stalled frontal boundary will increase convergence and lift. Any showers that form will move east quickly under the strong upper-level winds, which will limit rain totals. Up to 0.2 inches of total rain accumulation could fall over the favored northern San Juan and Southeast Mountains. The drier low-level atmosphere will favor this rain accumulation over the higher elevations as evaporation will likely lower rain accumulations for low elevation locations. No flooding is expected.

Primetime: 12PM to 8PM

FTB 09-19-2020: Showers Back in the Forecast with Gusty Winds

Issue Date: Saturday, September 19th, 2020
Issue Time: 10:00AM MDT

— Flooding is NOT expected today

Today a sharp trough will work its way into Colorado, further displacing the high pressure ridge to the south. The upper-level Low at the center of the trough is situated over the Washington/Oregon border as of this morning (see image below). Strong jet stream winds around this upper-level Low are creating plenty of lift and clouds over the Rocky Mountains, as seen by the blue, green, and yellow colors in the infrared satellite image below. This upper-level energy will give northwest Colorado some moderate chances for seeing precipitation today as strong southwesterly upper-level winds work their way into the state from the west. The morning sounding at Grand Junction shows a slight increase in upper-level moisture compared to yesterday with PW of 0.5 inches, but low-level moisture remains week with dew points in the 20Fs. This means any rainfall over the western half of Colorado will struggle with evaporation, limiting rainfall totals. The plains east of the mountains will likely stay mainly dry today due to downslope winds and a dry boundary layer, but a few showers could create virga over the Urban Corridor as they move off the higher terrain. Due to the dry near-surface air and more stratiform nature of rainfall today, no flooding is expected today.

A lee surface Low pressure is developing over eastern Montana, but this low pressure extends south along the eastern Rockies into the adjacent plains all the way through Colorado. This is creating a strong pressure gradient over the eastern plains of Colorado, which will increase surface winds to near 20 mph with gusts up to 35 mph. These strong winds are increasing the fire danger for the eastern plains today with continued warm and dry conditions favoring low humidity, which has caused a Red Flag Warning to be issued by the NWS. Strong surface winds can also be expected over northwestern Colorado as the trough works its way into the state. Additionally, showers that form over the western part of the state could create downdrafts and gusty winds up to 55 mph. This will create an elevated fire danger until precipitation can help moisten the near-surface air.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. 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:

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

Light showers are expected over the Northwest Slope and Northern/Central/San Juan Mountains as strong southwesterly upper-level winds associated with the incoming trough create lift. Some weak instability could create more convective showers, but rain rates should stay below 0.3 in/hr. The San Juan Mountains have the highest chances of seeing more convective showers, whereas the Northwest Slope will likely see more stratiform rainfall. Due to dry low-levels, any precipitation will have trouble reaching the ground for the lower elevations, so rain accumulation is most likely for high elevation mountain locations. Additionally, strong winds aloft will keep showers moving quickly. Several rounds of rain are expected starting just before noon and lasting into the overnight hours, so 24-hour rain totals could reach up to 0.4 inches for some of the northern mountain locations. Because of the dry surface layer, strong winds, and weak instability, no flooding is expected today.

The clouds and showers will help keep temperatures down a few degrees, but above normal temperatures are still expected for most locations. Smoke should lessen today, but it is still degrading air quality as of this morning across most regions.

Strong winds will make their way to the surface over the Grand Valley, Northwest Slope, and Northern Mountains with wind gusts likely up to 40 mph, which will increase the fire danger today and could cause some additional growth of the active wildfires in the state. Gusty outflow winds from convective showers could increase these gusts up to 55 mph. Some gusty downslope winds over the Front Range are also expected overnight. Be careful not to cause a spark, and keep an eye on current wildfire conditions today.

Primetime: 11AM to 6AM

Northeast Plains, Urban Corridor, Palmer Ridge, Raton Ridge, & Southeast Plains:

It should stay mainly dry and warm today, with temperatures approaching the 90F mark for many locations. Upper-level clouds will likely be present today, but they should not be thick enough to filter out too much sun. A slight chance for a few showers exists over the western Palmer Ridge and Urban Corridor as mountain showers move east. However, little to no accumulation is expected due to the dry surface conditions. No flooding is expected.

Strong surface winds will affect the eastern portions of the Northeast and Southeast Plains, with gusts up to 35 mph likely. Fire danger is elevated for these regions and a Red Flag Warning is in place. Activities that could initiate sparks should be avoided today. Smoke will continue to degrade air quality today as well.