Issue Date: Friday, September 20th, 2019
Issue Time: 9AM MDT

— Flooding is NOT expected today

Quite a bit happening this Friday with the upper level trough moving NNE throughout the day. It is currently over northern Utah and is forecast to be over the WY/MT border by this evening. This will put western Colorado under the influence of a jet streak (purple oval below). Thus, strong SW/SSW surface winds are forecast this afternoon (15 to 25 mph) with gusts between 35 and 40 mph anticipated. On top of that, a dry air mass begins to move over the area located at the base of the trough. This will cause elevated and critical fire weather over western Colorado and portions of the mountains. A Red Flag Warning has been issued by the Pueblo NWS WFO (Weather Forecast Office) from noon today until 9PM tonight for the Southwest Slope and Grand Valley. Additionally, very cold overnight temperatures are anticipated for the lower elevations of the Northwest Slope with northwest winds behind the trough, so a Freeze Watch is in place. Not thinking there will be much storm development over the mountains this afternoon with the drier air and strong winds aloft (shear the tops of the storms), but a couple isolated showers may be possible favoring the eastern San Juan Mountains and Central Mountains near and along the Continental Divide. Storms may produce some strong outflow gusts due to the temperature/dew point spread.

Over eastern Colorado, a surface low develops this afternoon associated with the upper trough. This will help set up a dryline over the eastern plains by noon with higher dew points on the east side of this line (green dashed line below). This will help produce lift for isolated storms, some which may become severe, over the eastern plains favoring the eastern Colorado border once again. With fast storm movement to the NNE, not thinking flooding will be an issue, but large hail (up to 1.75 inches) and strong outflow winds (35-40 mph) are anticipated with the severe storms that do develop. Additionally, the tightening pressure gradient will produce some strong southwest (west) and south (east) surface winds and gusts up to 35 mph, which will create elevated fire danger. This is especially true for the immediate adjacent plains over eastern Colorado where dew points are forecast to be lower. A stronger cold front drops through from the north again this evening in the wake of the trough, so overnight lows will drop into the low 50°Fs behind it with temperatures in the 70°Fs for the lower elevations tomorrow (minus the far southeast corner). Flooding is not forecast.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

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

Critical and Elevated fire weather are the main concerns this afternoon with the jet in place over western Colorado and the mountains. Not expecting much rainfall with a drier air mass moving in and strong winds aloft. However, a couple of storms may briefly form along the Continental Divide and southern San Juan/Southeast Mountains. Max 1-hour rain rates up to 0.2 inches will be possible (south), but most storms that form will produce under 0.1 inches. Strong outflow winds will be likely with storms that do form with higher winds more easily mixing to the surface. Overnight lows are expected to rapidly drop off once winds shift to the northwest and skies begin to clear. A Freeze Watch is in effect for portions of the Northwest Slope and western Northern Mountains. Flooding is not forecast.

Primetime: 11AM to 10PM

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

Elevated fire weather is anticipated this afternoon with a tightening surface gradient. This is especially true over the Urban Corridor, western Southeast Plains and Raton Ridge where dew points will drop off more rapidly with the dry air mass arriving sooner. A couple, isolated storms will be possible near the eastern border along a dryline that sets up. The main threats will be large hail and strong outflow gusts for the severe storms that are able to form. Max 1-hour rain rates up to 1 inch will be possible, and with quick movement to the northeast (~20mph), flooding should be avoided. Some light showers may be possible over this same area as the front drops through overnight, but only light accumulations are expected (under 0.05 inches).

Primetime: 2PM to 7PM

 

Issue Date: Thursday, September 19th, 2019
Issue Time: 8:30AM MDT

— A LOW flood threat has been issued for the 416/Burro burn area

Average to slightly above average low temperatures overnight across the state. There’s some dense, early morning fog being reported over the eastern plains near Limon out to the Kansas border (6:30AM MDT). This should quickly burn off with a little sunlight as the moisture is confined just to the surface. PW at Denver and Grand Junction was measured at 0.52 and 0.58 inches, respectively in this morning’s sounding. So not much change from yesterday. Expecting these values to increase over the next 24-hours from south to north, first over western Colorado with some really high values over eastern Colorado by tomorrow afternoon. This moisture surge has to do with the approaching low to our west. As the feature moves eastward and digs south throughout the day, upper level flow will turn from southwesterly to more southerly. This will allow a few disturbances to move through the flow and start to create a subtropical moisture surge (albeit weak).

One of today’s disturbances is marked over Utah with the “X”. This feature will first move east this morning, and then it will pull northwards throughout the afternoon. Not expecting much rainfall with this feature due to the low PW values, but an increase in cloud cover should mark its location throughout the day. It may help spark some weak afternoon convection over the northern high terrains near and along the Continental Divide with the diurnal flow, but more virga than measurable rainfall is forecast. Over the eastern plains, along the CO/KS border, additional thunderstorms may be possible this afternoon. The latest model runs are keeping the stronger storms to the east of the border, but there is a good chance for an isolated storm or two initiating along the line of convergence over southeast Colorado (weak dryline). Not expecting flooding from these storms due to the isolated nature and the quick storm motion to the northeast.

Another disturbance moves from south to north this evening, which starts over western Colorado. With the increasing PW throughout the day, expecting some weak showers over the south Central and San Juan Mountains to begin later this evening. Storms will be capable of producing totals up to 0.9 inches over a 2-3 hour period over the San Juans (earlier timing of the moisture surge), so a Low flood threat has been issued for the 416 burn area. Rain rates should be gradual enough they won’t cause issues elsewhere. The coverage should expand over the high terrains overnight as the feature moves north. Some weak showers may also be possible over the Raton Ridge/southern Southeast Plains associated with another shortwave tomorrow morning. As it moves north throughout the morning, expect more cloud cover than measurable rainfall. Despite the numerous rounds of rainfall across the state (today, tonight, and tomorrow morning), flooding outside of the 416 burn scar, is not anticipated.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

 

Zone-Specific Forecasts:

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

Very isolated max 1-hour rain rates up to 0.75 inches with totals up to 0.9 inches by morning are possible over the San Juan Mountains with totals decreasing to 0.5 inches over the Central Mountains. The 1-hour rain rates are reduced to 0.35 inches as you move north into the Northern Mountains and Northwest Slope. Most of the rainfall will be along and near the Continental Divide this afternoon and evening with some isolated activity over the higher elevations of the North/Southwest Slopes. Lowest elevations should stay dry today minus the southern tier Southwest Slope and San Luis Valley. Isolated totals up to 0.25 inches will be possible by morning. Thunderstorms this afternoon will again be capable of producing some gusty outflow winds, but they should not be quite as strong as yesterday and Tuesday. If storms continues to track over the 416 burn area tonight, flash flooding of local streams, debris slides and mud flows will be possible due to the moisture surge arriving. A Low flood threat has been issued for this reason.

Primetime: 6PM to 5AM

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

Some afternoon convection will be possible over the Front Range near the Continental Divide, but accumulations should stay low (up to 0.25 inches). Overnight convection along the Divide may cause totals up to 0.4 inches over the Front Range by morning. Additional thunderstorms over the Southeast Plains this afternoon could have rain rates up to 1.1 inches with large hail and strong winds also possible. As showers move across the southern border early tomorrow morning, max 1-hour rain rates up to 0.5 inches will be possible. Flooding is not forecast due to the low intensity of storms and quick storm motion of the stronger storms to the NE this afternoon.

Primetime: 4PM to 7AM

Issue Date: Wednesday, September 18th, 2019
Issue Time: 8:45AM MDT

— Flooding is NOT expected today

Much drier air has moved over the state, which is shown by all the yellow in the water vapor imagery below. Not surprisingly, PW has dropped to 0.55 to 0.60 inches at Grand Junction and Denver, respectively. Southwest flow aloft, which will turn more westerly this evening, will keep the drier air mass in place. Thus, not much afternoon shower and thunderstorm activity is expected with the minimal residual moisture in the atmosphere. The morning cloud cover over the western high terrains hints there is some pockets of moisture still over the mountains. So the best chance for an isolated weak shower or thunderstorm will be over the mountains favoring the Front Range as this plume is advected to the northeast. Flooding is not forecast.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Central Mountains, Northern Mountains, San Juan Mountains, Front Range, Southeast Mountains, Urban Corridor:

Best chance for rainfall today will be over the high terrains where pockets of residual moisture remain intact. While most storms will stop at producing just cloud cover and virga, a couple storms may produce light accumulations. Best chance for rainfall will be over the Front Range where totals up to 0.2 inches will be possible with the stronger storms. Most other storms will produce under 0.1 inches across the mountains. Activity may spill into the Urban Corridor this afternoon with the flow aloft (to the NE), but storms are expected to quickly dissipate. Max 1-hour rain rates up to 0.1 inches will be possible along with brief outflow gusts.

Primetime: 2PM to 8PM

Northwest Slope, Grand Valley, Southwest Slope, San Luis Valley, Raton Ridge, Palmer Ridge, Southeast Plains, Northeast Plains:

Slightly cooler temperatures are anticipated today behind a weak cold front that moved through overnight. The cold front is currently over the Southeast Plains and has stalled out back to the west. With low dew points across these regions, rainfall is not anticipated this afternoon. Surface winds over western Colorado may pick up to 10-15 mph (from the southwest) as the next trough begins to approach this afternoon. Critical fire weather is not forecast, but some pockets of elevated fire weather are anticipated.

Issue Date: Tuesday, September 17th, 2019
Issue Time: 9:05AM MDT

— Flooding is NOT expected today

The aforementioned trough from yesterday’s FTB begins to move to the east-northeast over the next 24-hours, which will bring some mid-level energy and moisture over the state for another round of garden variety storms this afternoon and early evening. Currently, PW is at 0.73 inches over Grand Junction, which is helping produce some showers, and snow at the highest elevations, over the San Juan and Central Mountains. This is associated with the first shortwave marked in the infrared satellite imagery below (orange “X”). Expect the southwest flow aloft to turn more westerly as the trough progresses eastward, so PW values will really start to drop off this evening as the dry air mass is advected from the west. Also, the downsloping (westerly) winds will help scour out moisture over eastern Colorado. Thus, only expected one or two weak storms, tops, over the far Northeast Plains or Raton Ridge area associated with the passing shortwave. The more likely scenario is increasing cloud cover as the shortwave passes overhead with a sunshine starting to peek through this afternoon as the moisture is scoured out/moves east.

Over western Colorado, morning cloud cover and rainfall will limit the amount of instability that can form this afternoon, so only expecting a couple scattered, weak thunderstorms with mostly showers. A couple of the stronger storms may produce gusty outflow winds and small hail. Despite this morning’s precipitation, there will still be a slight increase in instability with the daytime heating, so forecasting showers to pick up a little in intensity and coverage this afternoon and early evening over the high terrains. Best chance for measurable rainfall will be over the San Juan, Central and Northern Mountains, but with fast storm motion, only expecting light to moderate accumulations where multiple storms can track over the same area. The lower elevations may see a sprinkle or two, but accumulations should remain under 0.1 inches. Due to the low intensity of the rain, flooding is not forecast.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

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

Very isolated max 1-hour rain rates up to 0.25 inches will be possible over the high terrains with no more than 0.1 inches per hour for the lower elevations today. Multiple storms tracking over the same area may cause very isolated accumulations up to 0.5 inches by morning over the highest elevations of the western Central and San Juan Mountains. Strong outflow winds are possible this afternoon with weak thunderstorms that are able to form, along with small hail. Despite decent accumulations of rainfall over the last couple of days, rain rates should still be gradual enough that flooding issues should be avoided. Dry air quickly fills in from the west this evening, which should end rainfall just after dark. Flooding is not forecast.

Primetime: Ongoing to 8PM

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

Downsloping winds will really limit the chances for rainfall this afternoon, so the best chance for accumulation will be along the Continental Divide. Expect an increase in cloud coverage as the shortwave passes overhead, which may help keep the high temperatures down a couple of ticks. A couple weak storms may be possible as the clouds/light rain move east over the far Northeast Plains and Raton Ridge, but rain rates should remain below 0.25 inches. Clouds will start to break up late this afternoon as dry air works in from the west, so it should be a pleasant evening. A weak cold front passes over the eastern plains overnight behind the trough, but it’s more of a wind shift than a temperature change. Nonetheless, high temperatures may drop a couple degrees tomorrow.

Primetime: 2PM to 8:30PM