FTB 08-22-2019: Garden Variety Storms Forecast for Thursday

Issue Date: Thursday, August 22nd, 2019
Issue Time: 9:40AM MDT

— Flooding is NOT expected today

The trough continues its migration eastward today, which will start to bring some enhanced lift (“Jet” area) to the state this evening, and will help keep widespread, weak thunderstorms in the forecast for the next couple of days. The jet, associated with the trough, will also bring critical fire weather to the Northwest Slope. Moisture this morning remains limited over western Colorado (PW at 0.6 inches with the most moisture in the mid-levels) and increased southwest surface winds (tightening gradient) has caused a Red Flag Warning to be issued through this evening. There is a slight increase in the chance of measurable precipitation over the higher terrains of the Central and southern Northern Mountains (along with the San Juans) with the enhanced lift in the area. Overall, the threat is looking to be more cloud cover, virga and gusty winds. Light rainfall accumulations are also forecast with the highest QPF south.

Back east, PW has dropped off to 0.77 inches at Denver this morning with a pretty good dry layer above the surface, which means storms will have higher bases and produce less rainfall than yesterday. Best moisture and lift will be east of I-25, so expecting the higher accumulations of rainfall here. Storms are forecast to form over the high terrains just after noon, and westerly steering winds will help storms move into the adjacent plains by 1 or 2PM. Over the Urban Corridor/immediate adjacent plains, the largest threat will be gusty outflow winds from the storms and small hail. Parameters are indicating a severe thunderstorms or two are possible over the Northeast Plains (near the northern CO border) early this evening with the main threats again being gusty winds, local, heavy rainfall and severe hail. Better moisture is also hanging on near the CO/NE/WY border, so if a storm tracks in the state from the Cheyenne Ridge, it has the potential to drop some local, heavy rainfall. Not thinking this heavy rainfall will be widespread enough to issue a flood threat, and storms should be moving fast enough to the ESE that flooding thresholds aren’t likely to be exceeded by much.

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:

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

Scattered, weak thunderstorms are anticipated to develop over the higher terrains early this afternoon, driven by diurnal flow. Totals up to 0.8 inches will be possible over the southern Front Range and Palmer Ridge regions with totals to the north closer to 0.3 inches over the northern Front Range. Storms will quickly move off the high terrains into the Urban Corridor with the storms mostly producing cloud cover and some gusty outflow winds. Over the Urban Corridor (east of I-25), max 1-hour rain rates up to 0.5 inches will be possible. Heaviest rainfall is forecast over the Northeast Plains and northern Larimer and Weld Counties where max 1-hour rain rates just over 1 inch will be possible. Flooding is not anticipated.

Primetime: 12PM to Midnight

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

Critical fire weather continues for another day with a Red Flag Warning issued over the northwest corner of the state. Surface winds are forecast from the southwest and expected to be in the 10 to 20 mph range with gusts up to 35 mph. Mixed with low humidity values, fires will be in an environment they can spread quickly. So, please use caution with any open flames. Increasing cloud cover is also anticipated this afternoon, which will bring relief to temperatures that are still a few degrees above average for late August. Max 1-hour rain rates over the northern high terrains will be under 0.1 inches with isolated totals up to 0.4 inches over the San Juan Mountains.

Primetime: 12PM to 8PM

FTB 08-21-2019: Widespread, Heavy Rainfall Forecast for Late this Afternoon and Overnight

Issue Date: Wednesday, August 21st, 2019
Issue Time: 10AM MDT

MODERATE flood threat has been issued for the Front Range, Urban Corridor, Northeast Plains and Southeast Plains
A LOW flood threat has been issued for the Palmer Ridge, Junkins burn area and Spring Creek burn area

Widespread thunderstorms and heavy rainfall are forecast to the east of the Continental Divide today, and isolated storms will be possible again over the San Juan Mountains. The cold front that passed over the eastern plains last night has cooled down temperatures and created the cloud cover shown in the visible satellite imagery below. Expecting this to start to burn off throughout the day, which will build instability and set the stage for some severe thunderstorms and heavy rainfall this afternoon. Not surprising with all the cloud cover (indicates moisture return), dew points at Denver are in the upper 50°Fs with most areas over eastern Colorado currently showing 50°F+. PW at Denver jumped to 0.93 inches, which is in the 90th percentile for this time of year! A stationary front (red line below) has formed over the area and will be the primary focus for heavy rainfall tonight over the Southeast Plains.

Strong upslope flow to the north of the surface low, centered over the Southeast Plains, will provide moisture and lift for thunderstorm generation along the eastern mountains and Palmer Ridge. Additionally, two shortwaves (“X” below) are forecast to move across the state today, which will provide extra lift and increase storm coverage over the San Juan Mountains and Front Range/adjacent plains this afternoon. As the upper trough begins to move east off the Pacific Northwest coast today, the 500mb ridge access will slide east and upper flow (steering winds) will be from the northwest over eastern Colorado and more westerly over the western half of Colorado. This will allow upslope storms that form over the eastern mountains this afternoon to move into the adjacent plains. Easterly winds at the surface mean there will be some shear and decent CAPE where morning cloud cover can clear, so a couple severe thunderstorms are possible. Slow steering winds under the ridge will allow for some very heavy accumulations as storm motion will be to the southeast at 10-18 mph. Back building and trailing storms are also expected, which means the storms may linger over an area for more than an hour. As the storms move into the Southeast Plains, overnight heavy rainfall is also forecast along the stalled out boundary. Thus, this is an overnight threat.

Slight downsloping winds off the Southeast Mountains (due to the location of the surface low) will keep the highest moisture to the east. However, storms today will still be capable of producing 0.5 inches in an hour, so a Low flood threat has been issued for the Spring Creek burn area. Not thinking 1-hour rain rates will be high enough to cause any issues on the Junkins burn area. Though, totals in the area could reach 0.8 by tomorrow, so it is included it in the Low flood threat. Threats over burn areas include flash flooding of local creeks/streams, mud flows and debris slides. Severe storms today will be capable of producing large hail (quarter to up to half dollar) and strong winds from outflow boundaries. Flood threats include street flooding, local stream/creek/gulch flooding, flooding of low lying areas and field ponding. Arroyo flooding will be possible over the Southeast Plains.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Flood Threat Legend

 

Zone-Specific Forecasts:

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

Storms are forecast to begin over the San Juan Mountains by 1PM with the eastern mountains following a couple hours later. Max 1-hour rainfall rates up to 0.4 inches are possible over the San Juans with rates increasing to 0.5 inches over the Southeast Mountains. Thus, a Low threat has been issued for the Spring Creek burn area where mud flows, debris slides and flash flooding will be possible if a storm tracks over the area. Storm totals near the Junkins burn area could reach 0.8 inches, so the Low threat has been extended to this area.

Over the Front Range foothills and Urban Corridor max 1-hour rainfall rates up 1.9 inches along with isolated storm totals up to 2.5 inches will be possible (totals more likely closer to 2.25 inches). Over the western Palmer Ridge, max 1-hour rain rates up to 1.1 inches will be possible. Guidance is also hinting at outflow boundaries possibility initiating additional storms over the Palmer Ridge and southern Northeast Plains, thus the Low threat has been extended to this area. Max 1-hr rain rates up to 1.25 inches will be possible over this area if storms break the cap.  As the overnight rainfall sets up over the Southeast Plains and eastern Palmer Ridge, max 1-hour rain rates up to 2 inches are likely with storm totals just over 3 inches possible by morning. Flood threats include street flooding, local stream/creek/gulch flooding, flooding of low lying areas and field ponding. Arroyo flooding will be possible over the Southeast Plains. Please note this is an overnight threat. A Moderate flood threat has been issued.

Primetime: 1PM to 4AM

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

Moisture has not changed much since yesterday, so the chances of accumulations are slim to nil today.  Brief, weak thunderstorms are possible this afternoon along and over the Continental Divide in the Central Mountains, and isolated accumulations up to 0.1 inches will be possible. Higher elevations over the Southwest Slope may also see some light accumulations, but totals are forecast to remain below 0.1 inches. The main threat will be gusty winds as storms fall apart and lightning. High temperatures may be a degree or two warmer, so be sure to stay hydrated once again. Expecting an increase in cloud cover overnight as the trough moves east, so overcast skies are forecast for tomorrow morning.

Primetime: 2PM to 8PM

FTB 08-20-2019: Isolated Severe Thunderstorms Return to the Northeast Plains

Issue Date: Tuesday, August 20th, 2019
Issue Time: 10:50AM MDT

-A LOW flood threat has been issued for the Northeast Plains

The threat of isolated severe thunderstorms will return to the Northeast Plains this afternoon and early this evening with the main hazards being large hail, damaging winds and local, heavy rainfall. Overall, not much change to the location of the 500mb high, but there are a few new features that are expected to set the stage for severe weather over the Northeast Plains. As noted in the FTO, two shortwaves (“X” below) are expected to move over and to the north of Colorado throughout the day from Utah and Wyoming. These features will provide lift to break the cap later this afternoon and create more widespread storms over this area. Marked on the water vapor imagery below is a low pressure system that has developed over the Southeast Plains. Clockwise, surface flow of moisture around the low (green arrow) will bring in dew points in the upper 60°Fs to the Northeast Plains, which will increase PW to ~1.4 inches over the area. This moisture should reach back to the eastern boundary of the Urban Corridor, which is quite a big moisture change from yesterday. Upslope flow along the Front Range and Palmer Ridge will also provide a chance for some afternoon thunderstorms over these higher terrains. Some storms may drift over the Urban Corridor, however dry air will limit the heavy rainfall threat.

Southwesterly and westerly flow of dry air aloft will continue west of the Continental Divide today, which will limit storm potential over most of the area. Southerly flow of mid-level moisture this morning over the eastern San Juan Mountains and Southeast Mountains will provide a chance for some afternoon high-based, weak thunderstorms, but rainfall accumulations are forecast to be minimal.

The threats for severe storms today over the Northeast Plains include heavy rainfall, wind gusts up to 70 mph and large hail (up to tennis ball size).  Storms are expected to begin early to mid-afternoon over the eastern plains and continue until early tomorrow morning.  Storm motion is forecast to be between 10 and 15 mph, which is slow enough to allow for heavy rainfall over a 1.5 to 2 hour period.  A weak cold front dropping in from the north and a low level jet will be the forcing mechanisms (lift) for thunderstorms tonight.  Rain rates may decrease at this time due to limited instability, but some high totals will still be possible by tomorrow morning. Drier air over the Southeast Plains will limit chances for rainfall though some isolated storms are possible near the Kansas border.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

The severe storms are forecast to begin mid-afternoon over the eastern plains favoring the eastern Palmer Ridge and Cheyenne Ridge for formation. Isolated max 1-hour rainfall rates up to 1.75 are possible with 1.5 to 2 hours totals up to 2.25 inches near the CO/NE border.  Max 1-hour rainfall rates over the eastern Palmer Ridge are forecast to be just under 1 inch, so flooding is not expected. Downsloping flow over the Southeast Plains should keep storms isolated, but if they are able to break the cap near the Kansas border, totals up to 0.5 inches are possible. Storms that fire over the eastern mountains should produce only cloud cover and some brief gusty winds. Totals may increase to 0.15 inches if storms make it into the immediate adjacent plains and up to 0.6 inches over the Raton Ridge. Mostly expect another day of hot temperatures and broken records.

Primetime: 2PM to 4AM

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

Southerly flow of mid-level moisture this morning will provide a chance for afternoon thunderstorms over the eastern San Juan Mountains.  However, dry air at the surface will mix out most of the moisture keeping max 1-hour rainfall rates around 0.15 inches.  Some weak thunderstorms may be possible with the main threats being gusty winds and lightning. The rest of the forecast region is forecast to remain again today with a slight chance for isolated, weak thunderstorms along the Continental Divide. Afternoon high temperatures will be similar to yesterday, so be sure to keep hydrated.

Primetime: 2PM to 7PM

FTB 08-19-2019: Mostly Dry and Hot Weather Continues

Issue Date: Monday, August 19th, 2019
Issue Time: 9:40AM MDT

— Flooding is NOT expected today

Another day of hot, dry weather is forecast with a dry air mass remaining in place over Colorado. The upper level ridge overhead strengthened throughout the day yesterday providing strong subsidence (sinking motion) over the area. This has resulted in a PW decrease with Denver and Grand Junction measuring 0.52 and 0.41 inches this morning, respectively. A thin layer of moisture is present in the mid-levels producing some cloud cover, which can be seen in the white colors in the water vapor imagery below. This cloud cover will increase over the west again during the afternoon, but rainfall is not forecast. Chances for measurable rainfall will increase over the far eastern plains as a shortwave (“X” below) moves over the state from the southwest and mixes with a little more moisture. At best, there will be some weak afternoon thunderstorms just to the east of the dryline (orange line below). Dew points in the upper 40°Fs to 50°F will keep accumulations well below flood threat criteria and may produce some brief, gusty winds. Flooding is not forecast for today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

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

There is a slight chance for weak thunderstorms over the far eastern plains this afternoon to the east of the dryline.  A deep layer of dry air will limit max 1-hour rainfall rates to 0.25 inches.  Wind gusts up to 45 mph are possible in the storm’s vicinity.  High temperatures are forecast to increase 2-5°F with temperatures nearing or exceeding records in Colorado Springs and Pueblo. Flooding is not forecast over the area.

Primetime: 1PM to 7PM

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

High pressure has set in over the west this morning, which has decreased winds, and thus fire danger, over the area. However, the area will remain very dry so caution is still needed with open flames. Cloud cover is forecast to increase over the area throughout the day when compared to the last couple of days, which should provide some shade from the afternoon sun.  There is no precipitation forecast over the area today.