FTB 08-14-2018: Flood Threat Returns to Eastern Colorado

Issue Date: Tuesday, August 14th, 2018
Issue Time: 09:30AM MDT

— A MODERATE flood threat has been issued for Southeast Mountains and portions of the Front Range, Urban Corridor, Palmer Ridge and Raton Ridge

— A LOW flood threat has been issued for the Front Range, Southeast Mountains, Central Mountains, Urban Corridor, Palmer Ridge, Raton Ridge and portions of the Northeast and Southeast Plains

Quite a lot of fog over the eastern plains this morning with some very dense fog in spots. This means the low-level moisture has returned as the upper-level low traversed eastward into Kansas over the last 24-hours. The fog should begin to breakup throughout the morning, which should increase visibility. However, smoke from wildfires will limit the usual crystal blue skies again this afternoon. Expecting similar smoke conditions to yesterday. Today, the 500mb high begins to rebuild over the desert southwest, which will return weak, northwesterly flow aloft. A vorticity max over Utah is expect to rotate around the high, which should bring a little extra energy to the state for more widespread afternoon thunderstorm coverage. To our north, a passing trough will also push a shortwave near our norther border, which will help enhance thunderstorm activity over the area even more. Overall, looking at an active day with the threat of thunderstorms returning to the Front Range and Southeast Mountains.

As the shortwave passes over western Colorado early this afternoon, it should help kick off the thunderstorm action a couple hours sooner than usual over the higher terrains. Looking at widespread showers and thunderstorms similar to yesterday with Grand Junctions Precipitable Water (PW) around 0.75 inches. I wouldn’t be surprised if some of this moisture mixes out and rain rates drop off a bit from yesterday. This means thunderstorms that form today will again be capable of producing gusty winds and dangerous lightning. Increased cloud cover and integrated smoke will also help keep the high temperatures a bit cooler.

The shortwaves arrive to eastern Colorado around peak heating this afternoon, which means stronger thunderstorms and increased coverage along the Front Range and Southeast Mountains for initiation. PW values at Denver this morning were over 1 inch and with storm motion slowly to the southeast around 15 mph, the flood threat returns. Outflow boundaries from earlier thunderstorms will likely help kick off more thunderstorm action in the moist environment, so a couple rounds of storms over one area will be possible. Storms are expected to move into the adjacent plains later this afternoon, but the far eastern plains look to remain capped. Therefore, the flood threat should be confined west of Highway 71. There is also a possibility of a couple of storms moving off the Cheyenne Ridge into northern Weld County, so the Low threat has been extended into that area.

A Low flood threat has been issued for the eastern mountains and immediate adjacent plains. Burn areas will be need to be monitored closely as there is high confidence max 1-hour rain rates will exceed 0.5 inches/hour for all larger thunderstorms. Most of the problematic burn scars are included in the smaller area of the Moderate flood threat, which should suffice. Threats today include mud flows and debris slides over the steeper terrains and flash flooding of small streams. A quick 0.5 inches in 30 minutes is possible over urban areas, and with impervious surfaces, flooding of low-lying intersections and streets is possible. Storms will end a few hours after sundown, though some showers may linger in the Southeast Mountains overnight.

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.

Flood Threat Legend

 

Zone-Specific Forecasts:

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

Peak heating and the passage of a couple upper-level shortwaves coinciding will bring thunderstorms back to the eastern mountains and adjacent plains this afternoon. High low-level moisture has returned as well and with slower steering winds aloft, the flood threat returns. Instability and minimal shear will create an environment capable of producing some severe hail (up to 1.5 inches) and damaging winds under the strongest thunderstorms. Max 1-hour rain rates up to 1 inch/hour (north) and up to 1.25 inches/hour (south) are achievable. Further south, 2 to 3 hour totals up to 1.75 inches are possible. Threats today include mud flows and debris slides over the steeper terrains and flash flooding of small streams. This is especially true near recent burn scars, so these should be monitored closely this afternoon. A quick 0.5 inches in 30 minutes is possible over urban areas and with impervious surfaces, flooding of low-lying intersections and streets is likely.

Primetime: 1PM – 12AM

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

Similar setup to yesterday with rain rates dropping off a bit. Storm activity should be relatively widespread today and kick off a couple of hours sooner than usual with the passing shortwave arriving from Utah. Valleys may see some light rain again today, but gusty winds are the more likely scenario. Small hail is also possible under the stronger storms over the San Juan Mountains. Max 1-hour rain rates up to 0.1 inches (north) and 0.3 inches (south) are possible. Flooding is not expected.

Primetime: 12PM – 9PM

FTB 08-13-2018: Widespread Storm Activity for Western Colorado

Issue Date: Monday, August 13th, 2018
Issue Time: 08:50AM MDT

— Flooding is NOT expected today

Some scattered clouds to start the week over the Northwest Slope associated with the trough passing to the north today. The visible satellite picture also shows a bit of fog along the Urban Corridor and eastern plains thanks to the upper-level low over the southeast corner of the state. The low is also generating some moderate to heavy rainfall to our east over KS, OK and TX as it pulls in abundant subtropical moisture from the Gulf of Mexico. Some of this moisture may be pulled in to the far eastern plains today, but shouldn’t make it further west than the dashed green line. Thus, some showers and thunderstorms are possible over the Southeast Plains through this afternoon and tonight. With storm motion to the S/SW at 20-25 mph, flooding is not expected.

The center of widespread, thunderstorm action today will be over western Colorado with the bulk of the action occurring over the Northern, Central and San Juan Mountains. Moisture is not expected to be ample, but some additional low-level moisture will be transported around the high into western Colorado. There’s also upper-level energy passing around the high that will bring storm chances to the lower valleys. With a large temperature and dew point spread, the main threats today will be gusty wind and dangerous lightning. Slightly higher low-level moisture is forecast over the southern San Juan Mountains, so 24-hour totals will be greatest over this area. This slightly higher moisture may also creep into far southern Southeast Mountains. So for the eastern mountains, storm chances will be greatest over this area and along the Continental Divide. Flooding is not expected today, and storms are expected to end a couple of hours after nightfall.

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:

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

Storm activity should be relatively widespread today, which includes the lower valleys. The strongest storms are still expected over the higher terrains with storm motion to the south. The highest low-level moisture is forecast over the southern San Juans, so expecting the highest 24-hour values along the CO/NM border. A few storms may wander into the San Luis Valley that form over Saguache County. Threats today include strong outflow winds, small hail and dangerous lightning. Max 1-hour rain rates up to 0.4 inches (north) and 0.75 inches (south) are possible. Flooding is not expected.

Primetime: 1PM – 11PM

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

The low pressure over the southeast corner of the state may help pull in some higher low-level moisture values over the far eastern plains. There is a slight chance for some afternoon thunderstorms along a line over convergence in this area. With quick storm motions (20-25 mph) to the S/SW, widespread flooding should be sidestepped. Guidance hints at outflow boundaries from the KS convection possibly sparking some late night storms over the Southeast Plains as well. Isolated max 1-hour rain rates up to 1 inch are possible. Over the higher terrains, storm action should be confined to and along the Continental Divide and over the Southeast Mountains near the CO/NM border. Max 1-hour rain rates up to 0.3 inches are possible with most areas receiving 0.15 inches or less. Flooding is not expected.

Primetime: 1PM – 4AM

FTB 08-12-2018: Storms Scattered across the San Juans, more Isolated Elsewhere

Issue Date: Sunday, August 12th, 2018
Issue Time: 09:15AM MDT

— A LOW flood threat has been issued for the 416 and Burro fire burn scars.

The main synoptic feature across much of the American West continues to be a sprawling ridge of high pressure stretching from California to the Dakotas. Western Colorado is situated near the center of circulation at mid-levels this morning, with a small weakness in the ridge present over southeastern portions of the state. This weakness is due to a broad upper-level low currently over the Texas/New Mexico border approaching Colorado slowly from the southeast. A couple of smaller-scale upper-level circulations are embedded in the northern fringes of this feature, as is apparent on this morning’s water vapor satellite imagery (annotated below).

Moisture will be the main limiting factor, as precipitable water at the Grand Junction and Albuquerque, NM sounding sites is at-or-below average for mid-August. However, the approaching upper-level features, while weak, are expected to generate enough extra lift to cause some scattered convection across the southwestern mountains. Rain coverage and intensity are both open questions today, but a low chance exists for rainfall rates of 0.50-0.75” per hour over the recent burn areas in the San Juan mountains. These storms may be relatively slow-moving from NE to SW, with storm motions around 15-25 mph. Therefore, if a couple of cells line up over the recent burn areas, mud flows and debris slides may become an issue. The weak upper-level flow and dry surface air will also mean that convection may produce well-defined outflow boundaries, offering opportunities for scattered storms to continue well into the evening hours. The flood risk therefore extends until midnight tonight.

Elsewhere, storms are expected to remain fairly isolated. Dry surface air will limit storm coverage and also result in an abundance of virga showers across the southern tier of the state. Gusty outflow winds will be possible in the vicinity of convection today, with or without rain actually reaching the surface. Along the western slope, dry thunderstorms may be even more pronounced, with the risk of dry lightning strikes accompanying gusty winds. The potential for new fire initiation along the western slopes should be closely monitored today.

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.

Flood Threat Legend

Zone-Specific Forecasts:

San Juan Mountains, Southwest Slope, Central Mountains, Grand Valley, and Northwest Slope:

Clear conditions to start off this morning, then clouds will gradually increase over the higher terrain into the afternoon. Storms are expected to start developing over mountainous areas by around 1-2 PM, and may linger in spots until nearly midnight tonight. Rainfall chances are best in the San Juans, where rainfall rates of 0.30-0.75” per hour are possible, and total accumulations for the day may top 1” in spots. Further northwest, appreciable rainfall is unlikely along the Northwest Slope and Grand Valley, but convection may bring gusty winds and dry lightning, posing a fire risk.

Primetime: 1PM to Midnight

Northern Mountains, Front Range, Southeast Mountains, Raton Ridge, San Luis Valley:

Sunny to start with today with isolated storms building into the afternoon. Rainfall rates should be generally light, and most passing storms will produce just a trace to a few hundredths of an inch of rain. Dry lightning may be an issue, particularly across the southern half of this area.

Primetime: 1PM to Midnight

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

Seasonable summer weather for areas east of the mountains today. Along and north of the Palmer Ridge, temperatures will be generally a few degrees above seasonal normal with high temperatures in the upper 80s °F and lower 90s °F. South of the Palmer Ridge, a little more cloud cover is possible, keeping temperatures just a few degrees cooler than average. Highs will be in the mid-80s °F to around 90°F.

FTB 08-11-2018: Isolated to Scattered Storms Across the Southwest Corner

Issue Date: Saturday, August 11th, 2018
Issue Time: 08:45AM MDT

— NO flood threat today.

Subtropical ridging continues to dominate the flow over Colorado and surrounding states as we enter the final few weeks of the summer season. While it has been a robust monsoon season across states to the south, the monsoon has yet to take a consistent hold across Colorado. This remains the case today, with the sprawling ridge across the Great Basin and Northern Rockies ushering in dry northeasterly upper-level flow across much of the state, stifling any chance at afternoon convection for many. The exception to this may be across mountainous areas of the southwestern 1/3rd or so of the state, where moisture may be just sufficient to kick off some thunderstorms capable of lightning, gusty winds, and primarily light rainfall.

As was the case yesterday, moisture is the limiting factor. Low-level moisture is near or slightly below the seasonal average (as per the Grand Junction morning sounding) but is expected to slightly increase throughout the day. Any storms that do form should be limited to higher terrain and will have high cloud bases, allowing for much of the rain to evaporate before reaching the ground. Where this evaporation occurs most rapidly, gusty winds may develop, and lightning may be a threat even if significant rain is not present in a storm. Those near active fire areas should therefore be aware of the possibility of sudden changes in wind strength and direction today, and should be prepared for the low possibility of a very isolated thunderstorm affecting recently burned areas. With the threat of substantial rain so remote today, however, no flood risk areas have been issued.

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:

Southwest Slope, Grand Valley, Central Mountains, San Juan Mountains:

Sunny to start with today before clouds begin building over the high terrain by mid-day. Isolated showers are expected to produce gusty winds and lightning at times along with generally light rainfall. Max rainfall rates may briefly exceed 0.25” per hour in very isolated cells but will generally produce just a trace to perhaps 0.15” per hour. Storms should be moving from NE to SW at around 10-20 mph.

Primetime: 2PM to 10PM

Northwest Slope, Northern Mountains, Front Range, Urban Corridor, Northeast Plains, Palmer Ridge, San Luis Valley, Southeast Mountains, Raton Ridge, and Southeast Plains:

Generally a seasonally warm day on tap. Temperatures should max out within a few degrees either side of seasonal normals (relatively cooler to the south, warmer to the north), and cloud cover will be minimal. A very low chance for a brief shower exists in the highest terrain of the Front Range and Southeast Mountains, but rainfall will top out at just a trace to a few hundredths of an inch.

Primetime: 2PM to 9PM