Issue Date: Friday, May 18, 2018
Issue Time: 10:15AM MDT

— MODERATE/LOW flood threat for Front Range, Urban Corridor and Northeast Plains

— LOW flood threat for Southeast Plains and Palmer Ridge

Quite a change to the weather pattern to start this weekend. As previously discussed in the FTO, the continuous low pressure that has been spinning over Nevada will finally begin to breakdown and eject over Wyoming this afternoon. Currently, the low is located in NW Utah and marked on the water vapor imagery below. This afternoon a lee cyclone is expected to form over the Southeast Plains, which will help pull higher moisture into the state to its north. It will also help trigger thunderstorm development over the Southeast Mountains and Raton Ridge. Strong convergence occurs on the east side of the lee cyclone, so storms that track into this area will be the focus for severe weather. Threats include large hail, gusty winds and a possible tornado. West of the Continental Divide, the main focus will be increased shower potential and cloud cover over the higher elevations.

Over NE Colorado this morning, heavy cloud cover is present. Expecting cloud cover to modestly break up throughout the day, but increased lift associated with the trough and high moisture will likely keep the cloud deck somewhat intact over the Front Range. As the trough moves through this afternoon, it will help trigger widespread thunderstorms along the Front Range, Northern Mountains and Palmer Ridge. While these thunderstorms are expected to cluster into a MCS over the plains, a select few could become severe over areas with higher instability. The main threat with the stronger storms will be large hail, gusty winds and heavy rainfall. Rainfall is expected to continue overnight in the northern Urban Corridor and Northeast Plains. Over the higher elevations in the north, heavy snowfall is possible. For more details, please see the regional forecasts below.

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:

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

Scattered showers and thunderstorms are expected today and tonight. A few storms may become severe further east. Over the Northeast Plains, localized totals up to 3.25 inches are possible by tomorrow morning. Flood threats over the Northeast Plains and Urban Corridor include small stream flooding, street flooding and field ponding. Over the northern high terrain small stream flooding and debris slides are possible. Burn scars in the area should be monitored closely this afternoon. A Moderate flood threat has been issued.

Maximum rain rates:
Northeast Plains: 1.75 inches/hour
Urban Corridor, Palmer Ridge, Raton Ridge and Southeast Plains: 1.25 inches/hour
Front Range and Southeast Mountains: 0.85 inches/hour

Primetime: 12PM to 11 AM

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

More cloud cover and scattered showers over the higher terrains are expected today. Moisture for showers, possibly accompanied by a role of thunder, is best over the Northern and Central Mountains. However, gusty winds are more likely than measurable rainfall. Max 1-hour rain rates up to 0.1 inches. Flooding is not expected today.

Primetime: 1PM to 9PM

Issue Date: 5/17/2018
Issue Time: 8:35 AM

A LOW FLOOD THREAT IS FORECAST.

Two separate weather regimes will split the state today, as denoted in the image below. Southwesterly flow aloft has overspread much of the state, and this will stoke the development of a surface low pressure trough across the eastern plains. This low pressure trough will act as a dryline, with dry, westerly winds to the west, and moist, southeasterly flow to the east. This moisture axis will essentially split the state into two forecasts.

For areas along/east of the moisture axis, the influx of moisture will allow for significant instability to develop this afternoon. Coupled with favorable shear and surface convergence, isolated-to-scattered strong/severe thunderstorms are expected this afternoon/evening. The main threats will be large hail (up to 1.75” in diameter), strong winds (60+ mph), and heavy rainfall; an isolated tornado is also possible. There is a low flood threat issued due to the potential for training storms to develop during the evening and overnight hours, but the potential for this scenario (25%) is low enough to not warrant more than a low flood threat. More likely, this activity will occur just outside the state, over the plains of Nebraska. More details on rain rates and timing can be found in the zone-specific forecast discussions below.

For areas to the west of the moisture axis, dry and warm conditions will be the name of the game today, with gusty winds (thanks to a tightening pressure gradient) leading to critical fire danger, especially across the western slope and some areas along/adjacent to the southern Front Range and Southeast Mountains. Please check with your local National Weather Service Office for more details on any Red Flag Warnings.

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, jump below the map.

Zone-Specific Forecasts

Urban Corridor, Palmer Ridge, and Raton Ridge:

Downsloping winds will dominate the day with warm and dry conditions. High temperatures will push into the 80s and near 90 in some locations. Far eastern portions of the Palmer Ridge will attempt to get in on the early stages of thunderstorm development, so an isolated thunderstorm cannot be ruled out there. Maximum rain rates will be fairly low, around 0.25-0.5 inches/hour, as storms won’t mature until being east of the area.

Timing: 2 PM – 8 PM

Northeast Plains and Southeast Plains:

Isolated-to-scattered strong thunderstorms are expected this afternoon and evening, with a few becoming severe. Large hail, strong winds, lightning, and periods of heavy rainfall are the main threats, and an isolated tornado cannot be ruled out. Fairly brisk storm motions to the east will keep most rainfall below flash flood thresholds, but the possibility for training thunderstorms warrants the issuance of a low flood threat for portions of the Northeast Plains. Due to strong surface convergence along the dryline, an isolated thunderstorm or two will continue into the early morning hours tomorrow, mainly near the CO/KS/NE borders.

General Rain Rate: 0.5-1.0 inches/hour
Maximum rain rate: 1.25-1.5 inches/hour

Timing: 2 PM – 11 PM, with an isolated thunderstorm or two ongoing into the early morning hours near the CO/KS/NE borders

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

Warm and dry, with gusty winds leading to critical fire danger for lower elevations, mainly across the western slope. A few cumulus clouds will bubble during the afternoon over the higher terrain, but nothing will come from them. Be sure to check with your local NWS office for any information regarding Red Flag Warnings.

Issue Date: 5/16/2018
Issue Time: 9:45 AM

NO FLOOD THREAT IS FORECAST TODAY.

A sharp, upper-level ridge has taken residence over the state today. Along with warm, dry air aloft, this will cap off the atmosphere over Colorado, resulting in a warm and dry day statewide. During the afternoon/evening, a few fair weather cumulus will bubble over the higher terrain as low-level moisture remains plentiful, but aside from a few streaks of virga, it will not be able to muster any precipitation. The only possible exception to the dry rule will be across the eastern plains near the CO/KS border. An axis of instability will set up there as an influx of moisture arrives from the east/southeast, but even so, strong capping aloft and a lack of deep moisture will fight it every step of the way. The chance of isolated thunderstorms development is 20%. For more information regarding rain rates and timing, please see the zone-specific forecast discussions below.

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, jump below the map.

Zone-Specific Forecasts

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

Warm and dry, with plenty of sunshine to go around. A few streaks of virga will paint the afternoon/evening sky as a few cumulus clouds develop near the higher terrain. Near the CO/KS border, an axis of instability will provide a small chance (20%) that an isolated thunderstorm or two can break through the otherwise capped atmosphere aloft. Any storms will quickly move east, and a lack of deep moisture will keep rain rates from reaching flash flood thresholds. Maximum rain rates will be 0.5-1.0 inches/hour.

Timing: 3 PM – 8 PM

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

Scattered cumulus clouds over the higher terrain during peak heating this afternoon/evening will be all the atmosphere can muster on an otherwise warm and dry day. Winds will increase this afternoon as the pressure gradient tightens, so be careful with anything that may cause a spark and result in a fire.

Issue Date: Tuesday May 15, 2018
Issue Time: 10:15AM MDT

— LOW flood threat for Front Range, Urban Corridor, Northeast Plains, Palmer Ridge, Southeast Plains, Raton Ridge

The low pressure that has been sitting over Nevada this past week finally begins to break down throughout the day. This will release extra energy to help support hail producing thunderstorms again today. A moisture axis is marked on the water vapor imagery below. This shows the threat for rainfall will again be confined to the eastern portion of the state, especially along the Palmer Ridge and southern Urban Corridor due to convergence from moist southerly/southeasterly flow. Also marked are the high clouds over the Southeast Plains from the rainfall overnight. These clouds will limit upslope convection along the Southeast Mountains and Raton Ridge until stronger heating occurs this afternoon. It will also keep the heavy rainfall south over Las Animas and Baca Counties.

As mentioned, the threat for copious amounts of nickel to quarter-sized hail continues today with the thunderstorm that form along a line of convergence in the Urban Corridor and Palmer Ridge. This is due to freezing levels being just above 6,000 feet. Larger hail may occur as the storms move into the eastern plains, but will not be as large as yesterday (baseballs) due to limited shear. Also important to the forecast today are slower steering winds. Together these factors increase the flooding and heavy rainfall potential, so a Low flood threat has been issued for the Front Range, Urban Corridor, Palmer Ridge and Northeast Plains. Threats include small stream flooding, road flooding and field ponding.

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:

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

The strongest storms are expected to form in the early afternoon along the southern Urban Corridor and Palmer Ridge and grow into a large MCS as they move east into the plains. Later in the day, storms are also expected to form along the Southeast Mountains and Raton Ridge. A few storms might also sneak into the Northeast Plains from the Cheyenne Ridge, so the Low threat has been extended into this area. Max 1-hour rain rates to the north are 1.25 inches/hour. South and along the Palmer Ridge max 1-hour rain rates up to 1.75 inches are possible. A Low threat has been issued with small stream flooding, road flooding and field ponding likely.

Primetime: 12PM to Midnight

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

Best chance for showers today along the Central Mountains, but dry air from the southwest flow will limit rainfall totals to trace amounts. Temperatures will be a few degrees warmer than the past couple of days. Some cumulus are likely over the higher terrains and Northwest Slope. Flooding is not expected today.

Primetime: 1PM to 9PM