FTB 07-20-2017: Heavy Precipitation Accompanies High Moisture

Issue Date: Thursday, July 20, 2017
Issue Time: 10:45AM MDT


MODERATE flood threat for portions of San Juan Mountains, Southwest Slope

LOW flood threat for portions of Northwest Slope, Northern Mountains, Grand Valley, Central Mountains, San Juan Mountains, Southwest Slope, Front Range, Urban Corridor, Palmer Ridge, Northeast Plains


The upper level ridge begins to move to the east a bit today and settles over eastern Montana and Saskatchewan. Southerly surface winds over Colorado will draw some drier air into the southeastern portion of the state. Over the Northeast Plains, easterly winds will set up with the heat low and help pull in moisture from NE and KS. Precipitable Water (PW) is expected to rise to as high as 1.3 inches. The axis of moisture is marked in the water vapor image below. On the western portion of the state, PW is still over 1 inch in some places. A plume of moisture can be seen over the SW portion of the state in the water vapor imagery. PW isn’t forecasted to change much by this afternoon, so we can expected more widespread showers again today with local, heavy rainfall.

Storms will begin to fire in the mountains today around noon. The best access to moisture and lift will be over the Southwest Slope and San Juan Mountains. 3-hour storm totals could be as high as 1.6 inches with 1-hr rain rates up to 1.1 inches. Other parts of the high country will have 2-hour totals of 1.5 inches. Threats today include flash flooding, small stream flooding and road flooding, debris flows and mud slides. Burn scars should be monitored closely as a handful of storms in the Low flood threat will produce rain rates greater than 0.5 inches, which could lead to debris flows and mud slides on steeper terrain. The National Weather Service in Grand Junction has issued a Flash Flood Watch from noon until midnight for the southern and western portion of their forecast area.

The high elevation storms will drift into the southern half of the Urban Corridor later this afternoon. An isolated storms may also fire over the Palmer Divide.  Under the ridge, slow moving storms will drop local, heavy rain. 1-hour rain rates up to 1.1 are possible that could cause urban and small stream flooding and street flooding. Gusty winds and small hail are expected with storms. Outflow boundaries will likely provide lift for a couple isolated thunderstorms in the Urban Corridor and Northeast Plains. 1-hour max rain rates up to 1.8 inches are possible with the high moisture and 2-hour totals will be near 3 inches. These storms could become severe with hail up to 1 inch in diameter and wind gusts up to 60 mph. Field ponding and street and small stream flooding are expected. Storms over the mountains will begin to dissipate in coverage around 9PM and most all showers should cease by midnight.

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:

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

Sunny with highs near normal. Partly cloudy skies are likely with storms beginning to fire over the higher terrain around noon. High moisture and slow moving storms will warrant a Moderate threat today with 2-hr totals up to 1.6 inches. 1-hr max rain rates of 1.1 inch are possible over the San Juan Mountains and Southwest Slope. A Flash Flood is in effect until midnight tonight. Threats include flash flooding of small streams and road, debris flows and mud slides.

Primetime: 12PM to 11PM

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains:

Sunny this morning and very hot over the Northeast Plains. Storms will begin firing over the higher terrain around noon and an isolated storm or two will move into the urban corridor by late afternoon. Max 1-hr rain rates are expected to be 1.1 inches with 3-hr totals near 3 inches. Isolated thunderstorms over the Northeast Plains are likely with initiation from an outflow boundary. Expect heavy local rain, gusty winds, 1-inch hail, field ponding and urban and small stream flooding.

Primetime: 12PM to 10PM

Southeast Mountains, Raton Ridge, San Luis Valley, Southeast Plains:

Sunny and hot for the Southeast Plains today. Drier air should limit coverage of storms and heavy rainfall potential in the higher terrain. Storms that form over the higher terrain will have max 1-hour precipitation rates of 0.5 inches. Temperatures will be in the upper 90s for the Southeast Plains. There is no flood threat.