FTB 07-05-2019: Severe Thunderstorms for the Eastern Plains with Heavy Rainfall Forecast

Issue Date: Friday, July, 5th, 2019
Issue Time: 10:45 AM MDT

–A MODERATE flood threat has been issued for the Palmer Ridge and portions of the Front Range, Urban Corridor and Southeast Plains
–A LOW flood threat has been issued for the Southeast Mountains, Raton Ridge, Northeast Plains and the Arkansas River at Avondale

Upper level moisture is returning to the western half of the state after a few days of dry, hot air coming in from the southwest.  The high pressure ridge to our east has begun to break down as a series of shortwaves moved across the central and high plains yesterday. A weak shortwave trough is expected to move over the area today (yellow line below), which will provide upper level forcing for severe thunderstorms over the eastern plains this afternoon and into tonight.  Weak upper level moisture will return from the Pacific (blue in WV imagery below), but dry surface conditions west of the Continental Divide will keep chances of precipitation very low over this area.  Surface moisture over the northeast corner of the state is high this morning, as indicated by fog and cloud cover.  This is due to yesterday’s storms and the passage of a cold front overnight.  There is still a strong surface moisture gradient present from Denver eastward with PW values over an inch on the eastern plains.  Moisture will stick around and increase slightly throughout the day.

The low pressure system that brought us our weather yesterday remains over the southeast corner of the state this morning, bringing in deep, moist air from the south and east.  Dewpoints were measured in the 50°Fs and 60°F east of the foothills this morning.  There is a tight temperature and moisture gradient over the Palmer Ridge (orange line) as cloud cover, fog, and advection of cooler temperatures from the north have prevented temperatures from climbing rapidly.  Surface heating and warm air advection is expected to mix out the cloud layer by early afternoon, which will allow temperatures and instability to increase.  Severe thunderstorms are expected to form this afternoon over the Urban Corridor and Palmer Ridge and move eastward over the eastern plains early this evening. Storms are also possible over the northern border that will form and move off the Cheyenne Ridge. Storms over the far CO/KS border may persist overnight with support from the passing shortwave and LLJ. This will drop some heavy 2-3 hour rainfall totals, which could cause some flooding issues. Storm motion is forecast to be 10 to 15 mph to the east and back building/trailing storms will allow for some decent  2 to 3 hour accumulations.  Therefore there is a Moderate threat issued for the southern Urban Corridor, Palmer Ridge and portions of the Southeast Plains. A Low threat is issued for the majority of eastern Colorado. The Low threat is extended back to the Southeast Mountains to include the recent burn areas (Junkins, Hayden Pass and Spring Creek). Threats for today include heavy rainfall, large hail up to 2 inches diameter, strong wind gusts and a possible tornado.

Riverine:

The NWS Flood Warning continues for Avondale as dam releases at the Pueblo Reservoir are keeping the Arkansas River running at Minor flooding stage.  Minor, low-land flooding is forecast along the river, so the Low flood threat includes this area.  The La Junta gage on the Arkansas will also peak into Action stage this afternoon. Rivers and streams in the high country are running high, but are forecast to continue to decrease today.  However, dam releases are likely to continue, which may cause swollen rivers and minor flooding in downstream areas. Light showers are possible over the Continental Divide today, but are not forecast to increase river flows. Please tune into your local NWS for the latest on Flood Advisories and Warnings, but here are a list of the current Flood Advisories (as of 6:30AM): Los Pinos River below Vallecito Dam, Blue River below Dillon, Gore Creek and the Eagle River near Gypsum, Roaring Fork River and Fry Pan River.

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, Palmer Ridge, Urban Corridor:

High levels of surface moisture, moderate diurnal heating and slow steering winds are forecast to produce heavy rainfall over the area. A passing shortwave and low level jet is forecast over the area tonight, which will enhance the chances of heavy precipitation.  Surface convergence along the dryline and Palmer Ridge will provide the forcing for afternoon and evening severe thunderstorms over eastern CO.  These storms will be capable of producing isolated max 2-3 hour rainfall totals up 2.25 inches. Hail up to 2 inch diameter is possible.  A Moderate flood threat is issued over the Palmer Ridge, southern Urban Corridor and western area of the Southeast Plains.  Additional storms capable of producing max 1-hour rainfall rates of 1.25 inches are possible over the Northeast Plains. A Low flood threat is issued over the forecast area.

Primetime: 3PM to 5AM

Front Range, Southeast Mountains, Raton Ridge:

There is a slight chance for precipitation over the Continental Divide and foothills today.  Upslope flow of moist, warm air from the eastern plains will provide a chance for weak thunderstorms mid-afternoon.  Maximum 1-hour rainfall rates of 0.25 inches are possible.  A Low flood threat is issued, which includes the Junkins, Hayden Pass and Spring Creek burn scans.

Primetime: 2PM to 11PM

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

Moderate mid-level moisture will return to the area today as the flow gains more of a westerly component.  Drier air at the surface and weak upper level forcing will keep the chances of precipitation very low today.  Fire danger has decreased from yesterday due to the increase in midlevel moisture and cooler temperatures.  Afternoon cloud cover is forecast over the area this afternoon as upslope flow takes over and combines with upper level moisture.  High temperatures over the high country are once again expected to be in the 70°Fs and 80°Fs, with highs in the 90°Fs over the Grand Valley.