STP 06-27-2015: High Pressure Expanded Across Colorado, Confined Most Activity to the High Country

Issue Date: Saturday, June 27th, 2015
Issue Time: 9:00 AM MDT


With upper-level high pressure building across the western US, thunderstorm activity across Colorado was suppressed a bit over previous days. As temperatures warmed up during the afternoon, isolated-to-widely scattered showers and thunderstorms were able to develop, impacting mainly the high country. A couple of thunderstorms found themselves over the I-25 corridor south of Highway 50, producing lightning, gusty winds, and brief heavy rain.

An Areal Flood Advisory was issued for Teller and portions of Fremont County during the evening as a relatively slow moving thunderstorm drifted across the area. There were no flash flood reports in the area, however, and no other flash flooding was reported yesterday. As far as severe storm reports are concerned, the only report came in from Custer County; 1 inch hail fell in Rosita around 6 PM.

Storm Total Precip Legend

FTB 06-26-2015: Storm Coverage To Decrease As New High Pressure Builds To The West

Issue Date: 6/26/2015
Issue Time: 10:10AM

— Low flood threat for Palmer Ridge, Southeast Plains, Southeast Mountains and Raton Ridge

A new upper-level high pressure system is developing along the western North American coast and is easily seen on this morning’s water vapor imagery (see below). One the one hand, the high pressure will cause a 2-6 degrees Celsius increase in mid-level temperatures, which tends to suppress thunderstorm chances. On the other hand, this morning, as in the past two to three weeks, low-level moisture was very high with dewpoint temperatures in the 40s/50s west of the Divide and 50s/60s east of the Divide. Precipitable water values were in the 0.6 to 1.0 inch range across the state, but we expect these to lower gradually through the day. All in all, storms are expected to be confined to the higher terrain as well as the southeast portion of the state today.


We expect partly to mostly sunny skies this morning to lead to thunderstorms over the highest terrain of the Northern, Central and Southeast Mountains. Early storms will yield only light rainfall, up to 0.5 inches. As storms mature and move southeast, rainfall intensities will increase and a Low flood threat has been issued for parts of southeast Colorado. Activity may persist as late as midnight as stubborn outflow boundaries maintain convergence zones that could fuel storm activity. In the late afternoon and evening, storms may produce hail up to 1.25 inches in diameter over the Low threat area.

We have finally removed the flood threat for the Arkansas River as only very isolated stretches (notably near La Junta) remain in minor flood stage. Please check your National Weather Service products for detailed information on river stage levels.

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.

Flood Threat Legend

Zone-Specific Forecasts

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

Mostly sunny and turning warmer with high temperatures up to 100F in the lowest elevations by the Colorado River. A thunderstorm cannot be ruled out over eastern parts of the area, but rainfall will be less than 0.25 inches. No flooding is expected.

Central Mountains, Front Range, Urban Corridor, Northeast Plains:

Mostly sunny early, then turning partly cloudy with scattered thunderstorms forming over the highest terrain shortly after noon. Rainfall will be limited to about 0.7 inches, so no flooding is expected.

Palmer Ridge, Southeast Plains, Raton Ridge and Southeast Mountains:

Sunny early then partly cloudy with isolated to scattered thunderstorms developing by early afternoon over the highest terrain. Storms will move southeastward over the plains by mid-afternoon. Most storms will have light rainfall, generally under 1 inch. However, some of the strongest cells will be capable of 1.8 inch per hour rainfall. A Low flood threat has been issued for parts of the area. Hail up to 1.25 inches is also possible, mainly over the plains.

Activity will diminish after sunset, but isolated storms may last through 1AM over the Raton Ridge.

Primetime: 2PM through 9PM, except through 1AM near the New Mexico border

STP 06-26-2015: One Hail Of A Day

Issue Date: 6/26/2015
Issue Time: 9:03AM

Active weather continued across the state on Thursday, as unseasonably high low-level moisture levels produced many a thunderstorm. Although there was plenty of heavy rainfall to go around (more on that later), the theme of Thursday was: hail. Almost every single strong thunderstorm deposited hail. By far the most menacing of the storms was the cell near Keenesburg that produced multiple reports of tennis ball size, or 2.5 inch diameter, hailstones. Many broken windows were reported, though perhaps Mother Nature spared us the worst with the storm traveling over relatively sparsely populated areas.

Heavy rainfall was also observed across many parts of the state. Thankfully, most of the heavy rainfall occurred in very short spells, on the order of 15-30 minutes due to a general lack of wind shear. The highest observed 24-hour totals came in from the Northeast and Southeast Plains were Kit Carson and Prowers counties received over 2 inches. However, short-term heavy rainfall also occurred over the San Juan mountains and the Grand Valley, where many locations picked up 0.25 to 0.75 inches over a matter of 15-45 minutes.

Meanwhile, without widespread rainfall, the very high river levels continue to slowly fall across the state. This morning, only one gage officially remained in Minor flood stage: the Arkansas River at La Junta.

No official reports of flash flooding were received yesterday. For estimated rainfall in your area, be sure to check out the rainfall map below.

Storm Total Precip Legend

FTO 06-25-2015: Active Period Ahead

Issue Date: 6/25/2015
Issue Time: 1:45 PM


An active period awaits Colorado during the next 15 days, even while the western US stays entrenched under an upper-level ridge. The placement of this ridge will likely leave Colorado under northwesterly flow in the upper-levels and southwesterly flow in the mid-levels. This setup lends itself to a sufficient supply of moisture, providing two main event periods. So, without further ado…

  • Event #1: Friday (6-26-2015) through Sunday (6-28-2015)
    • Low- and mid-level moisture will be sufficient to produce diurnal rounds of showers and thunderstorms each day. Weak disturbances will likely be embedded in the NW flow aloft (originating in the purple-circled area), but the main focus will need to be placed on favorable low-level areas. More on this in the discussion below.
  • Event #2: Wednesday (7-1-2015) through Monday (7-6-2015)
    • Upper-level ridging will remain in place, but the surface will be active as a few lee surface troughs/cool fronts impact Eastern Colorado. This will act to siphon moisture northward from the Gulf of Mexico into Eastern Colorado. Additionally, a SW moisture surge will arrive between July 2nd-4th, bringing deep moisture and increasing the threat of heavy rain.


Event #1: Friday (6-26-2015) through Sunday (6-28-2015)

No Apparent Flood Threat, “Sweat the Small-Scale Stuff”

Typical of the summertime season, the upper-level ridge is in place across the western US, and diurnal rounds of showers and thunderstorms are expected during the afternoon and evening hours. Subtle disturbances will emerge in the NW flow aloft during this period, providing some additional support for developing thunderstorms. That is not where the main focus should lie – instead, the main focus should be in the low-levels. Surface upslope flow and/or lingering boundaries from previous days’ storms will be the focus for heavy rain threats. These features will be better addressed in the FTB, so be sure and stay up-to-date on the daily forecasts. Monday and Tuesday will also hold chances for diurnal showers and thunderstorms under the ridge, but moisture will be less as the next surge gathers for Event #2.


Event #2: Wednesday (7-1-2015) through Monday (7-6-2015)

Elevated Flood Threat as the Surface becomes Active and a SW Moisture Surge Arrives

The upper-level ridging is not going anywhere, but the surface will become increasingly active during this period. A few lee-side troughs/cool fronts will impact eastern Colorado, providing not only a focus for thunderstorm development, but also will siphon moisture from the Gulf of Mexico into Colorado. Precipitable water values will climb over 1 inch across the Palmer Ridge, Northeast Plains, and Southeast Plains, with various surges pushing 1 inch values into the Urban Corridor, as well. Prolonged events are not expected, as the surface features will be fairly transient, but upslope flow into the foothills will need to be monitored on a daily basis. The most active period will likely be July 2nd through July 4th, as a deep moisture surge arrives from the southwest, increasing the heavy rain threat.

Additionally, this period could be quite active in terms of severe weather for areas along and east of I-25. As details emerge over the next few days, the forecast will become clearer, so stay tuned for Monday’s FTO for an update.

As always, a reminder that this event is still coming into focus, and the precipitation map below will be subject to change as conditions warrant. At this time, this is what to expect from the information at hand.