FTO 07-31-2017: Drying Short-Lived As Moisture To Find Its Way Back Into Colorado

Issue Date: Monday, July 31, 2017
Issue Time: 1:30PM MDT
Valid Dates: 8/1 – 8/15

After quiet a memorable stretch of active weather, noticeable drying has begun across roughly the western half of our state. Before getting to today’s Outlook, we want to reflect on just how impressive the recent rainfall has been. Below are 30-day precipitation estimates from the NOAA River Forecast Centers. Isolated parts of southeast Colorado have received over 10 inches (even as high as 15 inches) of rainfall since July 1st. Even more impressively is that this has occurred through a prolonged stretch of days with very heavy rainfall, as opposed to one or two events. So how does this stack up with climatology? Average July precipitation depends strongly on elevation, but east of the Continental Divide, roughly ranges from 1.5 inches in the lowest elevations to as much as 4 inches in the foothills of the Palmer Ridge and Wet Mountains. Thus, over the last 30 days, isolated locations circled in the left map below, have experienced anywhere from 3 to 5 times their normal rainfall. Looking at NOAA Atlas 14 guidance, this equates to a 1 in 100-500 year event, depending on your exact location. For example, in eastern El Paso County, Atlas 14 estimates that a 30-day rainfall of 10.3 inches is a 1 in a 100 year event, with the 1 in the 1000 year event estimate being 13.7 inches.

Shifting the focus back to the Outlook, this afternoon’s water vapor image, below, shows some notable changes in the weather pattern across western North America. The upper-level ridge located south of Colorado, which has been responsible for moisture advection into the state, has expanded westward. The upper-level flow is currently switching to a more westerly component, which is now advecting much drier air from the west (see dryness over California). In addition, a strong surface high pressure is expected to move southward out of Canada over the next 72 hours. A cool (or even “cold”?!) frontal passage is expected across eastern Colorado that will limit high temperatures to as much as 15F below normal by mid-week. Frontal dynamics will likely support light to moderate precipitation that we label Event #1, but with limited instability, we currently do not expect a flood threat. Thereafter, a dynamic pattern will ensue with an upper-level trough expected to establish across the central United States. Frequent shortwave and surface frontal passages are expected across eastern Colorado, which should be accompanied by at least 24 hours of return flow to elevate moisture levels. We expect a multi-day elevated flood threat (Event #2) mainly across northeast Colorado. However, with fast steering winds, only isolated heavy rainfall (though with possible severe weather) is currently expected.

The forecasted Precipitable Water plumes from the GFS Ensembles, below, show that Denver will continue to experience average to above average moisture levels. Meanwhile, Grand Junction will remain mainly below normal. Overall, we expect a pause in the monsoon, and an increase in “action” for northeast Colorado.

We have identified two precipitation events for this Outlook, which are described in more detail below.

Event #1: Tuesday (8/1) through Thursday (8/3)

No Apparent Flood Threat though light to moderate precipitation will be possible

Elevated moisture levels will continue to be scoured out of Colorado as upper-level westerly flow advects in drier air. On Tuesday and Wednesday, scattered thunderstorms will be possible mainly across southern Colorado. However, with faster steering winds, total precipitation is expected to stay on the lighter side, in the 0.5 – 0.75 inch range (hence, no precipitation map). On Thursday, a strong, fall-like cool front will move southward across eastern Colorado, stabilizing the air mass and putting a cap on heavy rainfall coverage and intensity. Some upslope rain showers and perhaps a weak thunderstorm are expected, but flooding is not expected at this time.

Event #2: Friday (8/4) through Monday (8/7)

Elevated Flood Threat for isolated heavy rainfall with current guidance suggesting northeast Colorado as target

Summer-time cool fronts are fleeting and return southeasterly flow will quickly set up by Friday. Precipitable water is expected to return back into the 0.9 – 1.1 inch range (which is above normal). Isolated thunderstorms will return to our forecast then, and increase in coverage and intensity into the weekend as one or more additional weak frontal passages is possible. These fronts will act more of focal points for storms, with little temperature changes. The northeast quadrant of the state is expected to see the highest coverage of thunderstorm activity. At this time, we foresee max 1-hour rain rates up to 1.2 inches (west) and 1.6 inches (east). In addition, severe weather chances, are expected to increase as bulk wind shear ramps up to above 30 kts. We have labeled this event with an elevated flood threat mainly for isolated flash flooding. Stay tuned to the next Outlook for a better assessment of this event.

Legend

SPM 07-31-2017: Repeat of Widespread Rainfall

Issue Date: Monday, July 31, 2017
Issue Time: 10:20AM MDT

Summary:

Ongoing convection continued yesterday across the mountains and eastern plains. The San Juan Mountains had yet another consecutive day of rainfall and even Alamosa had some rain (0.42 inches). This is second rainiest July for Alamosa on record. Over the Front Range and Southeast Mountains, storms favored the areas that received more diurnal heating and had extra lift from elevated terrain. In the early evening, storms over the panhandle of Nebraska moved through the Northeast Plains and eventually merged along the CO, NE, KS border. Showers over the mountains quickly subsided near sunset and the threat over the plains ended by about midnight, although some showers lingered through about 2AM.

Over the San Juan Mountains, CoCoRaHS stations recorded around 0.5 inches. Radar estimates for this area were 1.1 inches. Over the Southeast Mountains, a CoCoRaHS station in Huerfano County (Walsenburg) recorded 2.33 inches. 2.28 inches of that total fell in 45 minutes. A bit further north, a gage in Fremont County recorded 0.75 inches in 20 minutes over Canon City. A gage in Manitou Springs also recorded 0.75 inches in 20 minutes. This same gage in Manitou Springs reported a storm total 1.3 inches (all of which fell in about 1 hour). Fountain Creek was running near action level again, with 1.96 inches reported falling at Fort Carson, but no major flooding of the river was reported. The river is expected to recover throughout the day. Storm totals in this area were estimated as high as 4 inches. There was another storm over Pueblo that dropped 2.2 inches of rain and there were several reports of flooded roads in town.

Over the plains, the larger swaths of heavy rainfall had 3-6 hour rainfall totals up to 3 inches. A CoCoRaHS station in Baca County recorded 2.15 inches. Over Yuma County, estimated totals were as high as 2.5 inches. Ongoing monitoring of the Arkansas River Basin will continue due to the heavy rainfall of the last couple of days. At this time, there is sufficient confidence that flood stage will not be reached.

To see how much rain fell across your area in the last 24-hours, check out the State Precipitation Map below.

Storm Total Precip Legend

FTB 07-31-2017: Drying Causes Downtick in Heavy Rainfall

Issue Date: Monday, July 31th, 2017
Issue Time: 09:40AM MDT

— LOW flood threat for portions of Central Mountains, Front Range, Urban Corridor, Southeast Mountains, Raton Ridge

An elongated High pressure system is oriented from northern California to northern Texas today that will turn the mid-levels to a more northerly flow. This northerly flow will help pull in the drier air located over WY and NE that will lower Precipitable Water (PW) values across the northern portion of the state. PW over 1 inch is still forecasted for the areas east and south of the dotted green line. The soundings over Colorado this morning are both about 0.85 inches and have drying in the mid-levels. The high dew points over the eastern plains are expected to decrease some throughout the day, and gusty winds are expected with storms that form. There is a Low threat for flooding today, and the threat will be much more isolated than the prior couple days.

As storms begin to initiate over the higher terrain with the diurnal heating pattern just after noon, they are expected to favor the Central, San Juan and Southeast Mountains. PW over the SW corner of the state should be high enough to see 3-hour totals up to 1.5 inches over the San Juan Mountains. Over the eastern mountains, 1-hour rain rates up to 1 inch are possible. Over the immediate adjacent plains along the Palmer Ridge, 1-hour rain rates up to 1.1 inches and 3-hour totals up to 2 inches are possible. Heavy rainfall over the last couple days along the southern Front Range and Southeast Mountains has saturated soils. With 3-hour totals up to 1.25 inches, a Low threat has been posted and burn scars should be monitored closely. Threats include gusty winds, small stream flooding and debris slides. Storm motion will be more southerly today, so there will not be widespread rain over the eastern plains like the previous two days. Showers should come to an end soon after sunset this evening.

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:

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

Sunny today with isolated showers over the mountains this afternoon. 1-hour rain rates up to 1 inch are possible with 1.1 inches over the Palmer Ridge area just adjacent to the mountains. 3-hour rates up to 1.25 inches over saturated soils warrant a Low flood threat. Threats include gusty winds, debris slides, small stream flooding. The threat should diminish by midnight.

Primetime: 1PM to 10PM

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

Sunny today with storms confined to the Southwest Slope and San Juan Mountains. 3-hour rainfall amounts up to 1.5 inches are possible.

Primetime: 1PM to 10PM

 

FTB 07-30-2017: Heavy PM Rainfall Expected Again

Issue Date: Sunday, July 30th, 2017
Issue Time: 10:30AM MDT

— MODERATE/LOW flood threat for portions of the Front Range, Urban Corridor, Palmer Ridge, Central Mountains, Southeast Mountains, Raton Ridge, Southeast Plains

—  LOW flood threat for Southwest Slope, San Juan Mountains, Northeast Plains

The monsoon moisture plume sticks around for another round of thunderstorms today. The Denver sounding this morning had 1.13 inches of Precipitable Water (PW), while the Grand Junction sounding had 0.93 inches with more drying in the mid-levels. Dew points over the eastern plains are as high as 65F and are also still high over the southwestern corner of the state. The high PW is marked in the water vapor image below. The mid-levels may dry out a bit over the course of the day with winds from the north, but not enough to limit heavy rainfall again this afternoon. Cloud cover continues over the southern and central mountains as well as the southern Urban Corridor, Palmer Ridge and Southeast Plains. The cloud cover will help limit temperatures again today, but has a much better chance of at least partially burning off by the afternoon allowing some instability to build unlike yesterday. With the ridge off to our north over Canada, light westerly flow will be present at the upper-levels.

Expect storms to start firing off the higher terrain just after noon to the north and a bit sooner over the south. Storms will begin to slowly move off the mountains onto the adjacent plains in the late afternoon likely favoring the higher terrain of the Palmer Divide and Raton Ridge. Storm motion will be 10-15 knots moving E/SE. Rain rates over the mountains will be as high as 1.3 inches in 1-hour with 3 to 6-hour totals up to 2 inches. There is a chance storms forming over the Cheyenne Ridge may move over the border into the Northeast Plains.

Over the San Juan Mountains, storm totals around 2 inches will warrant a Low flood threat with the antecedent rainfall. Heavy rainfall again today will create dangerous conditions over already saturated soils and burn scars. Threats include mud flows, debris slides, small stream and road flooding, so burn scars need to be watched carefully again today. Over the adjacent plains, expect storms to have max 1-hour rain rates up to 2.25 inches with 3-6 hour totals up to 4 inches. Threats include arroyo and street flooding as well as field ponding. Storms over the mountains should end a couple hours after sundown, while storms over the SE corner of the state will continue until 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:

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

Partly cloudy early with sunshine breaking through the clouds midday. Scattered to numerous showers and thunderstorms developing by early afternoon over the higher terrain. Max 1-hr rainfall up to 2.25 inches (adjacent plains) and 1.3 inches (mountains). Local 3-hour rainfall totals up to 4 inches over the plains. Threats include mud flows, debris slides, field ponding and arroyo and road flooding. The threat should diminish by midnight.

Primetime: 12PM to 12AM

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

Partly cloudy this morning over the southern high terrain becoming more sunny over the Central Mountains. Heavier rainfall is expected over the San Juan Mountains with max 1-hr rainfall up to 1.1 inch. 3-6 hour rainfall amounts up to 2.2 inches are possible. A Low flood threat has been posted with threats including isolated mud flows and debris slides over saturated soils.

Primetime: 11:30AM to 10PM