FTB 05-30-2020: Heavy Rainfall Forecast for the Mountains & Urban Corridor

Issue Date: Saturday, May 30th, 2020
Issue Time: 10AM MDT

— A MODERATE flood threat has been issued for the Front Range, Urban Corridor, Palmer Ridge, Northeast Plains and 416 burn area

— A LOW flood threat has been issued for the Northern Mountains, Central Mountains, San Juan Mountains, Southeast Mountains, portions of the Northwest Slope. This includes the Spring Creek, Deckers, and Lake Christine burn areas.

Same general pattern as yesterday with a few minor differences, which will be pointed out in the water vapor imagery below. There looks to still be plenty of moisture trapped underneath the ridging pattern, which is defined by the yellow line. PW in the soundings this morning was measured at 0.80 inches and 0.75 inches at Grand Junction and Denver, respectively, which means there is actually a slight uptick since yesterday. In the image, you can also see a surface low over the eastern plains as indicated by the blue arrows (surface flow). This will help reinforce and slightly increase surface moisture on its eastern and northern side (northeast Colorado). Dew points are forecast in the 50Fs, which is pretty juicy for Colorado.

Today, the 500mb low off the coast of California will start to move northward up the coast. While this feature is well to our west, there will likely be some enhanced lift over the state out in front of it, which will help storms that fire over the mountains this afternoon be more widespread than yesterday. Additionally, as the low starts to move towards the Pacific Northwest, the axis of the ridge will start to shift eastward. This will place the state under southwest flow loft today. This should allow storms that form (from the southwest to northeast over the mountains) to push into the adjacent plains this afternoon favoring the Palmer Ridge and Urban Corridor for accumulations. With storm motion slower than yesterday, paired with increasing moisture and trailing/pulsing storms, the flood threat increases.

A Low flood threat for the majority of the mountain areas, which includes the aforementioned burn areas above. A Moderate flood threat has been issued for the Front Range, Urban Corridor and Palmer Ridge as rain rates will increase in the moisture rich environment along a tongue of convergence and instability. Isolated 24-hour totals over the San Juan Mountains will likely reach 1-inch with the multiple rounds of rainfall through tomorrow morning, so for caution, a Moderate flood threat has been issued for the 416 burn area. Threats over all burn areas include flash flooding, mud flows and debris slides.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

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

Nearly stationary storms this afternoon will increase the chances for flooding over the Front Range. Other threats include severe hail and strong outflow gusts. Max 1-hour rain rates up to 1 inch will be possible with isolated storm totals up just over 1.50 inches by morning. This could cause local flooding issues such as mud flows, debris slides, field ponding and local stream flash flooding.

As storms move off the mountains, moisture increases, but the hail threat likely decreases somewhat. However, small hail will still be possible. With nearly stationary storms, very heavy, local rainfall is forecast. Guidance is indicating that the greatest threat for flooding will be from 3PM to 8PM. Max 1-hour rain rates up to 1.50 inches will be possible under the storm cores with isolated storm totals up to 1.90 inches possible. This would cause street flooding, field ponding and local stream/gulch flash flooding. Storms are expected to start dissipating around midnight.

Primetime: 12PM to 2AM

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

Although there was an inverted-V sounding west of the Divide (indicating strong winds with storms again), the lower atmosphere and multiple rounds of rainfall will increase totals for these regions as the boundary layer moistens. Isolated max 1-hour rain rates up to 0.60 inches will be possible with 24-hour totals up to 1-inch over the San Juan Mountains along with gusty outflow winds. Thus, a Low flood threat has been issued for the Deckers, Spring Creek, and Lake Christine burn areas. A Moderate Threat has been issued for the 416 burn area.

With 1-2 hour rain rates around 0.75 inches, a Low flood threat has been issued for other mountainous regions. Lower elevations over western Colorado and the San Luis Valley will likely get in on the rain action as well with instability rotating around the ridge. Totals between 0.15 and 0.25 inches are the most likely with strong outflow winds (40-50 mph).

Primetime: 12PM to 6AM

Raton Ridge & Southeast Plains:

Due to storm motions, not thinking there will be much rainfall threat over these areas today with the eastern Raton Ridge receiving the majority of the rainfall. Max 1-hour rain rates up to 0.50 inches will be possible. A couple overnight storms may be possible over the Southeast Plains, but lower moisture will reduce spatial coverage and totals. Isolated totals just under 0.50 inches by morning seem reasonable.

Primetime: 12:30PM to 3AM

FTB 05-29-2020: Warmer With Numerous High Country Showers And Storms

Issue Date: Friday, May 29th, 2020
Issue Time: 10AM MDT

— A LOW flood threat has been issued for the Spring Creek burn scar in Huerfano and Castillo counties
— A LOW flood threat has been issued for the Decker burn scar in Fremont, Saguache and Chaffee counties

The water vapor imagery on this Friday morning, shown below, could make one think that we are in the middle of monsoon season. A very strong ridge of high pressure is situated over the Four Corners region, being reinforced by a digging trough over the eastern Pacific Ocean. This early in the warm season, such a trough would typically be associated with very hot and mostly dry weather as moisture is typically lacking. However, such is not the case today as plenty of moisture remains “trapped” under the ridge. Precipitable water (PW) was estimated at 0.67 inches at Grand Junction this morning and is in the 0.50 – 0.85 inch range across the state (the Denver sounding appears to have sampling issues in the lower levels so the PW was suspect). The average PW at Grand Junction for late May is 0.45 inches and today’s value is roughly in the 90th percentile. It is important to note that most of this moisture is actually above the boundary layer, which will limit its effectiveness in contributing to heavy rainfall today.

From the perspective of dynamics, there does not appear to much happening today. A weak disturbance was noted over central Utah, but due to its north-northeast trajectory, it is unlikely to influence our weather today. However, sometimes one does not need dynamics when you have topography extending to 14,000 feet! With hardly a cloud in the sky this morning, expect a quick warm-up with scattered to numerous showers and storms developing shortly after noon. The highest coverage will be in the higher terrain of the San Juans, Central Mountains, Front Range, Palmer Ridge and Raton Ridge. Wind shear profiles suggest southeast storm motions in the 20-30 mph range. Individual storms will produce short-term heavy rainfall approaching, but not exceeding flood threat levels. Small hail and gusty downdraft winds will likely accompany the strongest storms today.

The main concern today are the two fresh burn scars in the Sangre de Cristo mountains: Decker to the north and Spring Creek to the south. With storm motion paralleling the mountain range axis, there is the good chance of multiple bursts of heavy rain over a period of a few hours. High-resolution rainfall accumulation guidance suggests better coverage than yesterday, as shown below. Thus, a Low flood threat has been issued for these two scars for the afternoon and evening hours. Remember, it does not take much rain to cause dangerous debris slides and mud flows over burn scars so please pay attention to the Pueblo National Weather Service updates later today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Urban Corridor, Front Range, Southeast Plains, Palmer Ridge, Raton Ridge, Southeast Mountains, Southeast Plains, San Juan Mountains, Central Mountains, San Luis Valley:

Sunny early then very warm with increasing cloudiness and scattered to widespread showers/thunderstorms developing by early afternoon. The highest coverage will be over the higher terrain of central and southern Colorado. Max 1-hour rainfall up to 0.8 inches, with max 3-hour rainfall up to 1.2 inches possible especially over the Palmer Ridge and Raton Ridge. Small hail and gusty winds are likely with the strongest storms. Primetime is noon through 9PM.

A Low flood threat has been issued for the Decker and Spring Creek burn areas from 1PM through 9PM today. Max 30-minute rainfall up to 0.5 inches, 1-hour rainfall up to 0.6 inches and 3-hour rainfall up to 0.9 inches could cause isolated flash flooding, debris slides and mud flows. Stay tuned to warnings from the Pueblo National Weather Service office.

Northern Mountains, Northwest Slope, Grand Valley:

Sunny early then partly cloudy and very warm is isolated to scattered showers and thunderstorms developing. Max 1-hour rainfall up to 0.3 inches. Flooding is not expected today. Gusty winds are possible with the strongest storms. Primetime for rainfall is noon through 8PM.

Northeast Plains:

Sunny early then much warmer with an isolated shower or storm possible especially over western areas. Max 1-hour rainfall up to 0.3 inches. Gusty winds are possible with the strongest storms. Flooding is not expected today. Primetime for rainfall is 2PM through 8PM.

FTB 05-28-2020: Scattered Storms Return to the Mountains

Issue Date: Thursday, May 28th, 2020
Issue Time: 9:15AM MDT

— Flooding is NOT expected today

Similar set up to yesterday with northerly flow aloft over the state as seen in the 500mb RAP analysis below. The main difference is that there is no distinct shortwave that will move through the flow to help enhance lift and increase moisture. Still expecting storms to pop over the mountains with upslope flow and residual moisture this afternoon. The northerly storm motion should keep storms mostly over the mountains, although they may move into the adjacent eastern plains favoring the higher elevations of the Palmer and Raton Ridges for accumulations later this afternoon and evening. With PW measured at 0.63 inches in Grand Junction and 0.74 inches in Denver, storms will have better coverage east of the Continental Divide and favor the Front Range, Southeast Mountains and eastern San Juans for development. Although dew points in the mid-40Fs will produce high-based storms, slow steering winds under the ridge will allow totals to increase more than they typically would. However, rain rates will not reach flood threat criteria.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

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

Afternoon upslope flow will increase storm activity over the mountains by early afternoon. Storms are anticipated favor areas along and east of the Continental Divide for accumulations. Max 1-hour rain rates up to 0.50 inches will be possible over the mountains with gusty outflow winds and small hail being the main threats under the stronger storms. As storms spill into the adjacent plains, totals over a 1 to 2-hour period could increase to 0.75 inches over the Palmer Ridge and just east of the Denver Metro with small hail possible under the stronger storms. With spotty coverage and lower rain rates, flooding is not forecast. Some late-night rainfall may be possible over the adjacent eastern plains tonight as well.

Primetime: 1PM to 1:30AM

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

Bit drier than yesterday over these regions, which means a downtick in afternoon storm activity. Scattered storms are still anticipated to develop over the mountains, and the storms will favor areas along and near the Divide. Expecting some gusty outflow winds, as seen yesterday with totals up to 0.15 inches possible. With southerly storm motion, the San Luis Valley may get some more light rainfall. Totals are not expected to exceed 0.10 inches. Storm activity should decrease around sundown.

Primetime: 1PM to 8PM

 

FTB 05-27-2020: Scattered Storms Return to the Mountains with the Passage of Another Shortwave

Issue Date: Wednesday, May 27th, 2020
Issue Time: 9AM MDT

— A LOW flood threat has been issued for the Deckers and Spring Creek burn areas

Below is a lower-level water vapor image with a 500mb pressure and wind analysis overlaid on top. You can still see the main low still to our southeast, and a distinct shortwave moving through the flow that is currently located over the Great Basin (dashed orange line).  North and northwest flow over Colorado will send this moderately strong shortwave across the state today, which will help produce scattered afternoon storms over the mountains and Northwest Slope. With shallow surface moisture and a lot of dry air aloft, storms for the most part are expected to be high-based, produce gusty winds and possibly small hail. Only moderate rainfall is forecast to accompany the strongest storms, but trailing storms may help increase totals over certain areas in a 2 to 3 hour period. Thus, a Low flood has been issued for the Deckers and Spring Creek burn areas, which have lower thresholds for flash flooding criteria. Storms will spill into the adjacent plains with the northwest steering winds, and the best chance for rainfall at the lower elevations will be over the Palmer Ridge and south.

Today is the annual sediment flushing exercise at Cherry Creek Reservoir, so please use caution along the trails. More information can be found here: Annual Sediment Flush – Cherry Creek

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northwest Slope, Northern Mountains, Front Range, Central Mountains, & Southeast Mountains:

Scattered afternoon showers and thunderstorms return to these forecast regions this afternoon. Over western Colorado, storms will favor the higher terrains and areas near the Continental Divide with 1-hour rain rates up to 0.40 inches possible. With coverage more spotty, trailing storms are less likely, but a few areas could receive just over a half inch by late Wednesday evening.

Coverage is expected to increase over the southern Front Range and Southeast Mountains due to slightly higher moisture values. Max 1-hour rain rates up to 0.60 inches will be possible with 2-3 hour totals up to 0.75 inches possible over isolated areas. Thus, a Low flood threat has been issued for the Deckers and Spring Creek burn areas as chances for trailing storms increase over these areas. Storms are expected to start dissipating over the Southeast Mountains and adjacent plains by midnight.

Primetime: 1PM to 1AM

Urban Corridor, Palmer Ridge, Northeast Plains, Raton Ridge, San Luis Valley, & Southeast Plains:

Activity over the mountains will likely spill over into the immediate adjacent plains later this afternoon favoring the Palmer Ridge and south for accumulations. Max 1-hour rain rates up to 0.60 inches will be possible with most lower elevations areas receiving under 0.15 inches and plenty of gusty outflow winds. A couple storms may wander off the eastern San Juan Mountains into the San Luis Valley with isolated 24-hour totals up to 0.20 inches possible by morning.

Primetime: 3PM to 1AM

Grand Valley, Southwest Slope, & San Juan Mountains:

An isolated, high-based storm or two may form over the western San Juan Mountains, but more storms are more likely to form over the eastern San Juans. Max 1-hour rain rates up to 0.40 inches will be possible, but most storms will produce less than 0.15 inches. Other lower elevation areas will remain dry this afternoon with high temperatures forecast to reach the mid to upper 80Fs.

Primetime: 1PM to 10PM