STP 07-28-2016: Isolated Strong-to-Severe Storms over the Plains

Issue Date: Thursday, July 28th, 2016
Issue Time: 9:00 AM MDT

Summary:

The High Country and Western Slope remained mainly dry yesterday as the atmosphere could only an isolated shower/weak thunderstorm or two. The main story over those areas were the hot temperatures, especially in the lower valleys. Further east, isolated strong/severe storms rumbled over the plains regions during the afternoon and through the overnight hours. Take a look at the severe storm observations reported to the National Weather Service:

70 mph thunderstorm wind gust: 9 miles NNE of Crook (Logan County)
2.50 inch hail: 2 miles E of Kersey (Weld County)
2.00 inch hail: 1 mile S of Galeton (Weld County)
1.75 inch hail: 1 mile ESE of Galeton (Weld County), 6 miles NNE of Nunn (Weld County), 4 miles E of Arriba (Lincoln County)
1.50 inch hail: 9 miles NNE of Crook (Logan County), S Arriba (Lincoln County)
1.00 inch hail: 6 miles NNW of Springfield (Baca County)
Tornado (brief): 1 mile E of Nunn (Weld County)

No flash flooding was reported. For a complete look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.

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Storm Total Precip Legend

FTB 07-27-2016: Substantial Drying For Most, A Few Strong Storms Possible Out East

Issue Date: Wednesday, July 27th, 2016
Issue Time: 10:10AM MDT

— Flooding is NOT expected today

An interesting weather pattern is noted this morning across the Rocky Mountain states, as shown by the water vapor image below. The first feature of interest is the high pressure that has developed over the Great Basin. This will quickly strengthen today, and its clockwise circulation will basically shut off the northward transport of new moisture from the south (including the outflow from Tropical Storm Frank). The result will be much lower rain coverage and intensity west of the Divide. The second important feature is a rather strong disturbance, especially by summer standards, that is currently located over Wyoming and Montana. This will race south-southeastward and clip eastern areas of our state, providing favorable dynamics for enhanced upward motion. Scattered thunderstorms will be likely for far eastern areas. Counteracting this will be the rapid drying as downslope winds usher in much lower dewpoints originating off the Cheyenne ridge and then spreading south and east. The disturbance will also promote fast storm motion, up to 40 mph, which will be a negative factor for heavy rainfall in a given location.

The highest low-level moisture will be locked up mainly along the Kansas border, which is where the highest rainfall rates will occur today. However, we expect rainfall to stay just below flood thresholds. Thus, flooding is not expected today.

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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:

Northeast Plains, Southeast Plains:

Sunny early then partly cloudy with scattered thunderstorms developing shortly after 1PM in northern areas, then spreading south and eastward. Max 1-hr rain rates up to 1.5 inches are possible. Large hail, up to 2.25 inches and gusty winds up to 65 mph will be possible with the strongest storm cells. Most activity will quickly diminish shortly after sunset, but isolated storms will be possible into the late evening hours. Max rain rates are expected to stay just below flood thresholds, so flooding is not expected today.

Primetime: 2pm to 8:30pm for most areas, through 11pm for far eastern areas

Front Range, Urban Corridor, Palmer Ridge, Raton Ridge, Southeast Mountains:

Sunny early then turning partly cloudy and hot. An isolated storm or shower is possible, especially for eastern areas. Max 1-hour rain rates up to 0.7 inches are possible. Gusty winds up to 55mph could accompany the strongest storm. Flooding is not expected today.

Primetime: 2PM to 7PM

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

Sunny early then turning partly cloudy and very warm with an isolated shower or storm possible, mainly over the higher terrain. Max 1-hour rain rates up to 0.4 inches are possible. Flooding is not expected today.

Primetime: Noon to 7pm

STP 07-27-2016: Similar to Monday, One Thunderstorm Complex Provided the Main Show

Issue Date: Wednesday, July 27th, 2016
Issue Time: 9:00 AM MDT

Summary:

The eastern plains were subjected to the strongest thunderstorms of the day, with isolated storms producing gusty winds, locally heavy rainfall, and lightning. Over the High Country and Western Slope, the majority of the isolated-to-scattered showers/thunderstorms produced gusty winds and lightning. Brief bouts with moderate rainfall accompanied the stronger (relatively speaking) storms over the San Juan Mountains and Southwest Slope, but not enough to produce any reported flooding issues.

The heavy rain event of the day occurred across the far Southeast Plains. The thunderstorm complex ramped up intensity after 8 PM MDT, dropping buckets of rain across portions of Kiowa, Prowers, Bent and Cheyenne counties, finally coming to an end/exiting the state by 4 AM MDT. The lone heavy rainfall report to the National Weather Service came from 8 miles ENE of Lamar (Prowers County) where 2.53 inches of rain had fallen from this activity.

For a complete look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.

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Storm Total Precip Legend

FTB 07-26-2016: Moderate Flood Threat Issued Along Eastern Border

Issue Date: 7/26/2016
Issue Time: 9:42 AM

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE NORTHEAST PLAINS AND SOUTHEAST PLAINS.
A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS AND PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, FRONT RANGE, AND SOUTHEAST MOUNTAINS.
A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, AND SAN LUIS VALLEY.

IPW values at all four of the normal reporting stations (Boulder, Grand Junction, Pueblo, and Schriever AFB) are below 1 inch this morning. This reflects the slight drying that has taken place as the lower-levels have become drier over the past few days. With that said, another swing upward is expected this afternoon and evening, pushing moisture values to levels where heavy rain remains a threat.

IPW_20160726

Over the mountains, storms will be scattered as the elevated terrain heats up and destabilizes the airmass. Most showers/storms will be garden variety, though, with the main threats being gusty winds and lightning, with brief light-to-moderate rainfall. There are two exceptions to this:

1) Front Range and Southeast Mountains: Low-level moisture is best here due to proximity with the eastern plains. Although westerly winds will likely dry the low-levels somewhat, enough of a concern exists for portions to be included in the low flood threat.
2) Southwest Slope and San Juan Mountains: The deepest moisture remains across southern zones as the drier air works in from the northwest. Access to better low-level moisture advecting in from AZ/NM increases the risk for heavy rain. Thus, the issuance of the low flood threat.

Near the eastern border, scattered strong-to-severe thunderstorms are expected, especially across Northeastern Colorado. The presence of a modest upper-level jet streak will be enough to support thunderstorm development, while strong daytime heating combined with sufficient wind shear will organize updrafts/downdrafts, promoting the development of large hail, strong winds, and heavy rain. Eastern portions of the Palmer Ridge may get in on this action, as well.

Over the Urban Corridor and western portions of the Southeast Plains and Palmer Ridge, the atmosphere will be generally capped off, but a few isolated-to-widely scattered storms will move overhead from the adjacent higher terrain. Enough moisture exists to warrant inclusion in the low flood threat area, but the lack of support aloft means it will be tough for storms to reach those thresholds

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.
20160726_MFT

Flood Threat Legend

Zone-Specific Forecasts

Northeast Plains, Palmer Ridge, and Southeast Plains:

Scattered thunderstorms, some will be strong/severe, are expected this afternoon through tonight. The main threat area will be across the far Northeast Plains and along/near the eastern CO border. For western portions of the Palmer Ridge and Southeast Plains, the storms will first develop over the higher terrain, and then move overhead. Elsewhere, it will be tough for storms to overcome a cap aloft, and only an isolated thunderstorm or two are expected. Maximum rain rates are as follows:

Northeast Plains: 2.0-3.0 inches/hour
Palmer Ridge: 1.2-1.8 inches/hour
Southeast Plains: 1.4-2.0 inches/hour

Timing: 2 PM – 2 AM

Front Range, Southeast Mountains, Urban Corridor, and Raton Ridge:

Isolated-to-scattered showers and thunderstorms expected, with the best coverage over/near the higher terrain. Enough moisture exists for a heavy rain threat, but the main threat from storms will be strong winds and lightning. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.8-1.2 inches/hour
Urban Corridor: 1.0-1.4 inches/hour
Raton Ridge: 0.5-0.8 inches/hour

Timing: Noon – Midnight over the mountains and Raton Ridge, 1 PM – 9 PM for the Urban Corridor

San Juan Mountains, Southwest Slope, and San Luis Valley:

Scattered showers/thunderstorms are expected over the higher terrain, moving generally eastward and over adjacent valleys. IPW values near 1 inch exhibit enough moisture for locally heavy rainfall, but drier low-levels will temper rain rates a bit until moisture can be mixed down, either by momentum or rainfall. Maximum rain rates are as follows:

San Juan Mountains: 1.0-1.8 inches/hour
Southwest Slope: 0.8-1.4 inches/hour
San Luis Valley: 0.6-1.2 inches/hour

Timing: 11 AM – 11 PM

Northwest Slope, Northern Mountains, Central Mountains, and Grand Valley:

Isolated-to-scattered showers/storms expected, but drier air will filter in from the northwest. This drier air will temper rain rates, keeping them below flash flooding thresholds. Activity will be garden variety, with the main threats being gusty outflow winds, lightning, and light-to-moderate rainfall. Maximum rain rates will be 0.3-0.6 inches/hour.

Timing: 11 AM – 9 PM for the Central Mountains, Northern Mountains, and Grand Valley, 1 PM – 7 PM for the Northwest Slope