FTB 07-30-2015: Moist Subtropical Flow Continues, Heavy Rain Threat Increasing

Issue Date: 7/30/2015
Issue Time: 10:15 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHEAST PLAINS, RATON RIDGE, SOUTHEAST MOUNTAINS, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, RATON RIDGE, SOUTHEAST MOUNTAINS, CENTRAL MOUNTAINS, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

The broad high-pressure ridge aloft will remain anchored to the south-central US, building slightly west and north throughout the day today. This will continue streaming subtropical moisture into Colorado, mainly south and east of a line from Logan County through Montrose County. North and west of that line, drier, westerly flow will continue and keep precipitable water values below average. This line that divides the moisture breaks the forecast into two distinct forecasts.

07302015_WV

To the north and west of the line, mostly sunny skies and warm temperatures will rule the day, with only a few isolated showers/weak thunderstorms possible over the higher terrain of the Front Range, Central Mountains, and Northern Mountains, with one or two moving over the northern portions of the Urban Corridor and Northeast Plains. Gusty winds and lightning will be the main threats from these storms, with modest rain rates (0.4-0.8 inches/hour).

The main action today and tonight will occur to the south and east of the line mentioned above. Deeper moisture, a passing disturbance, and decent instability will combine to bring a heavy rain threat to a relatively widespread area. That is not to say there will be widespread storms, just a widespread area where the heavy rain will be possible with any thunderstorm. Coverage is expected to be isolated-to-widely scattered across the lower elevations of eastern Colorado, with more scattered coverage across the higher terrain of southern Colorado and near the CO/NM border. Storm motions will be fairly slow, especially across mountain locations. A few showers and thunderstorms will linger into the early morning hours tomorrow.

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.
FTB_snapshot_20150730

Zone-Specific Forecasts

Northeast Plains south of I-76, Urban Corridor and Front Range south of I-70, Raton Ridge, Palmer Ridge, and Southeast Plains:

Isolated-to-widely scattered showers/thunderstorms expected, with coverage increasing from north to south. Heavy rain will be a threat, as deep moisture will be present, allowing for efficient rainfall production. Fairly slow storm motions will also be a factor, increasing the flood threat potential over locations that remain under a stationary, or near-stationary, storm. Maximum rain rates will be 2.0-3.0 inches/hour under the strongest storms, with 1.0-2.0 inches/hour being within reach for most storms.

Timing: 11 AM – Midnight, with a few showers and thunderstorms continuing into the early morning hours, mainly across southern zones.

Northwest Slope, Northern Mountains, Central Mountains, Grand Valley, Northeast Plains north of I-76, and Urban Corridor and Front Range north of I-70:

Mostly sunny and warm, with a few isolated thunderstorms possible. Drier air will be present, keeping rain rates relatively low (0.2-0.5 inches/hour over the mountains, 0.4-0.8 inches/hour over the Urban Corridor and Northeast Plains). Temperatures will be a few degrees warmer than yesterday.

Timing: 11 AM – 8 PM

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

Scattered showers and thunderstorms are expected, with fairly slow storm motions being a concern. The strongest storms will be capable of producing 1.2-1.8 inches/hour, or, with respect to slow-moving/stationary storms, up to 2.0-3.0 inches in 3 hours will be possible.

Timing: Primetime will be 11 AM – 10 PM, but showers and thunderstorms are expected to continue into the morning hours tomorrow.