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