FTB 05-17-2018: Moisture Axis Divides Colorado

Issue Date: 5/17/2018
Issue Time: 8:35 AM

A LOW FLOOD THREAT IS FORECAST.

Two separate weather regimes will split the state today, as denoted in the image below. Southwesterly flow aloft has overspread much of the state, and this will stoke the development of a surface low pressure trough across the eastern plains. This low pressure trough will act as a dryline, with dry, westerly winds to the west, and moist, southeasterly flow to the east. This moisture axis will essentially split the state into two forecasts.

For areas along/east of the moisture axis, the influx of moisture will allow for significant instability to develop this afternoon. Coupled with favorable shear and surface convergence, isolated-to-scattered strong/severe thunderstorms are expected this afternoon/evening. The main threats will be large hail (up to 1.75” in diameter), strong winds (60+ mph), and heavy rainfall; an isolated tornado is also possible. There is a low flood threat issued due to the potential for training storms to develop during the evening and overnight hours, but the potential for this scenario (25%) is low enough to not warrant more than a low flood threat. More likely, this activity will occur just outside the state, over the plains of Nebraska. More details on rain rates and timing can be found in the zone-specific forecast discussions below.

For areas to the west of the moisture axis, dry and warm conditions will be the name of the game today, with gusty winds (thanks to a tightening pressure gradient) leading to critical fire danger, especially across the western slope and some areas along/adjacent to the southern Front Range and Southeast Mountains. Please check with your local National Weather Service Office for more details on any Red Flag Warnings.

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.

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Palmer Ridge, and Raton Ridge:

Downsloping winds will dominate the day with warm and dry conditions. High temperatures will push into the 80s and near 90 in some locations. Far eastern portions of the Palmer Ridge will attempt to get in on the early stages of thunderstorm development, so an isolated thunderstorm cannot be ruled out there. Maximum rain rates will be fairly low, around 0.25-0.5 inches/hour, as storms won’t mature until being east of the area.

Timing: 2 PM – 8 PM

Northeast Plains and Southeast Plains:

Isolated-to-scattered strong thunderstorms are expected this afternoon and evening, with a few becoming severe. Large hail, strong winds, lightning, and periods of heavy rainfall are the main threats, and an isolated tornado cannot be ruled out. Fairly brisk storm motions to the east will keep most rainfall below flash flood thresholds, but the possibility for training thunderstorms warrants the issuance of a low flood threat for portions of the Northeast Plains. Due to strong surface convergence along the dryline, an isolated thunderstorm or two will continue into the early morning hours tomorrow, mainly near the CO/KS/NE borders.

General Rain Rate: 0.5-1.0 inches/hour
Maximum rain rate: 1.25-1.5 inches/hour

Timing: 2 PM – 11 PM, with an isolated thunderstorm or two ongoing into the early morning hours near the CO/KS/NE borders

Front Range, Northern Mountains, Northwest Slope, Central Mountains, Southeast Mountains, San Juan Mountains, Southwest Slope, Grand Valley, and San Luis Valley:

Warm and dry, with gusty winds leading to critical fire danger for lower elevations, mainly across the western slope. A few cumulus clouds will bubble during the afternoon over the higher terrain, but nothing will come from them. Be sure to check with your local NWS office for any information regarding Red Flag Warnings.