FTB 09-24-2018: Trough Continues Eastward Migration and will Bringing Scattered Showers and Thunderstorms back into the Forecast

Issue Date: Monday, September 24th, 2018
Issue Time: 09:30AM MDT

— Flooding is NOT expected today

Unsettled weather is expected today as the trough continues to travel through the state. Overnight, a cold front moved across the area, so temperatures this morning were a few degrees cooler with some cloud cover over the eastern plains. Expecting cloud cover to decrease with some morning heating, which will allow a bit of instability to build before the main axis of the trough moves through eastern Colorado this afternoon. There are also some showers present this morning over the northern high terrains associated with the upper-level dynamics of the trough. Expecting these showers to slowly dissipate through the morning hours. Shower and thunderstorms coverage will pick up over the San Juan and Central Mountains early this afternoon where instability is able to mix with some moisture. These high-based storms are expected to be rather scattered and weak in nature with some brief, gusty winds possible. Over the Northwest Slope, drier air will fill in behind the trough. This will bring back low relative humidity values, and paired with gusty surface winds and dry fuels, bring back critical fire weather. A Red Flag Warning has been issued through this evening with winds in the 15 to 20mph range and gusts up to 35mph possible.

As the cloud cover begins to burn off behind the front, a bit of instability will be able to build over eastern high terrains and the eastern plains north of the Palmer Divide. Enhanced low-level moisture behind the front and moisture associated with the trough will allow the return for showers and thunderstorms to the forecast. Precipitable Water (PW) at Denver this morning was measured at 0.45 inches with a nice dry layer between 500 and 350mb. This should be enough moisture for some scattered showers this afternoon and evening though totals will be a bit lighter with the dry layer. Expecting storms to initiate over the Front Range by midday and move into the adjacent plains by early afternoon. Some more isolated showers are expected over the Southeast Mountains. As storms move into the adjacent plains off the Front Range, better low-level moisture will allow rain rates to increase though decent storm motion should keep totals under 1 inch. The stronger thunderstorms this afternoon will likely produce some gusty winds. The highest totals are expected to be over the northeast corner of the state along the NE/CO border. A secondary cold front will surge south through the state overnight into tomorrow morning, so expecting more cloudiness and light showers tomorrow morning with some cooler temperatures. As anticipated, flooding is not expected today.


Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Northwest Slope, Southwest Slope, San Luis Valley, Grand Valley, Raton Ridge, Southeast Plains:

Not expecting much precipitation over these regions today. Dry air will fill in behind the trough over the northwest corner of the state, which will drop relative humidity values. With surface winds in the 15 to 20mph range, critical fire weather is expected. A Red Flag Warning is in place through this evening. Rainfall this afternoon should be confined to the southern high terrains for western Colorado, which means the Southwest Slope, Grand Valley and San Luis Valley will miss out on the action as well. The Raton Ridge and Southeast Plains are also expected to stay mostly dry with downsloping winds expected over the Southeast Mountain. A stray storm could wander into the Southeast Plains off the Palmer Ridge, but with dew points in the 30Fs cloud cover is more likely than wetting rainfall.

Urban Corridor, Palmer Ridge, Northeast Plains, Front Range, Southeast Mountains, Central Mountains, San Juan Mountains, Northern Mountains:

Some upslope showers are expected this afternoon over the higher terrains. Scattered storms over the southern and western mountains will be high-based, so max 1-hr rain rates should be limited to 0.15 inches with some gusty winds possible. Best moisture will be over the Front Range, so expecting shower coverage to be greatest over this area. Max 1-hr rain rates up to 0.3 inches are possible. Storms are expected to move into the Urban Corridor, Northeast Plains and Palmer Ridge early this afternoon. With a bit better moisture, max 1-hr rain rates up to 0.4 inches (south) and 0.75 inches (north) are possible. Expecting winds to pick up as well this afternoon with the upper-level jet mixing stronger winds down to the surface. Another cold front moves into the area early tomorrow morning, so some light showers may be possible over the Northeast Plains as the front slides south.

Primetime: 11AM to 12AM