FTB 08-13-2019: Severe Thunderstorms Forecast for the Northeast Plains Along with Overnight Rainfall for the Southeast Plains

Issue Date: Tuesday, August 13th, 2019
Issue Time: 10:25AM MDT

–A LOW flood threat has been issued for the Northeast Plains, Palmer Ridge, Southeast Plains and eastern Raton Ridge

Upper level flow has not changed much since yesterday, and with high pressure continuing to build over the desert southwest, westerly flow aloft will continue to pull in the dry air mass from Utah (see the lack of clouds in the visible satellite imagery below). There is some residual moisture over the San Juan Mountains, so some high-based light showers and weak thunderstorms may be possible this afternoon as a shortwave moves in from Utah and enhances weak upslope flow. The main threat from storms will be gusty winds. New to today’s forecast is a mid and upper level jet over the Colorado and Wyoming border, which will keep steering flows very fast over Northeast Colorado. It feature may also help break the cap this afternoon with the northeast corner of the state being in the right entrance region of the jet. The dry air from Utah and westerly winds off the eastern mountains will help reinforce a very strong dry line (marked in orange below) over the eastern plains. Dew points will drop by 30-35°F on the west side of this line, which will help cause convergence. Paired with convergence on a southward moving boundary, there is a threat for severe thunderstorms over the Northeast Plains from about 3PM through this evening. These severe thunderstorms will be capable of producing very large hail, strong outflow winds (>60 mph), an isolated tornado or two and brief, heavy rainfall. With the high shear, high CAPE environment and slightly lower freezing level than Sunday, storms have the potential to produce hail greater than the size of baseballs.

PW over Denver this morning measured 0.63 inches, but as mentioned above, it increases to over 1 inch over the eastern plains. This is a good indicator of the potential for heavy rainfall this afternoon. The flood threat for the severe thunderstorms over the Northeast Plains will only be about 1 hour due to storm motion between 20 and 25 mph to the southeast. There is the possibility for multiple storms to track over the same area of the Northeast Plains, but storms will likely dissipate due to lack of instability as they move into a cold pool. Thus, only the Low flood threat.

 

 

Interesting set up back to the west. The severe thunderstorms over the Northeast Plains look to send a strong outflow boundary back towards the Front Range this evening. This may cause an evening severe thunderstorm or two over the southern Urban Corridor. The one uncertainty is a slight inversion at about 600mb in this morning’s sounding at Denver, which would need to be broken for these isolated storms to occur. Not seeing a flood threat with these storms, due to higher moisture back to the east, but worth mentioning as to not catch anyone off guard.

Lastly, there is an additional Low flood threat issued for the Southeast Plains. The low level jet looks to set up over the area, which would help fuel overnight storms that form along outflow boundaries and other boundaries in the area. In a high-moisture environment, storms may be capable of producing local, heavy rain totals over a 2-3 hour period. There is a little uncertainty with instability (more gradual rain rates vs very high rainfall rates), so only issuing Low flood threat for this area. Flood threats today include arroyo flooding, road flooding, local stream/creek flooding and field ponding.

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:

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

Severe thunderstorms are forecast to begin around 3PM over the Northeast Plains. Max 1-hour rain rates up to 1.75 inches will be possible with the fast moving storms. The severe thunderstorm threat over the Northeast Plains is expected to end by 8PM, but a weak MCS over the Southeast Plains is forecast overnight. These storms will likely produce some smaller hail and gusty winds as well. As mentioned above, there is some uncertainty with the heavy rainfall potential with instability. However, there is a slight chance for some very local 2-3 hour rainfall totals up to 2.25 inches, so the Low flood threat has been extended south.

Back to the west, the outflow boundary will provide a chance for an isolated storm over the southern Urban Corridor and Front Range. A quick 0.5 inches in 30 minutes will be possible if the storms break the cap. Lastly, a few storms will fire this afternoon with upslope flow over the Palmer Ridge/Front Range intersect. Storms will produce mostly cloud cover over the mountains with max totals up to 0.3 inches possible over the Palmer Ridge.

Primetime: 2PM to Morning

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

A deep layer of dry air over the area will limit storm chances today. Slightly higher levels of residual moisture over the San Juan Mountains will provide a chance for afternoon thunderstorms over the highest elevations. Storm bases will be very high, so expecting rain rates to remain under 0.1 inches with gusty winds and a few lightning strikes possible. High temperatures look to be similar to yesterday and increasing, broken cloud cover from the passing shortwave is expected to help provide some shade from the heat.

Primetime: 2PM to 8PM