STP 07-28-2015: Isolated-to-Scattered Thunderstorms Broke Up the Otherwise Mostly Sunny Skies

Issue Date: Tuesday, July 28th, 2015
Issue Time: 9:00 AM MDT


An upper-level disturbance moved quickly across Colorado yesterday, providing a trigger for isolated-to-scattered showers and thunderstorms. Rainfall intensities were capped as dry air moved in from the west, causing precipitable water values to plummet to below 0.75 inches at all four of the normal reporting stations (Boulder, Grand Junction, Shriever AFB, and Pueblo). The result were thunderstorms producing mainly light rain and gusty winds. For a look at statewide rainfall, check out the radar-derived Storm Total Precipitation map below.

No flash flooding occurred yesterday. The only storm report was for a non-thunderstorm wind gust of 64 mph 26 miles North of Mack (Garfield County).

Storm Total Precip Legend

FTO 07-27-2015: Transition to Monsoon Season Underway

Issue Date: 7/27/2015
Issue Time: 1:20 PM


As we quickly move into August, it is a good time to remind everyone that this time period is when Colorado typically sees the Southwest Monsoon become a more frequent player in day-to-day weather. Monsoon surges arrive and are then cut-off, arrive and then cut-off, and so on and so forth. This FTO will certainly show no exception to this typical pattern. The first wet period covers Tuesday and Wednesday (07/28-07/29), and the second shows up for next week, Tuesday through Friday (08/04-08-07). Between the two periods, mostly sunny skies and warmth are expected to dominate most areas, with the typical summertime thunderstorms developing thanks to the presence of residual moisture under the upper-level ridge. After the second wet period, mostly sunny skies and hot temperatures are expected to return as the upper-level ridge builds overhead and cuts off the supply of subtropical moisture from the southwest. For more details, see the event discussions below.


Event #1: Tuesday (7-28-2015) and Wednesday (7-29-2015)

Elevated Flood Threat as Upslope Flow and Monsoon Surge Impact Portions of Eastern Colorado

Upper-low (marked by black #1) will drag cool front through eastern Colorado early Tuesday, and this boundary will stall/washout along the CO/NM border for Tuesday/Wednesday. Upslope flow will develop behind the front, and underneath a modest monsoon surge (green arrow), will bring an elevated flood threat to southeastern Colorado, namely the southern Urban Corridor, Palmer Ridge, Southeast Plains (near the interface with Southeast Mountains), Southeast Mountains, and Raton Ridge. Precipitable water values will climb to near, or just above 1 inch for those areas mentioned, which will result in efficient rainfall production. Check back in for daily FTB’s for the most up-to-date information.


Event #2: Tuesday (8-4-2013) through Friday (8-7-2015)

Elevated Flood Threat for Extended Wet Period

An upper-level low pressure system (marked by purple “#2”) will take its time getting its act together, and will be delayed arrival to the west coast until early next week. The approach will force the upper-level high back to the east, bringing another monsoon surge to Colorado. This is expected to bring an extended period of showers and thunderstorms, as the high will take a few days to re-position itself westward and cutting off the flow of moisture from the southwest.


FTB 07-27-2015: Fast Moving Storms Will Limit Rainfall

Issue Date: July 27th, 2015
Issue Time: 10:15AM

— Flooding is NOT expected today
Elevated Fire Threat for Northern Mountains and Northwest Slope

Today’s water vapor image, below, shows a large disturbance located over the Pacific Northwest, stretching southward into Nevada. Late last week, it appeared that as this disturbance crossed Colorado, it may have subtropical moisture support and pose a heavy rainfall threat. However, the subtropical connection did not materialize and heavy rainfall is not expected as the disturbance races by our state. Despite morning precipitable water values in the 0.8 – 1.1 inch range, there is very dry air just to the west. As this propagates eastward, it will put a cap on rainfall intensities. There are two additional limiting factors to rainfall today. First, the disturbance is moving very rapidly, meaning that storm motion is expected to be in the 25-40 mph range. Second, the surface low pressure noted over Wyoming this morning will induce downsloping flow east of the Divide. Climatologically speaking, downsloping flow is extremely unfavorable for heavy rainfall east of the Divide.

watervapor_20150727For today, we expect mostly sunny skies this morning to quickly give way to scattered thunderstorm activity by early afternoon. Coverage will be highest in the Central Mountains. Storms will race to the northeast across the Plains as the afternoon transitions into evening. Storm activity will quickly diminish around sunset. While rainfall amounts will be limited today, gusty winds, up to 60mph, will accompany many storms.

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

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

Sunny early with a few cumulus clouds developing by early afternoon. Scattered high-elevation storms will develop over the higher terrain and spread eastward. Max 1-hr rainfall will be 0.5 inches over the higher terrain to 0.9 inches over the far northeast. Wind gusts up to 65 mph will be possible with stronger storms. Flooding is not expected today.

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

Sunny to partly cloudy early with scattered afternoon storms possible across the higher elevations. Highest coverage is expected to be in the Central Mountains. Due to fast storm motions, max 1-hour rain rates will be 0.5 inches. Thunderstorms will be capable of wind gusts up to 55 mph. Across the Northern Mountains and Northwest Slope, the combination of very dry air coming in from the west and a sustained period of gusty winds of 40-50 mph will lead to a fire weather danger this afternoon. Please stay tuned to local news sources for specific information. Flooding is not expected today.

STP 07-27-2015: Lots of Action East of the Divide, Drier to the West

Issue Date: July 27th, 2015
Issue Time: 9:15AM


It was a two sided weather story yesterday across Colorado. While areas west of the Divide dried out during the afternoon, a passing disturbance ignited thunderstorm activity over the eastern Plains. Not surprisingly, with moisture increasing strongly as initial storms marched eastward, heavy rainfall rates were observed. The heaviest official rainfall was seen in Prowers and Lincoln counties where several locations picked up about 2 inches. As is common in the summertime, most of this rain likely fell in a period of about an hour.

In addition, many hail reports were received with the strongest storms. The largest hail of about 1.75 inches fell in Logan County, while many other reports ranging from nickel size to ping pong size were received in Washington, Morgan and Kit Carson counties. Finally, gusty winds were observed across many locations, from the foothills all the way to the Kansas and Nebraska borders. Widespread gusts up to 60 mph was noted as the initial thunderstorms merged into storm complexes over Yuma, Cheyenne and Kit Carson counties.

All in all, 14 weather warnings were issued by area National Weather Service offices. Six of them were flood related, 8 were severe weather related. As of this morning, no official flooding reports were received. However, it should be noted that some of the strongest rainfall occurred over rural regions and may have been missed.

For an estimate of 24-hour rainfall that occurred in your area, please check out our radar based map below. Since this is only an estimate, please be aware that some rain totals (for example, eastern Prowers County) may be greatly overestimated due to hail contamination.

Storm Total Precip Legend