STP 08-28-2015: Monsoon Moisture Fed Isolated-to-Scattered Thunderstorms

Issue Date: Friday, August 28th, 2015
Issue Time: 9:00 AM MDT


Monsoonal moisture continued its push across Colorado in association with a mid-level trough, provided enough fuel for scattered thunderstorms along and over the High Country, with more isolated coverage to the east. Fortunately, the timing of the trough was not favorable for widespread strong thunderstorms. Instead, storms relied mainly on the heat of the day to break the cap, with very marginal instability available. This kept most showers and thunderstorms on the garden-variety side of the scale, with only a couple stronger thunderstorms producing pea-sized hail and brief heavy rain. No flash flooding was reported, but street/field ponding did occur under the strongest thunderstorms.

According to National Weather Service Local Storm Reports, pea-to-dime sized hail was reported 2 miles S of De Beque (Mesa County), and pea-sized hail with heavy rain was reported 2 miles N of Black Forest (El Paso County). For a look at statewide rainfall totals, be sure and check out the Storm Total Precipitation map below.

Storm Total Precip Legend

FTO 08-27-2015: Three Events, No Apparent Flood Threats

Issue Date: 8/27/2015
Issue Time: 12:35 PM


There are 5 main features to pick out of the water vapor image below: Tropical Storm Erika in the Atlantic, Tropical Storm Jimena (right) and Hurricane Ignacio (left) in the Pacific, high pressure ridge over the central/western US, and the low pressure trough off the west coast of the US and Canada. Tropical Storm Erika does not mean much to our forecast, but it will be interesting to watch over the coming days with regards to its track, intensity, and longevity. Jimena and Ignacio will continue to move westward further into the Pacific, and are also without impact to Colorado.


The high pressure ridge and low pressure trough, and how they interact with each other, will be the main story of this FTO period. As hard as the low pressure trough tries, the high pressure ridge is not ready to give up ground. As those two systems duke it out, monsoonal moisture will come and go, along with associated disturbances. The low pressure trough will also eject a few shortwave troughs through the forecast period, but the best broad-scale support for thunderstorms will likely be too far north to really mean a prolonged/elevated flood threat for Colorado. In the event discussions below, I have highlighted 3 “event” periods, none of which provide an apparent flood threat. In a weather regime such as this, the daily Flood Threat Bulletin’s will be crucial because the amount of moisture/instability/support will fluctuate daily. Stay tuned.

Event #1: Saturday (08-29-2015) through Tuesday (09-01-2015)

No Apparent Flood Threat from mostly “Garden-variety” Thunderstorms

Weak south/southwesterly flow aloft will return as the ridge axis gets pushed east of Colorado by the upper-level trough. This will bring some moisture back into Colorado, providing an uptick in precipitation over and adjacent to the mountains. Low-level moisture east of the mountains will be a bit on the low side, so most storms will be garden variety with brief light-to-moderate rainfall. Over the mountains, moisture will be better overall, so locally heavy rainfall will be possible. At this time, precipitation totals from the 4-day event are not that spectacular, with most areas receiving a half-inch or less. No flood threat designation is warranted at this time.


Event #2: Thursday (09-03-2015) through Sunday(09-06-2015)

No Apparent Flood Threat with a Near-Repeat of Event #1

Nearly a repeat of Event #1, the upper-level trough off the west coast forces the ridge axis back to the east, bringing south/southwesterly flow aloft back to Colorado. This will bring an increase in moisture, once again, though how deep the moisture will be is to be determined. The upper-level trough will eject another shortwave during the second half of the period, but the best support for thunderstorms will remain north and east of Colorado. With the amount of moisture present still coming into focus, no flood threat designation is warranted at this time. The precipitation map below is what current observations/analysis suggest, and is subject to revision for the next FTO. Stay tuned.


Event #3: Tuesday (09-08-2015) and Wednesday (09-09-2015)

No Apparent Flood Threat as This Event is Still Coming into Focus

After a short-lull, Event #3 will be a quick-hitter. The upper-level trough that was anchored along the West Coast looks like it will finally get its way, pushing to the east in a fairly quick fashion. Following this event, it appears things may be quiet through the following weekend. Details will come into focus over the coming days, so check back in the next edition of the FTO on Monday.

FTB 08-27-2015: Plenty of Moisture Available, but Timing of the Trough Proves Crucial

Issue Date: 8/27/2015
Issue Time: 9:05 AM


The IPW graph below certainly shows that there is plenty of moisture available to showers/thunderstorms. Grand Junction, after peaking above 1.35 inches yesterday evening, has come back down slightly to just over 1.2 inches. As the monsoonal disturbance has rotated across Colorado, IPW has climbed at the other three of the normal reporting stations (Boulder, Pueblo, and Shriever AFB), as well. Moisture is definitely out there.

Fortunately, that amount of water is not coupled with proper timing of the mid-level trough currently moving across the Central/Northern Rockies. The trough axis sits roughly along the Continental Divide at this time, with the best forcing for showers/thunderstorms already across Eastern Colorado. The trough/associated forcing will continue to push east, targeting central NE/KS and points further east for stronger thunderstorms this afternoon/evening. Behind that trough is a weakly subsident airmass, designed to try and suppress strong convection. This will limit what storms can do along and north of I-70, with a more neutral environment south of I-70.


The best opportunity for storms will be over the higher terrain, favoring the Continental Divide and mountains to the east, near the mountains along the Urban Corridor and western extents of the Northeast/Southeast Plains, and wherever sunshine can produce enough daytime heating to break the cap across the Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge. Rain rates are not expected to be great enough to warrant a flood threat, but street/field ponding are expected under isolated, stronger thunderstorms. For more information on timing and rain rates, please jump below the map for zone-specific discussions.

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

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

Early morning showers and thunderstorms continue to wind down across the area at this time. By this afternoon, expect coverage to uptick again, with the main focus of scattered showers/thunderstorms along and east of the Continental Divide. There will be isolated showers/thunderstorms elsewhere with partly sunny skies. Instability will be lacking, which will limit rain rates, thus no flood threat is warranted. Maximum rain rates will be 0.3-0.7 inches/hour for locations west of the Continental Divide, and 0.6-0.9 inches/hour along and east of the Continental Divide.

Timing: 11 AM – 9 PM, with a few weak showers lingering into the overnight hours.

Urban Corridor, Northeast Plains, Southeast Plains, Palmer Ridge, and Raton Ridge:

Isolated-to-scattered showers and thunderstorms expected, with the best coverage along the Urban Corridor, Palmer Ridge, and western extents of the Northeast and Southeast Plains. Main storm threats will be lightning and gusty winds, but locally moderate-to-heavy rainfall will be attend stronger storms. Under the stronger storms, street/field ponding will likely occur, but nothing to warrant a flood threat, as storm motions will limit the impact. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 0.7-1.4 inches/hour
Northeast Plains and Southeast Plains: 0.8-1.2 inches/hour
Raton Ridge: 0.4-0.8 inches/hour

Timing: 2 PM – Midnight, with a few lingering showers/thunderstorms over the Southeast Plains into the early morning hours.

STP 08-27-2015: Moist over the West Slope, Dry over the Urban Corridor/Eastern Plains

Issue Date: Thursday, August 27th, 2015
Issue Time: 9:00 AM MDT


A monsoonal disturbance rotated through the high pressure ridge, moving into western Colorado during the afternoon hours. The disturbance produced abundant cloud cover and widespread showers/thunderstorms for locations along and west of the Continental Divide. The abundant cloud cover helped to keep the flash flood threat down due to a relative lack of instability to drive strong thunderstorms and torrential rainfall. Instead, most showers/thunderstorms produced light-to-moderate rainfall, with rates low enough that terrain/streams/etc. could handle it without any issues.

From CoCoRaHS observers, the following locations were the big winners in rainfall totals yesterday:

Mesa County: 0.94 inches
Gunnison County: 0.68 inches
Delta County: 0.54 inches
La Plata: 0.31 inches
Montrose County: 0.25 inches

With fairly sparse observations across the western slope, it is important to check out the STP map below which shows radar-derived measurements.

Storm Total Precip Legend