STP 06-27-2017: Scattered Showers over Southeastern High Country and Plains

Issue Date: Tuesday, June 27th, 2017
Issue Time: 10:27AM MDT

Summary:

The persistent weather pattern, marked by a weak upper-level ridge centered west of Colorado, continued its hold again yesterday. West of the Continental Divide, it was extremely warm and dry with Grand Junction reaching a high of 98F. East of the divide, the day began with very high low-level moisture. Dewpoints were 60F at Pueblo and surrounding regions at 9am. Over the Palmer Ridge and Northeast Plains, much of this low-level moisture quickly mixed out in the late morning hours limiting the number of scattered storms in the afternoon. However, a storm that formed in Wray reported ping pong sized hail around 4pm, causing broken windows and was accompanied by a small funnel cloud.

Upslope flow triggered a handful of severe thunderstorms storms south of I-70 and over the Southern high country where high moisture did not mix out. Near Durango, a spotter reported 0.61 inches in a 25 minute period. Most other storms dropped 1-1.5 inches of rain with storm centers closer to 2 inches. Under the stronger cells, 1-1.75 inch hail was also reported. There were two flash flood warnings and advisories issued over burn scars due to the slow moving storms. One warning over the Junkins Burn Scar and an advisory for the Waldo Canyon burn scar outside of Colorado Springs.

No flooding occurred yesterday (at least as of this morning). Please see the STP map below for a look at 24-hour precipitation totals.

Storm Total Precip Legend

FTO 06-26-2017: Elevated Flood Threat Later This Week, Then Heat Ramps Up

Issue Date: Monday, June 26th, 2017
Issue Time: 2:50PM MDT
Valid Dates: 6/27 – 7/11

After a roller coaster week featuring near record heat followed by a fall-like cold front, the weather has stabilized across Colorado over the past 48-72 hours. Return flow from the daily lower-level heat low (formed in part due to strong heating with clear mornings) has resulted in daily bouts of isolated to scattered thunderstorm activity, mainly in the southeast part of the state. Over the next 15 days, a variable weather pattern is anticipated, complete with a few frontal passages and associated thunderstorms, intense heat as well as a possible monsoon surge later in the period. Moreover, with Independence Day soon to come, there will be plenty of recreational activity across the state: make sure to check the weather forecast regularly for updates!

As the water vapor image shows, below, we have identified two precipitation events over the next 7 days. The upper-level ridge will be quickly replaced by a trough as a disturbance crosses into the US west coast. Initially, this disturbance will result in gusty, dry west winds overtaking the state, followed by a cool front passage on Wednesday. The cool front will provide some organized rainfall activity in the far northeast part of the state, but is otherwise unimpressive. Things turn more interesting later this week as another disturbance will quickly enter the US coast and partially combine with the first. A multi-day period of scattered to numerous showers and storms is expected, supporting an elevated flood threat. Fortunately, it appears that July 4th, itself, may be relatively dry as Event #2’s passage ushers in some drier air across most of the state. After Independence Day, guidance suggests a period of very warm/hot weather statewide, which is certainly not good news for the already parched western slope.

As shown in the forecasted Precipitable Water (PW) plumes from the GFS ensembles, the disparity between eastern and western Colorado will continue in full force. Note that Denver’s PW forecasts are on par, or even slightly above the long-term climatology. Meanwhile, the Grand Junction forecast continues to be well below climatology through the next 7 days. After Independence Day, there is some indication that monsoon moisture will enter southwest Colorado. However, at this lead time, the false alarm rate is high and caution is needed. Nonetheless, there is sufficient confidence to identify a third precipitation Event (#3). However, precipitation coverage and intensity have significant uncertainty.

Below we describe each of the three identified precipitation events in more detail.

Event #1: Wednesday (6/23) through Monday (6/26)

No Apparent Flood Threat as cool front enters the state

A brief window of time on Wednesday afternoon will juxtapose a cool front passage from the north with some elevated moisture content across northeast Colorado. Scattered shower and thunderstorm activity will be possible especially across the Northeast Plains. However, storm motions should be quick enough to limit heavy rainfall to less than 0.5 inches, though localized higher amounts are possible. With a favorable shear profile, severe weather (large hail and gusty winds) will be the bigger threat compared to heavy rainfall. Thereafter, dry, westerly winds overtake most of the state by Thursday.

Event #2: Friday (6/30) through Monday (7/3)

Elevated Flood Threat as another disturbance enters the state right on the heels of Event #1

Return moisture will begin to advect into Colorado on Friday and isolated to scattered thunderstorms will enter the picture in eastern and southeast areas. Things turn more interesting on Saturday and Sunday when a shortwave embedded in the jet stream will dive southeast and provide favorable upper-level dynamics for large-scale lift across Colorado. West of the Continental Divide, areas will simply be too dry to support any heavy rainfall. Instead, a prolonged period of gusty winds could lead to elevated fire danger. However, east of the Continental Divide, with sufficient moisture in place, scattered to numerous thunderstorms are expected both Saturday and Sunday. Highest coverage will be across eastern areas towards the NE and KS borders where up to 2 inches of rainfall will be possible (keep in mind, most of this will occur within a few hours on either Saturday or Sunday). Isolated flash flooding appears possible, hence the elevated flood threat. Severe weather is very likely to be a threat both days as favorable shear and sufficient moisture will overlap. By Monday (7/3), cooler, more stable air will overtake most of Colorado though residual moisture will likely support another day of isolated to scattered thunderstorms capable of short-term heavy rainfall across the Raton Ridge and surrounding plains.

Legend

Event #3: Friday (7/7) through Sunday (7/9)

No Apparent Flood Threat as monsoon surge may encroach on southwest Colorado

The large scale pattern after Event #2 will feature anomalously high heights over the US Central Plains. In turn, this will promote very warm to hot weather across most of Colorado after the 4th of July. Thereafter, there are indications that an inverted trough and/or shortwave feature from the west will cause a surge of moisture into the desert southwest. With this kind of lead time, and this early in the monsoon season, moisture is a key wildcard and currently looks marginal at best. Nonetheless, with climatology starting to favor diurnal storm activity in the San Juans combined with enough support from long range GFS ensembles, we identify an elevated precipitation threat for southwest Colorado by Friday 7/7. At this time, a flood threat is not anticipated.

Legend

FTB 06-26-2017: More Isolated Heavy Rainfall Likely, Over Same Areas

Issue Date: Monday, June 26th, 2017
Issue Time: 10:10AM MDT

LOW flood threat for parts of Palmer Ridge (including Waldo Canyon burn scar), Southeast Mountains, and Raton Ridge

Although aloft, the weather pattern of the last 48 hours has hardly changed, the details of the lower levels will be important today. The water vapor image, below, continues to show a seasonably strong high-pressure ridge over the Great Basin. This is causing very warm and dry weather west of the Continental Divide. East of the Continental Divide, a different headline is in store today: very moist low level air. For example, at Colorado Springs, the morning dew point temperature was at 59F – a very high reading especially above 6,000 feet. More of the same is true farther south and east, where yesterday’s rainfall is evaporating and will continue to do so as the strong June sun heats the surface. Isolated to scattered thunderstorms will once again be likely today across the higher elevations and foothills of southeast Colorado. While rainfall rates will be right in the neighborhood of flood-intensity levels, a complicating factor today is antecedent rainfall. Parts of southeast Colorado saw over 0.75 inches of rainfall yesterday, causing areas with saturated soil. Today’s precipitation probability map looks almost identical to yesterday’s observed rainfall map (see Storm Total Precipitation summary). Thus, a Low flood threat is posted for parts of southeast Colorado for isolated flash flooding, debris slides and mud flows.

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:

Southeast Plains, Palmer Ridge, Southeast Mountains, Raton Ridge, San Luis Valley, San Juan Mountains:

Sunny early then partly cloudy with isolated to scattered showers and thunderstorms developing by noon. Max 30-minute rainfall up to 0.8 inches and 1-hour rainfall up to 1.3 inches except up to 2.1 inches far southeast. Antecedent wet soils will increase the runoff threat today, thus a Low flood threat has been posted for isolated flash flooding, mud flows and debris slides. The Waldo Canyon burn scar should be monitored closely.

Primetime: 1PM to 11PM

Northeast Plains, Urban Corridor, Front Range:

Sunny early then partly cloudy with isolated showers and thunderstorms possible after noon. The chance of thunderstorms is slim today but if one manages to form, max 30-minute rainfall up to 0.8 inches and 1-hour rainfall up to 1.3 inches is possible. No flood threat is issued today due to low storm probabilities, but if a storm forms, isolated flash flooding will be possible.

Primetime: 2PM to 9PM

Grand Valley, Northwest Slope, Northern Mountains, Central Mountains, Southwest Slope:

Mostly sunny to partly cloudy and continued very warm today. An isolated shower or weak thunderstorm will be possible especially over southern areas. Max 1-hour rainfall up to 0.5 inches. Gusty winds and dry lightning will be more of a threat than rainfall.

Primetime: 12PM to 7PM

STP 06-26-2017: Scattered T’Storms With Heavy Rainfall Across Southeast

Issue Date: Monday, June 26th, 2017
Issue Time: 9:50AM MDT

Summary:

An accurate word to describe Sunday’s weather across our state is “persistent”. With a weak upper-level ridge centered west of Colorado, subsidence and generally clear skies continued to be the rule west of the Continental Divide.

Of course, the devil is always in the details. To the east, despite a persistent atmosphere aloft, slow and steady moisture return increased low level moisture and provided more fuel (compared to Saturday) for isolated to scattered thunderstorms. The focal points, and highest storm coverage, were off the south face of the Palmer Ridge and the Raton Ridge, especially the Wet Mountains. Several rounds of storms were observed, mainly limited to the foothills. Generally, rainfall was not overly impressive with 0.5 to 0.75 inches common under the storm cores. However, the exception was an isolated, severe storm in Baca County where a Local Storm Report noted 1.5 inches of rain in 10 minutes (!), a roughly 1 in 100 year event. In addition, the storm dropped copious amounts of small hail and prompted a tornado warning as soon as it crossed into New Mexico. A few other storms farther west produced over 1 inch of rain across large swaths of Las Animas and Huerfano counties. Although storms fell mainly over ungauged areas, the USGS gauge on the Purgatoire River near Thatcher (Las Animas County) showed an impressive spike (see below).

As of this morning, flooding was fortunately not reported from Sunday’s rains. For rainfall estimates in your area, check out the State Precipitation Map below.

Storm Total Precip Legend