Issue Date: Thursday, June 29th, 2017
Issue Time: 3PM MDT
Valid Dates: 6/30 – 7/13

An active period is in store across mainly eastern Colorado for the Independence Day holiday weekend. As the water vapor image shows, below, a large-scale trough is currently positioned northwest of Colorado. This is bringing a weak cool front southward during the afternoon hours and supporting some isolated to scattered storm activity. As the disturbance slowly moves eastward, return southeasterly flow will quickly regain control of eastern Colorado. This will push moist low-level air into our state, bringing about Event #1. Although we do not expect widespread heavy rainfall or flooding concerns, with the sufficient moisture, storms will have the ability to produce very heavy rainfall. Thus, an elevated flood threat. Just to reiterate: we expect this flood threat to be isolated in area on any given day.

A significant pattern change will occur right around the 4th of July when a strong upper-level ridge (currently off the Baja California coast) begins to dominate the western North American circulation. Initially, it will be sprawled across the entire western U.S., but eventually it will reposition farther north. The prime impact from this type of circulation will be very hot weather with temperatures of 5-10F above average. Yes, this implies a multi-day period of 100F+ temperatures at elevations below 5,000 feet. This is a concern for Colorado’s western slope, which has been hot and dry for weeks, with a prolonged period of elevated fire risk. This will continue for at least another 7-10 days.

As the high pressure ridge continues to consolidate in a more northward position, this will eventually allow for easterly flow to transport moisture into all of Colorado (not just our eastern half). The forecast Precipitable water plumes, below, show this nicely. Note that at Denver, values stay close to seasonal normal for the next 7-10 days. While at Grand Junction, after an initially very dry atmosphere, there are finally indications of higher moisture after July 4th.

Speaking of higher moisture in western Colorado, as we head into early July, we enter into an interesting period where average moisture tends to jump rather sharply. For example, looking at the climatological Precipitable Water percentiles at Grand Junction (source: Storm Prediction Center), below, note the quick increase in the median value (thin black line) from about 0.6 at the end of June to 0.7 in just a 7-10 day period. These kind of features can often be an important clue and complement to long range weather models.

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

Event #1: Friday (6/30) through Monday (7/3)

Elevated Flood Threat as at least isolated heavy rainfall is likely during the holiday weekend

Return moisture will begin to advect into Colorado on Friday with scattered showers and thunderstorms likely across the higher terrain east of the Continental Divide as well as the Plains to the east. Isolated heavy rainfall and a flash flood risk (keyword isolated) appear likely. Conditions will continue to support heavy rainfall 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 continue to stay too dry to support much, if any rainfall. Instead, a prolonged period of gusty winds could continue to maintain the elevated fire danger. East of the Continental Divide, with sufficient moisture in place, scattered thunderstorms are expected both Saturday and Sunday. Highest coverage will be across eastern areas towards the NE, KS and OK 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, with the main threats being large hail and strong winds, looks to be a threat on Friday and Saturday, then less so by Sunday as shear slackens. By Monday (7/3), 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 southeast quadrant of the state.

An important heads-up to all campers and hikers: isolated to scattered high elevation storms appear likely during the afternoon hours throughout this stretch. Even if a storm does not cause heavy rainfall, it could still produce hail, lightning and gusty winds. Furthermore, a storm can develop from “nothing” in as little as 20 minutes. Stay alert!

Event #2: Friday (7/7) through Monday (7/10)

No Apparent Flood Threat as monsoon ramp-up still looks on track to graze southwest Colorado

The large scale pattern after Event #1 will feature anomalously high heights over the US Central Plains. In turn, this will promote hot weather across most of Colorado after the 4^th of July with temperatures back to being 5-10F above normal. Thereafter, it continues to look like this strong ridge will migrate far enough northward to allow a feed of easterly disturbance that may be the first ramp-up of the 2017 North American monsoon. At this time, and with it being so early in the monsoon season, moisture is a key wildcard and continues to look marginal with Precipitable water staying below 0.75 inches. 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.

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 4^th, 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.

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 4^th 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.

Issue Date: Thursday, June 22nd, 2017
Issue Time: 3PM MDT
Valid Dates: 6/23 – 7/7

The Continental Divide frequently acts as a meteorological barrier, causing contrasting weather conditions in eastern and western Colorado. June has been a prime example of this, as shown in the temperature and precipitation anomaly maps shown below. While most of the state has been warm this June, generally 2 – 6 degrees F above normal, there has been hardly a drop of rain to the west of the Continental Divide. To be fair, eastern areas have seen streaky rain at best, but that is better than no precipitation at all. One of the key takeaways from this edition of the 15-day Flood Threat Outlook is the continuation of very dry conditions for northwest and western Colorado, raising some concern about the development of a flash drought. Fortunately, water supply appears to be in a stable state, which eases the stress of the lack of rainfall.

This afternoon’s water vapor imagery, below, is a good starting place for the flood outlook. A strong jet is positioned north of Colorado this morning, with a high pressure ridge offshore. A strong Pacific cool front has crossed most of the state providing a cool down and a brief surge of low-level moisture. For most areas, this moisture surge will be temporary, but for southeast Colorado, the front will never make it farther south than the Raton Ridge. Thus, a multiple day period of isolated to scattered thunderstorms is in store, marked as Event #1. This will be further boosted by a disturbance rounding the periphery of the ridge.

After Event #1, a strong disturbance will enter the US Pacific Northwest coast. This type of entry will initially dry out Colorado with strong westerly winds originating over the Great Basin. This can be seen in the forecasted Precipitable Water plumes, below (note strong drying in both Grand Junction and Denver forecasts). However, the drying will not last long east of the Continental Divide with a weak Atlantic subtropical ridge starting a period of moisture return by Friday, June 30. The result will be a prolonged stretch of at least isolated thunderstorm activity mainly in southeast Colorado, Event #2. Short-term heavy rainfall will be likely, but no organized flood threat is foreseen at this point.

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

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

Isolated heavy rainfall, but enough moisture to warrant an Elevated Flood Threat

Today’s cool front will stall along the Colorado / New Mexico border. As the surface high pressure (responsible for driving the front south) moves eastward, return moisture will begin to return to mainly southeast Colorado starting Friday and especially by Saturday. With Precipitable water values expected to exceed 1 inch, isolated heavy rainfall will be likely. The most favored region will be the Raton Ridge and Southeast Mountains. The Hayden Pass fire burn area will need to be watched for excessive runoff. Isolated flash flooding, debris slides and mud flows will be possible in the foothills and mountains. Farther northern, storm coverage will be more isolated and rainfall amounts are not expected to exceed 1 inch.

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

No Anticipated Flood Threat as return moisture will fuel daily rounds of popcorn thunderstorms

A ridge will re-establish itself to the west of Colorado, and return moisture will keep the atmosphere unstable during peak heating hours. The result will be daily rounds of isolated to scattered thunderstorm activity, mainly east of the Continental Divide. With slightly above seasonal amounts of low-level moisture, expect short-term heavy rainfall of 15-30 minute duration. Up to 0.75 inches is commonly possible with such storms. However, with little shear in place, storms cannot maintain themselves long. Nonetheless, with an active recreational period in anticipation of July 4^th, thunderstorm activity will need to be kept in mind for outdoor plans.

Issue Date: Monday, June 19th, 2017
Issue Time: 2:30PM MDT
Valid Dates: 6/20 – 7/4

A hot and relatively dry air mass has developed over the Great Basin, extending into western Colorado as of this afternoon (6/19). This is courtesy of a strong high-pressure ridge, the strongest of the young summer that will continue to strengthen over the next 24-48 hours. The result will be very hot temperatures, to the tune of 10-15 F above normal, and dry conditions through Tuesday. Thereafter, a low pressure system (currently in the Gulf of Alaska, as seen in the water vapor image, below) will temporarily displace the high pressure ridge and deliver a significant Pacific-origin cool front across eastern Colorado. This will be responsible for the first precipitation event noted in today’s threat timeline.

As shown in the forecasted Precipitable Water (PW) plumes, below, there will continue to be a significant disparity between conditions east of the Continental Divide and those to the west over the next 7 days. Event #1 will be generally limited to eastern areas. However, there are now indications that the season’s first push of monsoon moisture may begin to impact the southwest United States by early next week. More specifically, this appears to be due to a combination of two disturbances (one in the mid-latitudes, the other in the subtropics) that will merge before entering North America. At this time, the amount of moisture available with this event is highly uncertain, as can be seen from the very large spreads in the PW forecast by June 27^th, especially east of the Continental Divide. However, some precipitation appears likely over southwest Colorado. Thereafter, a period of drying looks to keep this quiet with seasonable temperatures.

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

Event #1: Wednesday (6/21) through Friday (6/23)

A multi-day period of enhanced precipitation, with one day of Elevated Flood Threat

A Pacific-origin cool front will come in from the northwest in combination with a southward movement of the main jet stream. Precipitation chances will first increase on Wednesday, with some scattered shower and storm activity likely towards the Kansas border. The highest threat, at this time, appears to be Thursday as Precipitable Water values exceeding 1 inch get drawn into eastern Colorado. Scattered to numerous showers and thunderstorms are expected on Thursday, with the highest coverage being in eastern Colorado. A severe weather threat, including large hail and tornadoes, is also anticipated. In terms of rainfall, expect hourly rainfall up to 1.25 inches per hour in the Front Range to 1.5 inches per hour farther east. An elevated flood threat will exist mainly for isolated flash flooding.

By Friday, most of Colorado will be under a more stable air mass, capping the potential for heavy rainfall. However, in the southeastern part of the state, residual moisture along the cool front combined with the diurnal upslope circulation will likely trigger isolated to scattered showers and storms. Rainfall amounts are expected to stay below 1.0 inch.

Event #2: Monday (6/26) and Wednesday (6/28)

No Anticipated Flood Threat as pulse of monsoon moisture arrives in southwest Colorado

After a temporary ridge following Event #1, two disturbance will combine to increase precipitation chances over Colorado. However, the placement is currently uncertain. A shortwave in the main polar jet stream will absorb a subtropical feature as it enters the western North America coast by late Sunday. By Monday, Precipitable Water exceeding 0.5 inch will support isolated to scattered shower and weak thunderstorm activity mainly over the San Juans and Southwest Slope. Rainfall up to 0.5 inches will be possible. On Tuesday and Wednesday, the disturbance will move eastward but the amount of low-level moisture remains uncertain. It is possible that heavy rainfall will occur both days east of the Continental Divide. However, at this time, the precipitation map below is limited to areas where there is reasonable confidence of widespread rainfall. Check back this Thursday for an updated look at this event.