FTO 08-03-2017: Very Active Weather Over The Next 7 Days

Issue Date: Thursday, August 3, 2017
Issue Time: 1PM MDT
Valid Dates: 8/4-8/18

Changes have been, and will continue to be in Colorado’s air for this edition of the 15-day Flood Threat Outlook. As shown in this morning’s water vapor image, below, an upper-level trough has established itself over the northern Great Plains. This has allowed for cool air out of Canada to be transported into eastern Colorado. For example, today’s high temperatures are up to 20F below normal in the northeast part of the state.

As shown in the 500mb GFS ensemble forecast maps, below, this trough is expected to persist not just in the short-term but possibly for the next 7 or more days. Meanwhile, Precipitable Water, while lower today than last week’s impressive string of elevated levels, remains above average. The positioning of the Great Plains trough will allow for a slew of mid-level disturbances to enter Colorado, especially the northeast quadrant of the state. In addition, many of these disturbances will be followed by weak to moderate strength surface high pressure systems such as the one currently moving southward. With some degree of elevated moisture remaining in the state, we foresee a prolonged period of active weather, mainly east of the Continental Divide.

Although active weather is expected, instability will not be as much of a given as in the past few weeks especially in northeast Colorado. Another important wildcard is that Great Plains trough will keep strong mid and upper-level winds around. For example, below is the forecast for the surface to 500-mb wind shear (keep in mind, this value can only be positive) at Denver. For reference, the average value for this time of year is about 15 – 18 knots. Instead, what we see is consistently strong shear with many days approaching or exceeding 30 knots. As some of you weather gurus know, there is only one implication of this in the summer: severe weather. Strong shear promotes tilted, possibly rotating storm updrafts that in turn support large hail and the transfer of momentum from the high mid-level wind speeds down to the surface. Thus, we foresee that severe weather will be a frequent component of eastern Colorado’s forecast for the next week.

Finally, most of the discussion has focused on eastern Colorado. Given that we are right in the middle of monsoon season, it would be amiss to not discuss the western slope. As shown below, while Denver’s PW will remain elevated for the next 7+ days, Grand Junction (and most of the western slope) will be near average to below average. The positioning of the upper-level ridge over the southwest US is not particularly conducive for moisture transport into western Colorado. Thus, although some isolated moderate to heavy rainfall will be possible especially over the shorter term, we expect drier than normal weather overall.

We have identified two precipitation events for this Outlook, which are described in more detail below.

Event #1: Friday (8/4) through Wednesday (8/9)

Elevated Flood Threat for daily rounds of heavy rainfall over mainly eastern Colorado

The combination of passing mid-level shortwaves and a one or more surface high pressure systems will allow for a very active weather pattern, especially in eastern Colorado. While the far northeast part of the state will experience very cool conditions, farther south, expect diurnal upslope flow to result in scattered to numerous showers and thunderstorms that will quickly move off the mountains and into the eastern plains. Over the next 5 days, widespread rainfall above 1 inch is likely. However, many locations will receive much more than that, with up to 4 or 5 inches not out of the question. On a daily level, max 1-hour rainfall up to 2.1 inches (east) and 1.4 inches (west) will be possible though 3 hour rainfall up to 3 inches could also occur. Severe weather will be possible nearly every day from Friday through Wednesday. Large hail will be the primary threat, with gusty winds also possible.

The main impacts from this event will be isolated flash flooding, higher elevation mud flows and debris slides (especially over fire burns) and street flooding in urban areas. However, we are also becoming increasingly concerned about riverine flooding within the Arkansas River basin where the past 30 days have seen near record rainfall over a wide area. Current flows from the headwaters downstream to the Kansas border are in the 75th to 90+ percentiles. The riverine flooding threat will increase as this event wears on. Please check back for Monday’s Outlook as well as daily Flood Threat Bulletins for the latest information.


Event #2: Thursday (8/10) through Saturday (8/12)

No Apparent Flood Threat for isolated heavy rainfall in far eastern parts of the state

Drier air will eventually encroach on Colorado from the northwest, which will reduce heavy rainfall potential following Event #1. However, with the Great Plains trough expected to stay in place, return flow could provide adequate moisture for isolated heavy rainfall in far eastern areas. In addition, the severe weather chances are likely to stay elevated. Currently, it is unclear if the flood threat will be within Colorado’s border, hence why this event is not assigned a threat yet.


FTO 07-31-2017: Drying Short-Lived As Moisture To Find Its Way Back Into Colorado

Issue Date: Monday, July 31, 2017
Issue Time: 1:30PM MDT
Valid Dates: 8/1 – 8/15

After quiet a memorable stretch of active weather, noticeable drying has begun across roughly the western half of our state. Before getting to today’s Outlook, we want to reflect on just how impressive the recent rainfall has been. Below are 30-day precipitation estimates from the NOAA River Forecast Centers. Isolated parts of southeast Colorado have received over 10 inches (even as high as 15 inches) of rainfall since July 1st. Even more impressively is that this has occurred through a prolonged stretch of days with very heavy rainfall, as opposed to one or two events. So how does this stack up with climatology? Average July precipitation depends strongly on elevation, but east of the Continental Divide, roughly ranges from 1.5 inches in the lowest elevations to as much as 4 inches in the foothills of the Palmer Ridge and Wet Mountains. Thus, over the last 30 days, isolated locations circled in the left map below, have experienced anywhere from 3 to 5 times their normal rainfall. Looking at NOAA Atlas 14 guidance, this equates to a 1 in 100-500 year event, depending on your exact location. For example, in eastern El Paso County, Atlas 14 estimates that a 30-day rainfall of 10.3 inches is a 1 in a 100 year event, with the 1 in the 1000 year event estimate being 13.7 inches.

Shifting the focus back to the Outlook, this afternoon’s water vapor image, below, shows some notable changes in the weather pattern across western North America. The upper-level ridge located south of Colorado, which has been responsible for moisture advection into the state, has expanded westward. The upper-level flow is currently switching to a more westerly component, which is now advecting much drier air from the west (see dryness over California). In addition, a strong surface high pressure is expected to move southward out of Canada over the next 72 hours. A cool (or even “cold”?!) frontal passage is expected across eastern Colorado that will limit high temperatures to as much as 15F below normal by mid-week. Frontal dynamics will likely support light to moderate precipitation that we label Event #1, but with limited instability, we currently do not expect a flood threat. Thereafter, a dynamic pattern will ensue with an upper-level trough expected to establish across the central United States. Frequent shortwave and surface frontal passages are expected across eastern Colorado, which should be accompanied by at least 24 hours of return flow to elevate moisture levels. We expect a multi-day elevated flood threat (Event #2) mainly across northeast Colorado. However, with fast steering winds, only isolated heavy rainfall (though with possible severe weather) is currently expected.

The forecasted Precipitable Water plumes from the GFS Ensembles, below, show that Denver will continue to experience average to above average moisture levels. Meanwhile, Grand Junction will remain mainly below normal. Overall, we expect a pause in the monsoon, and an increase in “action” for northeast Colorado.

We have identified two precipitation events for this Outlook, which are described in more detail below.

Event #1: Tuesday (8/1) through Thursday (8/3)

No Apparent Flood Threat though light to moderate precipitation will be possible

Elevated moisture levels will continue to be scoured out of Colorado as upper-level westerly flow advects in drier air. On Tuesday and Wednesday, scattered thunderstorms will be possible mainly across southern Colorado. However, with faster steering winds, total precipitation is expected to stay on the lighter side, in the 0.5 – 0.75 inch range (hence, no precipitation map). On Thursday, a strong, fall-like cool front will move southward across eastern Colorado, stabilizing the air mass and putting a cap on heavy rainfall coverage and intensity. Some upslope rain showers and perhaps a weak thunderstorm are expected, but flooding is not expected at this time.

Event #2: Friday (8/4) through Monday (8/7)

Elevated Flood Threat for isolated heavy rainfall with current guidance suggesting northeast Colorado as target

Summer-time cool fronts are fleeting and return southeasterly flow will quickly set up by Friday. Precipitable water is expected to return back into the 0.9 – 1.1 inch range (which is above normal). Isolated thunderstorms will return to our forecast then, and increase in coverage and intensity into the weekend as one or more additional weak frontal passages is possible. These fronts will act more of focal points for storms, with little temperature changes. The northeast quadrant of the state is expected to see the highest coverage of thunderstorm activity. At this time, we foresee max 1-hour rain rates up to 1.2 inches (west) and 1.6 inches (east). In addition, severe weather chances, are expected to increase as bulk wind shear ramps up to above 30 kts. We have labeled this event with an elevated flood threat mainly for isolated flash flooding. Stay tuned to the next Outlook for a better assessment of this event.


FTO 07-27-2017: Streak Of Heavy Rainfall To Continue, Then Substantial Drying

Issue Date: Thursday, July 27, 2017
Issue Time: 3PM MDT
Valid Dates: 7/28-8/11

If you have ever read either our Flood Threat Bulletin or Flood Threat Outlook discussions, you can probably quickly recall how much emphasis we place in analyzing moisture content when developing the flood threat forecast. The reason for this is that sometimes, especially in the summertime, moisture is as good or better predictor for heavy rainfall that any other guidance, including model Quantitative Precipitation Forecasts! For you technical readers, please refer to this paper in the Monthly Weather Review journal for more information. In short, it is not so much that moisture is a good predictor of where or when thunderstorms will develop, or even how long they last, but it is a good predictor of the upper limit of their potential rain rates.

So how has moisture fared across Colorado recently? With little shock, the table below shows significantly above normal amounts over (at least) the past 7 days, as measured by column integrated Precipitable Water at both the Denver and Grand Junction evening soundings (the asterisk in the measurements at Denver signify that the radiosonde did not sample the entire atmosphere, implying that this reading underestimates the actual PW). This was recently punctuated by values over 70% above normal, interestingly coinciding with one of the wettest 48-hour periods observed thus far this summer.

As this afternoon’s water vapor image shows, below, the same familiar players will once again be in charge of flood potential for the next 5 days or so. The first feature is the upper-level ridge that is currently centered just south of Colorado. As long as the western periphery of this ridge does extend too much west of the Four Corners region, this typically allows for the transport of low and mid-level level monsoon moisture into Colorado. This is expected to continue over the next 5 days, keeping elevated moisture and above normal heavy rainfall chances in the picture.

The second, and often overlooked feature, is the presence of surface high pressure features that act to (i) move southward “cool” fronts, especially across eastern Colorado and (ii) more importantly, provide enhanced pulses of moist upslope flow that can interact favorably with the climatological circulation to accentuate and expand the lifetime and eastward extent of the elevated convection over our higher terrain. As shown below in an interesting analysis of surface pressure anomalies available on the Tropical Tidbits model site, another surface high pressure is expected to move southward out of Canada. By Sunday and Monday, a stronger than normal fetch of moist, upslope easterly flow is expected across southeast Colorado. The result will be a pattern conducive for slow moving storms capable of producing very heavy rain rates, much like on Wednesday.

After the passage of this surface high pressure, guidance has now come into good agreement that the upper-level ridge will shift westward, which will drastically reduce moisture levels across most of Colorado. Heavy rainfall and precipitation chances are expected to lessen to below their climatological average accordingly.

For this edition of the 15-day Flood Threat Outlook, we have identified one prolonged Elevated/High flood threat event that is discussed in more detail below.

Event #1: Friday (7/28) through Tuesday (8/1)

Elevated/High Flood Threat for flash flooding, debris slides and mud flows, especially over higher terrain

Above normal moisture will persist statewide through early next week. Precipitable water is expected to remain in the 0.9 – 1.6 inch range, which will support daily bouts of scattered to numerous shower and thunderstorm activity from Friday through Tuesday. Rain rates up to 1.4 inches per hour (west) and 2.9 inches per hour (far east) will be possible, which will support an elevated flood threat. Highest coverage will be over the higher terrain, especially east of the Continental Divide as well as southeast Colorado. On Sunday/Monday, a cool front is expected to enhance rainfall coverage over southeast Colorado and amplify the intensity further, sustaining longer duration heavy rainfall. Current guidance suggests 3-hour rainfall up to 4 inches will be possible in the foothills of the Sangre de Cristo and Wet Mountains. Max 3-hour rainfall up to 2.4 inches will be possible in the San Juan Mountains.

As with the recent heavy rainfall event on Wednesday, this event will not have significant upper-level support implying that sustained heavy rainfall, beyond 3 hours, will not be likely. Thus, currently the main impacts of this event will be mud flows and debris slides in higher elevation locations, as well as small stream flooding associated with the stronger cells. Major riverine flooding is not expected at this time. Please stay tuned to daily Flood Threat Bulletins for more detailed information, especially if you plan on camping and hiking over the weekend.


FTO 07-24-2017: Prolonged Elevated/High Threat Initially Then Pattern Change Possible

Issue Date: Monday, July 24, 2017
Issue Time: 3PM MDT
Valid Dates: 7/25 – 8/8

High moisture content has been a staple across most of Colorado for the past two weeks. The map below shows the composite Precipitable Water (PW) anomaly across the southwest United States over the past 2 weeks, from NOAA ESRL’s excellent composite page. Note the broad positive anomalies (units are kg per square meter…multiply by 0.03937 to get inches), generally equating to anywhere from 10-20% above normal east of the Continental Divide to as much as 30% above normal across far western Colorado. Individual days have seen even higher anomalies.

After a rather dry start to July, rainfall has quickly accumulated across large portions of Colorado. This has not been so much in the form of organized events, as opposed to isolated to scattered storms that produce localized heavy rainfall as they slowly drift along in the weak steering winds. As shown below, this has allowed a large portion of the state to be above average for July precipitation, up to 300% above average in parts of the western slope as well as southeast Colorado. Interestingly, regions north of the Palmer Ridge and northeast Colorado have seen a pronounced minimum in precipitation due to a variety of reasons (which will unfortunately not be discussed here).

As shown in this afternoon’s water vapor image, below, we continue to foresee moisture levels to remain at elevated levels. This is being supported by two main features. First a weak upper-level ridge will provide a south/southwesterly fetch of very high moisture content into western Colorado (a.k.a. monsoon flow). PW values as high as 1.4 inches are expected over the next 48 hours, especially closer to the Utah border. Some of this moisture will be able to make it over the Continental Divide and provide heavy rainfall along the eastern foothills and mountains. In addition, a surface high pressure is expected to move south-southeast out of Canada starting Wednesday. This will deliver a weak cool front across eastern Colorado, providing a focus for thunderstorm activity. As the front slows on its movement southward, plenty of moisture will be available to the daily upslope circulation formed due to our higher terrain. Thus, although we do not foresee an organized flood threat, a High flood threat is in place for the next 72 hours for isolated, but very heavy rainfall capable of causing flash flooding. After this event wanes towards the end of the week, long range guidance is indicating a potential shift to a drier weather pattern. Thus, only one Event is identified for this Outlook. It is described in more detail below.

Event #1: Tuesday (7/25) through Monday (7/31)

High/Elevated Flood Threat for flash flooding, debris slides and mud flows, especially over higher terrain

Significantly above average moisture is expected to hang around Colorado for the next week or so. The original source of the moisture will be via a monsoon surge from the south/southwest. This will allow for daily rounds of shower and thunderstorm activity over the western slope Tuesday and Wednesday. Some of the season’s heaviest rainfall is expected with max 1-hour rainfall up to 1.5 inches and 24-hour totals up to 2.9 inches possible. Isolated flash flooding will be possible, but the bigger threat will be for mud flows and debris slides associated with short-term heavy rainfall as well as in areas with wet soil. The climatologically favored western parts of the San Juans, the Uncompahgre Plateau and the Roan Plateau/Cliffs will be the highest threat areas. However, with moisture at such elevated levels, expected heavy rainfall to make its way all the way down into the lowest valleys. Small stream flooding will be possible with the heaviest rainfall. However, major riverine flooding is not expected.

By Thursday, a surface cool front and surge of moisture is also expected from the northeast as a surface high pressure moves southward out of Canada. With PW up to 1.5 inches (towards the KS border) and up to 1.2 inches even along the foothills, very heavy rainfall will be possible, to the tune of 2.9 inches per hour (east) and 2.1 inches per hour (foothills). The best coverage is expected for the highest terrain of the Palmer and Raton ridges, Sangre de Cristos and Wet Mountains. Isolated flash flooding is expected with the heaviest rainfall. Without organized upper-level forcing, widespread heavy rainfall, and thus major riverine flooding, is not expected though small stream flooding will be likely. However, please check back to daily Flood Threat Bulletins for much more detailed information, since large storm complexes can develop even without substantial upper-level forcing in Colorado.