FTO 09-28-2017: Prospects Of Heavy Rainfall Continue To Hang Around, Later Than Normal

Issue Date: Thursday, September 28, 2017
Issue Time: 1PM MDT
Valid Dates: 9/29-10/13

Note: This is the last Flood Threat Outlook for the 2017 season. We will be back next spring, starting May 1.

After a fairly quiet start to September, a sudden recent change in the weather pattern has now put most of eastern Colorado in the above normal precipitation category for the month (see below). In fact, some locations in the Southeast Plains have accumulated 2-3 times their normal precipitation. The vast majority of the precipitation has fallen in relatively orderly fashion and flooding has largely been avoided (just like in the summer, thankfully). West of the Continental Divide, the San Juans have fared well with close to normal amounts, but other locations are below normal for September precipitation.

As shown in the Precipitable Water (PW) composite anomaly for the past 7 days, a large area of above normal moisture has existed in the Central and Northern Great Plains. Anomalies of up to 0.2-0.4 inches (the units below need to be multiplied by 0.04 to arrive at inches) have been found across OK, KS and NE. With even weak return (easterly) flow, pulses of this moisture have found their way into the Arkansas River valley, supporting the widespread generous precipitation.

As shown in the water vapor image, below, we expect a continuation of a very active pattern over the next week or so. The current disturbance, presently located over the Four Corners, will trek slowly NNE within the large scale trough in the subtropical jet stream. It will continue to provide mainly light to perhaps moderate precipitation through this weekend (Event #1). However, overall, this large-scale trough is not expected to move anywhere fast and a new shortwave, currently in the Gulf of Alaska, will help keep the western US in an unsettled weather pattern. This is identified as Event #2.

Unfortunately, the forecast for Event #2 has a very large amount of uncertainty with it. This can be easily seen in the forecasted PW plumes for Denver and Grand Junction, below. At both locations, not the sudden increase in spread beginning on Wednesday, 10/4. This arises due to uncertainty in the southern extent of the new trough, as well as how long of a return flow will exist from the Gulf of Mexico. In short, the range of PW forecasts is from well below normal to near October record levels approaching or exceeding 1 inch at both locations! Due to the continuation of much above normal PW in the Central Plains, a brief Elevated threat is warranted for eastern Colorado during Event #2. However, it is essential to stress that this may change. However, if a flood threat appears, we will be doing special Flood Threat Bulletin(s) as warranted.

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

Event #1: Friday (9/29) through Sunday (10/1)

No Apparent Flood Threat as precipitation coverage gradually steps down from Friday through Sunday

With enough moisture available, along with favorable dynamics, we expect pulses of rain and snow shower activity to continue through Sunday. The highest coverage will be Friday, favoring the upslope regions of the San Juans and Northern Mountains. Activity will subside by Saturday, though isolated to scattered showers will still be possible mainly over the higher terrain. By Sunday, there is a chance of enhanced precipitation coverage in far eastern Colorado as a warm sector is expected to setup here. At this time, the highest instability and low-level moisture is expected to remain in KS. Thus, while hourly rainfall rates up to 0.8 inches could occur along the KS border, this would not be enough to cause flooding concerns.


Event #2: Tuesday (10/3) through Friday (10/6)

Brief Elevated Flood Threat as another large system approaches Colorado; widespread rain/snow likely

After only a short lull following Event #2, another shortwave is expected to reinvigorate the western US trough, leading to statewide unsettled weather beginning on Tuesday. Widespread rain and snow showers are expected over the higher terrain mainly west of the Continental Divide on Tuesday, spreading eastward on Wednesday. Total precipitation (rain and snow) of up to 1.5 inches could occur in the favor upslope regions of the San Juans, Central Mountains and Northern Mountains. The biggest uncertainty at this time is for eastern Colorado. Some guidance is suggesting that return moisture advection will be strong enough to bring PW over 1 inch east of the Continental Divide. This will be supported by a relatively strong low-level surface cyclone expected to develop in eastern Colorado. Moderately strong instability, to the tune of Convective Available Potential Energy exceeding 750 J/kg, is possible in the eastern third of the state. This is where the highest risk of heavy rainfall and flooding will occur on Wednesday. At this time, the chance of 1-hr rainfall rates exceeding 1.5 inches and 3-hour rates exceeding 2.5 inches is possible, though not yet probable. If this materializes, we will be providing a special Flood Threat Bulletin(s).

After Wednesday, a gradual decrease in moisture is expected statewide. Precipitation coverage will accordingly decrease though scattered rain and snow showers will be possible, especially over northern Colorado, through Friday.


FTO 09-25-2017: Unsettled Weather Pattern Continues with a Cutoff Low Over the Desert Southwest

Issue Date: Monday, September 25, 2017
Issue Time: 3PM MDT
Valid Dates: 9/26 – 10/10

The slow moving, high-amplitude weather pattern that has influenced Colorado’s weather since late last week will continue to impact the state for the first portion of this forecast period. Today and tonight the first trough, that has driven the unsettled weather pattern over Colorado since late last week, will finally start to lift to the northeast. Another trough begins to dig south Tuesday and a closed Low forms over AZ/UT by Wednesday morning. This closed Low spins and remains nearly stationary over Utah until Thursday afternoon when it begins to lift northeast (Event #1). By Saturday morning, more westerly flow will replace the southwesterly flow aloft bringing warmer temperatures and should confine showers to the high terrains. A weak trough forms over the Pacific Northwest Saturday, and by Sunday afternoon/evening the disturbance makes its way into Colorado for Event #2. By mid-week, more zonal flow is expected, which will decrease the chances for heavy rainfall.

Currently average to below average Precipitable Water (PW) is present in Denver and Grand Junction. Beginning Tuesday, PW is forecasted to increase rapidly for locations east and west of the Continental Divide. The upturn in PW is expected to last through Sunday morning. This intensification of low level moisture occurs due to the counterclockwise rotation of the surface Low over the desert southwest. To the east of the Low (Colorado) there will be an increase in south/southeast surface winds, which will draw in moisture in from the Gulf of Mexico. The GEFS has PW values over 1 inch on Thursday over the Southeast Plains, which is 30-40% above average for this time of year. The uptick in moisture occurs in tandem with multiple shortwaves from the cutoff low. This will create an Elevated Flood Threat for Thursday and Friday. Although Event #2 occurs directly after Event #1, the atmosphere is expected to dry out quite a bit. This decreases the chances for heavy rainfall, and at this time there is No Apparent Flood Threat.

Below we describe the identified precipitation event in more detail.

Event #1: Wednesday (9/27) – Saturday (9/30)

Elevated Flood Threat with highest accumulations expected over the southern mountains and eastern plains

The high amplitude system over the western US continues to drop multiple troughs to our west. The next trough is expected to dig south Tuesday, and a cutoff Low forms over the desert southwest by Wednesday morning. This pattern is known for producing heavy rain in southern Colorado and along the Front Range. As is expected with a closed Low system, little to no movement occurs as it draws in moisture on its east side from the Gulf. Moisture is expected to increase rapidly both east and west of the Continental Divide starting Tuesday and reach its maximum Wednesday and Thursday. At the same time, multiple shortwaves and an upper level jet stream over the state are expected to increase lift. Wednesday, showers are expected to be strongest over the Central and San Juan Mountains. By Thursday showers and thunderstorms will spread to the east over the Southeast Mountains, Palmer Ridge, Raton Ridge and Southeast Plains. With high PW values, especially over the Southeast Plains, there is an Elevated Flood Threat for Thursday and Friday. Severe thunderstorms may be possible both days over the Southeast Plains should instability be able to pair with the available shear. The Low lifts to the north/northwest overnight Friday. Showers are likely again Saturday with residual moisture in the atmosphere, but accumulations will be confined to the higher terrains.


Event #2: Sunday (10/1) – Monday (10/2)

No Apparent Flood Threat as an upper trough develops over Montana and moves into the state

A second, weaker trough then develops Saturday following Event #1 over the Pacific Northwest. By Sunday afternoon, the trough moves into Colorado with the jet stream positioned over the northwest corner. Increased shortwave activity and upper level support will bring another round of showers to the state Sunday afternoon and Monday. Drier air works its way in from the desert southwest, so precipitation should be confined east of the Continental Divide. Flood is not expected at this time, but please check back to Thursday’s FTO to see if a special FTB will issued Monday.


FTO 09-21-2017: Powerful System To Bring The Entire Menu Of Weather, Including Early Flood Threat

Issue Date: Monday, September 21, 2017
Issue Time: 1PM MDT
Valid Dates: 9/22 – 10/6

This afternoon’s water vapor image of the eastern Pacific and western North America shows a high amplitude pattern over and to the west of Colorado. A high amplitude pattern sometimes implies that disturbances will be slow to move eastward, and such is the case today. The strong trough that has now entered the western US coast will very slowly trek eastward and provide a long duration precipitation event (Event #1) for most of Colorado. In addition, a particularly moist airmass has developed over the US Central Plains. Precipitable Water (PW) values up to 2 inches will stream northward into OK, KS and NE. While we certainly do not expect these kind of values to make it into Colorado (which would be record highs even during the summer), some of this moisture will be available to draw from.

Thus, we expect a very active period over the next 5 days or so. Initially, there will be an elevated fire danger across central CO due to high winds and low relative humidity. By Saturday, colder air will slowly over take the state from the northwest. Friday and Saturday (and possibly Sunday) will feature elevated chances of severe weather across the far eastern Plains, and possibly as far west as the Palmer Ridge. By Sunday, much cooler/colder air will overtake most of the state, changing the dominant precipitation to stratiform, as opposed to convective, rain and snow showers.

In terms of the flood threat, the forecasted PW plumes at Denver and Grand Junction, below, show a very large moisture gradient developing by this weekend. For example, by late Saturday, Denver’s PW could be as high as 0.9 inches while Grand Junction is expected to remain below 0.6 inches.

In fact, just these two sites do not tell the entire story. The forecasted PW at Lamar (below) shows values as high as 1.4 inches. While there is no long-term record of PW here, data from Goodland, KS (which is almost always moister than Lamar) shows that during late September, the 90th percentile of PW is about 1.1 inches. Thus, it is safe to say that the forecast values are well above average and likely close to record values. In addition, due to the slow movement of the trough, we expect a high potential of thunderstorm training. Although the higher flood threat remains just to the east of CO into KS and OK, an elevated flood threat will exist for southeast CO on Friday, Saturday and possibly Sunday for isolated flash flooding in southeast Colorado. Check back for our daily Flood Threat Bulletins for the most updated information.

Below we describe the identified precipitation event in more detail.

Event #1: Friday (9/22) through Wednesday (9/27)

Early Elevated Flood Threat for Southeast Plains transitioning to scattered rain and snow later in the event

A prolonged period of active weather is expected across Colorado starting on Friday. Multiple rounds of severe thunderstorms are expected in the Southeast Plains (and possibly Palmer Ridge) on Friday and Saturday, with decreasing chances by Sunday. Gusty straight-line winds and large hail (up to 2.5 inches) are expected to be the main threat. Isolated heavy rainfall will be likely in mainly far southeast Colorado on Friday and Saturday. Max 1-hour rain rates up to 2.5 inches will be possible, with max 3-hour amounts up to 4 inches. Training of storm cells could lead to a prolonged threat of heavy rain. Isolated field and road flooding appears likely, with isolated flash flooding also possible though only localized impacts are expected. Riverine flooding on the Arkansas River basin is NOT currently expected, but will be likely farther east in OK and KS.

By Sunday, cooler air will overtake most of the state leading to a sharp reduction in precipitation rates. Mountain snow levels will drop to valley floors especially north of I-70. Several inches of snow accumulation is expected, mainly north of I-70. Scattered rain and snow showers are expected to continue through Wednesday as the upper-level disturbance sticks around the state.


FTO 09-18-2017: Strong System Expected to Bring Rain and Cooler Temperatures This Weekend

Issue Date: Monday, September 18, 2017
Issue Time: 3:50PM MDT
Valid Dates: 9/19-10/3

A much drier and cooler pattern arrived to Colorado toward the end of last week. As the cold front moved through the state, the streak of September heat was brought to an end. While temperatures will continue to rebound today and tomorrow, another cold front is forecasted to bring more climatologically normal temperatures Wednesday to eastern Colorado. There is one precipitation event that has been identified for this Outlook. In the water vapor imagery below, this event can be seen over the Pacific Northwest. The strong trough is forecasted to dig south, west of Colorado, and produce an unsettled weather pattern over the state beginning later this week. After this trough moves through, a quieter, more zonal pattern sets up for the remainder of next week. While models hint at another trough moving through the area at the end of this Outlook, confidence in details are not high enough to issue any threat at this time.

Below average Precipitable Water (PW) is forecasted for Denver and Grand Junction through Thursday night. As the trough digs south, winds are expected to increase in speed and become more southwesterly at the surface. The low relative humidity and warmer temperatures will create extreme fire danger from Tuesday night through Friday morning. Please avoid outdoor burning as increased wind speeds can quickly spread fires. Towards the end of the week, PW values increase over the state with the eastern portion of the state remaining above average through the weekend. The increase in moisture occurs the same time the west coast trough begins releasing shortwaves into the state. The upper level dynamics and increased moisture could bring heavy rain and severe weather to the eastern plains and snow to the high country on Saturday.

Below we describe the identified precipitation event in more detail.

Event #1: Friday (9/22) through Tuesday (9/26)

Elevated/No Apparent Threat as increased moisture pairs with upper level dynamics across Colorado

Moisture returns to southwest Colorado and the far eastern plains later this week. Increasing SW and SSW surface winds will draw in moisture as the trough digs south, west of Colorado. On Friday, the main upper level trough will begin to eject shortwaves over the state that will cause enhanced lift and unsettled weather for southwest Colorado. Saturday, a cold front associated with the west coast trough is expected to move through the state and drop temperatures enough over the mountains that a couple inches of snow may fall at elevations above 9,500 feet. Over the plains, the increased dynamics paired with higher moisture is expected to create favorable conditions for severe weather and prolonged rainfall. This main trough moves northeast through Colorado overnight on Sunday. An Elevated Flood threat has been issued for Friday and Saturday with no apparent threat on Sunday.

A second, weaker trough then develops southwest of Colorado on Monday and Tuesday. Southwest flow aloft will allow the mid-level energy to move across the state during this period. PW is expected to decrease quite a bit after Sunday, but there should be enough residual moisture for daily rounds of rainfall. At this time, there is no apparent flood threat on Monday and Tuesday.