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.

Legend

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.

Legend

SPM 09-28-2017: Multiple Rounds of Rain for the San Juan and Southeast Mountains

Issue Date: Thursday, September 28, 2017
Issue Time: 10:50AM MDT

Summary:

Another gloomy and rainy day across the Front Range and southern mountains yesterday as the upper level Low over AZ provided lift for multiple rounds of showers. The Low continued to produce southerly winds over Colorado, which sustained the stream of moisture from the Gulf of Mexico. As the jet streak moved over the state with the northward movement of the Low, showers/snow continued overnight along the Front Range and Southeast Mountains. The jet also enhanced the intensity and increased the coverage of storms overnight. This morning, the jet continues to provide lift for the showers east of the Continental Divide over the Southeast Plains. Drier air began working its way into western Colorado during the evening yesterday, so showers over the SW corner of the state ended by 9PM and produced lower 24-hour accumulations.

Several CoCoRaHS stations across the Southeast Mountains recorded amounts greater than 1 inch. The USGS Fort Carson automated gage recorded 1.76 inches. Over Las Animas County, a USGS gage to the northeast of Trinidad, recorded 2.32 inches. A SNOTEL site in the southern Sangre de Cristo range recorded 1.4 inches of rain. Radar rainfall estimates show multiple areas receiving more than 2 inches with 2-hour rain rates up to 0.5 inches. To the west, Pagosa Springs recorded 24-hour totals around 1.15 inches. The highest radar rainfall estimates are over San Juan and eastern Dolores Counties where radar indicates just over 0.5 inches of rain fell. While river gages did record elevated flows due to runoff, there was no riverine flooding reported.

There were no reports of flooding yesterday as of this morning. To see how much rain fell in your area, scroll down to the State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas since 2012. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.

FTB 09-28-2017: Another Round of Showers for Eastern Colorado as the West Begins to Dry Out

Issue Date: Thursday, September 28, 2017
Issue Time: 10:40 AM MDT

— Flooding is NOT expected today

Showers continue this morning over the eastern high terrains and adjacent plains. The showers over the Southeast Plains are being support by the jet stream, which has lifted north with the Low pressure system. Dense fog was reported over Delta County with visibility down to 50 feet. Other automated stations over the mountains have also reported fog with the decreased temperatures and high dew points. Precipitable Water (PW) at Denver this morning was 0.77 inches and 0.64 inches in Grand Junction. The western portion of the state is better positioned in the dry slot today, so PW is expected to decrease throughout the day. The entrainment of dry air with the more southwest flow will limit rainfall totals over the higher terrains and valleys and decrease shower activity throughout the day.

Another day of unsettled weather and little sunshine as the upper level Low continues to slowly lift to the northeast. The jet stream and mid-level energy will provide ample vertical motion for showers and isolated thunderstorms over the mountains and eastern plains today. Moisture will remain above climatology with the southerly and southwesterly surface winds east of the Continental Divide. Expecting the first round of showers, currently over the Southeast Plains, to move northeast throughout the morning before the next round of showers this afternoon and evening. With cloudy skies, not much instability should build this afternoon, so the threat for thunderstorms and high hourly rain rates are marginal. Highest 24-hour accumulations are expected over the southern portion of the Southeast Plains with totals up to 1.5 inches. Over the higher terrains, the highest accumulations are expected over the Southeast Mountains and western Raton Ridge again. Expected more isolated showers over the northern high terrains this afternoon and evening. Snow will be likely again at the higher elevations with the snow line dropping to around 9,000 feet overnight. Overnight, showers will remain in place over the eastern plains and Southeast Mountains, but expecting little to no accumulation north of I-70. Flooding is not expected today.

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.

Zone-Specific Forecasts:

Southeast Slope, Grand Valley, Northwest Slope, Northern Mountains, Central Mountains, San Juan Mountains, San Luis Valley:

With most of the region being under the influence of the dry slot, showers are expected to decrease in coverage and intensity throughout the day. More isolated showers are expected over the higher terrains this afternoon with max 3-hour rain rates up to 0.7 inches (north), 0.4 inches (central) and 0.8 inches (southeast) possible. Flooding is not expected today.

Primetime: 11AM to 10PM

Front Range, Urban Corridor, Southeast Mountains, Raton Ridge, Southeast Plains, Palmer Ridge, Northeast Plains:

Cloudy with showers this morning over the southern mountains and eastern plains. Another round of showers is expected later this afternoon and evening after the first round exits the region to the northeast. Over the higher terrains, max 3-hour rain rates up to 0.6 inches (south) and 0.4 inches (north) are possible. Over the Southeast Plains, max 3-hour rain rates up to 1 inch are possible. Flooding is not expected today.

Primetime: 11AM to 7AM

SPM 09-27-2017: Showers for the Front Range and Southeast Mountains

Issue Date: Wednesday, September 27, 2017
Issue Time: 10:50AM MDT

Summary:

As the disturbance from the end of last week began to move eastward, general subsidence occurred across the state. The settled weather pattern did not last long as the next trough began to dig south over the desert southwest. By late afternoon, showers associated with the Low pressure over Arizona arrived to the southern mountains. At the same time, weak upslope flow initiated showers along the Front Range. Precipitable Water (PW) and instability were limited, so the precipitation yesterday was in the form of general showers and stratiform rain. Rain continued overnight and into this morning along the Southeast Mountains. Overnight low temperatures remained relatively warm due to the increased cloud cover.

Despite relatively low atmospheric moisture, persistent rainfall yielded some impressive 24-hour totals along the Front Range and Southeast Mountains. Over the Front Range, a handful of CoCoRaHS stations in Clear Creek, Jefferson and Park Counties recorded 0.65 inches. Over the Southeast Mountains, CoCoRaHS totals were in the 0.3 to 0.5 inch range. Radar estimated 24-hour rainfall just over 1 inch in Las Animas County. Drier air over the southwest Colorado state made showers more isolated in nature yesterday. SNOTEL stations over the San Juan Mountains recorded up to 0.3 inches. While there was no flooding reported Tuesday, it is important to note the 24-hour totals. Increased moisture is forecasted from Wednesday to Friday, and an uptick in rainfall intensity is expected. Antecedent rainfall can saturate the soil, which will increase runoff and the potential for mud flows and debris slides in the high country.

To see how much rain fell in your neighborhood, scroll down to the State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas since 2012. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.