FTB 09-30-2016: Moisture Spreading Eastward

Issue Date: 9/30/2016
Issue Time: 10:03 AM

NO FLOOD THREAT IS FORECAST.

REMINDER: THIS IS THE FINAL FTB OF THE 2016 SEASON. IT HAS BEEN A PLEASURE TO SERVE YOU. THE NEXT FTB SEASON BEGINS ON MAY 1, 2017.

The moisture plume that has been laying across western Colorado for a couple of days now is on the move eastward, evidenced in the IPW chart below. Grand Junction IPW (green line) remains elevated, at around 1 inch, while Boulder (blue line) and Schriever AFB (pink line) are on a steady march upward. All in all, there will be plenty of moisture to fuel another period of scattered showers/isolated thunderstorms across much of the High Country and Western Slope, with lesser amounts of activity spilling over the Urban Corridor, and western extents of the Southeast Plains, Palmer Ridge, and Raton Ridge. Generally speaking, areas east of a north-south line from Sterling-to-Flagler-to-Las Animas-to-Andrix will remain dry.

ipw_20160930

Most activity will be in the form of showers, with light-to-moderate rainfall being the main impact. A couple isolated thunderstorms will mix in, producing periods of moderate rainfall, perhaps brief bouts with localized heavy rainfall. Area burn scars such as Waldo Canyon and Hayden Pass should be monitored, but no flood threat is warranted due to dry antecedent conditions. Additionally, localized street ponding may be an issue on roadways, so be on the lookout for that, as well. For more information, including rain rates and timing, please see the zone-specific discussions below.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Palmer Ridge, Northeast Plains, Raton Ridge and Southeast Plains:

Scattered showers and a couple embedded thunderstorms are expected, with the most activity focused along and east of I-25. Isolated showers/thunderstorms will spread a bit further eastward, west of a north-south line from Sterling-to-Flagler-to-Las Animas-to-Andrix. East of this line, conditions are expected to remain dry, with more clouds than during previous days. Most rain rates will be light underneath showers, with efficient rainfall coming from more organized activity. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 0.6-0.8 inches/hour
Southeast Plains and Northeast Plains: 0.4-0.7 inches/hour
Raton Ridge: 0.3-0.6 inches/hour

Timing: 11 AM – 11 PM

Front Range, Southeast Mountains, San Luis Valley, Northern Mountains, Northwest Slope, San Juan Mountains, Southwest Slope, Central Mountains, and Grand Valley:

Scattered-to-numerous showers, and a few thunderstorms are expected, producing mainly light-to-moderate rainfall and gusty winds. One or two stronger thunderstorms will produce localized heavy rain, resulting in ponding on roadways and in low-lying areas. No flood threat is warranted as storm motions should keep activity moving at a decent clip, minimizing the threat from heavy rain. Maximum rain rates are as follows:

Southwest Slope and Grand Valley: 0.6-0.8 inches/hour
San Juan Mountains, Central Mountains, Northwest Slope, Northern Mountains, and Southeast Mountains: 0.4-0.7 inches/hour
Front Range: 0.5-0.9 inches/hour
San Luis Valley: 0.3-0.5 inches/hour

Timing: 11 AM – 9 PM for areas west of the Continental Divide, 11 AM – 1 AM for areas along and east of the Continental Divide

STP 09-30-2016: Scattered Showers/Isolated Thunderstorms Continued

Issue Date: Friday, September 30, 2016
Issue Time: 9:00 AM MDT

Summary:

REMINDER: This is the final STP product of the 2016 season. As always, it has been a pleasure serving you.

The subtropical moisture surge from the south continued across western Colorado, providing another period of scattered showers/isolated thunderstorms. Scattered showers continue this morning across the High Country, mainly north of I-70, with a few beginning to spill over adjacent portions of the Urban Corridor. Most precipitation at this time is falling as rain, with snow above approximately 10,000 feet. Overall, showers/thunderstorms produced light-to-moderate rainfall, with a few extended periods leading to rainfall totals greater than 0.30 inches. Below is the list of rainfall “winners,” according to CoCoRaHS observer reports as of 7:30 AM MDT:

Montezuma County: 0.34 inches
La Plata County: 0.29 inches
Routt County: 0.26 inches
Mesa County: 0.26 inches

No flash flooding occurred. For a statewide look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.


Storm Total Precip Legend

FTO 09-29-2016: Final FTO of the Season, 2 Events

Issue Date: 9/29/2016
Issue Time: 2:50 PM

threat_timeline_20160929

Well, the day is finally upon us to issue the final Flood Threat Outlook of the 2016 season. As always, it has been a pleasure to serve you and to provide these valuable products. Before jumping into the discussion of this FTO period, I wanted to take a look farther down the road and discuss how things look in the long-term. The image below is the Drought Monitor for the state of Colorado, released this morning (09-29-2016).

drought_20160927

While much of the state remains without drought, a large swath of abnormally dry conditions has taken hold across portions of Colorado, particularly for northern areas. In additional, moderate drought conditions are depicted along the Front Range/Urban Corridor, and across the Palmer Ridge region. A drought, while never a good thing, is not as alarming this time of year as compared to one developing in the spring time. This is due to the fact that, climatologically speaking, precipitation drops off sharply between August and September, and a bit more into October. If this drought were creeping up during the spring when Colorado typically sees its highest rain and snow, the cause for concern would be greater. This situation is something to keep an eye on moving forward, and will be important to monitor going into next spring, when its impacts could become more profound. Now, let’s jump back into the present time period and discuss this FTO period.

There are two events in the upcoming period, both occurring within the first 6 days. Event #1 is the continuation of an event already underway, associated with the weakening upper-level low drifting northward across the Great Basin. This upper-level low has brought a subtropical surge to western Colorado, highlighted by the green arrow in the water vapor image below. Eventually, the low will wash out in the mean flow, with the lingering moisture providing fuel for a few more days of lingering showers across the High Country and Western Slope. No flood threat is apparent with this event, as rain rates are expected to remain well below flash flood thresholds.

wv_20160929

Event #2 is depicted in the water vapor image as an upper-level trough currently positioned off the West Coast. This trough, and developing closed low, will shift into the Great Basin on Monday, bringing strong southwesterly winds aloft to the region. Concurrently, a surface low will develop, with an attendant cold front sweeping across the area. Along the front, a few storms may become strong over the High Country and Western Slope, owing to sufficient shear and instability. These storms alone are the cause for any flooding concerns, and this is the reason behind the gradient in the threat timeline at the top of the page. Any flood threat that may develop will be low-end, but is worth mentioning in this FTO, as there will not be an FTB produced due to it being out of season. Behind the front, on Tuesday and Wednesday, cooler air from the northwest will invade the state, dropping snow levels to around 6,000-7,000 feet across the Northwest Slope, Northern Mountains, Central Mountains, and Grand Valley regions. A potentially significant early season snow event may unfold across the higher terrain, so be sure and stay tuned to your local National Weather Service offices for more information.

After Event #2, high pressure is expected to build across the region for an extended period of time; at least through the end of this FTO period. This will lead to dry and pleasant conditions for the state by October standards. For more information on events #1 and #2, including precipitation maps, please jump below to the event-specific discussions.

Event #1: Friday (09-30-2016) through Sunday (10-02-2016)

No Apparent Threat as the Upper-Low Weakens/Washes Out

The weakening upper-level low will continue to drift northward across the Great Basin, eventually washing out into the mean upper-level flow. The subtropical moisture surge that it has brought to western Colorado, however, will continue, lingering across the region. This will provide the fuel for a couple more days of showers, and perhaps an isolated thunderstorm or two. Due to lacking upper-level support and instability, rain rates are expected to stay below flash flood thresholds.

Legend

Event #2: Monday (10-03-2016) through Wednesday (10-05-2016)

Elevated Flood Threat for Monday, Followed by Snowfall

The upper-level trough, and developing closed low, will shift into the Great Basin on Monday. The dynamics associated with this trough will lead to the development of a surface low and attendant cold front. The cold front will sweep across the area on Monday, with showers/storms expected ahead of, and along, the front. A few storms may become strong over the High Country and Western Slope, owing to sufficient shear and instability. These stronger storms are the cause for any flooding concerns, and this is the reason behind the gradient in the threat timeline at the top of the page. On Tuesday/Wednesday, behind the front, cold air from the northwest will invade the state, dropping snow levels to around 6,000-7,000 feet, potentially leading to a significant early season snow event across the higher terrain. In the precipitation map below, both the rainfall and snowfall are depicted as a liquid equivalent. Not all of what is depicted will fall as rain, and snow totals may reach over a foot in higher elevations. Stay tuned to your local National Weather Service office for more details as this event approaches.

Legend

FTB 09-29-2016: Colorado Contrast

Issue Date: 9/29/2016
Issue Time: 9:40 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHWEST SLOPE, GRAND VALLEY, AND SOUTHWEST SLOPE.

Stuck between an upper-level ridge over the central US and an upper-level trough off the West Coast, the flow across Colorado will be southerly today/tonight. This will continue to bring in subtropical moisture (associated with the weakening upper-level low) to western Colorado, while drier air remains in place for areas east of the mountains. This stark contrast between the two regions is illustrated well by the IPW graph below, showing the dramatic increase in moisture over Grand Junction (green line), while measurements from Boulder (blue line) and Schriever AFB (pink line) have remained fairly steady for the past few days.

ipw_20160929

The continued stream of subtropical moisture into western Colorado will lead to scattered showers/isolated thunderstorms throughout the day/night, producing mainly light-to-moderate rainfall. During the afternoon and early evening hours, thunderstorms will produce their most efficient rainfall. However, storms will be moving northward at a good pace, minimizing the flooding threat. Overall, a low flood threat is warranted, but it is important to note that any flooding issues will likely be localized ponding on roadways and in poorly drained areas.

Meanwhile, due to the southerly flow aloft, showers/storms are expected to stay confined along and west of the Continental Divide, with perhaps one or two spill-over showers into the Front Range/Southeast Mountains. East of those regions, the lower elevations will be controlled by high pressure and a stable air mass, keeping conditions dry and mostly sunny. For more details, including rain rates and timing, please see the zone-specific forecast discussions below.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Palmer Ridge, Northeast Plains, Raton Ridge and Southeast Plains:

Sunny and dry will once again be the main weather story today, with high temperatures pushing a few degrees above climatological average. Winds will be a bit gustier than previous days, so keep that in mind if you plan on being outdoors.

Front Range, Southeast Mountains, and San Luis Valley:

Mostly sunny skies and mainly dry conditions will be the name of the game today, with only one or two “spill-over” showers possible over the higher terrain. Even so, with near-surface levels remaining dry, any shower activity will be high-based, producing gusty winds and a sprinkle or two.

Timing: Noon – 10 PM

Northern Mountains, Northwest Slope, San Juan Mountains, Southwest Slope, Central Mountains, and Grand Valley:

Scattered showers/isolated thunderstorms are expected throughout today/tonight, with the best coverage occurring over westernmost regions. Rainfall will be most efficient during the afternoon and early evening hours, bringing the potential for localized flooding issues on roadways and poorly drained areas. Overall, the flood threat can be characterized as a “low-end, low flood threat.” Maximum rain rates are as follows:

Northwest Slope, Grand Valley, and Southwest Slope: 0.6-1.0 inches/hour
Central Mountains, Northern Mountains, and San Juan Mountains: 0.4-0.7 inches/hour

Timing: 11 AM – 11 AM