FTO 08-31-2015: Three Events, One Elevated Flood Threat On Tap

Issue Date: 8/31/2015
Issue Time: 12:40 PM

Threat_Timeline_20150831

True to form, the first thing to do when discussing the 15-day forecast is to take a look at the water vapor imagery and analyze the current workings of the atmosphere. First of all, the central Pacific Ocean remains active with three hurricanes (two pictured, circled in red; Ignacio left, Jimena right). While bearing no direct effects to Colorado, you can see the moisture connection between Ignacio and Colorado, as southwest flow aloft transports the moisture into the SW United States. The purple line represents the trough that will spend the better part of this week/weekend digging along the west coast. This will keep persistent southwest flow aloft across Colorado, maintaining the same sort of pattern that we have experienced over the previous 4 or so days. Moisture will gradually decrease Tuesday through Thursday, before increasing again on Friday and into the weekend. The drying will help break up the forecast between Events #1 and #2.

08312015_WV

By the end of the weekend/early next week, the upper-trough will push to the east, bringing an end to the persistent southwest flow and monsoonal moisture for a day or so. Then, almost like clockwork, another upper-level trough will enter the Pacific NW, placing Colorado under the influence of southwest flow once again. This trough will be a bit more transient than previous iterations, only bringing a two-day wet period before moving on and allowing Colorado to dry out for a few days to end this forecast period.

A LOOK AHEAD: After this 15-day period, the effects of the three hurricanes in the central Pacific will become more direct through indirect means; their recurving into the storm track across the northern Pacific will hold ramifications for our weather pattern. A favorable recurve/wave interaction could lead to another active period, but this is purely speculative at this point. It is way too early to tell, so be sure and stay tuned.

Event #1: Tuesday (09-01-2015)

No Apparent Flood Threat during Final Day of the Current Moisture Surge

Southwest flow aloft will remain over Colorado on Tuesday, combining with the lingering moisture plume to bring about another day of scattered showers/thunderstorms over the mountains and isolated coverage elsewhere. Instability values will spend another day on the low side, unable to unlock the full potential of the monsoonal moisture. For that reason, no apparent flood threat exists at this time. Depending on how today (08-31-2015) unfolds with respect to rainfall totals/saturated soil, a low flood threat may be introduced in tomorrow’s FTB. Be sure and check back in tomorrow morning for the latest.

Legend

Event #2: Thursday (09-03-2015) through Sunday (09-06-2015)

Elevated Flood Threat with a Near-Repeat Performance but an Additional Co-Star

Persistent southwest flow aloft will once again be the name of the game, bringing monsoonal moisture/disturbances into Colorado. The pattern is a near-repeat of the past 4 or so days, but an additional player will throw its hat in the ring; Tropical Storm Kevin. It has not been named yet, but will be in the next 24-48 hours. The remnants of Kevin will be pulled northward in the southwest flow, reaching Colorado by Saturday/Sunday. This extra moisture/associated disturbance will bring scattered-to-widespread showers/thunderstorms capable of heavy rainfall. By Sunday Night/Monday morning, the upper-level trough will push eastward, bringing the persistent southwest flow to an end for a day or so.

Legend

Event #3: Tuesday (09-08-2015) and Wednesday (09-09-2015)

No Apparent Flood Threat as This Event is Still Coming into Focus

As discussed above and in the previous FTO, Event #3 will be a quick-hitter. The upper-level trough will push to the east in a fairly quick fashion, bringing a short period of moisture and favorable dynamics for scattered-to-widespread precipitation. At this time, though, it appears the best large-scale support may remain north of Colorado, so the upcoming days will hold a lot of insight into how this event will ultimately play out. Please check back in the next edition of the FTO on Thursday for updated information.

FTB 08-31-2015: Monsoonal Moisture Plume Stretching Across Colorado

Issue Date: 8/31/2015
Issue Time: 9:20 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, CENTRAL MOUNTAINS, NORTHERN MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, AND SAN JUAN MOUNTAINS.

A late-summer monsoon day is in store for much of the state as deep moisture continues to overspread Colorado due to southwest flow aloft. The IPW graph below shows the increase over the last day or so, first for Grand Junction (green line) and then for Boulder (blue), Pueblo (pink), and Shriever AFB (red). Grand Junction has moderated just above 1 inch this morning, but that will uptick a bit again around lunchtime with the arrival of a new disturbance from the southwest. Additionally, the other three stations will see their moisture levels climb throughout the afternoon, as well.

IPW_08312015_2

Overall, isolated-to-scattered thunderstorms are expected across much of the state, with the best coverage over the Northern Mountains, Central Mountains, Front Range, San Juan Mountains, Palmer Ridge, Urban Corridor, and Northeast Plains. Storms are expected to maintain a good pace of motion, limiting the flood threat, thus only a low flood threat is warranted. For today, burn scars, low-lying areas, and urban areas susceptible to drainage issues will command the most relative attention, but the low flood threat will cover a good chunk of the state. For more details on 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

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

Isolated thunderstorms are expected over the Northwest Slope; isolated-to-scattered coverage over the Grand Valley, San Luis Valley, and Southwest Slope regions; and scattered coverage over the Northern Mountains, Central Mountains, Front Range, and Southeast Mountains. Areas that received good rainfall yesterday have been included in the low flood threat due to saturated soils exacerbating moderate rainfall rates. Maximum rain rates will break down as follows:

Front Range, Southeast Mountains, and Central Mountains: 1.0-1.6 inches/hour
Northwest Slope, Northern Mountains, Grand Valley, and San Luis Valley: 0.5-0.8 inches/hour
San Juan Mountains and Southwest Slope: 0.6-1.0 inches/hour

Timing: 11 AM – 10 PM, with a couple of showers lingering into the early morning hours.

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

Scattered showers and thunderstorms expected this afternoon and evening, with the best relative coverage for areas adjacent to the mountains, and across the Palmer Ridge and Northeast Plains. With moisture climbing, favorable position of an upper-level jet streak, and a mid-level disturbance, locally heavy rainfall and lightning will be the main storm threats today. As stated above, burn scars and urban areas susceptible to drainage issues will command the most relative attention with respect to flooding/ponding issues. Maximum rain rates will be 1.2-2.0 inches/hour, with locally heavier rain rates across the Palmer Ridge at 1.8-2.2 inches/hour. Storm motions will help limit the flood threat, so only the low flood threat is warranted.

Timing: 1 PM – 10 PM will be prime time, ending from west to east as drier air works in. Thunderstorms will continue until about midnight across the plains as they work their way into KS/NE.

STP 08-31-2015: Upper-Ridge Slid East, Deep Moisture Returned in the Southwest Flow

Issue Date: Friday, May 1st, 2015
Issue Time: 9:00 AM MDT

Summary:

As the upper-level trough pushed across the Northwestern US, the upper-level ridge that had been overhead for a few days was nudged to the east. This allowed for moisture to make a return to the state, especially west of the Continental Divide. The IPW graph below shows the return of the moisture, pushing precipitable water values at Grand Junction to nearly 1.4 inches (!) last night. This, along with other environmental factors, allowed for widespread showers and thunderstorms to return to the High Country yesterday, spilling over into the Urban Corridor and Northeast Plains this morning.

IPW_08312015

Instability was on the low side yesterday, so thunderstorms were unable to produce heavy rainfall resulting in flash flooding. Instead, areas received persistent light-to-moderate rainfall, with a few brief bouts of locally heavy rain. For a look at statewide rainfall totals, be sure and check out the Storm Total Precipitation map below.

No flash flooding was reported yesterday.

20150831_STPImage
Storm Total Precip Legend

FTB 08-30-2015: Approaching Trough Over Pacific NW Brings Changes to Colorado

Issue Date: 8/30/2015
Issue Time: 9:20 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE GRAND VALLEY, SOUTHWEST SLOPE, CENTRAL MOUNTAIN, AND SAN JUAN MOUNTAIN REGIONS.

The ridge of high pressure (blue “H” and dashed line) that has been overhead the past few days will be nudged eastward today by the trough (red “L” and dashed line) over the Northwest US as it moves over the Northern Rockies. As a result, southwesterly flow will increase over western Colorado through today/tonight, bringing increasing moisture to the area (green arrow). With the influx in moisture, precipitable water values will climb, reaching above 1 inch across the San Juan Mountains, Southwest Slope, and Grand Valley regions, thus the low flood threat in those areas. With that said, all regions across the High Country will see an uptick in thunderstorm coverage today; the main threats will be gusty outflow winds, lightning, and brief moderate-to-heavy rainfall. Rain rates and timing will be broken down in the Zone-Specific discussions below.

08302015_WV

East of the mountains, another hot and mostly sunny afternoon is in store, with temperatures increasing/clouds decreasing from west to east. Adjacent to the mountains, mid- and high-level clouds will increase through the afternoon as thunderstorms build over the mountains. A few isolated thunderstorms will attempt to move overhead, but dry air in the low-levels will mean strong outflow winds are the main threat. Gusts to 45-50 mph are not out of the question, accompanied by virga/light rainfall. By the nighttime hours, mid-level moisture will increase, keeping a low chance of thunderstorms in the forecast along the Urban Corridor and western portions of the Northeast Plains into the early morning hours.

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.

20150830_LFT
Flood Threat Legend

Zone-Specific Forecasts

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

Mostly sunny and hot, with high temperatures pushing into the upper-80s and mid-90s. Adjacent to the mountains, isolated thunderstorms will attempt to move overhead, holding a threat for strong outflow winds (gusts up to 45-50 mph) and virga/light rainfall. Overnight, and into the morning, the Urban Corridor and western portions of the Northeast Plains will hold a low chance (<20%) of one or two isolated thunderstorms as mid-level moisture increases from the west/southwest.

Timing: 2 PM – 8 PM, with the aforementioned isolated thunderstorms possible through the overnight/early morning hours.

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

Scattered showers and thunderstorms expected through the afternoon and nighttime hours, producing bouts with locally moderate-to-heavy rainfall, lightning, and gusty outflow winds to 45 mph. Maximum rain rates will break down as follows:

Front Range, Northern Mountains, Northwest Slope, and Central Mountains: 0.4-0.7 inches/hour
San Juan Mountains, Grand Valley, and Southwest Slope: 0.8-1.2 inches/hour
San Luis Valley and Southeast Mountains: 0.15-0.35 inches/hour

Timing: 11 AM – 10 PM, with a few showers/thunderstorms lingering into the morning hours.