FTB 09-30-2017: Widespread Showers & Weak Storms For Higher Terrain, But Rain Rates Limited

Issue Date: Saturday, September 30, 2017
Issue Time: 10:30AM MDT

— Flooding is NOT expected today

NOTE: This is the last scheduled Flood Threat Bulletin of the 2017 season. However, additional Bulletins may be necessary next week to cover the heavy rainfall threat anticipated in southern/eastern Colorado.

A broad upper-level trough continues to be positioned over the western half of the United States, as shown in this morning’s water vapor image, below. Despite the many, rather disorganized smaller-scale features, it is apparent that plenty of upward motion is currently occurring east of the trough’s axis in UT, western CO and northern New Mexico. In fact, showers and isolated weak thunderstorms are already occurring in the San Juan Mountains, showing that the system is bringing plenty of dynamical support. From the standpoint of heavy rainfall, the main limiting threat will once again be moisture, along with fast steering winds. This morning’s Precipitable Water (PW) measurements at Grand Junction and Denver are a rather subdued 0.54 and 0.48 inches, respectively. Higher values approaching 1 inch are found just east of the Colorado border into KS and NE. However, with only a weak easterly component of low-level flow, PW is expected to remain generally steady across the state.

With plenty of dynamical support, we expect a round of numerous to widespread shower and storm activity coinciding with peak heating this afternoon. Highest coverage will be over the higher terrain, mainly west of the Continental Divide. Rain rates are expected to remain well below flood threat thresholds over most of the state. Isolated higher rain rates are possible with storms that come off the Palmer and Cheyenne ridges. However, we expect that even these will stay just below flood threat intensity. Thus, flooding is not expected today. Nonetheless, widespread precipitation totals above 0.25 inches are expected over the central and western part of the state.

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:

Northeast Plains, Southeast Plains:

Mostly sunny this morning then becoming partly cloudy and breezy with isolated to scattered showers and thunderstorms this afternoon and evening. Highest coverage will be over western areas, towards the foothills. Max 1-hour rain rates up to 1.2 inches possible with the strongest storm cells, along with gusty winds up to 55 mph. Flooding is not expected today.

Primetime: 2PM to 9PM

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

Mostly cloudy and cool with numerous shower and weak thunderstorms increasing in coverage this afternoon. Max 1-hour rainfall up to 0.4 inches (north) and 0.7 inches (far south, along NM border). Snow levels 10,000 feet (north) and 11,500 (south). Gusty winds and small hail could accompany the strongest storms. Flooding is not expected today.

Primetime: 11AM to 10PM, with light rain and snow showers continuing overnight

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

Mostly sunny this morning, then increasing clouds with scattered to numerous showers and weak thunderstorms developing by early afternoon. Highest coverage will be over the foothills and higher terrain. Max 1-hour rain rates up to 0.75 inches (below 6,000 feet) and 0.6 inches (above 6,000 feet). Snow level will fluctuate around 11,000 feet, lowering later tonight. Gusty winds up to 50 mph and small hail could accompany the strongest cells. Flooding is not expected today.

Primetime: 2PM to 9PM, with light rain and snow showers continuing overnight

FTB 09-29-2017: Isolated Showers and Thunderstorms over the Higher Terrains and Adjacent Plains

Issue Date: Friday, September 29, 2017
Issue Time: 10:30 AM MDT

— Flooding is NOT expected today

As seen yesterday morning, showers over the eastern plains are being supported by the jet stream and mid-level energy rotating around the Low to our northwest. These showers will continue to move to the northeast and exit the state by late this morning. There is quite a bit of fog again this morning in the mountains and Urban Corridors. This should quickly burn off due to breaks in the clouds being more numerous today. Precipitable Water (PW) in Grand Junction was measured at 0.75 inches and has dropped to 0.65 inches in Denver. With more southwesterly flow today, decreases in moisture are expected today. This should make showers and thunderstorms more isolated in nature and limit accumulations.

The Low pressure is currently over the UT/WY border and will continue to move north throughout the day. Shortwave activity associated with the Low will provide enhanced lift one last time for another round of showers and thunderstorms this afternoon. Moisture will continue to retreat east throughout the day, but dew points over the southeast plains and southern mountains will be high enough for moderate rainfall further south. Breaks in cloud cover will allow some instability to build this afternoon, which will increase the possibility for thunderstorms. Isolated showers and thunderstorms are expected to form over the mountains by later this afternoon and move into the adjacent plains this evening. The storms are expected to dissipate as they move off the higher terrains, but brief heavy rain with gusty winds and thunder are possible. Overnight shower activity is not expected, but a few showers may linger over the southern mountains with weak upslope flow. 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:

Partly cloudy with some showers over the Northwest Slope. Drier air has allowed breaks in cloud cover over the region. Enough residual moisture and instability is expected for another round of showers and thunderstorms over the higher terrains this afternoon. Highest accumulations are expected over the southern San Juan Mountains; especially further east where moisture hangs on. Max 1-hr rain rates up to 0.5 inches (south) and 0.3 inches (north) are possible with 3-hr totals up to 0.8 inches (south). Flooding is not expected today.

Primetime: 1PM to 9PM

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

Cloudy and foggy this morning with rain over the eastern plains. Showers and thunderstorms likely over the higher terrains this afternoon, although they will be more isolated than the last couple of days. Highest accumulations are expected over the southern Front Range and Southeast Mountains. Max 3-hr rain rates up to 0.8 inches are possible. Showers and thunderstorms will move NE off the higher terrains this afternoon and may briefly produce heavy rain and gusty winds on the adjacent plains before dissipating. Max 1-hr rain rates up to 0.7 inches are possible.

Primetime: 2PM to 11PM

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

FTB 09-27-2017: Widespread Showers for Colorado as Upper Level Low Increases Moisture and Lift

Issue Date: Wednesday, September 27, 2017
Issue Time: 10:45 AM MDT

LOW flood threat for the Hayden Pass and Junkins burn scars

As expected, the upper level Low over AZ has drawn in a lot of low level moisture over Colorado as seen in the water vapor (WV) imagery below. The increase in low level moisture allowed showers and snow (at the higher elevations) to persist overnight and into the morning along the Front Range and Southeast Mountains. There is also an overall increase in cloud cover across the eastern portion of the state and the mountains, which kept low temperatures overnight warmer than normal. Precipitable Water (PW) this morning at Denver was measured at 0.73 inches, and in Grand Junction it was 0.4 inches. With southwest surface winds becoming more southerly throughout the day, moisture will continue to increase with the highest moisture expected over the southern portion of the state.

The upper level low is expected to slowly track north today and will be positioned over Utah by tonight. As the low tracks north, increased shortwave activity and upper air support from the jet stream will bring multiple rounds of showers and snow. Snow should be confined to elevations above 11,000 feet today, but as temperatures decrease tonight, the snow line is expected to drop to 10,000 feet. The initial wave of showers occurring now will be confined to the higher terrains, but by this afternoon, showers will increase in intensity and spread into the adjacent plains and valleys. Thunderstorm activity is expected to be marginal and more isolated due to cloud cover limiting instability.

The highest accumulations for the next 24-hour period are expected to be over the Southeast/San Juan Mountains and southern Central/Front Range Mountains. Over the highest peaks, greater than 6 inches of snow is possible, and a Winter Weather Advisory has been issued for elevations about 11,000 feet. With limited instability, max 1-hour rain rates are expected to be under 1 inch today. However, 3-hour rain rates up to 0.9 inches are possible. There is enough confidence that multiple areas over the Southeast Mountains will have 24-hour totals exceeding 2 inches, so a Low Flood Threat has been issued for the Hayden Pass and Junkins burn scars. Threats include road and small stream flooding, mud flows and debris slides. Shower activity is expected to decrease after midnight over the San Juan Mountains, but rain will continue into the morning over the other southern mountains.

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.

Flood Threat Legend

Zone-Specific Forecasts:

San Juan Mountains, Southwest Slope, San Luis Valley, Southeast Mountains, Raton Ridge, Central Mountains, Southeast Plains:

Cloudy this morning with showers over the high terrains. More widespread showers expected this afternoon. Showers are also expected to increase in intensity throughout the day. Max 1-hr rainfall rates up to 0.6 inches and max 3-hour rainfall rates up to 0.9 inches possible. 24-hour amounts up to 2.75 inches are possible over the Southeast and San Juan Mountains. Elevations greater than 11,000 feet will likely see all snow all day with the snow line dropping to 10,000 feet overnight. Snow totals greater than 6 inches are likely at the higher elevations. A Low flood threat has been issued for the Hayden Pass and Junkins burn scars for sufficient confidence 24-hour totals will exceed 2 inches over multiple areas. Threats include mud flows, debris slides and small stream and road flooding.

Primetime: 11AM to 7AM

Grand Valley, Northwest Slope, Northern Mountains, Front Range, Urban Corridor, Northeast Plains, Palmer Ridge:

Cloudy this morning over the high terrains and east of the Continental Divide. Currently, showers are confined to the higher terrains, but this afternoon are expected to increase in intensity and spread into the adjacent plains and valleys. Over the higher terrains, max 3-hour rain rates up to 0.8 inches with 24-hour totals up to 1.75 inches (south) and up to 1 inch (north) are possible. 24-hour amounts up to 0.8 inches are possible over the adjacent plains and valleys further south.

Primetime: 12PM to 12AM