FTB 09-26-2017: Lull In Between Systems

Issue Date: Tuesday, September 26, 2017
Issue Time: 10:40AM MDT

— Flooding is NOT expected today

As shown in the water vapor image, below, Colorado currently lies between two strong disturbances embedded in the larger-scale western North American upper-level trough. Over the northern Great Plains, we can see the current position of the system that affected our weather over the past 72 hours. That system is now a memory, though its footprint still remains across eastern Colorado in the form of a patchy low-level cloud deck. With subsiding flow in its wake, Precipitable Water (PW) values are bottoming out, with readings of only 0.47 and 0.30 inches at Denver and Grand Junction, respectively. However, just to our east, Dodge City, KS, has a PW of 1.00 inch, implying that this moisture has just barely been pushed east of our state.

Looking farther southwest, we see the makings of the next active weather producer. It is currently a shortwave with an axis roughly along the UT/NV border. As this feature “digs” south-southeast over the next 24 hours, it will cut-off from the main flow: a guaranteed headache for meteorologists! By the end of the day, we will begin to see some low and mid-level moisture return into southern Colorado. With only patchy clouds this morning, sunshine is expected to increase statewide (especially east of the Continental Divide). We expect some instability to be generated in the southern third of the state. Scattered showers and a few weak thunderstorms will be possible over the San Juans, San Luis Valley, Southeast Mountains and Raton Ridge. However, rainfall rates are expected to remain below flood threat intensity. Thus, flooding is not expected today.

Looking further ahead, the cut-off low will likely produce a period of very active weather for our state beginning tomorrow. Please check out the Flood Threat Outlook for a detailed overview of what we can expect, and stay tuned to daily Bulletins for updates on heavy rainfall and flooding chances. Remember that although we are in late September, the ingredients on the weather map suggest a continued possibility of heavy rainfall in the coming days.

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 Mountains, Raton Ridge, Southeast Plains, San Juan Mountains, San Luis Valley:

Patchy early low clouds, then sunshine returning by early afternoon. Isolated to scattered showers and weak thunderstorms will develop by mid-afternoon especially over higher terrain. Max 1-hour rainfall up to 0.4 inches with max 3-hour rainfall up to 0.7 inches. Activity could persist into the overnight hours, especially along the NM border. Flooding is not expected today.

Primetime: 2PM to 1AM

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

Mostly sunny this morning (though with some patchy low clouds east of the Cont. Divide) and turning warmer this afternoon. An isolated shower or weak thunderstorm is possible during the afternoon hours in the southern Front Range and Central Mountains. Max 1-hour rainfall up to 0.25 inches. Flooding is not expected today.

Primetime: 2PM to 7PM

FTB 09-25-2017: Below Average Temperatures Statewide with Rain East of the Continental Divide

Issue Date: Monday, September 25, 2017
Issue Time: 10:15 AM MDT

— Flooding is NOT expected today

Much cooler temperatures this morning with high temperatures expected to be 15-20F below normal. Overnight, some of the lower elevations over the Western Slope and San Luis Valley dipped below freezing. Still a lot of cloud cover over the eastern portion of the state, but the water vapor (wv) image below shows a decrease in the low level moisture statewide. This should give us a break from the heavy rainfall, although some rain is still expected east of the Continental Divide. Precipitable Water (PW) values at Denver and Grand Junction are 0.55 and 0.34 inches, respectively. Both have decreased a bit as the southwest flow aloft continues to advect drier air into the state. Today this southwest flow will get a bit more of a westerly component as the trough continues to move towards the northeast. Currently, the upper level low is located over Wyoming, and it is expected to pull north and reside over Montana/North Dakota by tomorrow morning.

The higher PW will also be east of Colorado today with the slow, eastward progression of the front. Another shortwave with upper level support from the jet stream is expected to move over the state. This will enhance vertical motion and help trigger showers over the eastern portion of the state. Northerly surface flow will get an easterly component this afternoon. Weak upslope flow and the associated showers are expected to be most numerous over the Front Range with more isolated showers over the Southeast Mountains. Some thunderstorms are also expected over the Northeast Plains this afternoon with the increased vertical motion. Due to decreased moisture over the western portion of the state, rainfall is only expected to be isolated and confined to the north where not as much drying has occurred. Again, expect higher elevations to see precipitation fall in the form of snow instead of rain. Tonight, showers are expected to end over the higher terrains around 11PM as subsidence behind the shortwave suppresses overnight shower and snow activity. There is no flooding 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:

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

Cloudy this morning with increasing showers and thunderstorms this afternoon. Heaviest accumulations in the mountains are expected to be over the Front Range. 24-hour precipitation totals up to 1 inch are possible with snow at the higher elevations. Showers and thunderstorms are also expected over the Northeast Plains this afternoon. 24-hour rain totals up to 0.8 inches are possible. Flooding is not expected today.

Primetime: 10AM to 11PM

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

Mostly sunny and cool with some clouds over the Northwest Slope. Dry air will limit shower activity to the north. Snow will likely fall at the higher elevations, but accumulations of both rain and snow are expected to be limited. 24-hour accumulations up to 0.25 inches are possible. Flooding is not expected today.

Primetime: 12PM to 8PM

FTB 09-24-2017: Cooler Temperatures and Continuing Rain as Upper Level Low Slowly Propagates East

Issue Date: Saturday, September 24, 2017
Issue Time: 10:20 AM MDT

LOW flood threat for the Southeast Plains, Raton Ridge, Northeast Plains

— Timing of flood threat is later than normal, and persists into the overnight hours

Heavy and widespread rainfall yesterday across the state. Much cooler temperatures in the wake of the cold front with 40s over the lower elevations and below freezing temperatures in the mountains. Heavy cloud cover over the eastern portion of the state with some convective showers still occurring over the Southeast Plains. A lot of fog was reported due to the increase in low level moisture and cooler temperatures. Currently, the western portion of the state is placed in the dry slot, so only a few clouds over the Northern Mountains. The upper trough will continue to move slowly eastward today and tonight. Colorado will continue to be under southwest flow aloft, which will draw in some drier air over western Colorado and inhibit rainfall. East of the Continental Divide, higher moisture will continue to reside, especially over the southeast corner of the state. Dew points over the plains are expected to reach 60F, so more heavy rain can be expected today and tonight.

Weaker showers are expected today over the higher terrains with snow at the higher elevations (9,500 feet). Upslope flow will continue behind the cold front with multiple upper level shortwaves enhancing lift. Expected scattered showers today and tonight. The highest accumulations are expected be in the Northern Mountains and northern Front Range. Over the southeast corner of the state, models hint at instability this afternoon with some decent shear. This could trigger a round of more convective storms capable of strong winds and small hail. Lift is expected to continue overnight, so expecting another round of showers in the moisture rich environment. High moisture paired with instability will aid in another 24-hour period of heavy rainfall accumulations. On top of accumulations yesterday, and increased runoff due to saturated soils, a Low flood threat has been issued. Threats include small stream and arroyo flooding, road flooding and field ponding.

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:

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

Cloudy this morning with increasing showers and thunderstorms over the northern, high terrains this afternoon. Some showers will move off the higher terrains into the Urban Corridor. Max 1-hr rain rates up to 0.5 inches possible. More convective rain is expected this afternoon over the Southeast Plains. Max 1-hour rain rates up to 1.5 inches are possible with isolated 24-hour rain totals up to 2.5 inches. Due to increased runoff from already saturated soils and high accumulations from yesterday, a Low flood threat has been issued. Threats include small stream and arroyo flooding, road flooding and field ponding. This is another overnight threat.

Primetime: 11AM to 7AM

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

Partly cloudy over the northern high country with sunshine increasing as drier air works its way in from the southwest. Some light precipitation and cloud cover is likely over the Northwest Slope as increased moisture rotates around the upper level Low. Max 1-hour rain rates up to 0.3 inches are possible over the northwest corner of the state.

Primetime: 11AM to 8PM

FTB 09-23-2017: Upper Level Dynamics and Moisture Combine for Widespread Showers and Thunderstorms

Issue Date: Saturday, September 23, 2017
Issue Time: 10:55 AM MDT

MODERATE flood threat for the Southeast Plains, Northeast Plains and Raton Ridge

LOW flood threat for portions of the Front Range, Urban Corridor, Southeast Mountains, Palmer Ridge, Raton Ridge, Southeast Plains, Northeast Plains

— Timing of flood threat is later than normal, and persists into the overnight hours

The visible satellite imagery below shows quite a bit of rainfall over the state as the trough moves in from the west. Beginning early this morning, the low pressure system over northwest Kansas began to pull in very moist air from Texas and Oklahoma into the eastern portion of the state. The dew points this morning over the Southeast Plains have already made it into the 60s, which is incredibly high for this time of year. Precipitable Water (PW) this morning at Denver was measured at 0.71 inches. Over Grand Junction, PW was 0.64 inches. Dodge City PW is probably a little high, but a better estimate for the SE Plains right now, which is at 1.26 inches. PW greater than 1 inch is marked in the map below. Aloft, expecting south and southwest winds, which will continue to pull moisture north and give storms a N/NE storm motion.

The upper low pressure system will slowly continue to slowly propagate eastwards with the majority of Colorado being under positive vorticity advection throughout the forecast period. Paired with high moisture, this lift will translate into the widespread showers over the state. Currently over the western portion of the state, the jet streak is creating a strong line of convection from north to south. Thunderstorms and widespread showers are likely through midnight until this feature begins to move to the northwest and western Colorado is under the influence of the dry slot.

As the upper low tracks to the northeast, showers and thunderstorms over the mountains are expected to increase. With colder temperatures behind the cold front, rain should transition into snow later this evening for elevations around 9,000 feet. With colder temperatures above 10,000 feet already, precipitation this afternoon will likely be all snow. That could mean a couple of inches by morning the higher elevations in the Northern and Central Mountains. East of the Continental Divide, the higher rainfall amounts will be over the southern portion of the high terrain due to the increase of moisture in this area. Southerly flow is expected to enhance precipitation along the Palmer Ridge/Teller County high elevation intersect. Over the Southeast Mountains, there is enough confidence that rain rates will reach 0.5 inches an hour near the Hayden Pass and Junkins burn scar, so storms that form in this area need to be monitored closely. A Low Flood threat has been issued for the higher terrains of the Palmer Ridge, Raton Ridge and Southeast Mountains. Threats include mud flows, debris slides and small stream and road flooding.

Over the eastern plains, enhanced thunderstorm and shower activity is likely this afternoon as the upper level energy, high low level moisture and lift along the slow moving cold front combine. With PW values over 1 inch, heavy rainfall is expected over the Southeast Plain beginning this afternoon. Rain is expected to continue over the far eastern plains into early morning, which warrants a Moderate Flood threat. Threats include stream and road flooding as well as field ponding.

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:

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

Cloudy this morning and increasing showers and thunderstorms throughout the day. Max 1-hr rain rates up to 0.7 inches possible (west) and 1.5 (east). Rain over the mountains will begin decreasing around midnight. Continuous rain is expected over the eastern plains through tomorrow making this an overnight threat. 24-hour totals up to 3.5 inches are possible over the far Southeast Plains. There is enough confidence 1-hour rain rates for storms near the Junkins and Hayden Pass burn scars will be over 0.5 inches. A Low Flood Threat has been issued. Threats include mud flows and debris slides (west), road and stream flooding (west/east) and field ponding (east). A Low/Moderate flood threat has been issued.

Primetime: 12PM to 7AM

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

Cloudy and rainy this morning with showers continuing to increase throughout the day. By late morning, more showers will form over the higher terrains. Southern high terrains could see 24-hour amounts up to 1.8 inches. Max 1-hour rain rates up to 0.5 inches are possible (south) and 0.3 inches (north).

Primetime: 11AM to 12AM