FTO 07-22-2019: Heavy Rainfall Threat Begins Over West Colorado and Moves East by Thursday

Issue Date: Monday, July 22nd, 2019
Issue Time: 2:20PM MDT
Valid Dates: 7/23 – 8/6

Active week ahead for this next FTO as the monsoon pattern sets up. Currently, the subtropical high is over the four corners region. Tomorrow the dry air marked in the water vapor imagery below will move through the state with NW/NNW flow aloft. This will decrease rain rates below flood threat criteria to the east, and the coverage of storms is expected to be patchier than the last couple of days. An Elevated Flood Threat (Event #1) has been issued as moisture will start to surge northward over the southwest corner of the state due to slight eastward movement of the high. This could be problematic for the 416 burn area as storms are expected to be nearly stationary over the San Juan Mountains. The PW surge over western Colorado continues on Wednesday and Thursday, with the low being pushed eastward by the incoming vorticity maximum. This shifts the ridge axis into a favorable position to advect a moist air mass over the state. Additionally, a weak cold front associated with this feature, drops through the eastern plains overnight on Wednesday. Post front upslope flow will cause a High Threat for Thursday.

By Friday, the ridge breaks down and the high moves into southern Arizona, which should decrease PW values statewide (Event #2). Scattered afternoon storms may be possible, but there is No Apparent Threat at this time. Saturday afternoon, a vorticity maximum passes over the state, which should aid in lift for more widespread storm coverage. There is an Elevated Threat for this day with the threat decreasing into Sunday. By early next week, NW flow will become more westerly and decreases the available moisture for rainfall. Temperatures will also be on the rise and with the jet brushing the northern border, critical fire weather may be possible sometime mid-next week. Please tune back into the FTO on Thursday of this week as details will likely change.

PW values look to decrease tomorrow after the dry air mass is pushed into eastern Colorado from the north. Different story to the west, where a PW surge begins. With PW well above climatology from Tuesday to Thursday, there is expected to be an uptick in afternoon and evening thunderstorm activity. The main threats will be gusty winds, severe hail and heavy, local rainfall. Slow storm motion under the ridge will also help increase totals. This will also put recent burn areas under the gun, especially if storms track over the scars more than one day in a row. To the west, a lot of disagreement between members after Wednesday about PW values. This likely has to do with the timing and intensity of the passing shortwave. Nonetheless, with upper level lift with the upslope flow pattern, there will be an Elevated Flood Threat on Saturday. PW looks to decrease statewide from Sunday into next week.

Event #1: Tuesday (7/23)Thursday (7/25)

High/Elevated Threat as the monsoon kicks off with the threat starting over western Colorado and moving east thanks to an increase in moisture behind a passing cold front.  

Looks like the monsoon will be in full swing for this event. Starting tomorrow, the slight movement of the ridge axis to the east will bring showers and thunderstorms back into the forecast for southwestern Colorado and decrease the heavy rainfall threat over eastern Colorado. Rain looks to be mostly confined to the San Juan Mountains and storm totals up to 1 inch will be possible. Although rain rates won’t quite reach flood threat criteria everywhere, if one of these nearly stationary storms moves or forms over the 416 burn area, flash flooding, mud flows and debris slides will be possible. Rainfall chances increase northward over western Colorado on Thursday with some thunderstorms likely over the valleys as well. Wednesday night, a cold front moves through eastern Colorado overnight, so there may be some weak thunderstorms and showers along this feature. Post upslope flow will create a High Flood threat for Thursday. More westerly flow aloft will allow these storms to move into the immediate adjacent plains and a couple of severe thunderstorms can’t be ruled out. Looking like the far eastern plains will be capped, but details like this are better forecast 24 hours before the forecast is valid. If there is rain over the eastern plains, expect the heaviest rainfall rates to be located along the elevated terrains of the Raton and Palmer Ridges. Please tune back into the FTB each morning for the latest heavy rainfall threat details.

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Event #2: Friday (7/26) – Tuesday (7/30)

Elevated Threat/No Apparent Threat as on and off upper dynamics and moisture move through the area with more zonal flow.

More zonal flow during this period should keep the heavy rainfall threat at bay for the most part. For Friday, the subtropical high is over southern Arizona. This will keep the moisture needed for heavy rainfall to our south and east. A passing shortwave on Saturday will combine with some higher PW values over eastern Colorado. This would produce more widespread thunderstorm coverage, especially if it passes during peak heating. A boundary over the eastern Colorado may set up with a weak lee trough, which could cause a severe thunderstorm or two. Expect showers to increase as the feature passes overhead and over the eastern plains. Still low confidence in this forecast at this time, so please tune back in on Thursday for the next FTO. After Saturday, things look to dry out for a few days and some hot temperatures return to the forecast.

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FTO 07-18-2019: The Heavy Rainfall Threat Returns for the Weekend with Relief from the Heat over Eastern Colorado

Issue Date: Thursday, July 18th, 2019
Issue Time: 2:10PM MDT
Valid Dates: 7/16 – 7/30

There is a two day break in rainfall statewide with some very hot temperatures forecast to start this FTO. Some relief from the heat will occur this weekend as a cold front, and increased moisture behind the front, will cause an uptick in thunderstorm activity (Event #1). Today, zonal flow aloft will continue to advect the dry air mass seen in the water vapor imagery below (yellow/orange) keeping conditions dry statewide. Flow aloft turns more southwesterly tomorrow as the ridge begins to build back to the west. A cold front is forecast to move through the state sometime between late Friday night and Saturday morning. Upslope flow in the moisture rich environment behind the front will return the flood threat to the eastern Colorado.  High accumulations are possible due to the slow steering flows under the ridge. This will also help drop high temperatures by 15-20°F over eastern Colorado. The building ridge will also increase storm activity over the southwest corner of the state (mountains) by allowing moisture to meander northward. By Monday, the subtropical high moves into the four corners region, which will reduce the flood threat by pushing the higher moisture to our south and west.

Event #2 begins on Wednesday and Thursday over western Colorado. The upper level low (marked below) gets pushed eastward from another incoming upper low to its west. This will create a broader ridging pattern along with slight eastward movement of the ridge axis. By next weekend, the ridge over Colorado breaks completely down and high moisture moves from western Colorado into eastern Colorado.

Well below average PW across the state for today and tomorrow. Behind the passing cold front, low level moisture has quite the return over eastern Colorado. Due to slow steering flows and high PW, a High flood threat has been issued. Reminder, this categorization means we expected to issue a threat higher than a Low flood threat in the FTB. Burn areas will need to be watched closely and there is also a chance for some nocturnal rainfall with post upslope flow. The front won’t affect temperatures over western Colorado, but a slight increase in moisture over the San Juan and Central Mountains is forecast, which would cause an increase in afternoon storminess. Additionally, the building ridge and increase in moisture will reduce fire danger. Going into next week, a more unsettled weather pattern is expected for western Colorado from Wednesday into the weekend. The monsoon looks to kick off at that time if models stay consistent with this forecast. The agreement in the plumes below (narrow space between gray lines) indicate the forecast isn’t expected to change much the next week.

Event #1: Saturday (7/20) – Monday (7/22)

High/Elevated Threat as the subtropical high starts to build back to the west and a cold front returns moisture to eastern Colorado.  

I think everyone is ready for a bit of a cool down, which eastern Colorado will receive this weekend. A little difference in timing between models, but sometime between late Friday night and Saturday morning a cold front will drop through eastern Colorado. This will cool temperatures off quite a bit and increase storminess with post frontal upslope flow. There is a High flood threat for the eastern mountains and immediate adjacent plains. Burn scars will be particularly susceptible with slow steering winds and a moisture rich environment slowing for high rain rates. Mud flows, debris slides and flash flooding will be possible over burn areas and steeper terrains. Decent CAPE over the immediate adjacent plains may cause a severe thunderstorm or two with the main threats being hail, strong winds and heavy rainfall. The eastern plains look to remain capped, so unless there is a strong shortwave moving through (lifting mechanism), not expecting storms over this area. The best chance for eastern plain storms will be along the Raton Ridge where increased convergence may allow a storm or two to traverse farther east. Sunday, conditions looked to be a little more capped as well, but with such weak steering flows, storms will still be capable of producing totals that may cause flooding issues over the mountains and burn areas. Thus, the Elevated flood threat.

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Event #2: Tuesday (7/23) – Sunday (7/28)

Elevated Threat as the monsoon kicks off with the threat starting over western Colorado and moving east.

If the models stay on track, the true monsoon looks to kick off by mid-next week. So a little bit of a late start, but no records for a late start have been broken yet. There is an Elevated flood threat over western Colorado as a PW surge begins early next week. The surge extends to the north, so there should be an increase in storms over the Northern Mountains as well. Burn areas are the main concerns (Lake Christine and 416), but there may also be a threat over the San Juan Mountains. An eastward shift in the ridge axis on Thursday allows moisture to rotate around the high into eastern Colorado. Elevated PW values and slow steering winds should keep the flood threat around through the weekend. Tune back into the FTO on Monday for the latest.

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FTO 07-15-2019: Hot Temperatures with a Decrease in Afternoon Storm Activity Beginning on Wednesday

Issue Date: Monday, July 15th, 2019
Issue Time: 1:40PM MDT
Valid Dates: 7/16 – 7/30

One more day with upper level ridging over the state before the shortwave trough (orange “X”) moves eastward and creates more zonal flow over the forecast area (Event #1). The approaching trough will push the subtropical high eastward and more westerly flow will begin to advect dry air into the state from the west. As far as shower and thunderstorm action, tomorrow looks to be the last day with widespread showers and thunderstorms. Moisture is expected to continue to decrease, so storms will be more isolated than today. Outside of some possible storms over the far southeast corner of the state on Wednesday, models aren’t showing much (if any) rainfall for Thursday and Friday. Temperatures will start to near the triple digit mark during this event across all the lower elevations, so be prepared for some hot temperatures Wednesday through Friday. Fire weather concerns return on Tuesday for the Northwest Slope. The jet will brush this area as the trough moves east and a tightening gradient could mix some strong surface winds to the surface both Tuesday and Wednesday afternoon.

Going into this weekend, the ridge begins to build back over the state. Strong southwest flow aloft across the state will create another PW surge on the west side of the high pressure system. By the end of this weekend into early next week, the high looks to center itself over the New Mexico and the Texas Panhandle. This is a favorable pattern for a PW surge both east and west of the Continental Divide. Therefore, there is an Elevated Colorado flood threat for Event #2 with the threat extending to recent burn areas. As the upper low marked in the image below moves east, a trough will push east and flatten the ridging pattern. This will likely bring another statewide break in heavy rainfall sometime at the end of next week.

The dry air mass seen in the water vapor imagery above has already begun to move into western Colorado. PW drops off to significantly below climatology by Thursday both east and west of the Continental Divide. Better moisture holds on over eastern Colorado on Tuesday, but storm chances look to decrease when compared to today. The GFS and NAM indicate some CAPE over the eastern plains on Tuesday, so it is possible for some stronger thunderstorms over this area during the late afternoon and evening. The main threats will be severe hail and gusty outflow winds with moderate rainfall. By Sunday night, the GEFS indicates a cold front moving through the state. If this occurs, it would cool off high temperatures and increase low level moisture for an uptick in storm activity on Monday. Models are quite uncertain with moisture return over western Colorado for Event #2, which is related to the placement of the subtropical high/ridge axis and moisture advection around the west side of this feature.

Event #1: Tuesday (7/16) – Friday (7/19)

No Apparent Threat as flow becomes more zonal and advects a dry air mass into the state from the west.  

Active weather for one more day before the atmosphere really begins to dry out with the zonal flow. Storms will likely be more isolated in nature tomorrow, although if they are able to make it to the eastern plains before dissipating, some stronger thunderstorms may be possible. Not anticipating a lot of flooding issues over the higher terrains during this event, but burn scars over the Southeast Mountains may be under a flood threat tomorrow. Be sure to tune back into the FTB for the latest tomorrow morning. The main threat for this period will be hot temperatures with triple digits anticipated as early as tomorrow for western Colorado. There is a Fire Weather Watch for the Northwest Slope for Tuesday and possibly Wednesday, so use caution with open flames. Southwest winds in the 10 to 20 mph range are forecast with gusts in the 35 to 40 mph range.

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Event #2: Saturday (7/20) – Wednesday (7/24)

Elevated Threat as zonal flow becomes more southwesterly with a building ridge and the next PW surge occurs statewide.

Lower confidence in the timing for the main threat for Event #2, although confident there will be another PW surge northward during this period. Zonal flow should turn more southwesterly as a ridge begins to build back over the state, which will increase moisture across the area. Should the high set up over NM/TX, a large PW increase statewide would be possible. Slow steering winds under the ridge would cause storms to have large accumulations similar to the event that occurred this weekend. Rotating shortwaves, if timed correctly, could help increase coverage with the diurnal flow, though it is too far out to identify these features. The GFS has a cold front moving through the state later this weekend. Increased upslope flow and moisture behind the front would cause an Elevated flood threat to start next week. It would also likely allow storms to develop over the eastern plains with some early morning fog and cloud cover. Please tune back into the FTO on Thursday as details will continue to change over the next several runs.

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FTO 07-11-2019: High Flood Threat Issued as the Ridge Axis Slides East and Produces a Moisture Surge

Issue Date: Thursday, July 11th, 2019
Issue Time: 3:00PM MDT
Valid Dates: 7/12 – 7/26

The water vapor imagery below denotes two features that will cause a High flood threat to be issued for Event #1. The first is the subtropical high, which is currently hanging out near the Four Corners region. The 500mb upper trough, marked below with an “X”, will position itself off the Pacific Northwest coast by this weekend. This will push the ridge axis to the east and allow a PW surge northward statewide. The surge will last west of the Divide through Sunday, and it will last through Monday/Tuesday to the east. With multiple days of heavy rainfall possible, soils may become saturated and increase runoff. The placement of the high over CO/NM will limit steering winds, which will also help large rainfall totals accumulate. Starting on Saturday, recent burn areas will need to be monitored closely.

From Tuesday to next Monday (Event #2) there is still some uncertainty in the forecast, but the GEFS is producing a weak ridging pattern over the state once again by the end of the week. A trough to our west may help keep the ridge axis to our east, which if this happens, would produce another PW surge statewide. The placement of the 500mb high each day will be key, since this will determine where the highest moisture (advecting clockwise around the 500mb high) will set up. Slow steering winds again will cause an Elevated flood threat to be issued, which includes recent burn areas. With the weaker atmospheric pattern, there will likely be changes model run to model run, so tune back into the FTO on Monday for the latest on trends from the runs.

Quite the ramp up in PW from the GEFS from Friday into Saturday. By Saturday evening, PW looks to remain above an inch over eastern Colorado through (possibly) Tuesday. For western CO, PW values are above climatology this weekend before models start to show a drop off in moisture. Expecting widespread showers over the western high terrains with an increase in activity over the adjacent valleys. The models likely show a drop off  in PW values due to advection of dry air from the west, which will scour out moisture and place higher PW values to the south/west. Wednesday and Thursday look to be mostly dry for western Colorado with more isolated afternoon storms possible on Tuesday afternoon. Same goes for eastern Colorado. Storm activity is forecast to decrease Wednesday and Thursday, so this should give us a break in widespread rain before the start of the next weekend (July 20-21; Event #2).

Snowpack/Riverine:

Peak flow from snowmelt this season has passed. That means for this weekend, flows are forecast to decrease statewide from decelerating snowmelt rates. This does not mean that rivers and streams will not be running high, so please use caution around rapid, flowing water. With heavy rainfall returning to the forecast for multiple days, some Minor flooding may be possible if storms track over the same areas more than one day in a row. Increased runoff would be expected in this case due to saturated soils. Please track the daily flood threat in the FTB and follow your local NWS office for all Flood/Flash Flood Warnings/Advisories. Note that all active riverine Flood Warnings and Advisories (at the time of the post) will be mentioned in the FTB.

Event #1: Friday (7/12) – Monday (7/15)

High Threat/Elevated Threat as the ridge axis slides east and a PW surge occurs statewide.  

Active weather weekend ahead with the eastward shift of the ridge. By Saturday, widespread showers and thunderstorms will likely produce flooding. A weak surface trough on Saturday may cause some storms to become severe over the eastern plains with the main threats being large hail and strong winds. Overnight rainfall may also be a concern on Saturday night, but this of course depends on the timing of shortwaves moving through the area. These small features are better forecast on a 24-hour scale. A stronger surface trough sets up on Sunday, so the risk for severe weather is a bit higher. Slow steering winds will cause heavy rainfall concerns with possible riverine flooding in the high country. Flows are still quite high from snowmelt and dam releases. Burn areas will also be under the gun with debris slides, mud flows and flash flooding being the main threats for storms that track directly over a scar. Once again FTB will be watching and tracking shortwaves rotating around the high as well, as this would help enhance convection and may cause more widespread activity over the eastern plains if they are timed with peak heating. Overall, we’re in for a wet weekend, and be sure to get off the high peaks early as the monsoon kicks off thunderstorms by the early afternoon.

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Event #2: Tuesday (7/16) – Monday (7/22)

Elevated Threat/No Apparent Threat as zonal flow becomes more southwesterly with a building ridge.

Once again, lower confidence in this forecast. Zonal flow at the beginning of the week should decrease the flood threat, especially to the west as dry air becomes entrained from the desert southwest. More isolated storm activity is expected by the middle of the week even though there is still some spread in the PW plumes after this weekend. The next ridge sets up by Friday, which may cause another PW surge from the south and east. Paired with slow steering winds, an Elevated flood threat has been issued. Please tune back into the FTO on Monday as details will continue to evolve and alter the forecast.

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