FTO 08-16-2018: Elevated Flood Threat before Cold Front Slides Through on Sunday

Issue Date: Thursday, August 16th, 2018
Issue Time: 1:15 PM MDT
Valid Dates: 8/17 – 8/31

As pointed out in the FTB this morning, the remnants of the last trough are now over the Midwest and the system will continue to push eastward. In its wake, an arid air mass has pushed into northern Colorado. Beginning Friday morning, increased moisture will arrive to this region (east of the Divide) as SE surface winds return with an approaching shortwave. With weak surface winds aloft, there is an Elevated Threat to start this FTO period. Moisture trapped under the ridge over southwest Colorado will again spark another round of afternoon thunderstorms favoring the San Juan and southern Central Mountains on Friday.

On Saturday, the upper-level trough marked in the water vapor imagery below begins to move inland and displace/breakdown the weak ridge. Positive vorticity advection (PVA) on the east side of the trough will provide a large amount of lift for widespread thunderstorms. Early Saturday, showers will begin over the Northwest Slope and increase north to south by early afternoon. By the mid-afternoon hours, the trough will arrive to the eastern mountains where upslope flow and daytime heating are occurring. A line of convergence is expected to produce a strong line of thunderstorms over the adjacent plains by early afternoon with plenty of thunderstorm action over the higher terrains as well. Threats include large hail, damaging winds and heavy rainfall. While moisture over the mountains won’t be quite as deep as over the adjacent plains, rain rates are expected to be high enough to cause flooding concerns near recent burn scars. Steering winds should be quick with the mid-level jet streak, so widespread flooding over the mountains is not a concern at this time. Though these details may change over the next couple of runs. Dew points over the eastern plains could reach 60F, so there are flooding concerns despite swift southeast movement of the storms.

Sunday, activity looks to calm down a bit as the trough drops a cold front over the state. This is expected to draw in some dry air with northerly flow, cap convection and bring much cooler temperatures. While no strong convection is forecast, there is a chance for some stratiform rain associated with the front. To start next week, the ridge begins to build north of Colorado once again. A second trough forms over the Pacific Northwest, but at this time, the models lift the trough north. This should keep northwesterly flow over the state and increase the chances for upper-level disturbances to pass through the state each afternoon through the end of the week. The big question each day will be the availability of low-level moisture. At this time there should be enough residual moisture under the ridge to the west and daily southeasterly flow to the east that mountain convection will be possible each afternoon. There is not enough confidence at this time to forecast anything more than No Apparent Threat.

Quite a jump in Precipitable Water (PW) for Denver starting tomorrow as southeasterly surface flow returns. Not much change for western Colorado. With steering flows aloft being light again tomorrow, burn scars will be monitored closely. On Saturday, PW begins to drop off statewide with the exception of the far eastern plains. Western movement of the 500mb high is expected to decrease rainfall chances over western Colorado until next week when the ridge begins to build and the 500mb high shifts east again. Not much confidence in low-level moisture after Tuesday as the GEFS shows a lot of variability both east and west. This likely has to do with how strong the ridge rebuilds and the location of its axis. All details that are nearly impossible to pinpoint this far out. Nonetheless, there should be enough moisture for daily rounds of thunderstorms with the diurnal flow.

Below we describe the identified precipitation event of this FTO in more detail.

Event #1: Friday (8/17) – Friday (8/24)

Elevated Threat/ No Apparent Threat for severe weather on Saturday and potential heavy rainfall over the eastern plains.

Elevated Threat for Friday as slow steering winds aloft and low-level moisture combine over eastern Colorado. Not much change in PW over the southwest corner of the state, but slow steering winds could cause problems for the 416/Burro burn areas. Saturday looks to have the best potential for flooding as the approach trough brings widespread PVA and strong upper-level winds. While steering flow will increase, large scale lift will promote widespread thunderstorm development. This will be most problematic over the eastern plains where high PW could lead to widespread, heavy rainfall, large hail and damaging winds associated with the line of convection. After Saturday, Sunday should bring a break in heavy rainfall as dry air moves in behind the trough. Some stratiform rain may occur with the passage of the cold front, but this is not expected to cause any flooding issues at this time. Monday to Friday of next week, the ridge begins to build north of Colorado. The strength and location of the ridge is a little hard to determine at this time, but there could be a couple days with a flood threat next week. There should at least be enough moisture for daily rounds of thunderstorms with the diurnal flow. Please check back to Monday’s FTO for more details.

 

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FTO 08-13-2018: Active Week Ahead with Possible Severe Thunderstorms for Eastern Colorado on Wednesday

Issue Date: Monday, August 13th, 2018
Issue Time: 1:25 PM MDT
Valid Dates: 8/14 – 8/28

It is mid-August already, which means climatologically we’ve started the descent from the peak of the North American Monsoon (NAM). At the beginning of the season (May), Climate Prediction Center predicted above average precipitation for June, July and August over western Colorado (40-50% probability). Over eastern Colorado, there was equal chance for above/below average precipitation. Unfortunately, the majority of western Colorado continues to be at 10-50% of normal precipitation. To further look into the overall pattern in July (the NAM), we took at a quick look at the NCEP/NCAR reanalysis data. Below is the 500mb geopotential heights for July 2018 (top) and July 1949-2018 or climatology (bottom). It shows that throughout July there was an anomalously strong high pressure center over the southwest US with the axis of the subtropical ridge extending further west than normal. This is similar to what we saw in observations for the Flood Threat Bulletin. For the most part, there were only slight movements with the center of the high to the east and west, which allowed for minimal subtropical moisture return or “monsoon surges” over western Colorado. There is still a chance for a couple more surges before the subtropical ridge shifts back south, but the 2018 monsoon season has been quiet when compared to season’s past.

There is only one identified event for this next FTO, which begins tomorrow and brings an Elevated Flood Threat to the state the rest of this week. The upper-level low over southeastern Colorado will begin to move to the east through tomorrow, and in its place, the subtropical ridge will begin to build again over the west. This should allow Gulf of Mexico low-level moisture to start to fill in over eastern Colorado. Thus, there will be an increase of storms over the eastern mountains beginning tomorrow. Flow aloft will become northwesterly again, which will promote an active weather pattern by allowing fronts to pass through the state. On Wednesday, the first boundary passes through the northeast corner of the state and brings a chance for some larger thunderstorms over the eastern plains. High instability and 40 knots of shear associated with a lee trough will promote possibly severe thunderstorms, which would be capable of producing strong winds, large hail and heavy rainfall.

By Thursday, the subtropical ridge will have re-established itself over the PNW with the center of the high pressure over the southwest US and 4-corner region. Flow aloft will become more northerly as well. From Thursday to Friday, the ridge axis will shift eastward and begin to pull in subtropical moisture to the southwest corner of the state. This would make the recent burn scars over the San Juan Mountains prone to flash flooding. The monsoon surge will continue its hold over eastern Colorado during this time with shortwaves possibly moving through each afternoon. By Saturday, the ridge begins to break down again and upper-level flow becomes more zonal. This would likely lower 700mb relative humidity and the threat for heavy rainfall.

Recent GFS runs have a cold front moving through the state late on Sunday, which could return an Elevated flood threat for this weekend. This would also start off next week a bit cooler if the upper trough materializes. Confidence in this solution is low at this time, so please check back to Thursday’s FTO. Further out, the GFS is starting to push some fall-like fronts through the state by the end of this FTO period. Again, confidence is very low in this solution, but it is a reminder that summer is coming to close.

Precipitable Water (PW) starts to return to above average both east and west of the Continental Divide beginning tomorrow. Little spread means there is higher confidence that elevated PW over western Colorado will remain through Friday. This may be an issue for the 416/Burro burn scars especially on Thursday and Friday. Be sure to tune into the daily FTB for more details. Less confidence and more spread in PW values after Thursday for eastern Colorado. Though all members have above average PW. The GEFS is also showing a drop of PW with the frontal passage on Sunday. Overall, looking at a very active week with heavy rainfall possible each day.

Below we describe the identified precipitation event of this FTO in more detail.

Event #1: Tuesday (8/14) – Monday (8/20)

Elevated Threat as a monsoon surge begins for eastern and western Colorado.

As the ridge begins to rebuild itself throughout the next couple of days, PW values will be on the increase. First, the threat will be over the eastern CO with more widespread storms returning to the Front Range and Southeastern Mountains on Tuesday. Recent burn scar flash flooding potential will be monitored and addressed daily in the FTB as small details will change with the forecast day to day. There is a possible severe weather threat on Wednesday over the eastern plains associated with lee troughing and a passing front. Heavy rain, damaging winds and large hail are all possible if the forecast stays on track. After this event, the subtropical ridge begins to slide eastward, which will pull in high low-level moisture to southwest Colorado. While widespread flooding is not forecast, this could be potentially dangerous for the 416/Burro burn scars as storms in the areas would have rain rates in the 0.75-1 inch/hour range. PW begins to drop off this weekend with a potential frontal passage on Sunday bringing cooler temperatures to start next week.

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FTO 08-09-2018: Rainfall Returns to the San Juan and Central Mountains over the Weekend

Issue Date: Thursday, August 9th, 2018
Issue Time: 2:30 PM MDT
Valid Dates: 8/10 – 8/24

Very dry air has worked its way over the western US as the 500mb high spins over the Great Basin. Extreme temperatures associated with this high has place many areas under an Excessive Heat Warning. To start of this FTO period, we will begin with Event #1 from Monday’s FTO. Starting Friday, the ridge will begin to move eastward as the closed low over the Pacific makes its way to the west coast. This will displace the ridge to the east, and a cut-off low, from the previous trough, will place itself over TX/OK. This weekend, the ridge completely breaks down with the passing of the trough to the north, and the cut-off low moves west and north into eastern Colorado where it slowly migrates east through Tuesday. This should keep the chances for rainfall over eastern Colorado minimal due to northeastly flow aloft. Tropical cyclone John will also move north this weekend, but shouldn’t affect the region until early next week by possibly bringing in some extra subtropical moisture. Slight eastward movement of the ridge Friday to Sunday may increase the chances for afternoon rainfall over the San Juan and Central Mountains. Widespread flooding is not anticipated though burn scars may be prone to flash flooding and mud flows.

From Tuesday to Thursday, a weak ridge begins to rebuild over the Pacific Northwest. Slight movements of the center of the 500mb high indicate there will be a chance of showers over eastern Colorado with returned low-level moisture to start Event #2. Tropical Cyclone John is absorbed into the westerlies as well, which has the potential to pull in a little extra moisture. An incoming trough over the Pacific Northwest pushes the ridge far enough east on Friday and Saturday that subtropical moisture is expected to return both east and west over Colorado. Showers will become more widespread, and at this time, there is an Elevated flood threat to start next weekend though there is still low confidence in a forecast this far out.

Precipitable Water (PW) continues to remain below average over western Colorado. A small increase is seen going into this weekend thanks to the slight eastward movement of the ridge. The GEFS continues to hold average PW over the next week, which will allow afternoon showers and thunderstorm to return to the San Juan and Central Mountains. For Event #2, the moisture should be far enough north to return afternoon rainfall chances to the Northern Mountains as well. Eastern Colorado remains quiet heading into this weekend though there is a chance for isolated showers over the mountains with residual moisture under the ridge. By mid-week the subtropical moisture begins to recover with PW returning to above average values. Quite a bit of uncertainty this far out, but the subtropical moisture trend is upwards, so there is the possibility of heavy rainfall returning to the forecast.

Below we describe the two identified precipitation event of this FTO in more detail.

Event #1: Friday (8/10) – Sunday (8/12)

No Apparent Threat as rainfall returns to western Colorado.

Slight shift in the upper-level ridge axis is expected to return some more low-level moisture to the southwest corner of the state. Widespread rainfall is not anticipated, but afternoon showers over the Central and San Juan Mountains are expected to increase. There is still a question as to how much subtropical moisture will return, though a flood threat will likely be unnecessary. Rain rates may exceed the 0.5 inch/hour threshold, which could cause issues with burn scars. Please tune into the daily FTB as those details will evolve day to day. Possible threats include mud flows, debris slides and flash flooding should a storm track directly over a recent burn scar. To the east, northeasterly flow associated with the cut-off low should prevent chances for afternoon showers and thunderstorms. A few storms may be possible along the Continental Divide and far southern Southeast Mountains.

 

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Event #2: Wednesday (8/15) – Sunday (8/19)

Elevated / No Apparent Threat as subtropical moisture returns with the ridge axis shifting eastward.

Event #2 of this FTO still has a lot of details that are in the works. The GFS continues to shift the upper-level ridge to the east, which would allow subtropical to return to the state both east and west of the Divide. GEFS moisture plumes indicate there is an upward trend in PW throughout the week, which would return the heavy rainfall chances. Tropical Cyclone John may also increase moisture into the area starting on Tuesday as it is absorbed into the next passing system. If this forecast continues to stay on track, the Event looks to return showers and thunderstorms chances to Northern Mountains. Trends in future model runs will be monitored closely throughout this weekend, and new details will be incorporated into Monday’s FTO.

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FTO 08-06-2018: Elevated Flood Threat Continues for Tuesday before the Drying Trend Gets Under Way

Issue Date: Monday, August 6th, 2018
Issue Time: 1:15 PM MDT
Valid Dates: 8/7 – 8/21

The satellite imagery below shows a closed low over the Pacific Ocean, which will control our weather the next week. Currently, the 500mb high is elongated is a bit south over New Mexico and Arizona making the winds aloft westerly. A bit of dry air can be seen over southern Colorado, which will help limit the rain rates along the border this afternoon. Throughout this week, the low will continue to progress slightly eastward and dig south. This will re-center the 500mb high to the northwest and progressively make the winds aloft have more of a northerly component. As the winds begin to become more northerly, this will pull in a dry air mass and provide a downtick in rainfall chances as the week continues. Expecting smoke to return with this flow, too. Residual moisture under the ridge from Tuesday to Thursday, along with northerly flow moving afternoon shortwaves through the state, will be enough to spark some afternoon thunderstorms. The main activity should be confined to the eastern high terrains and immediate adjacent plains. Due to increased convergence along the Palmer Ridge, this will be the most likely area for thunderstorms to survive as they move eastward in the afternoons.

By Friday afternoon, the center of the 500mb high is expected to be over the Great Basin and continue to build north through Saturday. The center of the upper high moves far enough north that it is expected to produce an easterly component to the winds aloft, which will keep the subtropical moisture well to the south of Colorado this weekend. The trough ejects eastward this weekend, but it should remain well to the north of Colorado. The trough re-deepens over western Canada at the beginning of next week, which will shift the ridge axis east and allow some subtropical moisture to return to the state (Event #2). The GFS has a cold front moving through the state on Wednesday of next week, which will help moisten the low-levels and promote upslope flow behind it. At this time, the details of the event will change quite a bit, so there is No Apparent Threat.

The GEFS PW plumes show above average PW to continue into Tuesday for eastern Colorado. Thus, the Elevated Flood Threat. The daily threat over the recent burn scars will be addressed in the daily FTB. After Tuesday, moisture drops off quite a bit with the arid, northerly flow aloft. This will reduce the flood threat; although, afternoon showers and thunderstorms are still likely Wednesday and Thursday with the upslope flow regime. Towards the beginning of next week, the ridge axis shifts slight west and may return the subtropical moisture to the state. There is quite a bit of spread as to how much moisture returns, so we will continue to watch trends from future model runs. Below we describe the two identified precipitation event of this FTO in more detail.

Event #1: Tuesday (8/7) – Thursday (8/9)

Elevated Threat / No Apparent Threat as high PW remains under the building ridge on Tuesday returning the heavy rainfall threat to eastern Colorado.

The upper-level high will begin to rebuild itself over the Pacific Northwest, but low-level moisture should remain high enough to return the threat of heavy rainfall to eastern Colorado on Tuesday. The threat will most likely be over the southern Front Range and Southeast Mountains and extend into the immediate adjacent plains – especially the Palmer Ridge area. At this time, activity looks to increase over the Southeast Mountains, so burn scars may need to be monitored closely tomorrow. Please check back to the FTB tomorrow morning for more specifics. After Tuesday, the drying out begins as arid air will be ushered into the state with more northerly flow. This should limit the heavy rainfall chances, though afternoon showers and thunderstorms will still kick off over the higher terrains in the afternoons on Wednesday and Thursday. If timed correctly with passing shortwaves, some stronger thunderstorms may be possible. This is especially true if they are able to survive and make it into the deeper moisture over the eastern plains. As of right now, the main threat will be gusty winds due to the dry upper-levels in the atmosphere.

 

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Event #2: Tuesday (8/14) – Thursday (8/16)

No Apparent Threat as subtropical moisture returns with the passage of a cold front mid-week.

No Apparent Threat for Event #2 of this FTO as there is very little confidence in this solution so far. The GFS is currently shifting the upper-level ridge to the east, which would allow subtropical to return to the state. It also shows a cold front passing through the state by mid-week, which will help prime the lower-levels with moisture and promote upslope flow behind it on Wednesday/Thursday for eastern Colorado. If this forecast continues to stay on track, the Event looks to return the flood threat to the western mountains as well. Trends with future model runs will be monitored closely throughout this week, and details are expected to change throughout the next couple of FTOs.

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