FTO 08-08-2019: Approaching Trough Maintains the Current Moisture Surge and Keeps Heavy Rainfall Chances Elevated through this Weekend

Issue Date: Thursday, August 8th, 2019
Issue Time: 2:45PM MDT
Valid Dates: 8/9 – 8/23

There are two events identified in this FTO. Event #1 from the last FTO is currently ongoing through tomorrow, but for simplicity sake, I tied it into the heavy rainfall event forecast this weekend since the dynamics are similar enough. Friday into this weekend, the low marked below will start to traverse east. This will push out some mid-level energy in front of it, which will also mix with shortwaves moving around the subtropical high. This will help create more widespread storm activity, especially if the shortwaves pass overhead during peak heating. At the same time, the subtropical high will be in a favorable position to maintain the PW surge, although values are forecast to drop off a bit from today. This PW surge will mostly favor locations east of the Continental Divide, but some higher PW values are expected to push northward over the southwest corner of the state Saturday and Sunday before the upper flow turns more westerly (dries it out). This could be problematic for the 416 burn area as rain rates will likely increase. Some cooler temperatures are forecast for this weekend, and much cooler temperatures are forecast for early next week after a cold front pushes south Monday into Monday night.

There is a lull in afternoon rain during the work week as the subtropical high becomes rather weak and broad over our area. This will allow more westerly winds, which will pull in dry air from the west (similar to the mechanism seen below). The next low looks to move through sometime near the weekend of August 17th. Not as much confidence in this forecast, but the narrow ridge would be pushed east allowing for a weaker monsoon surge northward. Models pointing to the monsoon season wrapping up after this event, which if it occurs, would lineup with climatology.

Incredible PW surge currently going on, which is always possible during the monsoon season. PW values over western CO are close to daily max PW records and, obviously, well above climatology. This surge is expected to last through this weekend before the model members start showing a decline in PW values. Expect the coverage of storms to follow this trend with isolated afternoon showers and thunderstorms likely only over the mountains from Tuesday forward. To the west, PW is currently declining due to the dry air (marked in the image above) rotating around the high and into western Colorado. Slight moisture surge on Saturday, which will be something to watch for the 416 burn area. Very low moisture by the end of next week and the approaching trough will cause us to watch for fire weather conditions. However, an increase in southwest winds is also anticipated, so the more likely scenario is that these winds will intensify the moisture; thus, lessening the chances for fire weather.

 

Event #1: Friday (8/9) – Monday (8/12)

Elevated Threat as the next trough approaches, it slides the ridge axis to the east and allows for an ongoing moisture surge.  

Heavy rainfall is anticipated for this event, although there is not a High threat issued. For one, the moisture sure isn’t as strong as this last surge; and two, faster steering winds should limit the areal coverage of the threat. Monday will be watched closely, because if the front drops south during the afternoon, it may cause some widespread heavy rainfall and severe weather over the plains. The main threats would be heavy rainfall, hail and strong winds. Friday and Saturday will have the most coverage of storms during this period due to upper level energy also moving into the state in the high moisture environment. Models have also been consistently showing cooler temperatures over northeastern Colorado for Saturday as well. Heavy rainfall over the eastern plains will be most likely on Monday, though there looks to be some gradual, heavy accumulating rainfall along the CO/WY border Sunday. Burn areas over the Southeast Mountains will be watched closely on Friday with eyes turning to the 416 burn area this weekend.

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Event #2: Saturday (8/17) – Monday (8/19)

Elevated Threat as the vertically amplified ridge shifts east with the next passing low, which returns high PW values to eastern Colorado.

Not a lot of confidence in the forecast as this is so far out. Thus, there is no precipitation map drawn below. Since it’s still monsoon season, thinking this will look similar to a typical monsoonal moisture event. However, it is getting pretty late into the monsoon season and the pattern isn’t quite textbook, so thinking it will be on the weaker end of the Elevated Threat at this time. Not horrible disagreement between models members on the Spaghetti plot (relative to a 216+ hour forecast), so fairly confident it will happen. However, there will likely be a lot of changes in the timing and details of the event, so please tune back into the FTOs next week for the latest details.

 

FTO 08-05-2019: Heavy Rainfall Threat Returns with the Frontal Passage on Wednesday

Issue Date: Monday, August 5th, 2019
Issue Time: 4:00PM MDT
Valid Dates: 8/6 – 8/20

There is lull in heavy rainfall tomorrow before a High/Elevate flood threat is issued through this weekend. Taking a look at the water vapor imagery below, quite a bit of dry air circulating up and around the Pacific and west coast. It is usually a bit more concentrated in nature with stronger drying near the equator. With the subtropical high over Arizona, some of that dry air (yellow/orange) has worked its way into western Colorado. This has decreased the chances for rainfall from Sunday to today, and it will continue to do so through tomorrow. Event #1 kicks into full gear on Wednesday as the ridge begins to get squashed and a cold front is pushed through eastern Colorado. This is due to multiple shortwaves rotating around the high/low (labelled Event #2), which will also help enhance lift for widespread coverage of afternoon and evening storms. Lee troughing on Wednesday and Thursday will also help return low level moisture to eastern Colorado on its north side. However, low level moisture on its east side will likely stay over Kansas. More westerly steering flows will allow storms to move into the adjacent and eastern plains, so some heavy rainfall and marginal severe weather may be possible on both days. Thus, there is a High/Elevated flood threat issued through Friday.

After Friday, the high begins to rebuild over Texas. This will pull more Gulf of Mexico moisture into eastern Colorado, creating a PW surge. At this same time, the low pressure system marked below begins to move inland. This is expected to increase shortwave activity and southwest flow this weekend. The associated jet may also create some fire weather danger concerns over northwest Colorado, but there is lower confidence in that forecast as this time. More zonal flow behind the trough will mix out moisture, so there is No Apparent Threat and a break in heavy rainfall forecast to start next week.

First thing that catches my eye about the images below is that average PW is starting to decrease (red line), which means climatologically, the monsoon starts to taper off. The second thing is the variability between model members after Wednesday (gray lines), which just means there is more uncertainty in my forecast for this weekend. With that said, there is quiet the moisture return after the passage of the cold front on Wednesday over eastern Colorado. Paired with an increase in dynamic (shortwaves, boundary, etc.), heavy rainfall will be likely and may last overnight on Wednesday over the eastern plains. The moisture sticks around on Thursday, but storms will likely be confined to the mountains due to capping over the eastern plains. With favorable dynamics in place over the weekend and Gulf of Mexico moisture likely over eastern Colorado (if the high reforms over Texas), thinking PW will be on the higher end of climatology. Thus, the Elevated flood threat.

To the west, there is a slight increase in moisture on Wednesday and Thursday as well, although not quite as much as the last couple of systems. The subtropical high will be in an unfavorable location to produce widespread heavy rainfall. Nevertheless, expecting these days to have the greatest coverage of storms over the higher terrains. Quite the downtick in moisture through this weekend into next week. If the moisture drops off before the approaching low (and associated jet stream), there might be an increase in fire danger over the northwest corner at the end of this weekend into early next week.

Event #1: Tuesday (8/6) – Friday (8/9)

High/Elevated Threat as post frontal upslope flow and numerous shortwaves combine for favorable heavy rainfall environment.  

Discussed this event quite a bit above, but I will add a few more details down here. On top of high moisture and favorable dynamics, storm motion is expected be very slow over the eastern high country and adjacent plains on Wednesday. Thus, some very heavy accumulations should be expected under storms that form, especially over the eastern plains (isolated totals > 2.5 inches). Surface based CAPE and decent directional and speed shear (shifting of winds with height) may cause storms along the Palmer Divide and east to produce some large hail and strong winds as well. The hail threat will decrease as instability lessens a couple of hours after sundown. Recent burn areas, especially the Spring Creek burn scar, will be monitored closely in the FTB throughout this event as rain rates could be high enough to cause flash flooding and mud flow issues. To the west, the best chance for flooding rain rates over the Lake Christine and 416 burn areas will be on Wednesday. More isolated coverage on storms on Thursday should decrease the overall flood threat to the west including the burn areas. Please be sure to tune into the FTO on Thursday for the latest and follow the FTB for the daily flood threat update.

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Event #2: Saturday (8/10) – Tuesday (8/13)

Elevated Threat as the ridge begins to rebuild over Texas, which returns high PW values to eastern Colorado.

With the more southern and eastern position of the subtropical high, this event will favor heavier rainfall over eastern Colorado. The ridge is weaker than usual over the state for this event, which mean weaker easterlies for upslope flow. The main flood threat for this event will be storms that make it further east under the westerly and southwesterly steering flows. If the peak in diurnal heating coincides with a passing shortwave over the adjacent plains, coverage of the heavy rainfall is forecast be much greater with some severe thunderstorms likely. The right entrance of the jet region will also be watched closely as this could cause some storms to linger overnight, which would increase accumulations. As mentioned before, lower confidence this far out and with varying model members. Please tune back into the FTO on Thursday as details will become clearer.

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FTO 08-01-2019: 2019 Monsoon Causes an On and Off Heavy Rainfall Threat through Next Week

Issue Date: Thursday, August 1st, 2019
Issue Time: 2:50PM MDT
Valid Dates: 8/2 – 8/16

There is an Elevated flood threat for both events of this FTO with breaks in heavy rainfall forecast every couple of days. Monsoon season is well underway, and since we are at the peak of climatological PW values, the active pattern is not surprising. By tomorrow, the subtropical high shifts to the west over the four corners regions, which essentially cuts off the PW flow over western Colorado limiting afternoon rainfall to scattered showers over the higher terrains. We will watch for any subtropical moisture sneaking into the southwest corner of the state (dependent on the placement of the high) as this could cause high enough rain rates for flooding issues over the 416 burn area. Post frontal upslope flow behind the front over the southeast corner of the state tomorrow may cause some flooding issues for recent burn areas over the Southeast Mountains, thus the Elevated flood threat. From Sunday into Monday, the ridge weakens with a passing trough. This will push a weak cold front through the state on Monday, which could trigger some heavy rainfall and a few severe thunderstorms over eastern Colorado. The eastern plains look to remain capped on Tuesday minus over the mountains where storms will initiate again with upslope flow. Not seeing a flood threat at this time, although details could easily change over this weekend.

For Event #2, the next northward monsoon push begins on Thursday. Not looking quite as strong as the current one, so at this time thinking coverage of the heavy rainfall threat will be less. Nonetheless, there will be an Elevated flood from Friday into Saturday. The low begins to move onshore next weekend, and once it moves through, more zonal flow is anticipated. This pattern typically pushes the high moisture to the east and suppresses the monsoon surge.

The GEFS moisture plume page hasn’t been updated yet today, so the image below is a little old. You can see the current PW surge, which peaks this evening. After this event, it looks like average PW through next Wednesday. Note the scale on the left (DEN) is slightly different than on the right (GJT). The slightly elevated levels of PW over eastern Colorado tomorrow may cause some heavy rainfall over the Southeast Mountains/Raton Ridge, but there will overall be a downtick in coverage and intensity through next Wednesday when compared the last couple of days. The burn areas over the Southeast Mountains will be watched carefully tomorrow for a flood threat, but elsewhere this time, thinking rain rates will be gradual enough to avoid flooding issues.

Below is a recap of the July’s temperature and precipitation. It was noted in the last couple of FTOs that the onset of the monsoon was a little slow this year. However, the majority of eastern Colorado received more than average rainfall with the 200-300% percent of normal area equating to about 2-3 inches. A bit drier back to the west, although it is important to note that August is their big rainfall month. Their 25-50% of normal precipitation equates to no more than 1 inch below normal. This below average rainfall did cause the D0 (Abnormally Dry) drought to be placed back on the drought monitor map over the northwest and southwest corners of the state. As far as temperatures, quite the turnaround from May and June, which were very cool. Most of the state was 1 to 3°F above normal with a couple of notable stretches of upper 90°Fs to 100°Fs. This put the cooling degree days (base 65°F) between 60-80% higher than normal along the Urban Corridor and the adjacent plains of the Southeast Mountains.

Event #1: Friday (8/2) – Wednesday (8/6)

Elevated Threat as the subtropical high shifts into the Four Corners Region and a cold front drops through the state on Monday night.  

On and off again moisture, but the threat for Event #1 is mostly over eastern Colorado and the mountains. There may be an Elevated threat over the 416 burn area on Sunday, but the placement of the high/strength of the ridge will be key (determines if the moisture can meander north). Monday to Wednesday, the threat moves into the eastern plains. This is especially true on Wednesday when the passing shortwave will help support thunderstorms over the plains and cause steering flows to have more of a westerly component as it breaks down the ridge. Not seeing any huge moisture surges, so an Elevated threat should suffice with a lee trough setting up from Monday to Wednesday. Of course if a passing shortwave teams up with peak heating while moisture is elevated, there may need to be a Moderate flood threat issued.

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Event #2: Thursday (8/8) – Sunday (8/11)

Elevated Threat as the ridge begins to rebuild before being shifted east causing the next moisture surge.

The ridge begins to build again during Event #2, and the axis will event slide east causing the next monsoon moisture plume to move northward when a shortwave moves through. The PW values look to peak on Friday and Saturday right now, thus the Elevated flood threat during this time frame. Lower confidence in this solution over western Colorado than eastern Colorado as well. Should be a fairly rapid event with the just brushing the northern border. This will also keep steering flows faster, which should limit rainfall accumulation.

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FTO 07-29-2019: Short Break from Heavy Rainfall Before the Next Statewide Monsoon Surge

Issue Date: Monday, July 29th, 2019
Issue Time: 3:00PM MDT
Valid Dates: 7/29 – 8/13

There is an Elevated/High flood threat for Event #1 of this FTO. As the high pressure slightly moves west into the TX panhandle Tuesday into Wednesday, the next monsoon PW surge occurs. The incoming low pressure system marked below is the instigator in the shift of the ridge axis to the east. Lee troughing will also help pull low level moisture northward over the eastern plains beginning on Tuesday afternoon. A cold front looks to drop through the state on Thursday, which often returns low level moisture, so a High flood threat is issued for post frontal upslope flow and overnight precipitation along the boundary over the Northeast Plains. The High threat continues on Friday, although the afternoon storm activity should be more of a mountain and adjacent plain flood threat due to capping over the plains. An Elevated flood threat continues through the weekend with the San Juan Mountains under the gun for western Colorado. Burn areas will be monitored daily throughout the event in the FTB.

For Event #2, the next trough digs south farther west than the last handful of systems. If this occurs, this will pull the subtropical high back to the west and put the state under more northwesterly flow aloft. Still lower confidence in the forecast this far out, but we will discuss it more nonetheless. This pattern would keep cooler temperatures in the forecast and prevent any long duration PW surges northward into the state. There is an Elevated flood threat on Tuesday in this scenario due to post frontal upslope flow. Faster steering winds should keep the threat over an limited area, but more of a westerly component to the steering flows would likely cause some rainfall over the eastern plains favoring the Southeast Plains, Palmer Ridge and Raton Ridge.

The next monsoon surge is quite visible in the GEFS PW plumes below (Event #1). By about Wednesday, PW values start to reach above climatology over eastern Colorado. This again is aided by a lee surface trough setting up on Tuesday. PW also creeps up over western Colorado by tomorrow night. So expect an increase in activity over the high country from Wednesday into Friday. The high then shifts into an unfavorable location for western Colorado rainfall and PW drops off drastically. Still some uncertainty in the forecast (west) after this weekend, but PW looks to remain at or just below climatology. So other than recent burn areas, the flood threat is anticipated to decrease.

Event #1: Tuesday (7/30) – Sunday (8/4)

High/Elevated Threat as the subtropical high shifts in to the Texas Panhandle and the next PW moisture surge begins.  

Should have one more quiet day with limited afternoon storm activity before the next monsoon surge begins. By Wednesday, PW values start to edge above climatological values, which should increase storm coverage during the afternoon. Expecting western Colorado to be impacted first with the threat moving east on Thursday. A passing cold front overnight on Thursday may cause some nocturnal thunderstorms capable of producing some very heavy rain over the northern Urban Corridor and western portion of the Northeast Plains. Post frontal upslope flow will likely impact and cause heavy rainfall over the southern Front Range, Southeast Mountains and San Juan Mountains on Friday. This is probably the day that needs to be watched closest for flooding over recent burn areas. Rain continues Saturday over the high terrains with some thunderstorms also possible over the Colorado/Kansas border. Weak thunderstorms are forecast each of the afternoon with a couple of severe storms possible over the eastern plains. The severity of the thunderstorms will likely be determined by passing shortwaves. If timed with peak instability, a couple of severe thunderstorms may be possible with some large hail.

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Event #2: Monday (8/5) – Thursday (8/8)

Elevated Threat/No Apparent Threat northwest flow aloft returns and shuts down rainfall activity over western Colorado.

Shift in the overall monsoon pattern if the GFS is right. The next low pressure system looks to dig south and west of California for this event. This would pull the subtropical high to our west and put eastern Colorado under northwest flow. It would really limit afternoon rain over western Colorado minus some weak storms and showers over the Central and San Juan Mountains. Cooler temperatures also look likely to start next week, which will be a nice break from the heat. A cold front may also be possible on Monday night, which could cause some heavier rainfall over the southeast quadrant of the state on Tuesday. This includes the Southeast Mountains and recent burn areas. However, higher PW values would quickly be pushed south and east under the flow aloft. This would only support scattered showers on Wednesday and Thursday capable of producing only light to moderate rain rates.

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