FTO 09-08-2022: Return of Rainfall & A Meaningful Cool-Down

Issue Date: Thursday, September 8th 2022
Issue Time: 2:45PM MDT
Valid Dates: 9/9 – 9/23

Heading into tomorrow, there should be some relief from this unseasonably hot start to September. After several days of broken temperature records, a cold front is expected to slide through the state tonight. This should help drop high temperatures several degrees over portions of eastern Colorado tomorrow, but to a lesser extent for areas west of the Continental Divide. A secondary and stronger cold front is forecast to push through late on Friday and into Saturday, which should more effectively reduce statewide temperatures for the weekend. This is all thanks to the passage of an upper-level trough which can be seen by the orange dashed line in the water vapor imagery below (Event #1). As the trough passes through, Hurricane Kay will continue to move northwards. Kay will likely help increase moisture across the southern border beginning tomorrow. On Saturday, the increase in dynamics from the trough and moisture from Kay may combine, which could cause a localized, low-end Elevated flood threat to be issued for portions of the southern high terrain. A few ingredients that would need to come together for the threat would be clear skies, warmer temperatures (south), and timing of the frontal passage/location of its associated dynamics. Additionally, near surface moisture will need to substantially increase to help produce more efficient rain rates.

The EPS has a slightly longer lull when compared to the GEFS before Event #2 begins early next week. It looks like another incoming, albeit weaker, trough will help to push the remnants of Hurricane Kay over the state from west to east. With a higher likelihood of the remaining moisture being located mainly in the mid and upper-levels of the atmosphere and the addition of faster steering flows and ongoing cloud cover limiting instability, means there is No Apparent threat issued at this time.

The PW plumes from the GEFS are now showing better consensus with the return of moisture for Event #1 (east) and for Event #2 (statewide). With the lower levels of the atmosphere likely remaining quite dry and less instability during Event #1, outside of the southern high terrain, there is No Apparent flood threat issued. As Event #2 begins, PW values are likely to rise to above normal values east and west, which translates into scattered to numerous storms returning to the forecast. At this time, not anticipating a lot of instability or too high of dew points, which means that the rain rates should only be light to moderate. So, there is No Apparent flood threat issued for Event #2.

Event #1: Friday – Sunday (9/9 – 9/11)

One day Elevated flood threat as some subtropical moisture and dynamics potentially combine over the southern high terrain.

The isolated to widely scattered storms on Friday into the overnight hours should mostly be confined to the northern and southern mountains. There may be some spillover possible near the elevated ridges with more zonal steering flows and dynamics present during the late afternoon and overnight hours. Best chance for precipitation accumulation will be across the southern high terrain, east of the Continental Divide where max 1-hour rain rates may reach up to 0.75 inches. By Saturday, cooler and more stable conditions along with widespread cloud cover are anticipated for much of the state. Best chance for some higher rain rates will be over the central and southern high terrain again with some spillover of storms possible into the immediate adjacent Southeast Plains. Rain rates may reach just over an inch an hour, so an Elevated flood threat has been issued. Elsewhere, light stratiform rainfall and mist is expected.

Event #2: MondayThursday (9/12 – 9/15)

Remnants of Kay should drift overhead from the west to east and cause an uptick in shower and storm coverage, but there is No Apparent flood threat issued.

Long-term guidance still has some differences with the timing of the increase in moisture from the remnants of Kay, but there is increasing confidence that there will be an uptick in storms and showers next week. The most scattered to widespread rainfall activity is likely to occur on Tuesday and Wednesday. Without much instability and surface moisture forecast at this time, not thinking there will be much of a heavy rainfall threat. Details about the final location of Kay and the timing of the next trough are likely to change, so be sure to tune back into the FTO on Monday as there might need to be a 1-day upgrade to the flood threat.

FTO 09-05-2022: Hot & Dry To Start The Work Week With A Cool Down Likely Friday

Issue Date: Monday, September 5th 2022
Issue Time: 1PM MDT
Valid Dates: 9/6 – 9/20

Yesterday marked the 85th consecutive day of precipitation occurring somewhere within the state. There were two weak mountain storms that developed during the late afternoon over Saguache and Gunnison County. Even though these storms likely only caused minimal accumulation, they helped keep the precipitation streak alive. Not sure how much longer the count can keep going since it’s going to be a hot and dry start to the work week.

The first part of this week will feature an anomalous strong upper-level High, which will promote subsidence and continue to pull in a dry air mass from the north. During this period, only very isolated showers over the mountains may be possible with limited residual moisture. With the High’s grip over the state, afternoon temperatures are expected to remain much above average through Thursday. In fact, several areas may break their daily record high temperatures throughout the week. By the end of the week, a trough is expected to drop a very welcomed cold front through the state, which should help cool temperatures and return precipitation chances to the forecast (Event #1). Sunday will likely serve as a transition day between events before the remnants of Tropical Cyclone Kay potentially move into the state (Event #2). Uncertainty continues in the forecast for Event #2, but there is increasing confidence that there will be an uptick in rainfall chances at the beginning of next week.

The PW plumes show below average values both east and west with the dry air mass overhead to start the forecast period (black line below red line). Even though the week is expected to start off quite dry and hot, lighter surface winds are anticipated under the ridge, which should help to suppress fire weather. While Event #1 looks to return moisture and some dynamics to the state, it looks like this event should be mostly confined to areas along and east of the Continental Divide. After Event #1, both plumes indicate that there is a good likelihood for a lull in moisture, and thus precipitation chances on Sunday. Then, Event #2 potentially kicks off as remnants of TC Kay move from west to east across the state. The large spread in ensemble members (gray lines) indicate uncertainty in the magnitude of moisture, but there will likely be an upward trend in PW, which should cause precipitation chances to increase next week. Currently, there is No Apparent flood threat, but we’ll be keeping an eye on the upper bounds of the potential moisture surge throughout the week.

Event #1: Friday – Saturday (9/9 – 9/10)

No Apparent flood threat with limited moisture and dynamics over eastern Colorado.

Best chance for precipitation during this event will be across the southern high terrain and elevated Palmer Ridge area where isolated max 1-hour rain rates may reach between 0.5 and 0.75 inches. There is a smaller chance for widely scattered storms over the adjacent plains, and with quick storm movement and limited surface moisture, any storms that do develop should pose more of a wind than heavy rainfall threat. Therefore, there is No Apparent flood threat issued at this time.

Event #2: SundayWednesday (9/11 – 9/14)

Remnants of tropical moisture may drift overhead from the west and cause an uptick in rainfall chances, but currently, there is No Apparent flood threat issued.

Long-term guidance is now showing TC Kay tracking up the Baja Peninsula. The remnants of TC Kay may linger to our west and slowly increase PW values over the state. However, if the moisture and energy is able to track more quickly overhead, by way of a shortwave pushing it eastward, there is a higher chance for a low-end 1-day threat early next week. The likelihood of that happening is not high, so it’s more likely that the moisture will slowly move overhead and cause an uptick in precipitation chances first west, then east. Be sure to tune back into the FTO on Thursday as there may be better synoptic and mesoscale clarity for when and where the remnants of Kay will track.

FTO 09-01-2022: Quick Rainfall Event To Start Labor Day Weekend, Then Possible Monsoon Surge Next Weekend

Issue Date: Thursday, September 1st, 2022
Issue Time: 2:30PM MDT
Valid Dates: 9/2 – 9/16

It’s officially meteorological fall today, although the well above average temperatures feel like it’s a continuation of summer. Above average temperatures are forecast through at least this next week with an upper-level ridge overhead. Event #1 is a quick 2-day event beginning tomorrow. Some diurnally driven storms are expected over the mountains with some isolated storms possible over the plains tomorrow. After Saturday, dry air should circulate around the building High, which should keep precipitation chances limited over Colorado next week. Event #2 is not expected to begin until the end of next week when flow could turn southerly and pull some tropical moisture northwards over the state. There is still a lot of uncertainty with this forecast as the plume could easily stay south and east of the state. Details should start to become clearer at the beginning of next week.

PW plumes are not overly impressive with values likely remaining below climatology through Thursday. There is a slight uptick in moisture over eastern Colorado tomorrow and Saturday behind a weak cold front. This front should help to produce isolated storms over the Southeast Mountains and possibly the adjacent plains tomorrow afternoon and evening. By Saturday, storms are expected to be confined to the mountains with northerly (north) and NNE (south) steering flows forecast. With only weak mid-level lift, quicker steering flows on Friday, and limited moisture, there is No Apparent flood threat issued for Event #1.

Event #1: Friday – Saturday (9/29/3)

No Apparent flood threat as weak dynamics and moisture produce isolated storms.

Isolated storms are forecast tomorrow over the mountains, Southeast Plains and elevated ridges. Storms that are able to develop over the adjacent plains along the boundary should have limited surface moisture, which means they could produce some strong outflow winds and plenty of virga. Over the mountains, isolated storms are most likely to develop along and near the Continental Divide and over the Southeast Mountains. There’s a better chance for widely scattered storms to develop over the southern high terrain on Saturday. During this period, storms should be short-lived and produce rain rates under 0.5 inches. Therefore, there is No Apparent threat has been issued.

Event #2: Thursday – Sunday (9/89/11)

Potential tropical moisture surge with added dynamics may cause an uptick in rainfall coverage, although at this time, there is No Apparent flood threat issued.

There’s a lot that can change over the next week and key ingredients that need to come together for Event #2. At the moment, long-term model guidance isn’t showing consensus either, so choosing to keep this forecast on the lower-end of the flood potential scale. A passing of a trough to our north, northward movement of the tropical moisture and the position/existence of a weak shortwave to our southeast will have to have the right timelines and tracks to get the moisture plume over the state. The likelihood of all the ingredients coming together is low, but stay tuned to the FTO on Monday as their will likely be an update to the forecast. At the most there would be a 1- or 2-day category upgrade to the event, but only if it looks more likely that the plume will move overhead.

FTO 08-29-2022: Mainly Dry and Warm Start To Fall, Then All Eyes Turn South…To The Tropics?

Issue Date: Monday, August 29th, 2022
Issue Time: 2:45PM MDT
Valid Dates: 8/30-9/13

After 79 straight days of precipitation somewhere across Colorado, the streak looks to finally end this week as drier air overtakes the state. At least briefly!

As shown in the water vapor image, below, a large-scale ridge is beginning to build across northwest North America. This will continue to evolve over the next 48 hours so that by mid-week, much above normal heat is expected along the US West Coast. Closer to home, however, the clockwise northerly flow around the expected ridge will promote subsidence. With only marginal moisture available, we expect a rather dry week for our state. At least compared to the past few months! Nonetheless, a couple of disturbances (Event #1 and Event #2) should support at least an isolated risk of shower and storm activity over the course of this week. Event #1 will occur on Wednesday, followed by another drier lull and then Event #2 this weekend. Flooding is not expected with either Event. Meanwhile, temperatures will continue to increase a bit and most of the state will see afternoon heat of 3-7F above normal values for early September.

Things turn interesting by next week as there is a surprising amount of consensus regarding the development of one or more tropical disturbances over the eastern tropical Pacific. As shown in the forecast GEFS PW, below, this will be occurring in a backdrop of higher moisture (see especially the Grand Junction forecast). Thus, showers and storms of modest intensity are likely to return to the Four Corners by early next week. Within Colorado, the best coverage will be along the climatologically preferred southern border, notably over the Southeast Mountains and San Juan Mountains. The flood threat will depend largely on the amount of tropical moisture that can trek up the western Mexico coast. In the absence of tropical moisture, it appears that the normal monsoonal dynamics and moisture will be too weak to support heavy rainfall capable of a flood threat. And with this situation being so far into the forecast, we do not expect flooding at this time. However, the next Outlook should be able to better assess the tropical moisture contribution, and whether a flood threat is needed.

The identified precipitation events are described in more detail below.

Event #1: Wednesday (August 31)

Isolated Showers and Weak Storms For Central Higher Terrain; No Apparent Flood Threat

Isolated to perhaps widely scattered showers and weaker storms are expected to return to the central higher terrain on Wednesday afternoon. Only 0.25 – 0.5 inches of precipitation is expected for lucky locations.

Event #2: Saturday – Sunday (September 3 – September 4)

Widely Scattered Storms For Central/Southern Higher Terrain; No Apparent Flood Threat

Isolated to widely scattered showers and weaker storms are expected on Saturday and Sunday afternoons. Best coverage looks to be over the southern Front Range and Southeast Mountains on Saturday, shifting towards the central and southwestern higher terrain by Sunday. Max 30-minute rainfall up to 0.5 inches looks possible, along with gusty winds.

Event #3: Tuesday – Friday (September 6 – September 9)

Showers and Storms Return, Mainly Along Southern Border; No Apparent Flood Threat At This Time

An prolonged increase in thunderstorm activity is expected by early next week as monsoonal moisture returns to the state. At this time, it appears there will be a strong gradient in the moisture, implying that only the southern one-third of Colorado looks to see meaningful precipitation. Widely scattered to scattered storms are expected mainly over the climatologically preferred Southeast Mountains and San Juan Mountains each afternoon and evening. At this time, max 30-min rainfall looks to be limited to 0.5-0.6 inches, thus flooding is not expected. However, the next Outlook should have a better assessment of whether a (low-end) threat is needed.