FTO 2021-05-06: A Cool Spring Continues With Another Round Of Prolonged Rain And Snow

Issue Date: Thursday, May 6, 2021
Issue Time: 2PM MDT
Valid Dates: 5/7-5/21

From a climatological perspective, the months of April and May are some of the most volatile for Colorado. Not only are there very large variations of precipitation and temperature at a given location during this time of year, but there is also drastic variation across the state at any given point in time. For example, the map in the Tweet below shows that climatologically, May is the wettest (and snowiest) month of the year for parts of the Front Range and Urban Corridor, while it is nearly the driest time of year for the southwest parts of the state (source: Brian Brettschneider, Twitter; @Climatologist49).

As shown in the water vapor imagery, below, we continue to see Colorado in the midst of a very active string of disturbances stretched across the eastern Pacific all the way into the eastern United States. As is quite common this time of year, these disturbances will continue to have a propensity to cut-off from the larger jet stream. This will first occur with the trough currently just off the northwest US coast, followed by a secondary disturbance that will reinvigorate the initial cut-off. In short, this will cause a prolonged 72-96 hour stretch of multiple precipitation events for most of the state.

Once again, the northeast quadrant of the state will likely collect the most precipitation, both in the form of rainfall and snowfall. However, almost the entire state will receive some measurable precipitation, with the possible exception parts of the San Luis Valley and lowest elevations of the Southwest Slope. Fortunately, we are currently seeing No Apparent Flood Threat with this event, given the general lack of atmospheric instability. As seen in the forecast PW plumes below, maximum values are expected to stay below 0.75 inch in Denver, which coupled with limited instability is not enough to cause concerns for heavy rainfall.

After Event #1 comes to an end during the middle of next week, we expect a quick rebound in temperatures to above normal level by Friday, May 14. This will cause a shift in attention to the snowpack for possible issues with snow melt. Unfortunately, as covered widely by local media, most of Colorado currently has below normal snowpack, thus limiting concern for snowmelt-induced runoff. However, one stark exception to this is the South Platte River basin, as shown below. Here, we see a snowpack that is just above normal for this time of year, with a further separation above normal expected due to the upcoming storm.

It is important to note that the image above is somewhat misleading, because it is derived by averaging many sites together including lower elevation ones that usually melt out earlier. If we focus on only higher-elevation SNOTEL sites, like Willow Park below, we see that deviations above normal are notably larger. For example, Willow Park is currently about 35% above the seasonal median, which could increase to 50-70% above median after the next precipitation event. This, coupled with the expectation of above normal temperatures by late next week means we will be watching out for snowmelt-induced flooding, most notably in the Big Thompson and Cache La Poudre basins.

Below, the Event #1 is outlined in greater detail.

Event #1: Saturday – Tuesday (5/85/11)

No Apparent Flood Threat as a couple shortwaves move through the flow and produce storms with residual moisture.

This event could begin as early as Friday afternoon with isolated showers and weak thunderstorms for parts of the southern Colorado, but will begin in earnest on Saturday. Multiple rounds of showers, with embedded thunderstorms are expected from Saturday through Tuesday. The highest thunderstorm coverage is expected on Saturday, with the availability of atmospheric instability. Strong-to-severe thunderstorms are possible in the Southeast Plains, towards the Kansas border. Reduced thunderstorm coverage is expected starting on Sunday, onwards, however, it is possible that the far southeast parts of the Southeast Plains could see another round of strong storms during the afternoons of Sunday-Tuesday. Over the course of the event, maximum one hour rainfall up to 0.7 inches is expected above 5,000 feet, with intensity up to 1.1 inch possible towards the Kansas and Nebraska borders. However, the duration of intense rainfall is expected to be too short and spatially confined to warrant a flood threat.

The snow level will begin above 8,000 feet on Saturday and gradually lower and oscillate between near 5,500 feet at night and 8,000 feet during the day beginning Sunday.

In all, isolated locations along the Front Range could see 2 inches to perhaps 3 inches of total precipitation from this storm. Because this will mainly be in the form of snow, flooding is not expected. A secondary maximum in precipitation, more associated with thunderstorms, is expected over the Palmer Ridge and Southeast Plains where total storm rainfall up to 2 inches could occur. However, given the spread out duration of this event and relatively low duration of high intensity rainfall, flooding is not expected.

Five of the six fire burn scars monitored by the Fire Burn Forecast page lie in high elevations and are not expected to be under threat for this event. However, we will be watching the Calwood burn scar, which sits between 6,000 and 7,000 feet in elevation, for possibly minor flooding concerns on Saturday.

FTO 05-03-2021: On and Off Precipitation Through Next Week

Issue Date: Monday, May, 3rd, 2021
Issue Time: 3:15PM MDT
Valid Dates: 5/4 – 5/18

The current shortwave affecting yesterday and today’s weather is marked with an orange “X” in the water vapor imagery below. As this system moves eastward, the High pressure system to our west will begin to build into the state, which will turn the steering flow more northwesterly. This will allow some mid-level energy to move through the flow tomorrow and Wednesday, which will likely produce scattered precipitation with the residual moisture.

On Thursday and Friday, the ridge should be fully intact overhead, bringing with it temperatures about ~10 degF above average and dry conditions. Snowmelt should increase during this 2 to 3 day warm period, before the next system (Event #2) propagates into the state sometime on Saturday. There is No Apparent threat for snowmelt during this period. Event #2 will again cool off temperatures and bring our next round of precipitation on Mother’s Day weekend.

PW values will begin to drop off from west to east as the current system moves eastward. Enough residual moisture looks like it will remain to help produce scattered storms over the Northern Mountains, Front Range and Central Mountains tomorrow. As the storms develop over the mountains and move southeast into the eastern plains, they could produce some dangerous lighting and brief outflow winds. However, there seems to be No Apparent flood threat at this time with PW values near average (Event #1). Looks like a bit of a rinse and repeat pattern for Wednesday.

Also shown nicely in the images below are the rapid drop off in available moisture (both east and west) as the ridge builds overhead on Thursday and Friday. With PW values likely dropping off to around 0.20 inches over western Colorado and surface winds picking up, there could be an increase in fire weather. By Saturday and Sunday, moisture looks to return behind a front. While there’s still quite a bit of spread in the amount of moisture that could become available, PW values on the higher end could mean a chance for severe weather and a flood threat over eastern Colorado. It’s still early in the season, so once Event #2 gets closer, precipitation type and amount will become clearer. An Elevated Flood threat has been issued for this event with eyes mostly on Monday.

There was a nice highlight of this season’s snowpack in Saturday’s SPM along with an update to the ongoing drought. Outside of the South and North Platte River Basins, snowmelt is well underway. Below is a look at SWE over the Gunnison Basin. Snowmelt here is looking a bit like 2018 and 2020, but with SWE amounts somewhere between these two years. The cooler temperatures today and next weekend may delay the snowmelt a bit, but overall, it’s look like full melt out will occur sooner than average. This is likely the case for most basins in the state. Streamflow will be interesting to watch this season due to very dry soils last fall. It is also likely that not all the melt water will make it into the major rivers.

Event #1: TuesdayWednesday (5/45/5)

No Apparent flood threat as a couple shortwaves move through the flow and produce storms with residual moisture.

Isolated precipitation totals may reach around 0.50 inches if storms are able to track over the same areas on Tuesday and Wednesday. With storm motion fairly quick to the southeast, local heavy rainfall is not anticipated at this time so there is No Apparent flood threat. As storms move off the Northern Mountains and Front Range, they may pick up in intensity over the eastern plains. The main threat from storms will be brief outflow winds, although they may produce some lightning and small hail as well. Severe storms are not anticipated at this time but check back in for full details in tomorrow’s FTB.

Event #2: Saturday – Monday (5/85/10)

Elevated flood threat with a cold front increasing moisture and dynamics and the possible development of a lee cyclone.

Low confidence in all the details this far out, but it looks like the next strong system will break down the ridge and move into the state at the end of next weekend. With it still being fairly early in May, it is fairly likely that there will be more snow with the system for the mountains. However, the snowline is hard to determine at this time. During the second half of this period, it looks like a lee cyclone could develop over the eastern plains. If this occurs, it could help pull in some higher moisture with easterly and southeasterly surface flow. This may lead to some severe weather with local heavy rainfall and more widespread precipitation for the mountains. Be sure to tune into the FTO on Thursday as details will no doubt change as there are large differences in the weather model ensemble members.

FTO 09-28-2020: Prolonged Dry Period with Fall-like Temperatures

Issue Date: Monday, September 28th, 2020
Issue Time: 2:30PM MDT
Valid Dates: 9/29 – 10/13

This is the last FTO for the 2020 season.

A prolonged dry period is the most on point way to end this flood season, due to the well below average precipitation that has occurred this year. An amplified ridge has set up over the western US, and this ridge will remain in place through early next week. This should keep a very dry air mass overhead. Northwesterly and northerly flow will push mid-level energy through the state on and off this week. Rather than increasing the chances for precipitation as they pass through, they will only help keep temperatures more seasonable. The first of the cold fronts drops through on Wednesday, and the next, over this weekend. Precipitation chances look like they may increase as the ridge begins to break down early next week (for a day). However, only very light precipitation will be possible along the northern Continental Divide, and there is an equal chance that no precipitation occurs. That means No Apparent threat is issued for the last event of the warm season.

It will be very dry this next week over western Colorado, where PW (right) remains well below average. This is due to the dry air mass (yellow above) being pushed overhead with the eastward movement of the High. During the first part of the week, surface winds will start to die down; however, by this weekend, elevated fire conditions are possible as a jet streak potentially sets up over the state again.

To the east, PW remains below average with some variation in model solutions by the end of the week. Either way, PW values will likely remain near or below normal (0.50 inches), which will keep rainfall out of the forecast. Stronger surface winds may mix down the surface later this week as the jet meanders, so there is a possibility for elevated fire conditions. This will be tracked in the daily FTBs through Wednesday.

Below is a quick look at the total precipitation that has fallen from May 1st to present. Much of western Colorado has received less than 4 inches of precipitation for the period, but a couple large precipitation events over the San Juan Mountains boosted them into the 6 to 8 inch range. Also, distinctive in the map is the bulls-eye over Yuma County. Extreme rainfall fell over that area on July 23rd, and 6.3 inches was observed in Wray. So, this added a significant amount to the total precipitation for the period.

Below the first image (middle) is the average precipitation from May to September from PRISM (30-year normal). Overall, Colorado had well below average precipitation for the season. This led to an extended and worsening drought (bottom image). At the start of the season, only 11% of the state was in D3-D4 category. That percentage has increased 38%, and now half of the state has reached the D3-D4 category. At the beginning of the season, only 23% of the state was under no drought conditions, but that dropped to 0% by August after a quiet monsoon season. Not surprisingly, this led to a very active fire season and two new fires (Pine Gulch and Cameron Peak) broke into the top 3 largest Colorado wildfires of all time.

 

Event #1: MondayTuesday (10/510/6)

No Apparent flood threat as the ridge breaks down and returns minimal moisture to state.

No map has been drawn since it is highly likely precipitation totals will remain below 0.50 inches, and possibly, no precipitation at all will be recorded for this event. Best chance for some accumulation will be along and near the northern half of the Continental Divide. A rain/snow mix may be possible due to cooler temperatures at the highest elevations. As expected, there is No Apparent flood threat issued.

FTO 09-24-2020: Elevated/Critical Fire Weather Forecast with only a Small Window for Precipitation

Issue Date: Thursday, September 24th, 2020
Issue Time: 3PM MDT
Valid Dates: 9/25 – 10/9

The main weather concern over the next couple of days will be critical and elevated fire weather as the shortwave trough and associated jet move inland. This will help tighten the pressure gradient over the state and produce windy conditions on both Friday and Saturday. Elevated and critical fire weather are forecast within the pink dashed line below where high temperatures will help create low humidity as this relatively dry system approaches. A Red Flag Warning has already been issued for tomorrow over the northwest quadrant of the state.

Sometime on Saturday or early Sunday, a cold front will dig south as an omega block begins to set up over the western US. This is expected to help lower the fire danger and cool temperatures off for Sunday. This will also be the only window for precipitation chances to increase. Colorado should be on the east side of this feature next week, so increasing northwesterly flow aloft will help disturbances pass through the flow. Very dry air fills in behind the front, so no precipitation is forecast, just more seasonable temperatures.

Below is the GEFS PW anomaly (green is positive) and 850mb winds forecast for Saturday afternoon. There is a slight moisture increase along and right behind the front, but a lot of dry air behind that. That translates to a limited window for increased precipitation chances. Light precipitation will be possible along and behind the front as it passes overhead. The GEFS keeps conditions drier on Sunday, but the ECM has been trending towards a wetter solution as a stronger front develops over the state. Either way, there is No Apparent flood threat as the rainfall will likely be more stratiform in nature. For this event, areas east of the Continental Divide will have a greater chance for accumulation.

Event #2, more or less, is just to mark when the omega block begins to break down. This will be when the chances for precipitation return, and likely marks the beginning of cooler weather. Until then, expect dry conditions with on and off temperature fluctuations as cold fronts ever so often pass through the northwesterly flow. At least it will begin to feel more like fall this next week.

Event #1: SaturdaySunday (9/26 – 9/27)

No Apparent flood threat as a cold front drops through the state and will help generate lift for some showers.

Not much precipitation is expected for Event #1, and the majority of the accumulation will likely be over eastern Colorado. It’s also likely that we won’t reach 0.50 inches as the map indicates below. This polygon just marks the area with the greatest chance for measurable accumulation. If the system follows the GEFS solution, little to no rainfall is expected and the best chances for accumulation will be over the far eastern plains.  If the developing front is a little stronger and wetter, some accumulation can be expected over the Southeast Mountains and elevated ridges. Either way, there is No Apparent flood threat due to more stratiform precipitation being forecast. Expect temperatures to be cooler on Sunday and into next week, but not before some 90Fs over the lower elevations on Friday and Saturday.