FTO 09-21-2020: Scattered Storms before Dry and Warm Conditions Take Hold

Issue Date: Monday, September 21st, 2020
Issue Time: 2PM MDT
Valid Dates: 9/22 – 10/6

Tropical Storm Beta can be seen in the water vapor imagery below near Texas. Beta should be making landfall in Texas later today/tonight, and slow storm motion is expected to produce local flooding issues over that area and also over southeast Louisiana. Over Colorado, a weak upper level ridge will begin to build in from the west today behind the departing trough, which can be seen over Manitoba below. Over the next couple of days, moisture and mid-level energy will rotate counterclockwise around the monsoon High to our south. This will allow scattered diurnal driven storms to develop over the mountains each afternoon. Little to no rainfall is expected for the valleys and adjacent plains as storms will likely dissipate as they move off the mountains. Additionally, a weak shortwave/trough (orange “X”) will move inland from the west, which could supply some added lift for more widespread storms on Tuesday. With steering winds becoming more westerly tomorrow, storms may spill into the immediate adjacent plains, but will likely only produce measurable rainfall over the elevated ridges.

The elevated PW values over the state last for a couple of days before they fall to below normal values for late September (image below), which will reduce the chances of precipitation. Moisture return for Event #1 also looks to be larger east of the Continental Divide, which will mean greater coverage of storms. With the surface layer remaining on the drier end, higher cloud bases will allow for more evaporation to occur, which should keep rainfall accumulations below 0.75 inches tomorrow. So, there is No Apparent flood threat issued. Linger moisture on Wednesday may allow for another round of isolated storms over the mountains, but little to no accumulation is expected.

The ridging pattern maintains itself for the rest of the week, so dry and warming conditions are forecast. A peak in high temperatures is anticipated on Thursday (west) and Friday (east), so expect temperatures to be a couple degrees warmer during this period. The next incoming trough (south of Alaska) will pass north of us this weekend, but it looks to be a rather dry system. This means the chances for precipitation will not increase, and high surface winds could cause elevated and critical fire weather conditions with another lee cyclone developing. Additionally, cooler temperatures are likely behind the trough as it drops a cold front through the state this weekend. Moving into next week, flow becomes northwesterly behind the trough, so the cooler temperatures are likely hang around. Reminder that the flood season ends on September 30th, so Monday’s FTO will be the last of this season.

Event #1: Tuesday – Wednesday (9/22 – 9/23)

No Apparent flood threat as a shortwave and weak subtropical moisture combine for showers and weak thunderstorms.

Event #1 continues tomorrow with scattered storms anticipated over the mountains and eastern ridges. With moisture a little bit higher east, there’s a better chance for a couple thunderstorms to develop over the eastern mountains and Palmer/Raton Ridge. Storms will again be smaller in size but greater in coverage when compared to today, but the higher totals in the storm cores should continue to be isolated. Brief, gusty outflow winds and lightning will continue to be the main threats from the stronger storms that develop. Valleys and the adjacent plains may see a few sprinkles, but the more likely scenario is just an increase in cloud cover. As instability drops off with the setting sun, storm coverage will quickly begin to decrease. There is No Apparent flood threat issued.

FTO 09-17-2020: A Couple More Days of Mild Weather Before a Pattern Change

Issue Date: Thursday, September 17th, 2020
Issue Time: 2:40PM MDT
Valid Dates: 9/18 – 10/2

The persistent ridge will begin to break down this weekend. This is expected to return scattered showers to the mountains, and there may be a little rainfall for the lower elevations next week. The Low to our west will finally start to move inland tomorrow (Event #1), but as it tracks eastward it was also lift to the north. So, this means less rainfall over the weekend, and light precipitation should be confined to the northern mountains and Northwest Slope. Mid-level energy moves into the Four Corners region behind the trough, which should keep scattered showers in the forecast through mid-next week. This area of lift and increased moisture looks like it will slide south Thursday and Friday, so some scattered precipitation may be possible over the southern mountains, but it should be rain-free elsewhere. A more fall-like trough moves across the state after that (Event #2), so expecting precipitation chances to pick back up across the state. Long-term models are showing a lot of disagreement with the details for Event #2, so confidence is fairly low in the forecast.

A couple more days of this mild and smoky weather before the Low arrives. This should increase moisture across the state to more normal values for this time of year, and help clear out some of the smoke with flow aloft shifting from the southwest. As the trough lifts north over the Great Basin, the jet stream could cause some stronger winds over western Colorado on Saturday (central and north). This can be seen in the 10-m wind speeds below reaching between 20 and 25 mph. While relative humidity will be a little higher further north, over central Colorado there could be some spotty elevated fire conditions. Lee troughing means that increased winds are also expected over the eastern plains. The plumes are also indicating a brief downtick in moisture on Sunday, which should keep showers mostly out of the forecast before picking back up over the mountains early next week. Overall, not looking like a ton of moisture with this system. The GFS continues to show the most rainfall activity, but there is No Apparent flood threat.

Event #1: Saturday – Wednesday (9/19 – 9/23)

No Apparent threat as cut off Low pushes the ridge south and brings scattered storms back to the mountains.

Looking like some increased cloud cover, slightly more seasonable temperatures, and an uptick in air quality this weekend. Scattered storms will return to the northern mountains and Northwest Slope Saturday evening, along with increased surface winds during the day. This could lead to some elevated fire conditions where relative humidity is lower. With the dry boundary layer, thinking rainfall efficiencies will be on the lower end, so there is No Apparent flood threat. A weak cold front on Saturday night will help to start to clear the smoke out and may cool temperatures a couple of degrees on Sunday over northern Colorado. Sunday will likely be pretty quiet on the rainfall front and storms that do form will likely produce little wetting rainfall over the mountains. Monday through Wednesday, scattered storms return to the forecast with some mid-level vorticity and moisture that linger in the area. Rainfall totals will again be on the lower end, but rain rates for burn areas will be watched on Tuesday and Wednesday. With steering flows gaining back a northerly and westerly component by Wednesday, storms may spill into the immediate, adjacent plains. Scattered storms may linger over the southern mountains on Thursday and Friday, but lighter totals are anticipated at this time.

Event #2: Saturday – Sunday (9/26 – 9/27)

No Apparent threat as the next fall-like Low drops south across the state.

Low confidence this far out for the details of this forecast, but all models are showing another stronger trough moving across the state. The strength of the Low is unknown, but it will likely be another fall-like cool off as the end of September nears. All models are showing some rainfall for this event, and the majority of the rainfall will likely be over the higher terrains. The GFS, as usual, is showing the most rainfall for this event. Be sure to tune back in next week as the forecast details will continue to change.

FTO 09-14-2020: Unseasonably Warm with Dry Conditions Until this Weekend

Issue Date: Monday, September 14th, 2020
Issue Time: 1:50PM MDT
Valid Dates: 9/15 – 9/29

It’s setting up to be a warm, dry, and uneventful week as an elongated ridge sets up overhead. Slight movement in the High tomorrow will start to pull smoke back into the state from the wildfires across the western US. So, expect a decrease in surface visibility by early afternoon across the northern border. As the near surface smoke wraps around the High, it will be pulled south throughout the day. Event #1 will start to set up to our west over the next couple of days as an upper Low becomes cut off from the main flow and spins off the west coast. Another incoming trough will push the Low inland this weekend, which will in turn, suppress the High to our south. This will return rainfall chances and bring more seasonable temperatures to the forecast until the next ridge begins to build overhead at the end of next week.

Dry air will continue to reside over the state to start the week (yellow above), so outside of some scattered, high-based light showers over the mountains, Colorado is not looking at any rainfall. This can also be seen by the well below average PW values until the incoming Low moves inland. There are still a few unknowns with the forecast for Event #1. One being how much moisture the Low will carry eastward, which is shown the spread in model members (gray lines). It may be able to interact with a northward moving tropical disturbance, and if it does, this would increase the chances of precipitation over western Colorado. If it does not, it becomes a drier forecast for the western border, so there’s a good chance for elevated fire conditions on Saturday as the system moves overhead. The other unknown is strength and timing of the disturbance. The GFS has a bit of a broader trough passing to our north and slightly faster than the GDPS and ECMWF. The GDPS and ECMWF also created a stronger amplitude trough, so we’ll have to see how the models come into agreement as the week progresses. Either way there is No Apparent Flood threat as scattered showers and storms return to the forecast for a few days.

Event # SaturdayWednesday (9/19 – 9/23)

No Apparent flood threat as a cut off Low moves inland and returns precipitation chances.

Long-term models are still in a disagreement with the finer details of this event, but it looks like cooler and wetter weather is on its way at the end of next weekend. The Low will arrive on Saturday, and as it moves east, expect a cold front to move through the state and cool temperatures down to more seasonable values for Sunday. As the front drops south, it may return light rainfall to the northern mountains. For Sunday and Monday, there’s an increase in rainfall chances over this same area, but chances also increase over the adjacent plains with a lee trough setting up. On Tuesday, a shortwave, with tropical origins, looks like it will move through the zonal flow. This will translate to more widespread showers over the mountains before the ridge builds overhead on Thursday. We’ll continue to keep watching this event as it may impact recent burn areas depending on its moisture content. Before the ridge begins to build overhead, it looks like there will be another surge of northerly flow on Wednesday associated with a system over the upper Midwest. This should keep temperatures more seasonable through the end of next week and push the remaining moisture south and east of the state. So outside of some light showers over the southern mountains with residual moisture on Wednesday, it should remain dry.

FTO 09-10-2020: Dry Weather Ahead with Temperatures Warming Back Up

Issue Date: Thursday, September 10th, 2020
Issue Time: 2:45PM MDT
Valid Dates: 9/11– 9/25

Event #1 is a quick one-day event as the upper Low continues to lift to the northeast. Some scattered, lingering snow showers are possible over the northern mountains tomorrow morning, but it should clear up by midday. There may also be some light, wrap around precipitation over the northeast corner of the state during the afternoon, but other than that, it should remain dry. There is No Apparent flood threat. Winds are also expected to pick up over the mountains and Southwest Slope during the afternoon as the jet moves overhead, so some elevated fire weather may be possible. Cooler temperatures may help alleviate this hazard, but be sure to tune into the FTB tomorrow morning.

A ridge begins to build back over the western US this weekend and will hang through next week, so mild weather is on tap. A lot of variation from model run to model run in strength and timing, but it looks like the next Low will drop in sometime at the end of next week (Event #2). This will bring precipitation chances back into the forecast.

As this ridge builds in from the west tomorrow, there will a sharp increase in temperatures across the state the next couple of days, which is shown below. There’s a bit more of a diurnal swing over the western Colorado (right), but overall, we are looking at temperatures returning to more seasonal values by Sunday. The early season snowstorm will seem like a distant memory by Tuesday as high temperatures will be back in the upper 80Fs. Feeling confident about the increase in temperatures due to good model member agreement through the forecast period.

PW really drops off after tomorrow morning (western Colorado) as the Low tracks to the northeast. With PW dropping to well below normal values going into this weekend, rainfall is not in the forecast. PW drops off over eastern Colorado after tomorrow morning, and subsidence behind the Low should keep rainfall chances low. It’ll be a stretch of warm and dry weather for most of the week. The change in the weather pattern can be seen on the tail end of the PW forecasts, but the large disagreement between members on moisture return means there’s a chance for very little rainfall. If this is the case, there may be some increased fire weather towards the end of the week. With PW looking to stay below 0.75 inches for the worst case scenario, there is No Apparent flood threat for Event #2 at this time.

Event #2 FridayMonday (9/18 – 9/21)

No Apparent flood threat as the next Low breaks down the ridge and returns some higher moisture to the state for some afternoon rainfall.

Some models are showing an open wave, while other models are showing another cut off Low. Either way, chances for precipitation will increase towards the end of this week. It will be a much warmer system than this last event, so expecting all rainfall at this point in time. Global models are also showing different timing for the arrival, but this will likely only be two to three-day event during this period. A lot of details will change due to it starting 160 hours from now, so there is Low confidence in the details. The precipitation map below shows a higher end rainfall scenario mostly from the GFS.