Issue Date: Monday, June 8th, 2020
Issue Time: 2:45PM MDT
Valid Dates: 6/9 – 6/23

Event #1 is already underway, and the forecast details can be found in today’s FTB. Tomorrow morning, ongoing showers and snowfall will start to move from the Front Range/Urban Corridor into the eastern plains. Rain rates should be gradual enough that flooding is not anticipated at this time, and precipitation should exit the eastern border by late afternoon into the early evening. Behind the front there will be some respite from the heat with northerly surface winds. Highs tomorrow afternoon are expected to be in the 70Fs over the valleys/plains, high 60Fs over the Urban Corridor and 50Fs over the mountains with skies gradually clearing. There’s a break in rainfall from Wednesday to Thursday as the ridge begins to rebuild northward and the Low strengthens over the Gulf of Alaska. There will also be an increase in high temperatures each afternoon with highs 10-15F warmer by Friday.

Event #2 begins on Friday as the Low moves into the Pacific Northwest and shifts the ridge eastward. This will allow some moisture to creep northward from the Gulf of Mexico, and resume diurnally driven thunderstorms over the southern high terrains. Additionally, a lee trough sets up over the eastern plains on Saturday and Sunday, so a couple severe storms may be possible over the Southeast Plains in this time frame. Of course, this will only be possible if the dry line sets up over Colorado and not to the east over Kansas.

You can see that nice moisture creep northward from the Gulf of Mexico for Event #2 in the Denver PW forecast (left). There’s quite a bit of uncertainty over western Colorado as to how much and how far north the moisture will travel. We’ll likely only see an uptick in moisture over the eastern San Juan Mountains with the highest rainfall totals along and near the Continental Divide.

Another noticeable feature in the images below are the below average PW values both east and west to start this week. With the base of the trough moving through the state tomorrow, high surface winds are anticipated once again. This will likely create Red Flag Warning conditions across southern Colorado (back west), so be sure to tune into the FTB and your local NWS offices for more details. A High Wind Watch is also in place over the eastern plains on Tuesday. Less wind is forecast for Wednesday and Thursday under the building ridge, so critical fire weather is not anticipated at this time.

Event #1: Tuesday (6/9)

No Apparent Threat as the base of the trough and associated rainfall moves eastward.

Not thinking very much instability will be able to build tomorrow. That means ongoing showers will only slightly increase in rain rate efficiency during the afternoon, and flooding should be avoided. Overall, it will be a nice wetting rain for the eastern plains with the most widespread rainfall over the Northeast Plains. Areas over the Southeast Plains will likely receive between 0.25 and 0.50 inches of rainfall with isolated pockets up to 0.75 inches. Totals further north are expected to be slightly higher. Also, prepare for severe surface winds over the eastern plains tomorrow afternoon. Northerly gusts up to 65 mph will be possible and surface winds are forecast to be in the 35 to 45 mph range.

Event #2: Friday – Sunday (6/12 – 6/14)

No Apparent Threat as next trough passes through Colorado, dislodging the ridge eastward.

This will be another system that moves through the state this weekend. Enough moisture should make its way northward for storms to return to the southern mountains each afternoon. Steering flow will likely push the storms into the adjacent plains. Best chance for wetting rains will be on Saturday, and a couple severe storms may be possible over the eastern plains associated with a developing surface low (lee trough). We will continue to watch and update the forecast in Thursday’s FTO. At this time, there is No Apparent flood threat from the rainfall due to limited moisture and fast steering flow. With the jet once again overhead, critical fire weather will likely return for northwest Colorado.

Issue Date: Thursday, June 4th, 2020
Issue Time: 11:15AM MDT
Valid Dates: 6/5-6/19

June has started where May left off: very hot with limited precipitation, despite above normal moisture in the air. The 7-day temperature departures from normal, provided by the High Plains Regional Climate Center, shows all of Colorado at least 5 degrees F above normal. Small portions of western Colorado are over 10 degrees F above normal!

Fortunately, as shown in this morning’s water vapor image, below, substantial change is on the way. A surprisingly persistent cutoff low has been stalled off the southwest US coast for days. As a large trough digs into western North America, it will finally force the cut-off low to move rapidly northeastward beginning Friday. The result will be Event #1: a 24-36 hour period of strong dynamical lift, along with a plume of well above normal moisture content and atmospheric instability. Much welcomed widespread precipitation will cover central and western Colorado, though only briefly. Following this event will be drastically cooler weather statewide, more reminiscent of early May.

As shown in the forecast Precipitable Water (PW) plumes, below, values of up to 1.0 inch are expected across most of central and western Colorado during the brief Event #1. This is about double the normal value at Grand Junction for this time of year. In fact, the record value for early June is right around 1.0 inch, so certainly an unusual amount of moisture. However, there are two main factors that will act to limit the rain rates, and thus, the flood threat. First, the storm motions are expected to be extremely fast, above 30mph due to a strong pressure gradient aloft. This, coupled with limited instability of perhaps up to 700 J/kg, implies brief heavy rainfall will be likely, but its duration will not be long enough to support flooding. Second, quite simply, climatology. It is very unusual for western Colorado to get heavy rain before the monsoon season due to a poor juxtaposition of required factors (such as limited instability, fast storm motion, insufficient moisture fetch, etc). Nonetheless, there will be an increased flood threat for burn areas as detailed in the specific discussion below.

Following Event #1, there will be a 3-5 day period of below normal temperatures and low moisture with limited, if any, rainfall statewide. Thereafter, another large-scale trough is expected to approach the western US coast. However, the amount of moisture it has to work with is uncertain, and while we have enough confidence to identify an Event #2, there is No Apparent flood threat at this time.

Below we describe each of the two identified precipitation events in more detail.

Event #1: Friday (6/5) through Saturday (6/6)

Elevated Threat on Saturday especially for the 416 and Lake Christine burn scars

A surge of moisture accompanied by strong atmospheric dynamics will result in scattered to widespread showers and thunderstorms beginning Friday afternoon and lasting into Saturday. Friday’s precipitation is expected to be benign, as the dynamical support will still be too far away. Up to 0.25 inches of rainfall will be possible across the western higher elevations.

On Saturday, expect widespread showers and weak thunderstorms mainly west of the Continental Divide. Max 1-hr rainfall up to 0.6 inches will be possible with isolated 24-hr totals of up to 1.25 possible across the San Juans and Central Mountains. There will likely be at least a Low flood threat needed for the 416 and Lake Christine burn scars on Saturday. However, at this time, it appears that other areas will avoid flooding. Check back to the daily Flood Threat Bulletins for more updates. In addition to the rainfall, there will be very gusty winds up to 65mph in favored higher elevation wind “tunnels” as well as with the strongest thunderstorms.

East of the Continental Divide, especially over southeast Colorado, expect very hot and windy weather. It is possible that Red Flag warnings may be needed, especially downwind of the Sangre de Cristo Mountains.

Event #2: Friday (6/12) through Sunday (6/14)

No Apparent Threat as dynamics present, but moisture return uncertain

After a prolonged stretch of cooler and drier weather, another large-scale disturbance will approach Colorado from the northwest. It will take time to advect high moisture into the state but scattered showers and storms look increasingly likely beginning Friday, 6/12. However, moisture content is uncertain and it does not appear this system will be able to tap into deeper moisture from the Gulf of Mexico. Thus, at this time, rainfall looks to stay below 0.5 inches and there is No Apparent flood threat.

Issue Date: Monday, June 1st, 2020
Issue Time: 2:40PM MDT
Valid Dates: 6/2 – 6/16

A look-alike omega pattern from last week’s FTO has set itself up with an elongated ridge extending from Utah to the Midwest. On the left side of the High, clockwise motion continues to pull moisture northward from the Gulf of Mexico (green arrow). While the higher PW values are to our southeast, there is still plenty of moisture trapped under the ridge to help produce daily afternoon storms over the state this week. Flow becomes more westerly aloft by tomorrow, which should help mix out the surface moisture as more dry air is entrained; thus, there is No Apparent Threat for the first part of Event #1.

By the week’s end, the Low off the coast off CA starts to eject northward. The ECMWF has a more northward track of the Low, and as the low lifts on Saturday, it quickly weakens. This would lead to a drier scenario later this week into this weekend. However, the GFS has the low ejecting more to the northeast, and with higher moisture, which would bring more widespread showers and a heavy rainfall threat to the state on Friday. So for now, there is only an Elevated Threat for Event #1 on Friday. The models will likely come into better agreement by Thursday, so be sure to tune back in as there may be an upgrade to a High threat if the GFS scenario looks more likely.

Increased southwest flow behind the Low will keep the weekend rather dry with the ridge access well to our east. Temperatures will be hot, too, with highs nearing the triple digits over the Southeast Plains. Antcipating an increase in fire danger over weekend with an uptick in surface winds (jet overhead) combining with a drier air mass. The Low over the Pacific Northwest then begins to traverse eastward to our north on Tuesday and Wednesday, which will push a cold front through the state and may help return low level moisture for storms. At this time, there is an Elevated flood threat for Event #2 during this period.

As expected by the GEFS, there is a large spike in moisture across the state as the Low tracks to the northeast and clips the Northwest Slope on Friday. Clearly, this is the most notable feature in the plumes below without much uncertainty from model runs. Also, notice the above average to average PW values to start this week. This shows there is plenty of moisture for storms to fire over the mountains each afternoon, although activity looks to decrease Tuesday into Thursday. Steering flow also intensifies tomorrow, so except the westerly flow to carry storms off the mountains into the adjacent plains and decrease the flood threat. As mentioned above, there is a quick decrease in moisture as the southwest flow begins behind the Low (Event #1). This will keep the weekend mostly, if not all, rain-free and increase fire danger as 10-meter winds increase in to the 15 to 25 mph over western Colorado in the GEFS (not shown).

Also, I wanted to take a quick peek at the May precipitation across the state, since it is the end of the month and start of meteorological summer. It has been very dry over the majority of the state with the largest departures from normal precipitation over the southeast border counties and lower elevations of western Colorado (top image). From March to May (meteorological Spring; bottom image), the Southeast Plains has seen 3.5 inches or greater below average precipitation. This is also true for the Conejos/Archuleta County area and western slopes of the Central and San Juan Mountains. It’s no surprise these areas have quickly fallen into the D3 and D2 drought categories. The year-to-year variability in Colorado’s precipitation never ceases to impress me.

Event #1: Tuesday – Saturday (6/2–6/6)

Elevated Threat as moisture and dynamics peak on Friday with current GFS runs pulling the Low to the northeast, which would bring widespread showers and the flood threat back to Colorado for the day.

An axis of instability and convergence looks to set up over the eastern plains tomorrow with a surface low. Higher areas of CAPE (northeast corner) may remain capped, but with dew points in the 50Fs on this side of the surface low, heavy rainfall over a small area is possible. CAPE further south is quite a bit less and with dry low levels, heavy rainfall is not anticipated. Best convergence (chance of rainfall) will be on the low’s south side (Southeast Plains), and the main threat will be strong outflow winds and small hail. Mountain storms look to favor the Southeast Mountains, but westerly flow aloft will likely scour out a lot of moisture. Thus, there is No Apparent flood threat. Wednesday into Thursday, coverage of storms over the mountains looks to be even less. We’ll have to see where the dry line sets up each afternoon over the eastern plains. If it remains within Colorado, and storms can make it to the border counties, some moderate rainfall may be possible. Friday, the threat increases with the aforementioned low moving northeast. There is still a lot of uncertainty, but the map below will illustrate the GFS solution (higher moisture and better dynamics on Friday).

Event #2: Tuesday – Wednesday (6/9-6/10)

No Apparent Threat as the Pacific low becomes an open wave and passes to our north.

Don’t want to get into too many details this far out, but storm chances ramp up as the next shortwave influences the region next week. Right now, Tuesday looks to be the most active as a cold front pushes through the state. There looks to be a nice break from the heat from earlier in weekend after the front passes. Guidance is showing temperatures returning to more seasonable values behind it, but I wouldn’t put too much weight into the models yet. There could easily be an increased heavy rainfall and severe weather threat as this event nears, so be sure to tune back into the FTO. A map has not been drawn for this event due to low confidence in the details/guidance not showing a half inch of rain for this event over any one area.

Issue Date: Thursday, May 28th, 2020
Issue Time: 2:35PM MDT
Valid Dates: 5/29 – 6/12

We’re currently in the semi-omega pattern discussed in Monday’s FTO. A persistent low continues to hold on over the Midwest, which has provided northerly flow across the state. Starting tomorrow, this low begins to merge with the vorticity max to the north of the border. Paired with the incoming low off the coast of CA, the omega pattern continues to hold on and places Colorado under a highly amplified ridge for Event #1 (part 1).  The low to our west begins to move towards the Pacific Northwest later this weekend and another vorticity max shoots south off the backside of the low, and lands off the coast of CA. This will keep the ridge intact over the state for Event #1 (part 2); however, Colorado will be on the west side of the ridge, so more southwest flow is forecast aloft.

Timing of moisture surges and strength is a little different between models, but it looks like moisture is maintained over the state through the end of next week as indicated by the average PW (black line) compared to climatological PW (red line). For Event #1 part 1, Saturday looks to have the heaviest rainfall potential due to higher PW values, the vorticity maximum ejecting northward to our west, and slow steering winds aloft. Thus, the Elevated flood threat.

There is a slight dip in available moisture on Monday due to the location of the ridge axis, but by Tuesday, PW increases statewide. It’s a bit difficult to tell how much and where moisture under the ridge will pool next week as indicated by the spread in PW values (gray lines). However, the above climatology values and pattern suggest rainfall each day. Slow storm motion, on and off shortwaves, lee troughing, and saturated soils by the week’s end all point to an Elevated flood risk for this event. This is especially true if storms track over recent burn areas for multiple days. Additionally, bankfull conditions over smaller streams and rivers are anticipated after this weekend’s warm up over northern and central Colorado. Any heavy and widespread rainfall may cause minor low-land flooding issues for these areas by the end of next week. This will be watched closely over the next week and the threat will be added to the daily Flood Threat Bulletin if it’s needed.

Event #1: Friday – Sunday (5/29-5/31)

Elevated Threat as moisture and dynamics peak on Saturday.

For the most part, this event looks like a mountain event with some activity spreading into the immediate, adjacent eastern plains. With storm motion more from the north, best accumulations over the eastern plains will most likely be along and south of the Palmer Ridge. More scattered storms with wind gusts are anticipated on Friday and Sunday. Saturday, there could be some small hail under the stronger storms, but the main threats from storms will be local, heavy rainfall and gusty outflow winds. Burn scars over the southern half of Colorado will be watched closely on Saturday. This includes the 416 burn area, although it is likely the higher moisture will be along and east of the Divide in the San Juan Mountains.

Event #2: Monday – Friday (6/1-6/5)

Elevated Threat as the amplified ridge continues to hold over the state and shortwaves move through the southwest flow.

It’s a bit difficult to tell which days will be the higher flood threat days this far out. However, dynamics (shortwaves, lee troughing, etc) could align equally on each of the four Elevated flood threat days. Right now, I’m not expecting to issue a threat each day, I just wanted to capture the potential time frame of the heavy rainfall threat. With rainfall anticipated over the mountain multiple days in a row, saturated soils also become a concern if enough subtropical moisture makes its way north. This is especially true over recent burn areas and areas experiencing peak runoff (north and central Colorado). The good news is this event will likely bring rainfall back into western Colorado and southern Colorado, which is currently in a D3 drought. If the southwest flow is stronger (ridge axis slightly east of the state), storms will likely move into the adjacent plains. This could bring another round of beneficial moisture to crops and drought areas as well as some severe weather. Be sure to tune back in on Monday as models will likely come back into better agreement, which will allow the details of the event to be better defined.