FTO 06-01-2020: Afternoon Storms Forecast this Week with an Increasing Flood Threat by the Week’s End

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: TuesdaySaturday (6/26/6)

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.

FTO 05-28-2020: Subtropical Moisture Moves In Under the Ridge

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: FridaySunday (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: MondayFriday (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.

FTO 05-25-2020: Afternoon Rainfall Returns to the Forecast with Hot Temperatures by Next Weekend

Issue Date: Monday, May 25th, 2020
Issue Time: 1:25PM MDT
Valid Dates: 5/26 – 6/9

There will be a break from storms on Tuesday before a fairly active rainfall pattern returns to the state through the start of next week. Event #1 begins as the Low to our north moves southeast and allows some mid-level vorticity to slide into the state with northwest flow aloft. Event #2 begins immediately after when the low off the coast of Mexico helps set up an omega pattern, which will place Colorado under the influence of a highly amplified ridge. This will return hot temperatures to the state by next weekend, and decent moisture could work its way up from the south over the as the ridge axis is slightly shifted eastward. Thus, the Elevated flood threat for next weekend.

Moisture has a strong return by Tuesday (west) and Wednesday (east) as seen in the PW plumes below. This is particularly true for eastern Colorado where PW increases about three-quarters of an inch in a 36-hour span. Looks like Wednesday could get interesting in regards to rainfall with PW close to an inch, so we’ll be watching the timing of the shortwave passage closely in the daily FTB. Moisture levels stay above average over eastern Colorado through Monday, thus the increase in chances in rainfall for this FTO. Over western Colorado, PW values are slightly less, but are still above climatology through next weekend. This should help lessen fire concerns along with calmer surface winds under the ridge, and bring scattered afternoon storms back into the forecast for the higher terrains. Really hoping the aforementioned moisture surge from the Gulf of Mexico happens for the southwest corner of the state and San Luis Valley region.

It looks like southern Colorado will reach complete melt out at gauged SNOTEL stations by tomorrow. Over northern Colorado, there could be another rise in water heights later this week with those hot, summer-like temperatures and increase rainfall chances. Not expecting any large-scale flooding, but low-lying areas and small creeks will likely be at bankfull conditions. Also, forecasts indicate the Yampa will be on the rise again with Elk River near Milner coming close to Action levels by next weekend.

Event #1: WednesdayThursday (5/27-5/28)

No Apparent Threat as moisture and dynamics return to the state with a passing shortwave.

A cold front looks to drop through the state behind a shortwave sometime on Wednesday night. Depending on timing, this could cause some heavy rainfall and severe weather for eastern Colorado with the main threats being strong winds and hail. However, good upper level dynamics remain to our north, so coverage is not expected to be widespread. As far as rainfall, right now, it’s looking like it is mostly a rainfall event for northeast Colorado and the Palmer Ridge; however, scattered storms will also be likely return to the Northwest Slope, Northern and Central Mountains as well. Overnight rainfall may linger over the plains. Storms should be moving fast enough during the afternoon hours that widespread flooding is not anticipated, thus the No Apparent flood threat.

Event #2: FridayMonday (5/296/1)

Elevated Threat as amplified ridge sets up over the state and allows Gulf of Mexico moisture to be advected northward.

This will be an interesting omega-like pattern for Event #2. Should the ridge axis set up nicely (slightly to our east) over a two-day period, rainfall could be widespread across the mountains – including southwestern Colorado. This would be very beneficial for the dry soils, worsening drought and early season melt out. Without much upper level dynamics in play, severe storms are not likely over the eastern plains, but the adjacent eastern plains could also see some decent rainfall with slower storm motions forecast. The Elevated Threat right now is mostly for recent burn areas, and details will likely change by Thursday’s FTO.

FTO 05-21-2020: High Fire Danger to Start Memorial Weekend before Storms Return Sunday

Issue Date: Thursday, May 21st, 2020
Issue Time: 12:30PM MDT
Valid Dates: 5/22 – 6/5

There will be two events for this FTO, and both will originate from the Low just off the coast of British Columbia/Washington. This Low is expected to slip southwards into the Great Basin Friday into Saturday. Initially, it will pull in a dry air mass and increase fire danger on Friday and Saturday. However, a cold front is expected to drop through Saturday night, which will help return moisture for widespread storm activity on Sunday into Monday. By late Monday into Tuesday, it should start to dry out with warmer temperatures forecast for mid-week when the ridge axis shifts east.

Long-range models are hinting at the mid-level energy from Event #1 hanging out somewhere over Texas and getting cut off from the main flow (Event #2). This would pull higher PW values northward on its east and north side (counter-clockwise motion) and return the chances for afternoon rain towards the end of the week. With a northwest flow likely returning aloft over the weekend, it may push another cold front south, which could interact with these higher PW values. Thus, the Elevated Threat for flooding during Event #2.

As anticipated and focused on in the last FTO, one of the main weather stories for this FTO is the critical fire weather forecast over Memorial Day weekend. The new drought map was released today which has increased the D3 area over the southern border, so it is dry! PW values at Grand Junction (and south) are in the 10th percentile for this time of year. Another SW to NE oriented jet streak will be over Colorado Friday/Saturday, which will increase surface winds into the 10-25 mph range with gusts 10 to 20 mph higher. Be sure to tune into your local NWS office for the latest and use caution with open flames while camping. There are already many counties with fire restrictions in place, so here is a map with those current restrictions. A Fire Weather Watch is already  in place for most of western and southern Colorado for tomorrow afternoon.

With that said, not much rainfall anticipated except for some scattered, high-based storms over the mountains on Friday and Saturday. Moisture increases behind the aforementioned cold front, but this looks mostly to be east of the Continental Divide. Storms should be mostly stratiform in nature Sunday into Monday, but if more convective storms can form on Sunday afternoon, we’ll have to watch the Spring Creek and Decker burn areas. At the tail end of the Denver PW plume is the increase in subtropical moisture mentioned for Event #2.

No change to the streamflow forecasts from the last couple of FTOs, but I just wanted to mention that the SWE from SNOTEL sites over the Upper Rio Grande Basin are showing complete melt out after this last hot stretch of temperatures. The San Miguel, Dolores, Animas and San Juan River Basins look like they will also melt out by early next week. This is very early (relative to climatology), which is typically in mid to end of June. Unless there is a major pattern shift over the next couple of months, we’ll be dealing with very high fire danger this summer due to the dry spring/end of winter.

Event #1: SundayMonday (5/24-5/25)

No Apparent Threat as moisture return from a cold front and surface low return widespread shower activity to the state.

Storm activity is expected to increase on Sunday with the main vorticity max moving into the state and a surface low forming over the Northeast Plains. Overall, there will be cooler temperatures with storms kicking off over the mountains by noon and activity spreading into the adjacent plains by mid-afternoon. The far eastern plains look to remain capped, so not expecting any severe weather at this time. A couple stronger storms could produce high enough rain rates (0.50 inches per hour) that would cause issues for recent burn areas over the Southeast Mountains, so we will be watching this closely. Storms may linger over the Southeast Mountains and immediate adjacent plains overnight, which would help increase totals. But that would be very welcomed with the lack of rainfall the last couple of months. Due to the anticipated stratiform nature of the storms, not thinking flooding is going to be an issue at this time, especially because it begins to dry out on Monday. Thus, there is No Apparent threat.

Event #2: Thursday – Sunday (5/28-5/31)

Elevated Threat as a “cut off low” pattern pulls subtropical moisture northward.

If the vorticity lobe is able to be cut off from the main flow as it passes through the state to start next week, it could create a nice surge of subtropical moisture northward into the state on its east and north side. Of course, this all depends on the strength, persistence and placement of that low. At this time, it’s a little too far out to know the details, but we should see an uptick in storms by the end of next week. Northwest flow will likely return to the state as the pattern begins to break up next weekend, which will likely drop a cold front southward. There will be an Elevated flood threat during that time if high moisture is intact over the state. Due to low confidence in the placement, strength and persistence of the low this far out, a map has not been drawn. Please tune back into the FTO on Monday as details will likely have evolved.