FTO 09-16-2019: Two Quick Moving Troughs Return Rainfall Chances and Cooler Temperatures

Issue Date: Monday, September 16th, 2019
Issue Time: 1:40PM MDT
Valid Dates: 9/17 – 10/1

Temperatures have been above normal with the high pressure dome over the state the last couple of days. This helped keep weather relatively rain-free since the last FTO. The exception is over the southwest corner of the state where a passing shortwave in the flow help produce some much needed rainfall on Sunday and again today. Isolated totals over the southeastern San Juan Mountains were just over an inch this morning with another round of rain expected today/tonight. Due to the quickness and lack of strength of the two events in this FTO, they have been grouped into one event. The first event begins overnight tonight as the trough moves northeastward and brushes the northwest corner of the state tomorrow afternoon. This looks to mostly affect the western high terrains and Northwest Slope. Behind the first trough, subsidence and a cold front Tuesday night should keep Wednesday rain-free with much more seasonable temperatures anticipated statewide. Some light, overnight showers will also be possible over the eastern plains on Tuesday night as the front drops south. Also, forecasting surface winds to turn from more southwesterly (west) to more westerly statewide over this time period as the trough approaches. Surface wind speeds are forecast to increase as well (20-30 mph) with the tightening gradient.

The next trough moves through the flow in similar fashion to Event #1 (Event #1 pt2) sometime on Thursday or Friday. This should return rainfall chances to the forecast. The timing of the trough’s passage will determine how much rainfall coverage there will be. If it passes through during peak heating, expect slight higher rain rates and coverage than if it passes through overnight. Behind the trough, a very dry air mass will fill in with another cold front pushing south on Friday night. With lower PW and a jet overhead (fast storm speed), there is No Apparent Flood Threat at this time. Storm chances return sometime at the beginning of next week (briefly) as the 500mb high sets up over the desert southwest. This will produce northwest flow aloft over the state and push another front through the eastern plains. A very dry air mass looks to fill in behind this, so it’s not looking like there is any heavy rainfall threat in the near future.

PW looks to remain elevated (above climatology) both east and west of the Continental Divide through Thursday/Friday. There’s a slight moisture increase with both of the troughs that move through, but nothing too spectacular. With PW looking to remain well below 1 inch, there is No Apparent Threat at this time. However, scattered afternoon showers over the high terrains is likely for this period (minus Wednesday). Not too worried about burn scars at this time, but the Lake Christine and 416 burn area will be watched closely on Thursday. Any threats needed will be added to the FTB.

It gets really dry going into Friday/next weekend with high temperatures doing a nice rebound from Sunday into Monday. PW drops to the 10th percentile if the forecast below verifies. With vegetation already dried out from the abnormally dry and hot weather, please use extra caution with anything that could spark a flame. While Red Flag Warning criteria does not look to be met for a prolonged period of time, there is still Elevated fire danger for this reason. A brief period of critical fire weather over the mountains will be possible on Friday afternoon before the trough fully exits to the east.

Event #1: Tuesday (9/17) – Friday (9/20)

No Apparent Threat as two troughs lift northward across Colorado and increase PW values enough for some brief, high elevation measurable rainfall.

There will be two quick rainfall events during this period as a couple troughs lift to the northeast. Not looking like either event will be more than one day with cool temperatures, dry air and subsidence filling in behind both. Right now, timing is making this look more like a western Colorado event, although that could change over the next couple of runs for Event #1 pt. 2. Expect an increase in surface winds as well as the troughs move though and tightens the gradient with the jet also possibility mixing down stronger winds to the surface. This may cause some critical fire weather (on top of dry vegetation) if dry air moves in before the jet moves out. Storms during this event are looking to be relatively high-based, so brief, strong winds will be possible along with small hail for the stronger thunderstorms. For eastern Colorado, the best chance for a couple thunderstorms will be over the Southeast Mountains and Raton Ridge on Thursday afternoon. There doesn’t look to be a severe threat over the plains as better thunderstorm parameters will be in Kansas.

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FTO 09-12-2019: Warm and Dry Weekend with Rainfall Returning Next Week

Issue Date: Thursday, September 12th, 2019
Issue Time: 12:40PM MDT
Valid Dates: 9/13 – 9/27

Should be almost completely rain free through this weekend, although there may be some isolated storms possible over the mountains during the afternoon hours. Zonal flow will produce westerly winds aloft, so a couple storms my wander into the adjacent plains. However, dry air will also be in place, so not thinking these storms will produce much, if any, rainfall. Temperatures also start to climb as the ridge builds over the state. A trough starts to develop and deepen to our west on Sunday, which will change flow aloft to southwesterly. This will possibly pull in disturbances on Monday/Tuesday, and with an increase in moisture, some weak afternoon thunderstorms will be possible over the mountains and western Colorado (Event #1 pt 1).

Wednesday is looking to be quiet as the trough pulls northward and subsidence is over the state. There will likely be cooler temperatures as well. The low re-digs back over the Great Basin for Thursday, which may push some mid-level lift into the state ahead of the trough. As the system pulls to the northeast sometime on Thursday and Friday, this will return the chances for more widespread showers and thunderstorms. Storm motion will again be swift (similar to this last system), and with limited moisture, there is No Apparent Flood Threat at this time. This may be upgraded for one of the days as confidence increases for the timing of the event. Linger moisture on Saturday my produce some showers over the mountains, and depending on where the convergence line ends up, the Southeast Plains may get a couple thunderstorms as well.

PW is quite low through the weekend for both Grand Junction (west) and Denver (east). The jet still looks to be mostly north of the state and without any mixing of strong winds down to the surface near the northern border, not thinking there will be any fire danger tomorrow and into this weekend. PW starts to creep up with the first part of Event #1 early next week. Thinking this will be more of a western Colorado/mountain event. However, a couple weak storms over the far Southeast Plains can’t be ruled out. Model members aren’t in full agreement with the moisture return, but right now thinking storms (to the west) will look similar to the system that just passed through. Tune back in on Monday as model agreement will likely better align.

Event #1: Monday (9/16) – Saturday (9/21)

No Apparent Threat as a shortwave and large trough moves across the state.

There will be a nice break in rainfall until the next system moves through and increases PW temporarily. Again, looking like a mountain and western Colorado event. Flooding is not anticipated at this time due to lower levels of moisture associated with the system. The main threats will be small hail and gusty winds from thunderstorms if the shortwaves line up with peak heating. At this time, thinking storm motion will be fast enough and rain rates low enough (fewer pockets of decent instability) that flooding will be avoided. Recent burn areas over western Colorado will be watched closely in the FTB on Monday into Tuesday.

The stronger system likely moves through sometime between Thursday and Friday next week. This will bring better chances of statewide rainfall. Not a ton of confidence in this forecast yet as it’s a bit far out. Right now, not looking like a huge moisture surge, and with fast storm motion once again, flooding should be avoided. Excepting the first day to be more of a threat over western Colorado, and the rainfall transitioning to the east on the second day. Rain rates look to have the potential to cause flooding issues if storms track over a recent burn area. So, this will be monitored in the FTB and updated in Monday’s FTO as confidence is again on the lower end for this forecast for now.

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FTO 09-09-2019: Quick 1-day Flood Threat Before the State Begins to Dry Out

Issue Date: Monday, September 9th, 2019
Issue Time: 2:10PM MDT
Valid Dates: 9/10 – 9/24

Event #1 should be short and quick with a trough moving swiftly from west to east on Tuesday and Wednesday. Expect the low (marked below) to dig southeast a bit before traveling east. Showers and weak thunderstorms are forecast on Tuesday afternoon over western Colorado as a shortwave travels through the state ahead of the trough. Storms will move to the northeast fairly quickly and with PW only around 0.7 inches, there is No Apparent Threat. A second set of storms may be possible over the Southeast Plains along a dryline. Wednesday, the main axis of the trough moves through the state. Thus, showers and thunderstorms are expected to be more widespread and stronger in nature. Timing will also be on the earlier end, so storms should start with lighter rainfall and intensify with a little daytime heating (building instability). Again, rainfall will favor the western high terrains with the highest accumulations anticipated along the Colorado/Utah border. Additional weak thunderstorms will be possible over the eastern plains along and north of a weak cold front. Depending on timing of the frontal passage, weak thunderstorms or no thunderstorms could be forecast in the FTB. Not thinking there will be a flood threat either way at this time.

Weather will be rain free on Thursday with subsidence and drier air filling in behind the trough. A weak shortwave may help produce some light showers on Friday over Colorado’s eastern border. However, after the trough passage, the upper level flow will be from the west. This will pull in a very dry air mass; thus, rainfall elsewhere is not anticipated on Friday and may help kill the convection chances over the eastern border. This very dry air mass hangs on through Monday with the westerly flow aloft continuing. So expecting no rainfall this weekend with temperatures ~5°F warmer than climatology.

Note that the PW scales below have been reduced now that we are moving into fall. The long-term averages are also continuing to show a negative trend. For western Colorado, moisture returns with the trough and shortwaves moving through the flow. With PW around 0.7 inches and multiple rounds of rain over an area, isolated totals up to 0.75 inches to 1 inch may be possible on Wednesday. The best chance for this type of accumulation would be over the high terrains along the western CO border. PW looks to drastically drop off after the event, and with the jet brushing the northern border, fire weather may be possible this weekend. Be sure to check in with your local NWS office and the daily FTB for up to date information over the weekend.

PW drops off to more climatological values over eastern Colorado this afternoon. There is a nice diurnal cycle of moisture shown with a fairly consistent message between model members. Higher moisture may hang on over the eastern border depending on where the dryline/cold front sets up, so a few higher totals may be possible if storms can break the cap. Flooding is not anticipated at this time for eastern Colorado.

Event #1: Tuesday (9/10) – Wednesday (9/11)

Elevated Threat as a trough moves across the state and produces showers and thunderstorms over western Colorado.

The next trough begins to traverse eastward on Tuesday into Wednesday. This should produce storms over western Colorado on both days. Storms are expected to be stronger on Wednesday and will favor the high terrains over the western border. With some dry air still expected in the low levels, brief wind gusts will be possible with the stronger storms along with small hail. Not sure how instability will look (has to do with timing of the trough axis) during the afternoon on Wednesday. However, if some decent instability is able to build, could see some higher rain rates. This will be especially important to track for the Lake Christine burn area. Additional storms may be possible each afternoon over the eastern plains. Not thinking these eastern Colorado storms will have flooding issues at this time with the highest dew points to the east.

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FTO 09-05-2019: Elevated Flood Threat Going into the Weekend

Issue Date: Thursday, September 5th, 2019
Issue Time: 2:45PM MDT
Valid Dates: 9/6 – 9/20

Finally, the high pressure that has been producing the above average temperatures will begin to break down as a series of troughs move through the western US. This more seasonal pattern will bring cooler temperatures and rainfall back into the forecast, which is probably welcomed by everyone at this point. The heavy rainfall threat returns to Colorado tomorrow after a cold front moves through the eastern plains this evening and helps return low level moisture (Event #1). There will likely be some high clouds behind the front and models indicated some light rainfall over the Northeast Plains tomorrow morning. Moisture remains high over eastern Colorado and elevated over the mountains for the duration of this event, and multiple troughs moving through the area at the same time will cause an Elevated threat to be issued through Sunday. Not quite as much moisture over western Colorado on Friday and Saturday, but there should be enough moisture for isolated afternoon showers and weak thunderstorms over the high terrains during the afternoon. The best chance for statewide rainfall accumulation will be on Sunday as a strong trough moves eastward just to our north.

The ridge begins to slightly build back over the state at the beginning of next week, which should pull in a dry air mass with westerly and southwesterly flow aloft. Thus, precipitation chances decrease until the next trough moves through sometime on Wednesday or Thursday (Event #2). Not very confident on the timing of this, but it will be a quick moving system. The GFS oscillates back and forth with the amount of moisture associated with this system, which is likely related to the changing strength and location with each run. However, due to the quick pace of the trough, there is Apparent Threat for Event #2 at this time.

PW over Denver is very elevated (for this time of year) during Event #1 with a slight decrease in moisture between the two passing shortwaves. To the west, not much change, which is to be expected with the high’s location. Expect more of the same for western Colorado for Event #1, but a slight increase in storm coverage on Sunday. Model members are trending downward in PW after the weekend both east and west, thus the decrease in flood threat. As for Event #2, quite a bit of spread in the models, which means it is a low confidence forecast. Please tune back into the FTO on Monday as details will likely become clearer.

Event #1: Friday (9/6) – Monday (9/9)

Elevated Threat as moisture returns to the mountains and eastern Colorado along with multiple shortwaves forecast to move through the westerly flow.

A cold front moves through the eastern plains tonight, which should return quite a bit of low level moisture. Guidance indicating some light showers may be possible over the Northeast Plains tomorrow morning, but at the least, there will be cooler temperatures and cloud cover behind the front. The heavy rainfall threat returns with more numerous and stronger storms anticipated over the higher elevations tomorrow afternoon. Storm totals over the mountains will likely be between 0.5 and 1 inch for the stronger storms or where multiple storms track over the same area. As storms move into the adjacent plains, unsure what the environment will look like. If the plains are capped or have limited instability (morning cloud cover doesn’t burn off early enough), not thinking a heavy rainfall threat will exist. We’ll also have to look at where the shortwaves are in tomorrow’s water vapor imagery to see if they could help break the cap over the plains.

On Saturday, there is a chance for some severe thunderstorms over the Northeast Plains (and heavy rainfall) with more coverage of storms over the mountains. Threats for the severe thunderstorms include severe hail, strong winds and localized flooding. Over the mountains, recent burn areas will also need to be watched closely. Sunday is now looking like the best chance for statewide rain and heavy rainfall. Once again, recent burn areas will need to be monitored closely.

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Event #2: Wednesday (9/11) – Thursday (9/12)

No Apparent Threat as a quick moving, possibly strong trough moves across the state on one of these days. Behind the trough, large scale subsidence should cause a break in rainfall chances.

Not a ton of confidence in this forecast yet as it is a bit far out to judge the timing, strength, and location of the next passing trough. Temperatures will likely more seasonable during this period with a slight increase on Monday and Tuesday with the building high pressure. If this a strong trough, as the latest model runs have been indicating, high temperatures may drop into the low 70°Fs behind the front. Otherwise, looking at highs behind the system to reach the 80°Fs and 70°Fs over the mountains and higher elevation valleys, respectively.

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