FTO 09-19-2019: Scattered Afternoon Showers & Thunderstorms Forecast for Friday, then a Drying Trend Heading into this Weekend

Issue Date: Thursday, September 19th, 2019
Issue Time: 1:20PM MDT
Valid Dates: 9/20 – 10/4

Active FTO with 3 events, although there is No Apparent Threat for Event #1 and #2 as quick moving upper level disturbances push through the state. Look at the water vapor imagery below, there is a lot of tropical activity right now. There is a chance that Tropical Cyclones Mario and Lorena get pulled into the westerly flow of Event #3 (a low deepening over the Great Basin). The high pressure over the Pacific is fairly consistent through this period, and builds off the coast of California after Event #2. This will keep the flow more amplified and help produce northwesterly and westerly flow aloft over the state, which should keep temperatures more seasonable. Especially when compared to the last couple of weeks. Guidance showing building high pressure to the west and east of the trough for Event #3 that drops south and sets up somewhere over the Great Basin. This will help keep the low in place for a longer period of time, which may help pull a nice subtropical plume northward over the state. Thus, the Elevated Threat, although there is low confidence in the models this far out.

PW looks to drop off from west to east after Friday with the main axis of the trough moving to the northeast of the state. There is a slight moisture return behind a cold front for far eastern Colorado on Friday night, so there is still a chance for some scattered afternoon storms for this area if the shortwave passes overhead at peak heating. Expecting some strong south and southwest surface winds on Friday, and with a dry air mass overhead, critical fire weather is likely. Please tune into the FTB as this will be covered in the daily discussion. PW looks to rebound ahead of the next trough (Event #2), although the GEFS is showing quite the spread between the ensemble members. Rainfall looks more promising for the western high terrains with more scattered activity anticipated for eastern Colorado and the adjacent plains. Surface winds will likely increase as well with the tightening gradient, so if moisture return is on the lower end, there may be pockets of enhance fire weather on Monday. Unsure what PW will do after Event #2, but if it’s on the lower end, critical fire weather may return as portions of the jet move overhead on and off throughout the week.

Event #1: Friday (9/20) – Saturday (9/21)

No Apparent Threat as a trough lifts to the northeast with minimal PW return behind a cold front.

Event #1 begins with the trough to our west, which will pull to the northeast tomorrow afternoon. This will brush the northwest corner of the state and northern mountains bringing with it more wind than measurable rain. Scattered rainfall is anticipated over the northern high terrains with a dry air mass quickly moving in behind it. A surface trough sets up over eastern Colorado, so some severe weather may be possible over the Southeast Plains along a dryline. Unsure where the dryline will set up as there is equal chance it is in Kansas. Nonetheless, should storms form in Colorado, they should be moving fast enough with the jet overhead that flooding will be avoided. Max 1-hour rain rates up to 1.25 inches may be possible. Overnight, a cold front drops south, which returns a little moisture to eastern Colorado on Saturday. A second wave moves through the state sometime on Saturday, so a few weak thunderstorms and showers may be possible over the far eastern plains if the cap can break. Thus, we’ll need subsidence to be on the lower side and the wave passing overhead at peak heating. Another cold front drops through the eastern plains on Saturday evening behind the trough, which may bring additional light showers and overnight cloud cover to the plains. The ridge begins to rebuild on Sunday, and with a dry air mass in place, rainfall is not anticipated. Should be a cooler period with temperatures closer to seasonal values.

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Event #2: Monday (9/23)

No Apparent Threat as a quick moving pushes from the northwest to the southeast across the state. Scattered storms are anticipated for the mountains, favoring the western high terrains.

Another quick moving trough looks to slide through on Monday afternoon. There looks to only be a slight increase in moisture with the system, but it should be enough to return showers and thunderstorms to the forecast over the western high terrains. There is a slight jump in PW over eastern Colorado as well, so perhaps the mountains and adjacent plains can see a little rainfall activity. Cool, dry air moves in behind the fast moving trough, then it looks to remain quiet through the end of next week. Northwest/west flow aloft during this period should keep high temperatures down. Fire weather may be likely if PW values drop off due to the jet overhead or near the northern border.

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Event #3: Friday (9/27) – Sunday (9/29)

Elevated Threat as a trough digs south and sits over the Great Basin. This could create a PW surge over the state, which may last a couple of days.

Confidence is very low for Event #3 as a low looks to dig south and sit somewhere over the Great Basin. Unsure about where the low will sit (and for how long) with guidance slightly different each run. It looks like remnants of Tropical Storm Lorena/Mario may get pulled into the system if the track is further south. With a deepening low, if upper level flow can turn more southerly over Colorado, there could be a nice subtropical moisture plume advected northward for an extended period of time. While a lot of details can change, it’s worth mentioning and issuing an Elevated Flood Threat at this time. PW could remain elevated past Sunday, so be sure to tune in next week to follow this developing system. The map below is my best guess from the latest couple of GFS run assuming there is a PW surge. Reminder, the last FTO will be next Thursday, but the FTB will be extended beyond September 30th as needed.

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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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