FTO 09-27-2018: Elevated Flood Threat as Rosa Remnants Bring High Moisture to Western Colorado

Issue Date: Thursday, September 27th, 2018
Issue Time: 11:00 AM MDT
Valid Dates: 9/28 – 10/12

A series of cold fronts this week has finally brought more seasonable temperatures, so it’s safe to say that fall is officially in full swing. A blocking pattern over the Pacific has kept the elongated trough over the eastern US and Colorado under northwesterly flow aloft. Drier air seen the water vapor imagery below has been circulating over the state and keeping conditions mostly dry. The cold fronts have produced some moisture return behind them, but the limited low-level moisture has keep post-frontal upslope showers and thunderstorms more scattered in nature and never more than one day in a row. There is slightly higher moisture over the CO/NM border, which has helped produce some high-based light showers to the area the last couple of days, but totals have generally remained below 0.1 inches. A change in the weather is expected at the end of this weekend as the cutoff low over the Pacific gets absorbed into the westerly winds and progresses eastward. Tonight, it is worth mentioning that a cold front will move south through eastern Colorado. Expecting high clouds to start increasing from the north to the south this afternoon with the approaching system. By tomorrow temperatures will be coolest over the plains and lower foothills. Not much moisture or deep cold air with this surge, so only expecting increased cloud cover and possibly a few sprinkles. Temperatures by Saturday are forecast to spring back with highs reaching the mid-80Fs over the plains .

As the upper-level low gets absorbed into the main flow beginning tomorrow, it first moves eastward, and then north once it reaches California. This will turn the northwesterly flow aloft to westerly beginning tomorrow and last through this weekend. The westerly flow will promote dry weather with slightly above average temperatures for this weekend. Stronger winds aloft will likely mixed down to the surface on Saturday, so a fire weather watch has been issued for northwest Colorado. On Sunday, the cutoff low passes north of the state and some extra mid-level energy will pair with a slight increase in moisture for some showers over the western high terrains. This is when the forecast gets interesting.

The trough pattern over the Pacific continues after the cutoff low moves eastward beginning tomorrow. The vorticity max marked below settles into the same general region as the prior cut off low, which will control the upper-level flow. At the same time the trough moves into place, Tropical Cyclone Rosa (green in the image below) will be moving north along the Baja. The upper-level low will push the northern moving cyclone east, which could bring abundant tropical moisture over and near the western portion of the state in a normally quiet precipitation month. Paired with upper-level dynamics and topographic influences, heavy rainfall may be possible the first half of next week over the southwest corner and south Central Mountains. Though specific timing is still a little uncertain, an Elevated flood threat has been issued as models have become more consistent with this solution in their runs over the last two days. We saw a similar situation early this year with Bud, but generally speaking, these hurricane remnant rainfall events are very rare for Colorado. Towards the end of the week, the trough will transition eastward, which will keep showers and thunderstorms in the forecast. However, the flood threat drops off after the remnants of Rosa move through the state.

Diving into the moisture plumes from the GEFS, not much threat until early next week as the tropical cyclone moisture tracks into the area. The European model is also projecting a similar solution with Rosa, but the timing is a bit more delayed. We’re just now entering the 5-day forecast, which is more accurate in the sense model members come into better agreement about the atmospheric set up. This means there will be a better idea of the timing and location of the rainfall later this weekend, though right now it looks like a western Colorado event. PW looks to possibly reach just below 1 inch over Grand Junction, which is above the 95th percentile for this time of year (October record – 1.04 inches). With the largest PW values expected over the San Juan Mountains and southwest corner of the state, expecting the highest rainfall totals here as well. There is still quite a bit of spread with the moisture return between the model members, which has to do with the uncertainty in the exact path of Rosa. Although the Flood Threat Bulletin comes to an end on September 30th, special forecasts will be issued next week if there is a flood threat for a region or recent burn scar(s). Below we describe the identified precipitation event of this FTO in more detail.

Event #1: Sunday (9/30) – Friday (9/5)

Elevated/No Apparent Threaas tropical moisture associated with Hurricane Rosa is pulled into the state.

As mentioned above, still quite a bit of uncertainty with this forecast. At this time, global models are pulling the remnants of Rosa near/over the state. Still quite a bit of uncertainty as to where the best moisture will end up, but this is looking like a significant October rainfall event for western Colorado. Currently, models are keeping the moisture confined to western Colorado, but this may change with future runs. The GFS has the moisture plume reaching the southwest corner of the state by Monday, whereas the ECMWF (European model) is holding off another 24-hours. Models will likely come into better agreement later this weekend. Heavy rainfall is anticipated despite fairly quick storm motion as PW values will be near record values for October. Threats include mud flow, debris slides and local stream flooding with burn scars being the most susceptible to these threats. The FTB ends on September 30; however, if a flood threat appears, we will be doing special Flood Threat Bulletin(s) as warranted.



FTO 09-24-2018: Autumn Temperatures Arrive with a Series of Cold Fronts

Issue Date: Monday, September 24th, 2018
Issue Time: 1:20 PM MDT
Valid Dates: 9/25 – 10/9

Fall has officially begun and temperatures this week will finally start to feel more seasonable. Overnight, a cold front dropped through the state associated with the upper-level trough that created some early morning cloud cover and cooler temperatures. There were also some showers over the higher terrains due to upper-level dynamics associated with the trough. This afternoon, expecting showers and thunderstorms to return to the forecast as the rest of the trough pushes across the state. Behind the trough, northwest flow has pulled in dry air over the Northwest Slope, which paired with high surface winds, has created critical fire weather. The entrainment of dry air will limit widespread precipitation to one day, though there may be some isolated showers tomorrow afternoon over the Southeast Mountains (Event #1). The upper-level jet over northern Colorado will continue to produce gusty winds overnight on Monday into Tuesday. For the rest of this week, northwest flow will be present over the state as an elongated trough remains up over the eastern US. This will usher in a series of cold fronts; hence the cooler high temperatures for the first part of this FTO. The first of these fronts will pass through early tomorrow morning and may have some light showers associated with it. Temperatures over the state tomorrow are expected to be in the 60Fs to mid-70Fs, which is a welcomed treat after the long stretch of 90F+ high temperatures.

Looking below to the water vapor imagery, very dry air remains present over the desert southwest and Great Basin. This dry air is expected to work its way into the state with the northwest flow aloft the next couple of days. Without much moisture, only isolated, high-based showers and cloud cover are likely over the higher terrains until this weekend. Starting Thursday, a vorticity max starts to move over the west coast. As the cutoff low gets picked up into the main flow later this week and helps turn winds aloft to southwesterly on Saturday. This will allow a little more moisture to return to the state and bring showers and thunderstorms back into the forecast for the western mountains this weekend (Event #2). By Sunday, drier air replaces the weak moisture plume, which will in turn reduce showers and thunderstorms over the higher terrains. So storms on Sunday will be more isolated than scattered in nature. Finally, the upper-level jet on the front side of the trough moves into western Colorado on Sunday, and moves over northern Colorado on Monday. There will likely be enhanced fire weather these two days as increased surface winds pair with low relative humidity values and dry fuels. Please tune back into Thursday’s FTO for changing details.

After showers on Monday afternoon, Precipitable Water (PW) values look to drop off both east and west of the Continental Divide as NW flow aloft ushers in dry air. With such low PW values over western Colorado, there will be enhanced, and moments of critical fire weather, throughout the week. Please use caution with any activity that can produce a spark. Looks like a little better moisture east after Monday, though outside isolated showers and weak thunderstorms are expected over the south high terrains on Tuesday. After that, only expecting afternoon cloud cover during the afternoon and evenings. PW looks to increase this weekend as the cutoff low moves into the area. Still quite a spread in the model members, but there should be enough low-level moisture for showers and thunderstorms to return to the forecast on Saturday and Sunday. Please tune back into the FTO on Thursday as model members come into better agreement about moisture return. With PW values remaining below 1 inch during this FTO, there is No Apparent Threat at this time.

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

Event #1: Tuesday (9/25)

No Apparent Threat as residual moisture initiates some isolated showers and weak thunderstorms over the Southeast Mountains.

A secondary cold front will drop through the plains overnight into tomorrow morning. There is a chance for some light showers along the front as it surges south during the morning hours. Expecting some gusty winds to accompany the front as well. High temperatures for Tuesday will be in the 60Fs to mid-70Fs, so convection will likely remain capped north of the Palmer Ridge. Weak upslope flow is forecast for the Southeast Mountains during the afternoon, which may initiate some isolated showers and weak thunderstorms. A weak shortwave in the area will help these storms survive as they move east into the Southeast Plains. Totals are expected to remain below 0.5 inches, so a precipitation map has not be drawn.


Event #2: Saturday (9/29) – Sunday (9/30)

No Apparent Threat as a cutoff low gets picked up by the westerly flow and returns some moisture and upper-level energy to the state.

A cutoff low begins to move onshore Thursday this week. By Saturday, southwest flow is expected to return to the state, which will bring an increase in moisture. Shortwave activity is also expected to be on the rise during this period, which if timed correctly, will help with better rainfall coverage. The increase in moisture and upper-level dynamics should bring showers and thunderstorms back into the forecasts for the western, higher terrains this weekend. Still quite a bit of spread between the members as to how much low-level moisture will return, but PW values are expected to stay under 1 inch. That means there is No Apparent Threat at this time.



FTO 09-20-2018: Next Two Events Expected to Usher in Traditional Fall Weather

Issue Date: Thursday, September 20th, 2018
Issue Time: 1:30 PM MDT
Valid Dates: 9/21 – 10/5

The mid-September heat wave has finally ended with cooler temperatures filling in after the passage of a cold front. Below is the US Drought Monitor that was updated on Tuesday. Not many changes to eastern Colorado, but the Grand Valley region went from extreme drought to exceptional drought. The area experiencing extreme drought in Colorado is now up to 12.6%, which is up about 3% higher than the September 11th map. While some beneficial rain fell over this area yesterday, chances are it was not enough to lift the extreme drought conditions. The climatology of this region shows October receiving more rainfall than September; however, the CPC precipitation outlook has predicted a 33 to 40% chance for above average precipitation at the beginning of October. Hopefully, this pans out as the area desperately needs the moisture.

Taking a look at the water vapor imagery below, you can see some very dry air in the desert southwest and west of California. This dry air is currently working its way in behind the trough over Colorado with WSW flow aloft. The strong orange/blue gradient shows just how different these air masses are and the dry air is quickly mixing out any remaining moisture and producing blue skies. Friday into this weekend, flow aloft will turn more westerly with a slight southerly component at the end of the weekend. This will continue to entrain the dry air seen in the water vapor imagery below and without much moisture to work with, not expecting much rainfall (if any) this weekend. Some isolated weak, high-based storms may form over the mountains likely near or along the southern portion of the Continental Divide. By Sunday, the next trough begins to move inland form the west coast (Event #1). Only a minimal amount of moisture is expected to move into the state from west to east with the trough. Without a strong moisture sure anticipated, there is No Apparent flood threat.

The trough begins to propagate through the state on Sunday night starting over the northwest corner, so some high-based, light showers will be possible overnight. The NAM has the trough arriving a little later, so showers may start Monday morning. As the main axis of the trough moves through Monday, expecting storm chances to increase. A weak cold front associated with the trough will drop through the eastern plains on Monday. Not expecting much moisture return behind the front, but this will likely increase the chances for rainfall over the eastern mountains and adjacent plains on Monday. On Tuesday, northwest flow aloft will pick back up and general subsidence behind the trough should limit afternoon rainfall. With northwest flow over the state, another trough will be ushered into the area mid-next week. This system does not look to have much moisture with it, but is expected drop a strong cold front through the area overnight on Wednesday. At this time, high temperatures look to drop into the 60Fs over eastern Colorado for Thursday. After Thursday, northwest flow begins again, which should bring another break in rainfall activity through the weekend as dry air works its way into the state.

Just how dry is that air mass in the water vapor imagery above? Precipitable Water (PW) values are expected to drop about 0.7 inches across Colorado. This is an incredible drop off in moisture over a 24-hour period. Both PW plumes shows a little bit of moisture return after the frontal passage on Sunday, through PW values only return to around 0.5 inches. This may help to increase storm activity a bit with the help of upper-level dynamics, but these more seasonal PW values will likely increase afternoon cloud cover and produce gusty thunderstorms. This forecast is still a bit far out, so please tune back into the FTO on Monday as details in the forecast may change.

As far as fire danger, expecting surface winds to pick up over northern Colorado starting Sunday night with the trough passage and associated upper-level jet. With warm temperatures still expected on Monday and low relative humidity values, critical fire conditions may be met over this area. Please tune back into the FTB on Monday for more details. For a longer outlook, should the next trough drop into northern Colorado on Wednesday and Thursday there will likely be enhanced fire weather. The GFS is producing a very strong system, so if the intense upper-level jet mixes down to the surface it would produce some very gusty surface winds over northern Colorado. This portion of the forecast is still quite far out, but worth mentioning due to the strength of system in the models.

Below we describe the identified precipitation event of this FTO in more detail.

Event #1: Sunday (9/23) – Tuesday (9/25)

No Apparent Threat as northwest flow aloft ushers in another cold front that is expected to produce high-based storms and light snow over the highest elevations.

Beginning on Sunday night, the next trough begins to move into the state. With limited moisture, not expecting much rainfall though some light showers may initiate over the northern high terrains Sunday night into Monday morning. As the trough transitions eastward, a weak cold front will drop through the eastern plains on Monday. Not expecting deep moisture return behind the front, but there should be enough to increase rainfall activity over the eastern high terrains and adjacent plains on Monday. Some showers may linger over the higher terrains and eastern plains on Monday night into Tuesday, though totals are expected to remain under flood threat criteria. Drier air begins to move in on Tuesday, but there will still be a chance of some high-based showers over the mountains in the afternoon. Monday’s FTO will give a better idea of the spatial extent of the expected rainfall and update totals as model members better align with one another.


FTO 09-17-2018: Fall Temperatures Finally in the Forecast

Issue Date: Monday, September 17th, 2018
Issue Time: 1:05 PM MDT
Valid Dates: 9/18 – 10/2

The heatwave will continue for the first part of this week with more seasonal temperatures expected at the end of the week to welcome Fall. The number of 90F+ days in Denver for 2018 is now at 57 days, which is 4th overall in the 150 year record. The reason for the current heat wave is a persistent trough to our west, which is creating a ridging pattern and southwest flow over the state. Dry, warm air is being entrained from the desert southwest and the 500mb high location has been suppressing the better moisture to our south and east. Taking a look at the water vapor imagery below, Event #1 will occur as the trough finally makes it way eastward on Wednesday and Thursday. The eastward movement in the ridge axis should help to pull a bit more low-level moisture into the state, though this is not expected to be a full monsoon surge. Unfortunately, trends in long-term models indicate the weak monsoon season has come to an end. The best chance for some wetting rain this week will be on Wednesday with more limited showers and thunderstorms on Thursday after a more stable air mass moves into the area behind a cold front. At this time there is No Apparent flood threat. Expect temperatures on Thursday to return to more seasonal values. After Thursday, the extremely dry air mass in Nevada (see below) will make its way into Colorado for the weekend. This should again nix the chances for rainfall next weekend, though high temperatures are only expected to be 5-10F above normal. This will be a nice break from the record heat and perfect weather for leaf peeping.

As the trough begins to move eastward, the jet will sag over the northwest corner of the state on Tuesday. Without any mid-level moisture, as seen the last couple of days, critical fire weather is expected with surface winds speeds in the 15-25 mph range over western Colorado. On Wednesday, there should be an increase in moisture with the trough passage, so critical fire weather will likely get downgraded to enhanced fire weather over the Northwest Slope and Central Mountains. After Event #1, the dry air from Nevada (mentioned above) will move into the state, but with the jet to the north and west, critical fire weather is not expected this weekend. By Monday, another trough moves into the Pacific Northwest, which may set up the jet over western and north-central Colorado again. If this occurs, this would bring enhanced and/or critical fire weather back to western and north-central Colorado. Please tune into the FTB for the daily fire outlook as details will likely change.


Quite the drastic GEFS moisture plumes below. Still seeing the diurnal signal over eastern Colorado with strong low-level moisture return on Wednesday. Regrettably, this looks to be short-lived with high, post-frontal Precipitable Water (PW) values only remaining over southern and southeastern Colorado on Thursday. There will likely be heavy cloud cover on Thursday morning with spotty rainfall, limiting instability for afternoon storms on Thursday. More seasonal PW values expected after Thursday, which should produce some afternoon cloud cover and possibly some isolated, high-based thunderstorms over the mountains. For western Colorado, PW values look to increase on Wednesday across the southern half of the region associated with the trough passage. This will bring some high-based thunderstorms to the San Juan Mountains capable of gusty winds and dangerous lightning. Increased cloud cover is more likely over the northern high terrains as well on Wednesday. After Wednesday, the extremely dry air mass drops PW values well below normal. This could cause some enhanced and critical fire weather through the beginning of next week.

Below we describe the identified precipitation event of this FTO in more detail.

Event #1: Wednesday (9/19) – Thursday (9/20)

No Apparent Threat as the trough finally moves east and brings fall temperatures to the state.

On Wednesday the upper-level trough begins to move through the area. At this same time, a cold front starts to push through the Northeast Plains. This should help return some low-level moisture with surface winds from the south/southeast for some wetting rains on Wednesday. Expecting afternoon showers to initiate with upslope flow over the eastern mountains and move into the adjacent plains during the early evening. Steering winds look to be fairly quick and moisture only moderate, so not anticipating flooding at this time. The front stalls out over the Palmer Ridge, but has a resurgence on Wednesday night. This means some overnight showers may be possible over eastern Colorado with cloud cover and light showers to start Thursday morning. The passage of the main cold front should bring a cool start to Thursday morning and return high temperatures to more seasonal values. Although the upper-trough passes through fully on Thursday afternoon, a more stable air mass will be in-place. Models are currently showing some weak CAPE over the southern Front Range/Palmer Ridge intersect as well as over the Southeast Mountains, so this is the most likely area for afternoon rainfall activity. It is also possible for some rainfall over the Southeast Plains along a line of convergence associated with the pass trough. Although steering winds will be slower, widespread flooding is not expected at this time due to limited instability. Please tune back into the FTB on Wednesday and Thursday for an update on the rainfall activity as models are showing some variation in timing of the trough and frontal passage. This could affect what areas receive rainfall on Wednesday and Thursday.