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.

 

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 11^th 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.

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 4^th 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.

 

Issue Date: Thursday, September 13th, 2018
Issue Time: 11:40 AM MDT
Valid Dates: 9/14 – 9/28

Expecting critical and enhanced fire weather to continue through the beginning of next week. Dry, warm air continues to be entrained from the desert southwest (see the water vapor imagery below) and stronger winds aloft are mixing down to the surface. With relative humidity values in the teens to single digits across the state, conditions are ripe for rapid expansion of fires. Starting Saturday, low-level moisture begins to return to the lower-elevations as the persistent trough to our west begins to move eastward. This should decrease the fire danger some though it take a while for the trough to move east of the area. As the trough moves east, the 500mb flow gradually changes from southwest to more westerly. This will entrain some higher Precipitable Water (PW) values, so by Sunday the critical fire weather should downgrade to enhanced fire weather or no threat.

Also expecting to see showers and thunderstorms to start to return to the forecast from Sunday into Monday as the 500mb high moves into the Texas Panhandle. West of the Continental Divide, showers and weak thunderstorms will be most likely over the San Juan Mountains and near the Divide. Over eastern Colorado, storm coverage from Sunday into Monday is expected to increase over the eastern high terrains. Not expecting much rain from these storms due to lack of low-level moisture and decent steering winds. With adequate temperature/dew point spreads, some gusty outflow winds will be possible. Early Tuesday morning, the GFS is showing a weak cold front moving through the northeast corner of the state. Questionable as to how much moisture will return behind the front, but Tuesday’s temperatures will likely be a bit cooler over this area. Overall, should see an increase in afternoon thunderstorm coverage over northeastern Colorado from Monday into Tuesday. Wednesday, downsloping flow will pick back up, so a downtick in thunderstorm activity is expected as drier air becomes entrained into the low and mid-levels of the atmopshere. Currently, the GFS shows a stronger cold front dropping through the state on Thursday, which if this occurs, will give us our first taste of fall after a long, unseasonable warm stretch of weather. At this time, there is No Apparent Threat for flooding during this FTO period.

Starting on Saturday night, PW plumes start to show a return to more seasonal moisture values from west to east. This should first cause an increase in cloud cover on Saturday, and then an increase in storm coverage over the higher terrains on Sunday into Tuesday. Low-level moisture doesn’t last long over western Colorado, with a return to below average PW values after Monday. Enhanced/Critical fire danger will likely return to the forecast over this area should PW values drop below 0.4 inches as surface winds are currently forecast to be in the 15-20 mph range. Over eastern CO, there are elevated PW values from Sunday through Tuesday. Quite a bit of spread after this, which likely has to do with how much moisture returns behind the front. Tune back into the FTO on Monday to see how the members are trending for the end of next week. If PW values are able to stay above about 0.5 inches, we will likely still see some scattered storms over the mountains in the afternoon and evening hours on Wednesday and Thursday. If the cold front does drop through on Thursday, snow may be possible over the highest elevations.

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

Event #1: Sunday (9/16) – Wednesday (9/19)

No Apparent Threat as moisture returns with eastward movement of the ridge for scattered afternoon showers and thunderstorms.

Expecting low-level moisture to increase from the southwest to northeast on Saturday into Tuesday. At first this should only increase cloud cover, but by Sunday and Monday there should also be an increase in shower and weak thunderstorm coverage over the higher terrains. Early Tuesday morning, a cold front may move into the Northeast Plains, which could cause an increase in storm activity over this area during the afternoon. The low-level moisture is short-lived as downsloping winds begin to mix dry air back into the low- and mid-levels of the atmosphere on Wednesday. Expecting enough residual moisture under the ridge for some isolated, high-terrain showers on Wednesday. At this time, unsure about a cold front moving through on Thursday as there is quite a bit of disagreement between GFS members. Should this occur, showers and thunderstorms may be possible (with snow in the highest terrains) over the mountains and adjacent plains on Thursday evening. Please tune back into the FTO on Monday, and for now, we will leave this out of the rainfall map below.