FTO 07-09-2020: Heat Wave Continues & Storms Return to Start Next Week

Issue Date: Thursday, July 9th, 2020
Issue Time: 3PM MDT
Valid Dates: 7/10– 7/24

One can safely say that the models did really well predicting this week’s heat wave a little over a week out. Highs each afternoon have been up to about 10F above normal, and will continue to stay that way for the near future. Other than a couple storms over the Northeast Plains this afternoon, it should continue to stay dry. Scattered storms are expected to return this weekend beginning on Sunday as the ridge to our south is displaced eastward with an incoming Low and lee troughing occurs each afternoon. For the most part, it looks like the measurable rain will stick to the eastern plains, but weak scattered storms may be possible over the mountains at the beginning of the work week. So Event #1 will have No Apparent flood threat.

There will likely be a break in rainfall sometime mid-week as flow briefly becomes more zonal and mixes out remaining surface moisture with downsloping winds. However, long-rang models are showing a strong ridge quickly building to our east. If this ridge is able to set up in the right place, there is a chance for a strong PW surge across the state. Fingers crossed that this moisture surge, from both the Gulf of Mexico and Gulf of California, occurs and sets up a temporary monsoon pattern. This week’s warm temperatures and dry afternoons are definitely not improving our drought situation.

It’s still quite dry over western Colorado with PW values near the 10th percentile, which will keep fire danger high over the next couple of days despite lower surface wind speeds. The GEFS shows good consensus with the next moisture surge beginning Saturday morning. Still not quite enough moisture for a lot of wetting rainfall, and the dry boundary layer will likely also inhibit moderate accumulations; however, some scattered weak storms may be possible along and near the higher peaks of the San Juan and Central Mountains. Winds aloft (and at the surface) are expected pick up during the zonal flow period from Tuesday into Wednesday, and with PW beginning to drop off after Sunday night. This will likely translate to Elevated fire danger next week.

Over eastern Colorado, the weak moisture surge will return PW values closer to climatology. Thus, there may be some high-based storms over the mountains to start the work week. PW values look a little better over the eastern plains early next week due to surface lows developing in the afternoons. Thus, the best chance for wetting rainfall will be along the eastern border and over the Raton Ridge due to its position further south.

Event #1: Sunday Wednesday (7/12 7/15)

No Apparent Threat as lee troughs pull moisture over the eastern border, and a slight shift in the ridge axis increases moisture over the mountains.

Flooding is not anticipated this weekend into next week even though scattered storms return to the forecast. Not much measurable rain is anticipated over the mountains, western Colorado, or the adjacent plains, but the extra afternoon/evening cloud cover will be some nice relief from the heat. The near normal PW values are short-lived over western Colorado, and some dry thunderstorms Sunday into Monday may pose a fire threat. Fire danger will likely increase at the start of next week under the zonal flow pattern, which will dry out the atmosphere and help increase surface winds. Be sure to follow your local NWS office and check back into Monday’s FTO for the latest information.

Highest chance for wetting rainfall and a few thunderstorms early next week will be over the Northeast Plains. There should be better moisture in this area from the lee troughing and passing cold fronts/outflow boundaries, which should allow some higher instabilities to build. Additionally, rainfall will be possible along the southern Raton Ridge due to its vicinity of higher moisture (south) and passing (weak) shortwaves around the High. The flood threat looks low, but some decent (welcome) rainfall will still be possible.

Event #2: Friday Monday (7/17 – 7/20)

Elevated Threat as we look at our first (short-lived) monsoon moisture surge.

It’s a bit too far out to get too excited, but with the potential ridging pattern to our east, we could get a nice PW surge from the south across the state. Of course, this all depends where the ridge sets up and how long it sits in place. These sorts of details will likely evolve and change as the event nears. However, the last few long-term model runs are pointing to the start of a (temporary) monsoon pattern, so it’s worth mentioning. As far as the typical start date for the monsoon, we have arrived to the highest PW values of the year. So, the onset is already expected to be slightly delayed from normal. Don’t put too much emphasis on the map below as the details will change.

FTO 07-06-2020: Summer Heat Wave

Issue Date: Monday, July 6th, 2020
Issue Time: 2:25PM MDT
Valid Dates: 7/7– 7/21

Temperatures are on the rise with a moderately strong ridge overhead, and dry air is beginning to settle in across the state. This can be seen in the water vapor imagery below by the yellow shades over Colorado. Tomorrow will be the last shot at some weak thunderstorms and showers as the trough (orange “X”) transitions east and propagates some mid-level energy across the state. Due to the lack of boundary layer moisture, storms are likely to produce more wind than rainfall. High fire danger will return to western Colorado, the western mountains, and portions of the Southeast Mountains as southwest surface winds are expected to increase and couple with low relative humidity values. The best chance for wetting rainfall will be east of the mountains across the adjacent plains where a little better low-level moisture should exist.

Behind the trough, the heat is on as the ridge rebuilds over the state and westerly flow continues to scour out any of the remaining moisture. This is likely to be our first real heat wave of the summer with highs reaching the upper 90Fs to 100F across all the lower elevations by tomorrow. There may be some weak cooling on Thursday, but it will hardly be noticeable with highs still in the 90Fs. The extreme heat is expected ramp right back up, and temperatures over the eastern plains will push 105F by the week’s end. Expect the heat to last through the weekend.

Storm chances return Sunday into Tuesday (Event #2), which should help some lucky areas cool off during the afternoon over south, central Colorado. The increase in storm activity occurs due to the ridge axis shifting slightly east, and moisture  wrapped around the High moving over the state. There looks to be a (potential) tropical cyclone that forms over the eastern Pacific and travels northward along the Baja Peninsula. This could help pull some decent moisture northward on the west side of the high, so this interaction with the High and storm track/intensity of the potential TC will be watched closely throughout the week.

I’ve added the 10th percentile climatology to the GEFS PW plumes below to show just how dry it’s going to get on top of the heat. Record low values will be possible over western Colorado, which is not good news as this area is already experiencing a drought. PW also drops off significantly over eastern Colorado as well and hovers around the 10% climatology by Wednesday morning. All in all, please use extreme caution as fire danger will be elevated with already dry fuels and very hot temperatures forecast. The nice ramp up in moisture can be seen at the end of the week, and there is pretty good consensus among the different model members this far out. Saturday may allow for a few scattered storms over the southern mountains, but the boundary layer will still be quite dry, so not expecting much rainfall.

Event #1: Tuesday (7/7)

No Apparent Threat as the trough pushes some mid-level energy overhead and it mixes with limited low-level moisture to produce scattered storms over eastern Colorado.

The boundary layer is expected to be even more dry tomorrow, which will really rainfall efficiency with most of the droplets evaporating before the reach the surface. There is a slight chance that rain totals could reach 0.50 inches along the Raton Ridge, but this would be very isolated in nature. Small hail is also not out of the question under one of these stronger storms. Storms that are able to form tomorrow will likely produce gusty outflow winds and lightning, which seems to be the main threat from storms lately. Flooding is not expected so there is No Apparent Flood threat issued. A Red Flag Warning has been issued for western Colorado with a Fire Weather Watch for the majority of the mountains. Tune into your local NWS office and tomorrow’s FTB for more information.

Event #2: SundayTuesday (7/12 – 7/14)

No Apparent Threat as moisture gets wrapped northward around the High and moves into the state with a slight eastward shift of the ridge axis.

After a very dry and hot week, moisture begins to return to the state on Saturday. It still looks to be pretty dry over western Colorado, so expect storms to be confined to the Central and San Juan Mountains along and near the Continental Divide starting on Sunday. Better storm formation is expected over the Southeast Mountains and southern Front Range. Without much upper dynamical support, storms will likely be pulse-like in nature. A few may wander into the immediate adjacent plains with the counterclockwise flow, which is typical with this diurnally driven pattern. If any shortwaves arrive at peak heating, coverage of storms will likely increase over the eastern mountains and over the immediate adjacent plains, but it’s too far out to know any of those details yet. As for the rainfall chances, best guess now is that coverage will be greatest further south and east. As is typical, the GFS has a lot more moisture than the ECMWF, so we’ll continue to watch model trends throughout this week.

FTO 07-02-2020: Rainfall Returns for July 4th Weekend; Intense Heat To Follow

Issue Date: Thursday, July 2nd, 2020
Issue Time: 2:25PM MDT
Valid Dates: 7/03 – 7/17

The afternoon’s water vapor imagery, below, shows an amplified pattern over the eastern Pacific Ocean and stretching into the central United States. Amplified patterns sometimes result in active weather, but such is absolutely not the case in this situation. A large ridge, currently centered just east of Colorado, will be almost stationary over the next 7-10 days, resulting in little to no movement of water vapor in or out of the state. Meanwhile, a large-scale trough over southwest Canada will attempt to usher in shortwave ripples in the flow. Ultimately, the ridge will remain in control, but not before a 72-96 hour event of increased precipitation chances mainly across central and eastern Colorado. This is the only precipitation event identified during this Independence Day Weekend-edition of the Outlook.

Looking at the PW plumes from the GFS ensemble shows a marked difference across the Continental Divide. While Denver will see a prolonged stretch of above normal moisture, Grand Junction will stay generally below normal. Thus, expect to see little to no precipitation below about 8,000 feet west of the Continental Divide, with precipitation limited to about 0.5 inches for the higher elevations of the San Juan Mountains and Central Mountains. The area likely to see the highest rainfall amounts will be across the far eastern and southeastern Colorado border. There, plenty of residual moisture will be trapped with PW up to 1.3 inches along the southern border with Kansas. Although synoptic scale support will be rather weak, there will be sufficient instability for a 48 hour stretch of isolated heavy rainfall, warranted an Elevated flood threat for the Saturday-Sunday timeframe. This is certainly good news given the recent upgrade to extreme drought status for parts of southeast Colorado.

The aforementioned ridge is expected to take firm control of Colorado’s weather beginning Tuesday of next week (July 7th), as is clearly seen in the GFS ensemble forecast of 850 mb temperatures for Denver. Expect a 3+ day stretch of much above normal temperatures, reaching 100F even for elevations up to 5,500 feet. The prospects for widespread rainfall look very slim from Tuesday July 7th, onward. However, residual moisture could still produce higher terrain showers and storms depending on how much moisture is available in the boundary layer. However, at this time, precipitation amounts above 0.5 inches do not appear likely.

Event #1 is outlined in more detail.

Event #1: Friday (6/26) – Monday (6/29)

Elevated Threat on Saturday and Sunday with heavy rainfall possible along the Kansas border

Rainfall coverage will increase on Friday, with the highest coverage over the higher terrain as well as northeast Colorado where isolated severe storms will be possible. On Saturday and Sunday, the southeast quadrant of the state is expected to see the most activity, with one or two rounds of afternoon and evening showers and thunderstorms possible. Rainfall rates in the higher terrain are expected to stay at or below 0.5 inches so no flooding is anticipated, though the Decker and Spring Creek burn scars will need to be watched. Farther southeast, rainfall rates up to 2.4 inches per hour look to be possible across far southeast Colorado, as PW approaches 1.3 inches. Exact placement of storms will depend on mesoscale boundaries and the dryline position. Check back to daily Flood Threat Bulletins for updates.

FTO 06-29-2020: Drying Trend before Rain Returns for the Holiday Weekend

Issue Date: Monday, June 29th, 2020
Issue Time: 1:20PM MDT
Valid Dates: 6/30 – 7/14

High fire danger will continue on Tuesday as the Low remains to our west and mixes high wind speeds down to the surface. The main difference from today is that it will be cooler across the state due to a cold front moving through overnight. Not much low-level moisture is associated with the front, and it quickly mixes out tomorrow, so wetting rainfall is not forecast for Tuesday. Outside of some very light rainfall over the northwest corner, it should remain dry. The Low will begin to lift northward and a ridge will build from east to west across the state the rest of the week. So enjoy the cool tomorrow as it won’t last for more than a day or so. By Friday, Event #1 begins as the High becomes centered over the desert southwest. This will start to pull higher moisture northward and move weak disturbances across the state (dirty ridge). The ridge becomes dislodged and begins to move east by early next week as the next trough approaches from the west. Thus, rainfall chances should start to decrease.

As far as moisture, the GEFS shows it really starting to drop off after today both east and west as the southwest flow aloft entrains a very dry air mass. Values over western Colorado are near record low values, so very high fire danger is anticipated on Tuesday. Tune into your local NWS office for the latest on Red Flag Warnings. The uptick in moisture associated with the building High can be seen later this week as well. We’ll watch how much moisture moves northward across the southern border over this week, but at this time there is No Apparent Threat. If the PW values are on the higher end of the GEFS solutions below, it is likely that on Friday or Saturday an Elevated Threat will be issued. Please tune back in on Thursday for the latest as details will likely change. We’ll also be able to better place an Elevated flood threat for recent burn areas this weekend. Nevertheless, slower steering winds under the ridge later this week will allow storms to drop beneficial, wetting rainfall.

Event #1: Friday (7/3) – Monday (7/6)

No Apparent Threat as the High sets up over the Desert Southwest and advects subtropical moisture northward.

It’s going to be hot this week with the 5880m marker moving north of the state by Thursday. The aforementioned uptick in moisture should bring storms back into the forecast by late this week. Weaker showers on Thursday will likely help moisten the boundary layer, which will allow more substantial rain to fall on Friday/into the weekend. Without much steering flow at the upper levels, storms should mostly stay confined to the mountains and immediate adjacent plains. This will also help accumulate rainfall over certain areas. At this time, the best chance for widespread showers and thunderstorms will be on Sunday, but a lot of details can change over the next few days. Tune back into the FTO on Thursday for the latest. Lastly, I just want to quickly mention there is an Elevated flood threat for Event #1 over recent burn areas due to the slower steering winds aloft. I only mention that now due to the holiday weekend and the increase in mountain recreation. It’s a bit too far to say much more than that due to low confidence in the details.