Denver, Colorado

Tuesday PM Update: Heavy Rain, Flooding Concerns Through Wednesday

Posted by @brendansweather

Radar loop Colorado

Radar Loop 2pm to 3pm

Heavy rain reached northeast Colorado early this afternoon, and has really grown in intensity over the last hour. It is expected to continue off and on through Wednesday. A Flash Flood Watch is in effect for nearly all of Colorado through Wednesday PM, including the city and county of Denver.

The overall synopsis has not changed since
our post yesterday. Heavy rain with slow moving storms will mean an increased threat for flooding over the next 24 to 36 hours. Rainfall totals for northeast and northcentral Colorado are expected to be anywhere from 0.5 to 3 inches by Wednesday night, with locally heavier amounts possible.

We have already seen several Flash Flood Warnings posted north and northwest of Fort Collins, and expect more to come over the next day or so. Please be weather aware, and heed any warnings that are issued for your area.

Complex setup for pinpoint rainfall totals
It’s proving very difficult to pinpoint exactly how much rain any single location will receive. Models have been fluctuating greatly run-to-run both with regard to timing and locations of heaviest precipitation. What is certain is that the ingredients are in place for a widespread heavy rainfall event, but expect great variation in totals when it’s all said and done.


The 12z model suite has largely continued to insist much of the heaviest precipitation from this event may occur Wednesday, especially for southern Colorado. If you happen to see less rainfall this afternoon and into this evening, you’re not out of the woods yet as the rainfall threat continues tomorrow. Here are the 12z GFS precipitation totals through Thursday morning for Colorado, the EURO looks similar.

GFS precip totals
The NAM continues to “dry-slot” much of Denver proper and the southwest suburbs, shunting the bulk of the precipitation north and east of Denver. We’ll see how it handles things, as both the GFS and EURO are a bit more impressive in the city, as well as the higher terrain to the west. As we typically do, we’ll be weighing the NAM a bit lower, especially its 18z run. (We’ve already seen more than it’s 18z run in central Denver with cell currently overhead!).


Really any way you cut it, someone’s getting a lot of rain over the next 24 to 36 hours. We’ll watch carefully and update as needed. Warnings will be posted on our Twitter feed as well.

Monday Update: Addressing Flood Potential Across Colorado

Posted by @brendansweather

The cooler air we’ve been promising for some time now has arrived, and will continue to bring temperatures well below seasonal norms through the end of the week. Wednesday looks to be the coolest day of the week with highs in the low to mid 70s for Denver, and likely a few locations in the 60s across the Plains. (Some spots could see near record-cool highs for the date.)

The focus over the coming days will not be the temperatures, however, as the threat for flooding rain will be our primary concern through midweek and beyond. Models have fluctuated in where the heaviest rainfall totals will occur through Wednesday, but there will be a few regions we will be watching very closely, and most of the state should at least see some rain.

General setup
The setup for the coming days is not atypical for this time of year in that we’ll have ample moisture to work with through the middle of this week, and quite possibly continuing off and on well into next week. Take a look at this water vapor animation of the last 24 hours across the United States. Notice the stream of “moist” air flowing into the southwest and into Colorado. Late in the frame you can see dark colors over southeast Colorado, these were the storms that brought flooding to this part of the state last night.

Water vapor loop United States

We’re expecting the water vapor loop to look very similar over the coming days. You’ll also notice the large trough digging into the upper midwest. This is responsible for the cooler-than-average temperatures being ushered into the middle of the country.

The combination of this moisture stream and a stalled front over the coming days will mean a heightened flood threat for the region. The greatest threat for heavy rain across Denver, northeast Colorado and the higher terrain west of I-25 appears to be Tuesday afternoon through Wednesday when PW values are highest. That said, ingredients are in place again today (similar to yesterday) to support some strong, slow-moving storms this afternoon -- so that’ll need to be closely watched. The greatest threat for storms today will be west of I-25 and through the Colorado high country.

Flash Flood Watch
At this time there are no flash flood watches issues for northeast Colorado and the northern Front Range. This could very well change in the coming hours, as again, Tuesday and Wednesday look potentially very wet.

A flash flood watch is in effect for most of southern Colorado, however, and will likely remain so through much of the next several days. The current watch goes through midnight tonight, and includes all of the southern I-25 corridor, including the high flood risk burn scar areas like Waldo Canyon.

Flash Flood Watch Map | Mesomap

There’s been a lot of buzz over recent days with how this setup compares to last year’s September floods. While we’re watching the flood threat very closely, it seems irresponsible to make that connection at this point. Remember, this is the time of year where it’s typical to get flooding and flash flooding in Colorado, and the atmospheric setup is not exactly the same.

Unlike during the early summer weeks, storms in late July and August are moving much slower -- which increases the flood threat. Rather than focusing on how this does and does not compare to late last summer, let’s use that as a reminder of the threat flash flooding does pose to our region, especially at higher terrain west of I-25 and over burn scar areas. Please heed all watches and warnings over the coming days.

The NAM/EURO/GFS differ on who gets the heaviest rain over the coming days, but all broad-brush most of the eastern mountains, I-25 urban corridor, and into the eastern plains with 0.5 to 3 inches of rain, with locally higher amounts. Where storms stall, or train, we could see certainly see higher totals. Today’s 12z suite of both the GFS and NAM appear to try and pull the heaviest precipitation further north than previous runs. We’ll see.


12z GFS total precipitation through Wednesday

GFS precip forecast

12z NAM total precipitation through Wednesday

NAM

SREF means (KDEN)

SREF precip means

 

We’ll keep on top of everything and offer updates as needed. Again, greatest storm chances today will likely be over higher terrain to the west, but could see a few storms push east through the afternoon and evening hours. A greater threat for heavy rainfall begins Tuesday afternoon, and will likely continue well into Wednesday, if not through the day Wednesday.

To stay ahead of the storm, subscribe to Weather5280. Also find us on Twitter (@weather5280) where we’ll have our most frequent updates over the coming days.

The State of the Atmosphere: Saturday, July 26, 2014

Posted by @MattMakens247wx

Another cool/wet week ahead
Highs Saturday will climb into the low 90s again for most across the metro area, but should be the last time we hit the 90 degree mark for several days. Sunday's highs will be several degrees cooler, with even cooler temperatures expected by the middle of the week. We'll see another round of storms this afternoon across the Front Range and eastern Colorado, with an even better chance for storms as we head into the coming week. 

Easing the drought
The first half of 2014 was slightly cooler than average for eastern Colorado, including Denver. However, the western half of the state balanced that out by being slightly warmer than average.

Maximum temperature anomaly from 1981-2010 mean United States
Precipitation was the same balanced story. Despite a lot of snow, actual moisture accumulation was slightly wetter than average for the north, meanwhile drier than average across southern Colorado.

divisional precipitation ranks 2014 Colorado
The drought monitor is following suit:

United States drought monitor 2014
The current drought is a vast improvement to that of last year for the same time. Remember the drought monitor takes into account not only recent precipitation, it includes surface water, longer term precipitation accumulation, temperature trends, etc. Here’s a look at where we were on July 23, 2013.

US drought monitor 2014

The big improvement to the drought situation in Colorado has been our consistent moisture supply this summer. As you probably have noticed, this summer has been stormier and cooler than the past few years. The stormy pattern usually takes a few weeks off in late June through late July when the monsoon kicks in. This year’s monsoon kicked in early and has been the source to all of our daily storms.

The storminess has helped keep temperatures closer to average.
Remember, drought feeds heatwaves. When Colorado’s water supply is low the ground is dry and that can heat the surface air more effectively. With so much moisture in the ground this year the air temperatures have been on “par”. We’ve also noted a more northwesterly flow the past several months to keep the flow of cold fronts near the region. We had a colder push of air a couple weeks ago (remember the 70s on the 16th and 17th?) that will be a very similar pattern to what is taking shape this weekend.

The “moisture train” to continue
During the weekend the persistent ridge that brought us the 90s will be breaking down slightly with a large area of energy diving south out of Canada toward the eastern US. This will drop colder temperatures in from the north starting Sunday. Rain chances continue as monsoonal humidity collides with that colder air over the state. In fact, the upcoming week will bring valuable, additional rainfall. One model, the GFS, is bringing the country’s greatest moisture totals into our region, and especially those areas that need it most -- southern Colorado into New Mexico. Unfortunately more of the moisture won’t hit the western US for the sake of California’s drought and the Pacific Northwest wildfires over the coming week.

GFS precipitation forecast Colorado
GFS precipitation forecast United States

This moisture coincides with the forecast given here on Weather5280 with Brian’s last post on the monsoon. The climate projections indicated a wetter end to July and start to August. This is that projection verifying.

Further, the below average temperatures also forecast are on the way. The end of July and start of August will be much cooler than average. For Denver, 70s and 80s for the week versus low 90s.

As the cooler air arrives and rain chances climb we will dissect the week with a variety of posts on the system as it progresses.
Subscribe to Weather5280 for email updates, and be sure to follow us on Twitter as well (@weather5280)!

 

El Niño Status and Forecast

Posted by @BrianBledsoe

It has been a while since we’ve talked about El Niño, so I figured I would give you an update. Here is what the most recent sea surface temperature (SST) anomaly map looks like:

Sea Surface Temperatures

As you can see, a warmer than normal pool of water is located off the west coast of South America in areas referred to as Niño Regions 1+2. Farther west across the Pacific the water is actually a bit cooler than normal until you get to the western Pacific Ocean. In order for the El Niño to develop in a traditional fashion, the water must cool in the western Pacific Ocean allowing the Southern Oscillation Index to drop and the subsequent oceanic and atmospheric coupling to take place. Sounds like a lot of geek speak…but the way things are structured right now, a traditional El Niño will struggle to form. Recently the SOI has started to drop a bit, but has a long way to go to where it indicates a true El Niño:

SOI Phase
So, if a traditional El Niño doesn’t look as if it’s going to develop does that mean we won’t see an El Niño? No, just a different type. Modoki is the Japanese term for “similar, but different”. A Modoki El Niño is a specific sub-type of El Niño pattern that has the warmest water in the central and western Pacific Ocean. This is what a Modoki El Niño would look like in terms of sea surface temperature anomalies:

Modoki El Nino Example Map
Notice where the warmest water resides in the equatorial Pacific Ocean, right in the central/west central region. Comparatively cooler water is located just off the west coast of South America, where the warmest water exists as of right now. The transition that takes place in the next few months will likely allow the water off the west coast of South America to cool, while the central Pacific warms.

Does a Modoki El Niño mean the same as a traditional El Niño? Not exactly, but for some areas it can be very beneficial with moisture. One model that seems to have a nice handle on the overall transition and setup is the JAMSTEC Model. This model is out of Japan and is a very good model. Looking ahead, here is what is what it suggests will occur in terms of sea surface temperature anomalies for the next several months:


JAMSTEC Sea Surface Temperature Anomalies Sept-Nov:

SON Jamstec
JAMSTEC Sea Surface Temperature Anomalies Dec-Feb:

DJF Jamstec

JAMSTEC Sea Surface Temperature Anomalies Mar-May:

Jamstec MAM

Notice how the core of the warmest water stays mainly in the central Pacific Ocean, instead of emanating solely off the west coast of South America (as in a ‘typical’ El Niño). Another thing I am watching is that most of the Pacific basin is somewhat warmer than normal. This likely means we will keep a relatively positive/warm PDO for the next several months. Remember, a positive PDO usually helps us out in the moisture department (it has been positive since January).

JAMSTEC Temperature Anomalies Sept-Nov:

SON
JAMSTEC Temperature Anomalies Dec-Feb:

DFJ Temperatures jamstec
JAMSTEC Temperature Anomalies Mar-May:


Per the maps, red means warmer than normal and blue means cooler than normal. Usually during a Modoki El Niño, the east and southeast 1/3 of the country is cooler than normal for the fall, winter, and spring. This version of the JAMSTEC has the main cool signal over a smaller area of the Eastern US than usual, and then keeps most of the US cooler than normal into the spring months. Overall, I think the temperature profile looks spot on as to how a Modoki El Niño impacts the US.

JAMSTEC Precipitation Anomalies Sep-Nov:

JAMSTEC SON precip
JAMSTEC Precipitation Anomalies Dec-Feb:

jamstec DJF
JAMSTEC Precipitation Anomalies Mar-May:


The green color means wetter than normal conditions, and the brown means drier than normal conditions. The JAMSTEC has much of the Western US wetter than normal for the late summer and fall. I believe this is due to what will be an active Eastern Pacific hurricane season. Some of that moisture will enhance our monsoon, and maybe keep us wetter longer than usual.

It then has much of the southeast half of the US drier than normal for the heart of the winter. This is somewhat unusual, considering that the same area is progged to be cooler than normal during the same timeframe. Cooler than normal would imply an active storm track or at the very least, several major cool shots dropping into the eastern half of the country. When that happens, the pattern is usually wetter than normal.

The spring then turns around and shows up wetter than normal for much of the US, especially some of the same areas that are progged to be so dry during the middle of winter. Kind of a weird outcome if you ask me… Why? Because my favorite analog year for what is to come is 2009-10. During that time, the southern tier of the US was very cool and very wet. This was also at the same time that a Modoki El Niño episode was occurring. In fact, check out the map below that shows the temperature anomalies across the U.S. from September of 2009 through March of 2010:



All of that blue and purple is much colder than normal air that dominated much the US…especially the southern tier. Moisture was in no shortage for the area either. The map below shows wetter than normal conditions (blue shading) for many of the same areas that were cool too.

NOAA NCDC

This is kind of why I am skeptical of the dryness the JAMSTEC is showing for the Dec-Feb timeframe, but not necessarily for Colorado. I am skeptical of the dryness that shows up in the model farther east. Does that matter? Maybe, maybe not, but in terms of giving the model a higher weight of accuracy, I think this is important. For Colorado in late 2009 and early 2010, most of that above normal precipitation fell in the fall and early winter (October 2009 is legendary for the Wet and Sangre de Cristo Mountains of Southern Colorado). Then we kind of dried out during the heart of winter and into the spring. Per our discussion above, the JAMSTEC has us wet in the fall, somewhat dry in the winter, then wet in the spring again. Is that really that out of the ordinary? Do we ever see big winter storms in December, January, or February? Sometimes, but keep in mind that it is those early and late season storms that are usually the biggest and most beneficial to us east of the mountains. Plus, the PDO wasn’t as warm in 2009-10 as it is now, and I think that is important. A warm PDO is usually kind to Colorado when it comes to moisture and I believe it has been instrumental in helping erase the drought during the first half of 2014.

I know this is a lot of information, but for those that read us, you know that is what we are all about. Making sense of the chaos and helping you understand what is to come in the long range is something we love doing. Needless to say, we’ll be watching it closely for you.