Denver, Colorado

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.
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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:

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 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.


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.

The State of the Atmosphere: Monday, July 21, 2014

Posted by @brendansweather

It was a hot, but hopefully not unbearable, weekend across much of Colorado. In many respects, we all knew it had to come at some point, as this summer to-date has been fairly tame temperature-wise, and downright wet for some. The heat sticks with us through the week, though isolated to scattered afternoon storms will (like this weekend) may help knock down temperatures late in the day in some spots.

MOS guidance actually hasn’t been terrible with temperatures over the last few days. GFS has been a bit warm, NAM at times a bit cool, with highs generally leveling out somewhere in between. Both models keep us in the mid to upper 90s through the end of the week -- though we can’t entirely rule out a 100-degree reading -- with possibly a return to more climatological highs by next weekend and early next week.

There will be some moisture around through the period, which will keep storm chances in the forecast. Should see a general uptick in chances as we progress through midweek, and then again into next weekend.

Northwest fires, Colorado haze
You may have noticed the hazy conditions this weekend across Colorado. This was due to many fires burning in the northwest states where this summer’s heat has been far more relentless. NASA’s MODIS satellite captured several striking images of the fires over the last week, including this one from July 18. Several fires burning
across Oregon and Washington are visible, with dense smoke (brown-gray vs clouds) being captured over the Idaho mountains, then transported southeast.

MODIS northwest fires
Recent heat over the middle of the country has been at least brief reprieve for the northwest. Cooler than normal temperatures, or at least temperatures near average should continue through much of the week in this area, but for the pattern looks to repeat itself by the end of the week and into next.

Here are temperatures through the first 19 days of July. Denver sits at about average, the central U.S. well below normal, and it’s been baking in the northwest:

July 2014 Temperature Anomalies

June 2014 warmest on record
While temperatures have been at or below normal for a good portion of the United States this summer, as we’ve seen in the Pacific Northwest, this hasn’t been the case everywhere. In fact, according to the National Oceanic and Atmospheric Administration (NOAA), June 2014 was the hottest on record for the globe.

Below are the ocean and land temperature departures from normal for June. Notice the cool sector over the north central U.S. Expect this to look similar in the U.S. for July, with cool pool perhaps a bit further east.

June 2014 Warmest on Record

Pattern Reloads
While the next few days will remind us it is actually summer, it looks like the pattern reloads for next week, with another shot of cool air pushing into the middle of the country. This will mean a better chance for monsoonal moisture to return to the state, as well as cooler temperatures.

The question will be again how far west the cold air will make it, but expect at least a return to closer to normal temperatures as we head into the latter half of this weekend and next week.

Both the GFS and EURO ensembles have another strong trough digging into the east for next week. The GFS 00z run looked a bit stronger, and even a bit further west, which would help support cooler than normal temperatures again east of the Divide. The EURO ensemble control run looks very similar.

ECMWF 500 ensemble
And the CFSv2 continues to
insist cooler than normal temperatures across the central United States to start August. Latest runs actually look more impressive than they did last week for the first 10 days of August (degrees C).

CFSv2 temperature forecast August 2014

Enjoy the summer weather, and try and stay cool! Plenty more to come this week, so be sure to
subscribe to Weather5280 if you’re not already. We’re now less than 12 weeks away from our average first snowfall in Denver!

Flooding Rains Ease, but Here Comes the Heat

Posted by @brendansweather

Our drought-reducing progress will go on pause for a while as warmer and drier weather returns to the region. Temperatures over the next week will climb above normal levels for the state as high pressure builds in. The best precipitation chances over the next week look to be in the mountains, with very little over northeast Colorado and the Plains.

Rainfall totals
It’s been a heck of a run so far this this summer, however, and most of the monsoon season is still ahead of us. According to the latest U.S. drought monitor released Tuesday (July 15th), now 58% of the state is drought free. That’s up from 38% just three months ago.

While the worst drought in Colorado still persists across the southeast part of the state, we’ve seen some relief there as well. Take a look at the rainfall totals across the United States over the last seven days. Most notably, over southeast Colorado, New Mexico, and back into Arizona.

united states 7 day rainfall totals
While July is statistically a rainy month for Colorado, analysis run by the National Weather Service shows much of the urban corridor and southeast Colorado at 300 to 600% of normal precipitation over the last week. Some locations picked up more than 5” of rain during this period. (Though it’s worth pointing out that Denver International Airport has only recorded 0.92” of rain so far this month, and at more than halfway through the month, has only seen 30% of normal total monthly precipitation.)

rainfall as percent of normal Colorado
Temperatures remained below normal Thursday, with afternoon highs predominantly in mid-to-upper 70s the across the metro region. We’ll climb into the low 90s for Friday and Saturday, with even warmer temperatures possible by Sunday and into early next week. We’ll keep a chance for storms going for the next few days, but those chances will be much lower than they have been. Brief downpours, lightning and wind will be possible with any storms that manage to develop over the coming days.

Temperatures by early next week look to be hot. MOS guidance has highs in the upper 90s by Sunday, and staying so through at least Wednesday. We’ll stick with highs a few degrees cooler than guidance for now, but will see how the trend sets up over the next few days. In any case, the next week looks noticeably different than the last few days across the region. Here’s the culprit.

EURO 500

Models differ on just how long this area of high pressure sticks around for, but it does appear that overall the last 10 days of July will be warmer and generally drier than average. We’ll see how things progress, but given the pattern, hard to imagine we won’t see another break from the heat before too long. A quick peek at the CFSv2 shows it keeping us hot through day 15 (left), but brings cooler temperatures right back by the start of August (right).

CFSv2 Temperature Outlook

More to come on all of this. For now plan on a warmer and drier weekend, with well above normal temperatures possible by Sunday and into next week.