Denver, Colorado

Denver’s June-August Recap; Welcome to (Meteorological) Fall

Posted by @brendansweather

Although the calendar won’t tell you it’s fall for another few weeks, meteorological fall began on Monday, September 1st. Meteorological fall runs from September 1st to November 30th.

Summer Temperatures 2014
It certainly wasn’t the coolest summer Denver has ever experienced, but also a far cry from recent years, as the region overall experienced near-normal temperatures over the last three months. According to Climate Central, Denver ended up somewhere just right of center in this chart of coldest to warmest summers on record.

Denver summer 2014 temperature ranking

Official temperatures (recorded at Denver International Airport (DIA)) were very close to normal for June and July, but notably cooler than normal in August. June ended -0.1 below normal for the month at DIA, while July ended 0.3 degrees above normal. August, however, was an impressive -1.9 degrees F below normal! In general, northeast Colorado saw temperatures at or slightly below normal this summer, while southwest Colorado was at or above normal.

90 degree days were hard to come by over the last few months, at least when compared to recent years. Denver recorded 27 days at 90 degrees or above between June 1 and August 31. That’s down from 47 days in 2013 and 64 days in 2012. Not coincidentally, this is the fewest number of 90 degree days during the three month period that Denver has
seen since 2009.

How many 90 degree days Denver, Colorado

A similar story played out for precipitation. During the June through August period we saw generally wetter than normal conditions across northeast Colorado, with drier than normal conditions across the southwest corner of the state. DIA recorded 2.73 inches of precipitation in August, which is 1.04 inches above (162% of) normal.

August was very wet for much of the Rocky Mountain west. Northern Colorado ended the month well above normal, as did portions of Utah, Nevada, Idaho, Wyoming, and Montana. There were even a few spotty locations in California that recorded *some* rain…

August precipitation totals map USA
For the summer months (June - August) we see that the core of precipitation shifted a bit further east than it was over the last month, with much of eastern Colorado seeing near to above normal precipitation through the period.

Summer precipitation totals United States

Some drought relief
Colorado took a step in the right direction this summer when it comes to drought relief as well. According to the U.S. Drought Monitor Colorado started the summer with 44.71% of the state not experiencing drought conditions, and that number now stands at 60.21%. While it would have been nice to see more improvements in hard-hit southeast Colorado, there have been marginal improvements there as well. As of May 27, 2014 more than 12% of the state (all in southeast Colorado) was experiencing Extreme to Exceptional drought, that number is now down to 2.67%, with no drought conditions classified as Exceptional in the state.

United States drought monitor | Colorado

The month of September is typically fairly quiet for eastern Colorado. Our monsoon season comes to a close during the month, and the days tend to be warm and nights are cool. While nothing quite beats a perfect September day in this state, the month can also offer extreme weather. Next week will be the one year anniversary of last year's historic flood event across northern Colorado.

Related: High Water | Colorado Floods, 2013

September is Denver’s fourth warmest month of the year (behind July, August and June), it is also the first month of the snow season for the city. According to the National Weather Service, the warmest temperature on record in September for Denver is 97 degrees F, while the coldest on record is 17 degrees F.

While September averages 1.1 inches of snow for the month (long term average from 1882-2014), the city has not seen any snow in September in 14 years. In 2000 0.2 inches of snow were recorded at DIA, and in September 1999 3.1 inches fell. Here’s a look at the snowiest Septembers on record in Denver:

  • 17.2 inches 1971

  • 16.5 inches 1936

  • 12.9 inches 1959

  • 11.4 inches 1895

  • 8.7 inches 1985

Needless to say, there is no snow in the forecast this week with daytime temperatures primarily in the 80s, with the exception of Wednesday, when highs may soar well into the 90s, perhaps even flirting with the record high for the date of 95 from 1995.

Average Daily Maximum Temperature (F) from 1979-2011 by County

Posted by @brendansweather

Last week, @MetricMaps put out this awesome animated GIF of average maximum temperatures by day for the United States. The image is a great example of how visualizing time-series can be fun, but also difficult. One of the hardest parts of creating an effective time-series visualization is that the size of the data can get large quick.

So this week I teamed up with my good friend and colleague Chris Helm (
@cwhelm) to explore a different way to built a very similar visualization.

In order to build our application we began by whipping up a little script that scraped data from
the NLDAS. After some processing of the data we were able to use D3.js to build what we think is a pretty snazzy little example of mapping long term climate data.

You can find the full version with county interaction 
here. Tell us what you think!

What the Hurricane?

Posted by @brendansweather

Animated GIF shows GFS day 9 forecast flop
The moment we had been waiting for all summer finally arrived last week. The single model run to throw the social media world into a frenzy: the day 9 ‘dream’ hurricane from the GFS.

GFS 9 day forecast Cristobal
12z GFS August 19, 2014. Source: WeatherBell Analytics

It was amazingly viral for a landfall that was never to be. It was quite possibly the best non-storm setup we had seen in years, and, it was set to make landfall in New Orleans on the 9 year anniversary of Katrina. Of course, this was ridiculous and in the hours and days that followed many came out and said as much. I’ll resist the urge to continue a rant here about media and hype around storms though -- we’ve done that already.

The problem with the frenzy that erupted last week is that models have virtually
no skill at nine days out in forecasting development, strength, and track of a system like this. At the time the system was only a tropical easterly wave east of the Caribbean islands. The amount of hype surrounding this system prompted the National Hurricane Center to remind us all to be cool:

The bottom line really is: be alert, be prepared, but also be wary of long-range projections that go beyond what the science can offer. And make the NOAA National Hurricane Center your calm, clear, and trusted source for official forecast and warning information on tropical cyclones.

Rather than call out the worst offenders here, I’ll just say this: It’s irresponsible, cut it out. Folks living along the Gulf and Atlantic coasts are understandably on edge this time of year, so offering bogus warnings to push your agenda is a really gross way to get followers, views, and clicks. And for those sharing these reports -- please pay closer attention to what sources you are using, and make some effort to find a reliable, trustworthy one.

As forecasters, we should use this type of run to help better understand the pattern, and see what models are trying to sniff out. Why we’re compelled to share images of hurricanes making landfall over a week out is baffling -- especially on Twitter where there aren’t enough characters to explain that it’s a) just a single model run, b) there is a GREAT amount of uncertainty, c) we’re “gearing this to our meteorologist friends that understand these uncertainties” (we’ve all already seen it), d) it’s not in line with its ensembles or other models like ECMWF, and d) it’s the GFS.

Since it’s naive to think we can all agree to stop sharing 10 day forecast maps from a single model… maybe we can agree to try a few things when doing so:

  • Provide context, stress uncertainty over risk.

  • Refrain from using ALL CAPS and instigating words like “WARNING” and “URGENT” (it’s not).

  • Avoid reference to other major natural disasters (i.e. Katrina landfall). That’s not context.  

  • Forecast, not model-cast.

  • (why didn’t anyone show the EURO from that day? #clicks).

So, how on-target was this pot-stirring model run on 12z Wed, August 19?
The forecast was for 12z this coming Thursday, August 28 (pictured above). By the time Thursday rolls around, now hurricane Cristobal is expected to be far out to sea in the Atlantic (though we may have another disturbance to contend with in the Gulf). So how well did it do? Wrong body of water, no U.S. landfall, and about 1300 miles from where it was forecast to end up.

GFS forecast flop
Below is an animated GIF which shows the forecast progression for 12z August 28, 2014. The initial frame shows what kicked things off last week. Projected landfall: southeast New Orleans.

The consecutive images show all 00z and 12z
model runs since then, all forecasts for the same time on August 28, 2014. The first three runs kept the system in the Gulf on the 28th, but even here we see fluctuations of several hundred miles run to run. The following several days the GFS was ushering the system far out into the Atlantic by the 28th, before again trending back west toward Florida. If nothing else, this should help demonstrate the type of confidence any single model run should be given so many days out.

Finally we see some consistency from one run to the next when a center is found by hurricane hunters on August 23. The track at this point shifts back east a little, away from Florida and the eastern United States. Fish storm.

GFS forecast GIF Cristobal

Stormy Start to Week for Eastern Colorado

Posted by @weather5280

The Storm Prediction Center has added a Slight Risk for severe weather this afternoon across northeast Colorado as strong storms are expected to develop this afternoon after we had a rather quiet day Sunday.

Today’s storm risks includes the threat for tornadoes, as well as large hail and damaging wind. The SPC puts Denver in their 2% tornado probabilities with any storms that do develop today.

Current conditions page

The best potential for strong storms should occur just east/southeast of Denver proper, though cannot rule out a strong storm developing over the city. Best ingredients for severe weather will be from just south of I-70 extending east, north to I-76. Hi-Res models continue to try and develop strong storms near Agate and Limon, CO this afternoon. Along the Palmer Divide and north/northeast will have the best chance of seeing severe weather. HRRR shows storms initiating by mid afternoon near Denver, then pushing northeast into the evening.  Flash flooding doesn’t appear to be a huge concern with fairly rapid storm motions. The caveat will be any areas that see training or repeated rounds of heavy cells.

HRRRsource: WeatherBell

We expect a few areas to get good rainfall with storms that develop today. As is typically the case, best precipitation numbers will vary greatly from one location to the next due to the scattered natured of storms, but anywhere from a few tenths of an inch to >1.5” will be possible.

We keep a good chance of storms in the forecast through Wednesday in Denver across much of the plains, with temperatures below normal. We’ll have updates here as needed to get through the next several days.