Denver, Colorado

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 www.hurricanes.gov 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.
 

SPC
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.

Related:
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.

USA Pro Challenge Weather Forecast

Posted by @brendansweather

Sunday the USA Pro Cycling Challenge wraps up in Colorado with Stage 7 taking cyclists from Boulder to Denver along a 78 mile (126 kilometer) course. The weather appears as though it will mostly cooperate for the final day of this seven day event.

Storms will be a bit more numerous today across the northern Front Range a
nd eastern plains however. Storms should develop along the higher terrain to the west by early afternoon, before pushing across the plains by mid to late afternoon. Coverage and intensity will not be as great as what we say Friday, but a few storms could become strong -- with strong wind being the greatest threat.

Most HiRes models have the greatest chance for storms along and north of I-70 across eastern Colorado which seems plausible. Precipitable water values and CAPE will be greatest across northeast Colorado, with drier and more stable air south of I-70. Storm chances in Denver, as one might expect, will very hit and miss. Best chances could develop just outside the city as they did yesterday, but certainly cannot rule out a cell pushing through this afternoon. HRRR produces rainfall totals from a few tenths to nearly an inch across north central and northeast Colorado through tonight.


Saturday PWAT and PRECIP outlook Coloradosource: WeatherBell Analytics
 

Drier Sunday for Pro Challenge
Storm chances decrease across the plains Sunday, but there will still be a chance for a few isolated storms to develop with a good amount of moisture still around. Best storm chances for Sunday should stay west of Denver and Boulder, providing a beautiful late August day for the race.

As for temperatures… We’re expecting most locations across the Denver area and northeast Colorado to the 70s Sunday. MOS guidance for highs range from 80 to 85, but these temperatures have been overdone in recent days. I like the NAM idea for 18z Sunday, with mostly 70s along the I-25 urban corridor, and warmer temperatures across southeast Colorado.

Colorado temperature forecast map
source: WeatherBell Analytics
 

Cold shot for the west
We’ll continue to see chances for storms off and on over the next week, as well as below normal to well below normal temperatures across much of the west. A strong upper level trough will bring unseasonably cool temperatures well into next week for the west, and much of Colorado. You can see that strong trough digging in at the upper levels here:


With that, temperatures will be much cooler than normal, bringing a taste of fall for many from Utah and Colorado north to Montana and the Dakotas. Here is the CFSv2 temperature anomalies for the next five days across the United States.

CFSv2
All that cold air this time of year means... snow! Take a look at what the ECMWF is showing over the next week. Tonight we may see a few locations above 11,000 feet pick up some snow in Colorado, then again this week as things remain unsettled. Take a look at northwest Wyoming through Friday… with a few spots of blue showing in Colorado.

ECMWF snow forecast map Colorado

Long-Range Outlook: What Weather Awaits Colorado and the U.S. This Fall and Winter?

Posted by @BrianBledsoe

About a month ago I talked a lot about the course we are likely on as we head into fall. I addressed the potential for an El Niño and what type, and reiterated that our top analog for the next several months is late 2009 and early 2010. What happened in late 2009 and 2010? See for yourself…

SEP-MARCH 2010 Temperatures
The above map shows the temperature anomalies (or departures from normal) from September 2009 through March 2010. All of that blue and purple represent colder than normal temperatures. Conversely, all of the yellow, orange, and red represent warmer than normal temperatures. Basically, the southeastern two-thirds of the country was quite cold during that time, with Colorado being on the northwest fringe of the coldest air. What about precipitation?

SEP-MARCH 2010 precipitation United States Map
The map above shows the precipitation anomaly for the same period. Blue represents wetter than normal conditions, while yellow, orange, red represent drier than normal conditions. During the winter of 2009-10, Denver received 60.6” of snow (~105% of normal). Half of that came in October (17.2 inches) and March (12.8 inches).

Given the pattern that we are in and how things played out in late 2009, my gut says that we will hit fall pretty early with the potential for some very active weather in October. Now, we base our forecasts more than just on gut feelings, so let’s see what one of our better computer models thinks of this theory.

The JAMSTEC Model is based out of Japan and seems to have the overall pattern nailed, ocean temperatures included, which is a major key to this puzzle.


JAMSTEC Temperature Forecast:

September - November:

jamstec temperature forecast map

The blue shading represents cooler than normal temperatures and the red shading represents warmer than normal temperatures. If you look at Colorado, you see that the state is shaded in light blue. This means the model is calling for cooler than normal conditions for the September through November period.

December - February:

DEC-FEB Temperature forecast United States
 

Get a load of all that blue across most of the United States! The model is going crazy with colder than normal weather, especially for the central and southern part of the country. This includes Colorado, which is shaded in more than just light blue. The darker the blue, the colder the forecast. This would suggest a higher than normal probability of below normal temperatures during the heart of the winter months for Colorado.

March - May:

may-march temperature forecast united states

The spring of 2015 is also showing up as being colder than normal for the same locations that the model believes will have just endured a very cold December through February. To me this suggests three things: 1) a steady tap of cold air from Canada, 2) an active storm track bringing several rounds of snow, and 3) the snowpack on the ground being VERY reluctant to melt. All three of those things in combination make a perfect recipe for a colder than normal winter. Speaking of snowfall, lets look at that (by way of precipitation anomalies) a bit more closely.

JAMSTEC Precipitation Forecast:

September - November:

SEP-NOV precipitation forecast United States

On this map, the green represents wetter than normal conditions, and the brown represents drier than normal conditions. As you can see, the model has a lot of the United States wetter than normal for the fall. This includes Colorado. Plus, do you notice the slightly darker shade of green from just off the coast of Mexico into Arizona and Colorado? That enhancement means that those locations will likely be even wetter than the surrounding areas. I have been saying for a while now that the active Eastern Pacific hurricane season MAY supply us with a tropical connection of moisture that may meet up with a storm coming in from the west or northwest. That “phasing” -- in effect, when two jet streams link up -- has the potential to produce a major late summer flooding event or an early fall big snow event...or both. The pattern suggests that it is possible, and certainly something to watch.

December - February:

DJF precipitation forecast

The heart of the winter shows up with generally normal moisture for Colorado, with a slightly wetter than normal signal for far Southern Colorado. I think most of us would take a winter with normal snowfall, as they have been hard to come by of late. Either the mountains get hammered and the lower elevations don’t, or vice versa. Those winters when both the plains and mountains see good snowfall have been rare to say the least. However, the signal the model is set on is dry from central California into the Pacific Northwest (bad) and wet from Eastern New Mexico into the Mid-Atlantic. Look familiar? Pretty identical to the second map I showed you in this post.

March - May:

March - May precipitation forecast

The spring looks very similar to what the model is forecasting for the fall. It shows Colorado as being pretty wet, along with all of the Western High Plains. Getting good and consistent spring moisture has also been a rarity for Eastern Colorado lately. However, we had excellent spring moisture in 2010, and Denver even measured 1.3 inches of snow in May. Not unheard of, but is a testament to how long the cooler than normal weather hung around that year.

I always try to give a confidence reading with the forecast, and I must say that my confidence in this forecast is higher than normal. I’ve been talking about it for months, I haven’t seen any of the major drivers change much, and something else...can you feel a difference in the weather already? The nights are getting more crisp, some of the Aspen trees are already starting to show color, and we will see additional cold fronts move through during the next 10 days. Fall is coming, and it is likely coming early…