Denver, Colorado

Better Chance of Storms This Afternoon

Posted by @brendansweather

As we mentioned in this week’s State of the Atmosphere, it’ll be an up and down week for storm chances along the Front Range and eastern Colorado.

We saw several strong storms develop Monday across the eastern plains, with only a few scattered brief storms during the early afternoon hours in the Denver area. Today those storm chances go up a bit, with 30-40% chances for eastern Colorado.

While the best chances for strongest storms will likely be west and again east of the Denver area, our chances will be greater as well. Precipitable Water values will be higher than what we saw on Monday -- up from ~0.6 inches Monday afternoon to 0.8 - 1.0 inches today (models have been slightly overdone for Denver). Greatest PW according to the 18z NAM will extend from Denver northeast into southwestern Nebraska.

NAM PWATSource: WeatherBell Analytics

CAPE will also be greater across northeast Colorado, with pinpoint for greatest values across the northeast corner of the state where values could climb to 1500-2500. We’ll need to watch for capping across the eastern plains, but should see the cap break later in the afternoon as outflow boundaries push east.

NAM CAPESource: WeatherBell Analytics

As for precipitation totals… Both the GFS and EURO are pretty spotty with QPF through Tuesday night across northeast Colorado, generally ranging from 0.5 inches across the Front Range foothills, to a few tenths out east. We expect enough ingredients to be in place to allow for better storm coverage -- and in combination with the time of year, could see spotty cells put down heavy rainfall at times. As is always the case, impossible to pin down exactly how much any single location will see with these widely scattered storms. Overall severe threat should be low today, but can't rule out a few storms becoming severe across eastern Colorado. 

As for timing, Denver’s best shot at getting a storm will be during the early to mid afternoon hours Tuesday, decreasing into the evening. A few storms may linger in the foothills into Tuesday evening, as we also see the best chances for strongest storms developing across the northeast corner of the state by late in the day.

Storm chances decrease a bit for Wednesday, but enough moisture should be in place to keep a chance for scattered storms going. In fact, we have a chance for at least isolated storms in the forecast through next weekend, with slight better chances returning for Thursday and Friday. 

Measuring Forest Moisture Content With the U.S. Forest Service

Posted by @MattMakens247wx

SNOTEL Colorado
I recently went out with a couple of Forest Service firefighters to learn how they calculate the fire threat across Colorado. My interest in the Forest Service’s efforts to gauge wildfire threat levels started this winter when I was at a SNOTEL site measuring the snowpack atop Berthoud Pass. My co-worker and fellow meteorologist Cory Reppenhagen was with me when we saw the far greater than average snowpack and made the assumption that the fire threat will be much lower as a result… which was an erroneous assumption to make. As Cory and I left the SNOTEL site I thought, “I wonder how they know how much water actually gets into the trees. Lots of snow doesn’t mean that the water is actually getting into the trees, does it?”

As it turns out, the season snowpack can help keep the wildfire threat low, but only so long as there is snowpack. As soon as the snowpack melts, the trees rapidly dry out. Coming out of dormancy and going through the summer, all fire fuels (like trees, grass, and ground cover) need a steady supply of moisture to maintain that low fire threat.  I should point out, a good snowpack can lead to quick growth of the entire vegetation, not just the trees, so consistent rain is needed to keep that vegetation from turning into fuel.

The Forest Service measures moisture content in these fuel sources, and does so regularly. Cory and I went along with a crew in the Pike National Forest in early August to see what it was all about.

Forest Service moisture content research
We hit their Trout Creek fuel site, one of many spread throughout the forest. Samples were taken of the ground cover (litter), the ground layer just beneath a tree canopy (duff), living pine needles, and 3 various sizes of a tree (large twigs, medium to large branches/trunk). The collected samples are then “baked”.

This process meant adding all the collected fuel samples to a convection oven and baking them at 170 degrees for about 24 hours. The weight of the fuel sample is then compared to its weight before going into the oven. The difference in weight is the moisture (water) loss during the dehydration, which indicates the fuel’s moisture level.

These samples are collected at dozens of sites in Colorado, and hundreds across the Western US every two weeks. The compilation of the sites produces quick outlook images such as the one below, showing the moisture levels in the small to medium size branches (the 100-hour fuel as the Forest Service calls them). This image is from early August when Colorado’s steady supply of rainfall had high moisture content in this fuel source, and all other fuels measured quite wet as well.

0bs 100 hour Forest Service Wildfire
I asked the Forest Service how long a period of dry weather it would take to see the fuels trend drier, increasing our fire threat. Their response: only a week!

I had been waiting for a good period of drier days to see the fuel moistures change. This past week, and especially weekend, was just what I needed. Statewide temperatures were near to above average, and rainfall was near to below average. See the comparison in the fuel moistures to two weeks ago.

100-Hour Fuels (the small to medium branch sizes):

wildfire risk across United States comparison
Notice the drying that occured in both fuels, in a time period of two weeks. It makes sense that the smaller fuel dried in a more pronounced way, but even the large fuels showed drying across Colorado.

It is the combination of these fuels that make up our fire threat. These fuel samples help determine staffing for the Forest Service as well. On the day of our visit, several firefighters from that station were already working the fires in the Pacific Northwest where the threat was clearly greater. Yes, the fire threat in Colorado remains low, but that can change more quickly than I had previously thought.

As my colleagues and fellow Weather5280 bloggers have pointed out, the
moisture and temperature outlook will change very little over the next few weeks. However, if continued drying occurs, these fuels will be more prime to ignite. Most of Colorado’s fires are started by lightning, though careless campfires are also a threat.

For the next week, the most likely region for an elevated fire threat (including factors of dry thunderstorms, wind, and heat) will be in Southern Wyoming to Northwestern Colorado.

My thanks to Lawrence Lujan, Jay Karle, and Samuel Urffer with the Forest Service.

The State of the Atmosphere: Sunday, August 17, 2014

Posted by @coloradowx

Looking back at the first half of August, 2014
Now that more than half of the month of August has passed, it is prudent to take a look back on the conditions we experienced during the first half of the month in order to see how the month is shaping up overall and to inform our forecast for the remainder of the month.

On August 2, our
average high started falling once again (from 90 to 89), and on August 9 our average overnight low also started falling once again (from 60 to 59). Our average high/low as of Monday is now 87/58, and that will fall rather quickly to 84/54 by the close of the month.

So far this month, Denver International Airport (DIA) has experienced slightly below normal temperatures (-1.4 ) month-to-date, and has recorded ~0.80” of rain versus full-month average of ~1.7”. (As is typical of hit-or-miss summer thunderstorms, however, some spots across central Colorado have picked up several inches or more of precipitation.)

Sunday marked just the 3rd day this month with 90 -temperatures, and the 26th so far this season. The average number of >90 days Denver sees per year is 40, so it appears likely that we will fall well below this number by the end of the warm season -- perhaps about 30 days total (or 25% fewer >90 days than is typical.)

CPC’s analysis of monthly temperatures shows the much of the country has seen slightly below normal temperatures month-to-date (including Colorado), with California and the Pacific Northwest having seen well above-normal temperatures.

August 2014 temperature anomaly map United States
It would be fair to say that up through press time, most of Denver metro has seen slightly below normal temperatures with near to somewhat above-normal precipitation month-to-date. That said, we've seen several days in a row (including Sunday) that have featured slightly above normal temperatures, and we’ll look to continue that trend over the next 48 hours or so.

Looking Ahead
GFS ensembles show that we’ll experience a modest step down in temperatures through the remainder of the month: above-normal over the next couple days; near-normal for several days thereafter; finally, the probability of somewhat below-normal temperatures to finish up the last week of the month.

GEFS temperature outlook Denver
To add confidence to our forecast, let’s also take a look at what the CFSv2 has for the next 10 days in terms of both temperatures and precipitation. The model shows metro Denver just on the cusp of slightly below-normal temperatures. The southern extent of a push of cooler-than-normal temperatures during the last week of the month is somewhat unclear at this point, so I think the remainder of the month may either end up near-normal or slightly below-normal for temperatures. When we put that all together, I believe the August, as a whole, will wind up just slightly below normal for temperatures -- perhaps no more than -1 for the month.

CFSv2 August temperature outlook
As for precipitation, the CFSv2 suggests that Denver will be very close to near-normal values for the remainder of the month. There are some positive precipitation anomalies (though not a terribly strong signal). I think we’ll end up near- or just slightly below-normal for precipitation through the rest of the month. When we put that all together, I believe August, as a whole, will end up very close to ‘normal’ for monthly precipitation: likely slightly below in some areas and slightly above in others.

CFSv2 Precipitation Outlook
In short, we’ll experience a blend of above-, near-, and below-normal temperatures (likely in that order as the rest of the month proceeds), with a fairly seasonally-normal precipitation pattern. The week ahead should feature daily afternoon storm chances (some greater than others), with an eye on how models want to handle the potential for a cooler and wetter few days by next weekend.

Quick glance at September, October and November
Looking further ahead, I will leave you with a tease of what may be in store for us for September → November (SON) period according to a constructed SST anomaly model run by CPC. We see a zone of below-normal temperatures over the central part of the country, but the western extent is somewhat unclear. Other models show this zone of cool a bit further west to include the eastern half of Colorado.

SST forecast
And here’s what our top analog year -- 2009 -- looked like during the September → November (SON) period. You can see a small area of below normal temperatures over the central and southern Plains, including eastern Colorado. The Weather5280 team will have more in-depth look at how the fall months may shape up for us over the next couple of weeks.

2009 SON Temperature Anomalies

America is Not Underwater, Literally

Posted by @brendansweather

Let’s get a few things straight. Detroit, Baltimore, heck, even the entire northeast is not “America”. Throw in a few isolated locations in Arizona, maybe Seattle... still not America. The United States of America includes all 50 states, yes, even those west of the Mississippi.

Titled “
America Is Under Water, Literally”, today’s post on the Slate weather blog blurs over this reality, and was filled with exaggerated statements. Climate change debate aside (outlier != trend), let’s at least agree on this: America is not under water, literally.

It’s true, it rained a LOT this week in some places. Islip, NY recorded a whopping 13.26 inches of rain in 24 hours as of 9:30 this morning.
Baltimore and Detroit have also seen record rainfall this week. Warm season rains are spotty in coverage -- depending on where training sets up can greatly determine if records are set. Observation stations will often miss on heaviest precipitation, and vise versa.

Rainfall has been less impressive elsewhere. Here are the 24 hour rainfall totals for the U.S. as of this morning. Data is pulled from the NWS Advanced Hydrologic Prediction Service, and visualized by WeatherBell Analytics.

24 rainfall totals United States
No doubt impressive across the mid Atlantic and portions of the northeast. Domain max for the lower 48 during this period was 11.4 inches, with a spatial average of 0.1 inches. The average over the last week for the lower 48? 1.5 inches. Also of note, monsoonal rains in the west continue to dig into precipitation deficits. Take a look at Arizona in the map above, great news as it’s experiencing its 6th driest year-to-date on record according to the NCDC. No doubt welcome news, not panic.

month to day precip NCDC
Here are the rainfall totals as a percent of normal over the last seven days across the United States. While many locations have seen above normal rainfall, literally not everyone has.

United States seven day rainfall totals NWS
Flash flooding events in the are quite typical this time of year, especially when following a period of below normal precipitation in the west. Storms in late July through August tend to move a lot slower than during the late spring and early summer months, and PWAT values are higher too.

To pick out a few localized heavy storms in the northwest (with the majority of several states under a Red Flag Warning), tie that into the event in the east, and conclude there are flooding rains everywhere seems irresponsible. Most of the western United States continues to experience abnormally dry to extreme drought conditions.

Drought US Map

Ironically, the Capital Weather Gang posted this Tweet just as I got done reading about America being underwater. Keep your head above the water California, drought relief will come.