This is something we frequently deal with living near the mountains and having a wide range of elevations to forecast for. We cover from over 14,000′ mountain tops (one which has a paved road all the way to the top and is open everyday of the year when weather permits) to around 3,600 feet along the Arkansas River near the Kansas border. Many times, even during the winter months, one of the big focuses of our forecast work goes into figuring out where the rain/snow line is going to be.

As a starting point, one of the data sets that we use to determine the rain snow line is the temperature on the 700mb temperature pressure surface (I talked some about this on the 29th) because we use it to determine what the temperature will be at ground level. This data set is presented in Celsius (the rest of the meteorological world uses metric too!) and for the Colorado Springs metro area, we very closely watch if we are warmer than or cooler than -5 degrees Celsius at this level. If we’re colder than that, its pretty safe to assume we’ll get snow (whether it sticks or not is an entirely different issue!) and warmer than that we’re likely to have rain.

In the fall/spring when we get rapid deep lift from convection (the same process that produces thunderstorms) this rule gets a little gray. For example, I’ve seen snow come down heavy with really large flakes over downtown Colorado Springs in a strong convective cell when the 700mb temperature is -2 degrees C. This rapid lift to a level higher than 700mb, which is usually around 10,000′, will pull even colder air downward, sometimes producing snow when we would normally expect rain using the 700mb rule of thumb.

At noon Thursday, the 700mb temperature over Colorado Springs is expected to be around -4 degrees Celsius. With an atmosphere unstable enough to support convection tomorrow, we do expect some snowflakes to mix in at times with any showers over Colorado Springs and higher elevations. It isn’t unreasonable to think that the heaviest showers may be all snow.

It’s quite smokey/hazy out there this morning.  Check out this morning’s views from Denver and Briargate respectively.  On a clear day Pikes Peak would be sparkling in the background.  We often show this view from Denver as it shows Pikes Peak behind and just to the right of the Denver skyline.  You can see Pikes Peak from Briargate, but it is clearly quite hazy. 

No temperature inversions (cold air near the surface) today (just look at the temperatures shortly after sunrise), the air is very dry so moisture isn’t to blame either, and this environment does not efficiently trap pollutants.  That leaves us with smoke as the lone cause.  Where exactly the smoke is coming from is the question really.  Smoldering of the fully contained fire near Mueller State Park, smoke from the California wildfires is likely being caught up in the southwesterly flow overhead, and a prescribed burn is underway near Lake George, all are probably contributors…

I’m curious as to the timing of a prescribed burn on a day when the fire danger will be so high.  Maybe there is a good reason for it, possibly a fire won’t burn well enough to clear the vegetation on a cooler, moister day, I don’t know.  We are hoping to talk with officials about this later this morning, and if we do you’ll hear about here on NEWSCHANNEL 13 at noon…

And adding to what Matt said earlier about the big temperature change coming our way, we’re really close with tomorrow’s storm to getting some snow, and it may actually snow for a time in Colorado Springs as the cold air spills over the area.  It’s even possible a little snow will accumulate over the higher terrain in the Pikes Peak Region.  Get ready to remove the hoses and drain the sprinklers again too, behind this storm we expect temperatures to fall below freezing for a couple of nights…

Its about as big a temperature drop from one day to another that we see around here as we go from mid 70s/mid 80s Wednesday to mid 40s/mid 50s on Thursday. We’ve been tracking this thing for more than a week now and you can see the noticeable temperature contrast that FutureCast is portraying Wednesday afternoon. Below, you’re looking at the temperature contours at the 700mb pressure surface (which runs at about 10,000′).

Why do we care what’s happening at 10,000′? We use the temperatures at this level of the atmosphere to determine what we end up with at the surface. In addition, for our varied terrain in the Rocky Mountain region its also useful for tracking the airmasses as the move in and out of here. The mountains can often make the leading edges of new airmasses difficult to determine as it moves through the valleys in pieces. Its very clear in the example above that the dividing line between the warm airmass and the colder airmass lies over the Great Basin by Wednesday afternoon. This dividing line also traces out where the jetstream is, noted in the image by the red/orange arrows. As this gets closer tomorrow afternoon and an elongated area of low pressure develops east of the mountains, we’ll transfer some of this strong windflow from this level down to the surface. Not only will this “mixing” help to warm our temperatures, but we’ll see high fire danger develop in the afternoon.

By Thursday morning the jet will be shifting south of us and we’ll be under the effects of this new, much colder airmass. 30 degrees colder in much cases being a big enough change, we’ll really notice it as wind speeds between 20 and 30 with gusts up to 30mph. A lot like Saturday, it should be a pretty raw day for most of us.

Being that severe weather season is underway (more tornadoes yesterday, this time in Virginia), we will often be exposed through various media outlets to statistics related to severe weather, specifically tornadoes.  These statistics will be used to compare this season with past seasons in order to make some conclusions about the severity of the season at hand.  Sometimes these statistics will be used to support or refute some large scale conclusions related to climate change.  Without getting into this argument directly, there are some things to consider when evaluating statistics, specifically as they are related to tornado reports.

I’ve included below an image from the Storm Prediction Center (SPC) in Norman, Oklahoma.  SPC is the central “data hub” for all reports of severe weather nationwide.  The red triangles denote locations where somebody reported actually seeing a tornado, or damage that was likely due to a tornado.  That said, the locations of the reports may not exactly match the actual location of the tornado that was reported.  For example, if someone sees a tornado that happens to be 10 miles away (quite possible), the report will show up as a triangle on this map, and that triangle will be plotted as the location of the person making the report.  This is done to reduce the errors associated with estimating the location of a tornado in view… 

I’ve chosen May 4th of last year because it was a memorable one and will help me illustrate a point.  This was the day that the town of Greensburg Kansas was essentially destroyed by an EF5 tornado.  Total tornado reports tallied 33 on this day, and through the vast majority of media outlets it was reported that 33 tornadoes had occurred.  This number then went into a seasonal “tally” that may eventually be used to compare the 2007 season to another.  Keep in mind that the total number you see (33) are of the number of “reports” of tornadoes, and this number is not detailed enough to sort out whether the reports received are from several people observing the same tornado, versus individual reports of a number of distinct tornadoes.  Quite frankly, only damage surveys conducted by the National Weather Service can determine how many tornadoes were actually present in an area where a “cluster” of reports are received.  In this case a thorough damage survey was conducted and we have some conclusions.  The line of reports through south central Kansas were of 2 distinct tornadoes that occured with the storm reponsible for destroying the town of Greensburg, Kansas, there were 22 “reports” received that were directly attributable to these 2 tornadoes.  Another tornado was well documented by storm chasers in northern Oklahoma, there were 2 distinct reports received from that event.  Further examinations of the other reports this day suggest it is likely that the cluster of reports in Illinois were of a single tornado, as were the reports from northern Kansas and South Dakota.  There was a single report of a tornado in Colorado. 

The conclusion here is that on a day when 33 “reports” of tornadoes were received, it is likely that there were actually only 7 tornadoes nationwide.  The numbers of reports received on any given day will vary depending on several factors.  Some notable factors to consider include:

- The areas affected.  Higher population centers will generate more reports as more eyes are available to see the tornadoes on the ground.  As our population grows and previously uninhabited areas become developed, it is at least plausible that more “reports” of tornadoes will result, there are suggestions this may be happening in eastern Colorado.  Higher numbers of cameras (video or otherwise) help document more events than in previous years…

- Overall visibility.  Night tornadoes may not be seen by most people.  Some tornadoes are wrapped in a thick curtain of rain and hail, obscuring their view in many instances.  Other tornadoes may have little precipitation associated with them, and in the abscense of trees may be visible for tens of miles.  Yet others may form over heavily forested areas which would limit visbility and probably reduce reports…

The bottom line is that we have heard often already this year that we are in the midst of a record tornado year, and that possibly climate change is to blame.  While I’m not completely ignoring an active start to the season, I don’t believe that comparing the number of “reports” of tornadoes from last year to a running tally of “reports” of tornadoes this year, and then assigning “reports” to actual numbers of tornadoes is an accurate way to compare the 2 seasons. When you hear that we have had 400 plus tornadoes already this year be thankful that the truth is a number far less, I’d hate to consider how much more damage would have occured if in fact that many tonadoes had already reached the ground…

There are several “scales” that we use in meteorology to indicate the length of time and geographical space that a weather event/phenomena occurs in and can be measured on. These range from global ( Polar Jet, Subtropical High - several weeks to several months), synoptic (a mid-latitude cyclone that can last a week or more), meso (hurricanes, squall lines - several hours to a couple days) and micro (individual cumulus clouds, dust devils - several seconds to an hour).

While on the synoptic scale as a cold front approached and the jet lifted off to the northeast, today did not appear to be a huge tornado threat day across the mid-atlantic. There was plenty of fuel for thunderstorms however as warm and moist air ahead of the cold front was lifted by the front itself and an area of lift underneath the exiting jet-streak (an enhanced area of stronger wind within the larger jetstream flow). Straight line wind gusts appeared to be the biggest threat until an area of surface low pressure developed in the mid-afternoon on the meso scale and played a very important part in the tornadic activity that as of this writing has injured around 200 people. This low in southern Virginia acted to back, or turn, the low-level wind enough that several tornadoes were produced in southern Virginia in the areas marked with red triangles on the map.

As of this writing, the report from Suffolk, VA where dozens of people were injured is still listed as a report of Wind Damage. The local National Weather Service office will do a damage survey, probably Tuesday morning, and will determine whether it was a tornado or straight line wind that damaged several buildings and cars in addition to the injuries.

The meso scale is very important when we forecast back home in Colorado. Our terrain will cause small scale areas of low pressure to develop that can help to enhance low-level wind turning, similar to what happened today in Virginia. One of those areas of low pressure is part of the Denver Convergence and Vorticity Zone (DCVZ), a phenomena that develops when there is stable and uniform flow out of the southeast on the eastern plains. It sets up just to the north of the Palmer Divide and the tail end of it can cause thunderstorms to develop and sometimes produce tornadoes in the northern portions of our viewing area. I’ll address the DCVZ in a later post or maybe Marty will post about it Tuesday morning if he doesn’t have another idea already stewing.

“High fire danger” has been a common phrase lately and we’ll be saying it again by the middle of this       week!  Monday will be sunny and about 15F warmer than over the weekend.  Also, very low dew points will produce minimum humidity levels below 15%.  So why hasn’t the National Weather Service issued a Fire Weather Watch or Red Flag Warning?  The answer:  not enough wind to cause critical fire danger.

All of the ingredients need to be there for the NWS Pueblo Office to issue a Fire Weather Watch or Red Flag Warning.  Here’s the necessary criteria for these watches and warnings to be issued in southeastern Colorado:  (1) Fuel characteristics have to be favorable for large fire growth as determined by fire management using the Red Flag Fuels Support Page: click here   (2) Wind:  Frequent wind gusts of 25mph or greater (3) Relative Humidity:  15% or lower

In addition to the basic criteria above, a combination of other elements may result in Red Flag conditions.  Haines Index of 5 or 6, wind shifts associated with cold fronts, microburst winds, first significant lightning (wet or dry, 15% coverage of thunderstorms or more) event after an extended hot and dry period, and poor RH recovery.

It looks like these necessary ingredients could be present on Tuesday and Wednesday!  STORMTRACKER13 will keep you updated on the latest fire danger in your area.

-Stacey Kaiser 

I commented this morning how this Saturday is going to be quite different than the past couple of weekends. We were really spoiled for the most part on weekends this April with plenty of sunshine and above average temperatures. That’s not the case today with below average temperatures for the day and freezing temperatures in store for tonight. Temperatures will generally be in the 20s with temperatures in the teens for the mountain communities.

A viewer called me asking if it is normal to see freezing temperatures this time of the year. The answer is–YES! According to NWS records, we can still see freezing temperatures in June. For Colorado Springs, the latest recorded freeze was June 2, 1919. For Pueblo, the latest recorded freeze was June 3, 1951. The mean dates for Colorado Springs and Pueblo are May 2 and May 4, respectively. Remember, with freeze warnings it is always smart to protect crops and vegeataion and drain exposed water pipes.

-Josh Poland

The next wave in the atmosphere moves through Colorado on Saturday sending another cold front into the area and more wind. It will be east of us on Sunday as a north to south oriented jet-streak, represented by the yellow and orange colors across the Dakotas and Nebraska.

A ridge of high pressure (represented by the bumps in the white lines which are the 500mb pressure surface height contours) centered over eastern Nevada will be moving in from the west. This ridge will move overhead Monday as temperatures continue to warm and the wind will be relatively light. By Tuesday afternoon, the axis of this ridge will move eastward and the wind will start to pick up again as southwesterly flow increases (the green colors over northern California and Oregon) ahead of a large trough that will move in toward the tail end of next week.

As of now it looks like the heart of that trough moves just to our north, with most of the moisture in northern Colorado, Wyoming and Nebraska. If it ends up a little farther south, we’ll have to up our precipitation chances versus what I had in the 7 Day forecast tonight at 10 and temperatures will have to be dropped to. These are the types of things we’re looking at when we say we’re tracking the storm on the newscasts. We don’t have time to go into this much detail, but we really are tracking these things pretty closely. Have a great weekend!

-Matt Meister

Matt posted about a Supercell Thunderstorm that was beating up the countryside in northern Kansas.  Included is a plot of severe weather reports, the vast majority of them were directly related to this storm.  Some real eye openers here with baseball to SOFTBALL sized hailstones and several reports of tornadoes.  The first tornado reports were of brief touchdowns, not much damage.  But the late reports from just north of Beloit (just west of Concordia) were of a larger tornado and were accompanied by reports of damage including one house that was destroyed.  Thankfully the latest reports are that nobody was hurt.  These reports came well after the images that Matt posted last night.  I’ve left the text associated with one of the reports of giant hail..

As is often true in spring, conditions like this often result from a system driving colder air into an area of warm, moist air.  Thursday was no different, and as the colder air settled in a relatively small area of snow showers developed over the Palmer Divide during the night.  No more than a dusting to a half inch on the cars and grass, but interesting nonetheless.  Here is a shot of radar at the time the snow showers were moving over the Palmer Divide, and also an image from this morning of traffic moving along at I-25 and Tomah Road just north of Monument Hill, notice a little snow surrounding the roadway…

It won’t be too much longer before the severe thunderstorms replace the snow around here either, and when that happens you’ll find that STORMTRACKER 13 is your best local resource for weather on television, radio and the web…

- Marty Venticinque

When reports came in around 6pm of a wildfire burning east of Pueblo tonight, concern was high for two reasons. One, it was about 10 miles southwest of Ordway, an area that dealt with a 9000 acre wildfire and loss of several buildings a little more than a week ago. Two, conditions are ripe for rapid fire growth this evening ahead of a cold front that is advancing southward. In the image above, taken at 7pm, you see the position of the cold front advancing southward over the Palmer Divide and a deepening area of low pressure along the Colorado/Kansas border. As the area of low pressure has strengthened this afternoon and early evening, westerly wind ahead of the front has increased, occasionally gusting to 40mph. Behind this front the wind is out of the north and still gusty. As of 8pm, I have word that the fire is pretty much contained, which is good news given the conditions. Notice how dry the atmosphere is ahead of the front with the humidity levels in the single digits and teens. The good news is behind the front as cooler air moves in overnight humidity levels will go up and the strength of wind gusts will decrease somewhat. Fire danger will be much lower through the weekend as another front on Saturday will keep us cool through Sunday.

At the same time tonight in northwestern Kansas along the triple point (where the dryline and warm front intersect) a strong supercell thunderstorm is making a bee line toward Hill City Kansas from the northwest. You see some of the storm attributes that we’ll use when severe storms are in our area, indicating a 43% probability of a tornado on the ground at 8:43pm local time. However, the image on the right shows the velocity (wind field) inside the circle two minutes later. On our velocity colortable, green colors represent wind flow toward the radar (located in Goodland, Kansas - west (left) of where this image is) while red colors indicate flow away from the radar. Strong outflow wind is dominating the storm and thus a tornado likely isn’t on the ground, but this outflow is blowing at over 70mph and power flashes are being reported just north of Hill City. Large hail between golf-ball and tennis balls are also occuring with this storm and it is moving directly toward several other small towns.

-Matt Meister