Denver, Colorado

Weekend System Stays Strong, but Favors Northerly Track

Posted by @mattmakens247wx
This weekend's storm system remains impressive and model trends are beginning to become consistent. This consistency will continue to improve now that the center of the system is moving onshore the Pacific Northwest.


There are a few changes since our last post. First, the models are more consistent with a northern track and that results in various changes for our area. All three big global models have similar locations for the center of the system by Sunday. Notice the ECM is strongest, but the position is similar to GFS and GEM.


With the position of the 500mb low looking more consistent, the other pieces to this puzzle are falling into place. Assuming the low doesn't change path or speed too greatly we are looking drier here in northern Colorado than we once did. A path like this will favor very heavy snowfall, over a foot, for the north-central mountains, and rain for the eastern plains, well over 1 inch rainfall possible. However, for the Denver area we are likely to be in the dry slot of the storm. This will keep us with rain/snow above 6500 feet and rain below. Amounts will vary greatly, but 1/4" to 1/2" will be considered "good" for this system in Denver, less than that in the Springs. But in between, the Palmer Ridge has the best chance for moisture of over 1 inch in both rain and snow.

The timing of this moisture will begin Saturday with isolated thunderstorms statewide. Sunday the low moves into its position over northeastern Colorado; the heavy snow will begin in the mountains and a steadier rainfall for the lower elevations. The biggest impact of this system will be on the region, and the severe weather threat it poses on the plains to the east.

The system's speed is slow slow that it will slowly move toward Minneapolis by the end of the week. With that slow motion, Colorado stays on the cold side with areas of rain and snow to continue through Tuesday. Beyond, the system will weaken and is far enough away that we dry off and begin to warm back to average.

Another round of data comes in tonight, we'll be watching carefully for any changes to the track and update as necessary.

Another similar system will develop for early May that will be the next system to keep tabs on.

Rude Awakening to Severe Weather Season?

Posted by @GMoore34
SPC Day 3 Risk (MesoMap)SPC Day 4/Day 5 Risk

This tornado season is off to a record slow start, but an active severe weather pattern is setting up for this weekend. The storm that we've been tracking for the coming weekend which, depending on the eventual track, should bring a return to cooler and wetter weather here on the front range, will also bring an outbreak of severe weather across the central and southern plains.

Saturday afternoon looks to bring the first round of severe weather, including large hail, high winds and isolated tornadoes. The threat will extend along the dryline from Texas clear through to Nebraska, where dew points could climb into the 60s by late afternoon, and at least briefly enhance tornado threat.

GFS 10m Wind 2m Dew

At this time, it appears that Colorado won't be able to advect enough low level moisture back into the state to get in on the action on Saturday afternoon except maybe across the eastern border out into Kansas. The dry line (intersection of moist air and dry air) will shift out into Kansas and become the focus for discrete supercells that will have the potential for large hail and a brief tornado.

On Sunday, more ingredients come together to bring us the potential to see an outbreak of strong tornadoes across central Kansas, Oklahoma, Texas and points eastward.

Surface based CAPE across Kansas, parts of Nebraska and Texas is nearly 3500j/kg! That, combined with dew points near 60-65 degrees will provide the fuel for severe storms to fire along that above mentioned dryline and then moving E/NE.

Each day the threat should shift a bit further east, eventually encompassing parts of the southeast. Keep your weather radios handy and be weather aware Saturday afternoon through Monday afternoon as this widespread event unfolds.


On the back side of this system comes our chance for rain and snow in Colorado. We'll have an update shortly on where things stand there -- still plenty of wobbling in eventual storm track -- and continue to monitor what promises to be a very active few days of weather across the United States this weekend.

Weekend Storm Track a Mess, Better Consensus Elsewhere

Posted by @BrendansWeather
It's been a wild week of 'data watching' with regard to the next big upper level trough due in this weekend. When we last updated, the GFS was extremely bullish, the EURO was faster and more open, and the Canadian was intriguing, but not all that convincing for moisture along the immediate Front Range.

Yesterday's 12z GFS run was 95% drier than the 12z run on Monday. Last night it recovered a bit, tracking the low further south and enhancing upslope for northeast Colorado. This was inline with the EURO as well, which increased confidence in a more southern solution over a northern solution.

This morning things look a bit different again. All three models (EURO/GFS/Canadian) have a different idea of how the upper level low moves across the state this weekend. While there is decent consensus in timing, and all three models show the upper level low closing off, the eventual track continues to be the big question. At the moment, the EURO might be the best compromise between the solutions -- it's deeper and slower than it was Monday and it tracks the low somewhere between the northern GFS track and southern Canadian track.

Take a look at 500mb for Sunday afternoon. The 0z ECMWF (left) and the 06z GFS (right) are actually in pretty good agreement with where the upper level low will be -- over northeast Colorado. The Canadian (below) also has a closed low over eastern Colorado, but is tracking that low much further south.

EURO 0z 500mbGFS 06z 500mb

CMC 500mb

There are still plenty of issues regarding the eventual placement of the surface level low. Like we've seen in the upper level pattern since yesterday, the surface low trend is also south. Should the trend continue, it will likely mean better placement for upslope along the northern Front Range, and less of a concern for downsloping killing our precipitation chances.

Take a look at where the GFS was tracking the surface low yesterday (left) as compared to today's 06z run (right). Yesterday it trended away from previous solutions and took the surface low well north into Nebraska and South Dakota, the 06z run tracked a 986 low straight across Burlington, Colorado.

12z GFS April 2206z GFS April 23

Now we look at this morning's 12z GFS run of the same period. It's trended back north, though not nearly as far north as what it was trying to do yesterday. Note, however, that should this track verify, it'll mean much less precipitation for eastern Colorado (notice no green or blue east of the Front Range) as it favors downsloping winds.

GFS 12z Surface 042314

The differences here are significant. Should the surface low track too far north, we simply won't see a widespread big precipitation event across eastern Colorado. If the EURO and Canadian have a better handle, and we see the GFS come south once more -- then big time precip is once more in play.

If we were to take a compromise between the northern solutions of the GFS/EURO yesterday, and the southern solution which the Canadian has been pretty consistent on, the middle of-the-road low we see on the EURO might be correct. If so, we'll definitely be watching for flooding concerns, especially along the Front Range foothills. All three models bring spotty QPF of over 1 inch to portions of northeast Colorado through Monday, here's what the rather bullish Canadian has in mind. . .

Canadian Total Precip

A lot to watch over the coming days, as it's clear model consensus is not there yet, though it feels like we're getting closer. Things are bound to swing in different directions several times over the next few days, but confidence is pretty good that there will be a large closed low traversing the west this weekend, the question is exactly what track it will end up taking.

More updates to come. . .

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

Posted by @BrendansWeather
With another beautiful spring weekend under our belt, it's time to start looking at what the coming week has in store for us. The coming week looks to start warm and windy across eastern Colorado, but big changes could be on the way by the middle of next weekend, with a chance for isolated storms across extreme eastern Colorado for the middle of the week.

NAM 10m Wind Wed | WeatherBell
Warm and Windy
We've got Tuesday as the pick day of the week temperature-wise this week, though none of the next five days look particularly chilly. The average high this time of year is 63 for Denver, we'll better that each of the next several days, with highs near 80 degrees on Tuesday.

The big downside to the warm temperatures through the middle of the week will be the breezy conditions across the state. I would expect we'll see a Red Flag Warning get hoisted for portions of eastern Colorado by the middle of the week as temperatures will be warm, humidities low, and winds gusty. There is already a Fire Weather Watch in effect for much of southeast Colorado for Wednesday.

Severe Weather Threat
Despite all the mild temperatures, our storm track remains active. We saw some rain this last weekend, and there's a chance we see more by the coming weekend -- even an outside chance we see a good soaking storm.

First we'll be tracking a severe weather threat across Texas, Oklahoma, and Kansas as we head into the Wednesday (day 3) timeframe. As it stands right now, impacts on the severe weather front from this system appear low for Colorado -- but proximity reminds us it's the time of year when severe weather starts to become more common here in Colorado.

The Storm Prediction Center has issued a "slight risk" outlook for the areas outlined below.

SPC Day 3 Risk MesoMap

The biggest threat at this time appears to be for hail and damaging wind. That said, there will very likely be tornadic activity with this severe weather outbreak -- so if you live within the highlighted risk area, or have travel plans that take you across this area Wednesday, please make plans accordingly. We'll continue to update as the week progresses.

Weekend Soaker?
Our focus across northeast Colorado quickly shifts to next weekend after Wednesday. The GFS operational has been remarkably consistent in bringing a big upper level trough across the state this weekend and plentiful moisture. In fact, it's 12z brings more than 1.5" QPF to DEN Sunday through Monday night. This would mean a big time rain event for lower elevations and likely a good late season snow for higher elevations.

Plenty of uncertainty remains with regard to this system. That being said, even the deterministic ECMWF appears to be trying to come around to a bit wetter solution for the state after being largely dry previous runs. The GFS ensemble control run also looks more promising with it's 12z run, though ensemble members have been rather all over the place over recent days. Here's a look at the GEFS ensemble control run at 500 hpa for this coming Sunday -- a mean upper level low tracking across Colorado. Eventual track will be a huge factor with this system.

GEFS Ensemble 500mb

The differences between the GFS and EURO are significant enough that it's worth a good degree of caution before getting too excited about this system. Also let's keep in mind we are still nearly a week out, though this trough has been on the radar for some time.

Here's a look at the 0z EURO (left) and 12z GFS (right) at 500mb for next Sunday. Notice how much deeper and slower the GFS is compared to the EURO.

EURO 06z Mon 500GFS 06z Mon 500

Somewhat surprisingly I might actually be favoring a deeper solution with this system as the GFS is trying to do over the faster more open solution on the EURO. One reason for this is it's clear the EURO has been trying to move toward a deeper solution over the last few runs. Another is that there might be enough blocking out in front of this system to slow it down. Even the Canadian that comes in rather unorganized, but then slows and deepens as it moves across Colorado Sunday -- a solution that would actually favor eastern plains over Front Range communities.

A lot to watch over the coming days. It's fair to say the State of the Atmosphere is active! We'll push updates throughout the week.