The State of the Atmosphere: Sunday, April 24, 2016

After a few warm and relatively dry, albeit windy, days there is another change set to move in. This change follows a pair of the warmest days of the year so far. Both Friday and Saturday hit 77°, which is the warmest temperature recorded so far this year, and the second and third time that temperature has been recorded to date.

This week expect a bit more of an unsettled weather pattern with on-and-off chances for rain and snow statewide.

Setup
For the past few days we've had some warmth, yet a storm system had moved through and sits just to our northeast. This system sent us the wind, clouds, and snow showers in the mountains this weekend.

SSEC Satellite

As this system moves east, watch what happens to the overall pattern over the Western U.S. as additional systems develop and move through (the circles that move over Colorado just left of the center of the screen).

Weather5280 Models

Each of those circles represents the center of a storm system in the middle of the atmosphere and, for us, this will mean periods of wet weather.

Unsettled week
A look at the GFS (American model) will give you a general idea of the week ahead. This is just one model solution, but is consistent with others, and we will release our official snowfall forecast(s) (if needed), as the week goes on.

For Monday into Tuesday expect the best potential for precipitation to occur across the northern portion of Colorado, unless the track of our first system digs a bit further south. The GFS shows the best precipitation Monday evening west and north of Denver:

Weather5280 Models

As this first wave moves through, there doesn't seem to be a lot of cold air in place. The flow remains primarily out of the south and southwest, which tends to promote downsloping for Denver. Here is the 12Z GFS forecast sounding for Denver very early Tuesday morning. Note the very dry low-levels.

12Z GFS Forecast Sounding for Denver, 09Z Tuesday, April 26

So, we'll keep a chance of storms in the forecast for Monday, and rain (even a bit of snow at higher elevations across the metro) for Tuesday and Tuesday night, but for now those chances are generally low for Denver, with better chances north and west.

For Wednesday and Thursday we see those rain and snow chances continue across the state:

Wednesday through Thursday | Weather5280 Models

By the weekend we may even see a better chance for rain and snow across eastern Colorado:

Friday, Saturday, Sunday | Weather5280 Models

Temperatures
Monday will likely be the warmest day of the week, before we start to see a more unsettled pattern take shape. Temperatures should top out around 70°F in Denver Monday afternoon, which is still a few degrees above our average high of 64°F.

12Z GFS 2m Max Temperatures| 00Z April 26

But then our temperatures will cool, and generally stay so through the remainder of the week. This is all due to a series of shortwave troughs that will dig southeast and bring chances for rain and/or snow starting Monday night and into the weekend. You'll see in the animation below our temperatures beginning the week above normal (yellows and reds), but then lots of blue taking over for the remainder of the week, indicating temperatures dropping to near-normal and below normal for the period:

Weather5280 Models

High temperatures on Tuesday and Wednesday will likely be 10-15°F cooler than Monday's highs due to increased cloud cover and precipitation.

Central plains severe weather threat
With the series of shortwave troughs that will be affecting our weather throughout the week, there will be a considerable severe weather threat further east. This is partly due to waves ejecting over the High Plains, deepening, and gaining more spin due to an increase in the column of air over lower elevations as opposed to over the mountains. This is referred to as lee cyclogenesis, which will be a contributing factor to the severe weather threat this week.

These lows act as a source for synoptic-scale ascent, deep-layer shear, as well as bringing steep lapse rates further east from over the higher terrain. The placement of these steep lapse rates over the moist air at the surface at lower elevations to the east, is what contributes to convective instability, a key ingredient for severe weather. This layer of steeper lapse rates within the 700-500 mb layer is called an elevated mixed layer because it forms over elevated terrain.

We can see this moisture is already starting to push north, with dewpoints (green numbers) well into the upper 50s and low 60s extending as far north as Nebraska and Iowa.

Surface plot showing moisture return across the Central Plains, 1943Z April 24

Sufficient moisture combined with favorable dynamics will result in a significant severe weather threat on Tuesday over much of Kansas, Oklahoma, Texas, and Nebraska. Here is a forecast sounding for Hastings, Nebraska at 20Z Tuesday afternoon:

12Z NAM Forecast Sounding for Hastings, NE, 20Z Tuesday April 26

This is a quintessential "loaded gun" sounding with steep lapse rates overtop a moist layer, 3200 J/Kg of mean-layer CAPE (a measure of instability), and wind shear in the lowest 3 km of the atmosphere that is more than sufficient for tornadoes. Very large hail is also possible given the extreme amount of instability, along with the temperature and moisture profiles.

The Day 3 outlook from the SPC shows this storm risk, with the greatest potential extending from Dallas through Oklahoma City into north-central Kansas. The hashed red shows the greatest area of concern for severe weather on Tuesday from the SPC:

If you have travel plans through the plains this week of family that lives in the area, you'll want to stay tuned to the latest forecasts as we head into Monday and Tuesday.

For us, we'll continue to track the unsettled pattern shaping up for the week, and offer updates as needed.

Weather5280

This is a collaborative post from multiple authors at Weather5280.

Denver, Colorado
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