March 2, 2012 Severe Weather Outbreak

Posted on March 2, 2012 by dbikos

The severe weather event of March 2, 2012 was forecast well ahead of time by the Storm Prediction Center to be associated with significant severe weather:

An animation of GOES-13 visible imagery can be found here:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=dev/lindsey/loops/2mar12_vis&image_width=1020&image_height=720

Sufficient clearing took place over a large portion of the High and Moderate risk areas during the day, allowing for heating to take place to increase CAPE values.

Numerous supercell thunderstorms developed across this region which resulted in many severe weather reports.

The early imagery between approximately 1500-1700 UTC depicts the storms that led to tornadoes in northern Alabama.

Further north in the warm sector, skies were primarily partly cloudy  in Kentucky, central and western Tennessee, southern Indiana, and southern Illinois.  This allowed insolation to occur and allow for the buildup of CAPE.

Convective initiation occurred in Missouri and Illinois in the early portion of this loop.  Some thunderstorms developed along the cold front (further west), while other storms developed further to the east along a pre-frontal boundary.  The southernmost of these storms developed in southern Illinois and and soon after initiation moved more to the right (eastward) compared to the other storms.  This can be indirectly observed by the orientation of the anvil cirrus, which is more east-west oriented compared to other storms further west which have a northeast-southwest oriented anvil cirrus.  This right moving storm went on to produce numerous tornado reports with significant damage in southern Indiana, northern Kentucky and southern Ohio.  This is a classic example of a long-track tornado.  Although there may have been broken segments in the tornado damage path (where the tornado may have lifted), the path of the supercell is readily detected.

The storms that developed in western Tennessee by 2030 UTC also appear to have initiated along a pre-frontal boundary, just east of the cold front.  Between 2030 and 2125 a left-moving storm is observed in central Tennessee (note the storm with the northward storm motion).  An east-northeast to west-northwest oriented outflow boundary is produced by this storm, which appears to have interacted with the pre-frontal storm mentioned earlier in the vicinity of Nashville.  These outflow boundaries can intensify other storms via enhanced horizontal vorticity that becomes tilted by the storm’s updraft into vertical vorticity, so long as the magnitude of the cold pool isn’t so great that it stabilizes the air mass the storm is ingesting.

There are numerous other boundaries that we see in the visible later in the loop in Mississippi and Alabama.  Some of these are obscured by cirrus from the sub-tropical jet moving into the area by the end of the loop.

Posted in Severe Weather, Tornadoes | Comments Off

Puget Sound Convergence Zone in Action

Posted on January 17, 2012 by admin

By J.Braun

An impressive overrunning snow event is on tap for much of western Washington Tuesday, January 17th with bands of snow showers moving inland across northwestern Washington.  The (mainly) snow shower activity has increased in coverage as mid level a disturbance rotates around a weak low west of Cape Flattery.  In addition to this, more organized (and heavier) shower bands are merging with a Puget Sound Convergence Zone (PSCZ) and are bringing moderate to heavy snow to the region around an Everett to Port Angles line.  The PSCZ set up is characterized by flow around the Olympic Mountains (red arrows in image above) through the Strait of Juan de Fuca on the north and the Chehalis Gap on the south.

Associated with the  with the mechanism  in place forming the PSCZ is a snow/rain shadow downwind of the Olympic Mountains (dotted yellow arrow pointing to orange oval, south of the convergence zone).  This area typically gets only a third to half as much precipitation as the rest of the local region.

Forecasts issued Tuesday morning called for about 5 to 10 inches of snow in the Seattle metropolitan area through Wednesday…with 1 -3 inches today and 3 – 7 inches tomorrow. Mostly like the kids are home from school for at least a couple of days.

Our VISIT Orographic Effects Session has more details on this as well as other convergence zones around the country:  http://rammb.cira.colostate.edu/training/visit/training_sessions/satellite_interpretation_of_orographic_clouds/

Posted in Coastal Effects, Orographic Effects, Winter Weather | Comments Off

Snow and Cloud discrimination with GOES-R Proving Ground Product

Posted on November 1, 2011 by dbikos

The GOES-R Proving Ground serves to demonstrate products that will be available on future satellites that are part of the GOES-R series.  One of the Proving Ground products developed at CIRA is the GOES Snow / Cloud discriminator.

The utility of this product can be shown with the snow event that affected Colorado on October 26, 2011.  The GOES visible imagery the following day:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/27oct11_vis&image_width=962&image_height=911

depicts a field of clouds moving over snow covered land in portions of Colorado.  Due to snow and clouds having similar colors, it can be difficult to discriminate between snow on the ground and clouds, particularly for a still image.

Now let’s look at a animation of the GOES Snow / Cloud discriminator product for the same time period:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/27oct11_snow-cloud&image_width=962&image_height=911

In this product, snow cover on the ground appears red, low-level clouds (i.e., stratus) appear off-white, and high-level clouds (i.e., cirrus) appear bright magenta.  Note that this is a daytime only product.

A loop of other channels and proving ground products may be displayed at the same time for comparison purposes:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/27oct11_loop&image_width=962&image_height=911

Feel free to stop the loop and move through the slides using the arrow keys or the < and > buttons at the top.

Slide 1:  GeoColor Imagery

Slide 2: GOES 10.7 um (IR)  imagery

Slide 3:  GOES Low Cloud / Fog imagery

Slide 4:  GOES Snow / Cloud discriminator imagery

Slide 5: MODIS visible imagery

Posted in GOES-R Proving Ground, Miscellaneous | Comments Off

The pre-storm environment for the 27 April 2011 tornado outbreak

Posted on May 2, 2011 by dbikos

Knowledge of various air masses and low-level convergence boundaries in the pre-storm environment is critical for a severe weather event.  This blog post will examine the various air masses and boundaries associated with the 27 April 2011 tornado outbreak in the southeast.

First, we will examine the GOES 10.7 um infrared (IR) imagery during the overnight to early morning hours (0545 – 1315 UTC):

http://rammb.cira.colostate.edu/visit/web/27april11/loop_ir_early.asp

An MCS tracked across Mississippi and Alabama during the overnight hours.  The coldest cloud tops tracked from central Mississippi northeastward through central Alabama and into Georgia.  Usually the outflow boundary associated with an MCS will exist just south of the region of coldest cloud tops as this corresponds to the southern edge of the thunderstorm complex.  Near the end of the loop, we see another thunderstorm complex developing in southeast Arkansas moving into northwest Mississippi.

We’ll pick up where the IR loop left off with the higher resolution GOES visible imagery (1332 – 1725 UTC):

http://rammb.cira.colostate.edu/visit/web/27april11/loop_vis_early.asp

Focus on central Alabama early in the loop, there is a distinct line between clear skies and low level cloud steets moving northward.  As we get to the later segment of the loop, you can see north-south oriented lines within this region of low-level cloud streets.  Some of these features are low-level convergence boundaries, others are gravity waves associated with a strong upper level jet that moves over the region.  We’ll discuss this more later.  The convection we observed in the earlier IR loop in northwest Mississippi is moving northeast with a distinct outflow boundary being left behind on its southern flank:

Let’s examine the air mass characteristics via this visible loop with METARs:

http://rammb.cira.colostate.edu/visit/web/27april11/loop_vis_obs.asp

Behind the outflow boundary in northern Mississippi and Alabama, we see a much more stable air mass.  However, as we move forward in time, this outflow boundary is moving north as a warm front, note the strong southerly winds to the south of the boundary.  Note the observation of 67/64 with north winds on the 1825 UTC visible image in north central Alabama, by the 1932 UTC image, this site is now 75/72 with south winds.  A similar trend is seen in northeast Mississippi.  Look further south in central Alabama, the whole region has destabilized considerably and we can see numerous north-south oriented lines which we will examine next.

This is the GOES visible imagery from 1545 – 2132 UTC:

http://rammb.cira.colostate.edu/visit/web/27april11/loop_vis_late.asp

South of the MCS outflow boundary mentioned earlier, we see numerous north-south oriented lines in Mississippi and Alabama.  Some of these are gravity waves associated with an upper level jet that are moving over the region, others are low-level convergence boundaries, some of which may be associated with the overnight MCS activity.  One way to discriminate between gravity waves and low-level convergence boundaries is that convection may develop along the convergence lines while convection does not develop along the gravity waves since they exist at a higher level.  Careful insepction of looping imagery can be compared with these labeled images to help identify the various low-level convergence boundaries:

Labeled visible image at 1825 UTC:

Labeled visible image at 1902 UTC:

Note the low-level convergence lines across central Alabama.  Careful inspection of the looping imagery allows one to detect these features after the gravity wave passes to the east.  The 0.5 degree tilt base reflectivity loop from the Birmingham WSR-88D also shows both of these boundaries in the vicinity of Tuscaloosa and Birmingham:

http://rammb.cira.colostate.edu/visit/web/27april11/loop_radar.asp

The boundaries are easier to see in the early portion of the loop but they become more subtle in the radar loop as times goes forward, and in the visible loop are obscured by anvil cirrus.  Their location and orientation to storm motion would strongly indicate that they played a role later in the afternoon.

This case illustrates the importance of identification of low-level convergence boundaries.  Some of these boundaries were relatively easy to identify, others were much more subtle.  Be sure to use multiple sources of observational data when identifying boundaries, particularly when they are more subtle.

Posted in Convection, Severe Weather, Tornadoes | 4 Comments

Synthetic IR imagery for 19 April 2011

Posted on April 21, 2011 by dbikos

This blog entry will consider the synthetic IR imagery from the NSSL 4-km WRF-ARW model for 19 April 2011.  There were many severe weather reports on this day:

http://www.spc.noaa.gov/climo/reports/110419_rpts.html

The synthetic IR imagery from the WRF-ARW model from 1200 to 2300 UTC (from the 0000 UTC 19 April 2011 model run) is given here:

http://rammb.cira.colostate.edu/visit/web/19april11/loop_synthetic_ir.asp

The synoptic scale feature of interest is a trough that is moving eastward across the Plains.  A surface low (not shown) is forecast to move across Missouri with an attendant warm front towards the northeast and a trailing cold front to the southwest.  There are indications of the cold front in the synthetic imagery, note the arc of colder brightness temperatures extending southwest from Missouri to north Texas then bending northwest across the Texas panhandle by later in the day.  A dryline is forecast to move eastward across Texas, it intersects the cold front near the Oklahoma border by late afternoon.  Model brightness temperatures are warmer on the dry side of the dryline during the afternoon.  The model has partial clearing in the warm sector ahead of the surface low in eastern Missouri and portions of Illinois.  Initial afternoon thunderstorm development is forecast in this area.  Shortly thereafter, thunderstorm development is forecast by 2200 UTC in southeast Oklahoma and northeast Texas.  By 2300 UTC, thunderstorms are forecast in between these two areas from northwest Arkansas into southwest and central Missouri.

Let’s look at what happened by examining the GOES IR imagery during the same time period:

http://rammb.cira.colostate.edu/visit/web/19april11/loop_goes.asp

After the departure of a morning MCS, clearing took place in eastern Missouri and the southern half of Illinois which allowed destabilization to occur south of a warm front and east of the surface low.  The cold front and dryline in Oklahoma and Texas do show up during the mid-afternoon hours but the lack of contrast by later in the day makes them more subtle than the synthetic imagery.   Toggle on the surface winds (from the RTMA) so you can confirm where the various low-level boundaries are located.  The initial afternoon storms developed in southeast Oklahoma along the intersection of the cold front and dryline with thunderstorm development in eastern Missouri soon thereafter.  Later, convection develops in between these two regions in northwest Arkansas / southwest Missouri.

The synthetic imagery from the WRF-ARW model depicts a similar evolution to thunderstorm development compared to what actually happened.  Monitoring cloud cover trends and identifying the various low-level boundaries throughout the day can assess how much confidence one should have in the model forecast.  Be aware that at times, various low-level boundaries may show up differently in the synthetic imagery compared to GOES imagery.

For more information on severe weather applications of the synthetic imagery from the NSSL 4-km WRF-ARW model, you may take this VISIT training session:

http://rammb.cira.colostate.edu/training/visit/training_sessions/synthetic_imagery_in_forecasting_severe_weather/

Posted in Convection, Destructive Winds, GOES, GOES R, GOES-R Proving Ground, Hail, Satellites, Severe Weather, Tornadoes | Comments Off