With the new calendar year, it is probably a good time to reflect back on 2011. The previous year will be remembered as an infamous year for deadly tornadoes in the US. The featured chart presents the yearly total of NWS issued severe thunderstorm and tornado warnings dating back to 1986 along with the total area covered by these warnings. The area value is expressed in normalized units of the size of the continental United States. For example, enough warnings were issued this past year to cover the CONUS seven times over. The total number of warnings came in slightly behind 2008. The large drop in areal coverage of warnings is due to the change to storm based warnings in 2008, which warns for only partial areas of counties.

Voting:
Good = 9
Bad = 2

Tags:   2011   nws   warnings  

A winter storm watch was in effect for portions of Iowa this evening and tomorrow for an approaching winter storm. The watch was upgraded to a winter weather advisory with a few inches of snow expected. The featured map presents the frequency of winter storm watches for the past six years. Most of the state is shown in the 4-8 watches per year.

Voting:
Good = 20
Bad = 2

Tags:   nws  

The remnant of Tropical Storm Lee has been causing all kinds of grief over the southeastern US. A number of tornadoes were reported on Monday along with 99 tornado warnings issued. The featured map presents the number of county based tornado warnings issued so far this year. The largest total is 35 for Madison County, Alabama (Huntsville). More tornadoes are possible today over the Carolinas.

Voting:
Good = 14
Bad = 6

Tags:   nws   warnings  

Following up on yesterday's feature, today's looks at a comparison of tornado warned storm motion versus the severe thunderstorm warnings near in both space in time. Sometimes the sign of a storm that can produce a tornado is when the storm's path deviates from others around it. Typically, this deviation is a slight turn to the right (called right turners). Based on forecasted storm motions, just slightly more than half of the storms exhibit some bit of right turning. Another aspect to "right turners" is a decrease in speed as the storm's circulation strengthens. Again, just over half have forecasted speeds below those of warned severe thunderstorms around it. This is by no means an exhaustive look at this topic, but hopefully an interesting chart nonetheless. The bottom chart does contain an interesting dual maximum (25kts and 40kts) that may be a sign of a separation between isolated supercells (slower speeds) with linear convective complexes (typically faster moving).

Voting:
Good = 12
Bad = 5

Tags:   nws   warnings  

A severe thunderstorm developed over northcentral Iowa Tuesday afternoon and marched mostly south. This storm motion is not what we typically expect in Iowa of west to east moving storms. When the NWS issues severe thunderstorm warnings, they include a storm motion. The featured chart summarizes these storm motions issued by NWS offices with jurisdiction over Iowa. The upper chart presents a comparison between storm direction and date of year. It shows the favored direction of travel from the southwest in June to more west- northwesterly in July. The bottom chart compares storm speed with direction of travel. The fastest storms are clearly from the southwest with the largest concentration around 30 knots from the west. So our storms from the north-northwest on Tuesday are a bit rare, but not nearly unheard of like storms travelling from the east to west!

Voting:
Good = 16
Bad = 2

Tags:   nws   warnings  

With the entire state under a blizzard warning during portions of Saturday and Sunday, one may wonder if the entire state experienced blizzard conditions. The term blizzard is typically applied to a period of at least three hours where visibilities are less than 1/4 mile due to snow or blowing snow along with wind speeds at or above 35 mph. People typically do not stand outside during strong winter storms, so it is hard for them to meet the three hour requirement and only report an instantaneous blizzard. Automated weather equipment struggles at low visibilities... The featured chart shows a comparison between reported three hour visibilities and wind speeds. Both plots contain two sets of data. The 'best wind' dataset includes the variable combination when winds were the strongest. The 'best vis' contains the observations when visibilities were the lowest. The x-axis is shown in log format, so to visually see smaller values. The left hand chart uses max/min values over the three hours, while the right hand chart uses simple averages (helping out the wind criteria). Even using the more lenient method, only ~20% of the sites in Iowa hit blizzard criteria in this qualitative form. For those that were outside, they probably considered the weather blizzard like.

Update: NWS Directive has the blizzard warning criteria as "Sustained wind or frequent gusts greater than or equal to 35mph accompanied by falling and/or blowing snow, frequently reducing visibility to less than 1/4 mile for three hours or more." (but no definition of what "frequently" means)

Voting:
Good = 39
Bad = 10

Tags:   winter1011   blizzard   nws  

The featured chart displays the total number of tornado and severe thunderstorm warnings issued in Iowa up until 21 May since 1986 as computed by the IEM. While 2010 has seemed quiet, our pace is ahead of the past two years. There has certainly been a lack of tornadoes with the red bar showing one of the lowest totals in the past 10 years. The Storm Prediction Center has Iowa in a slight risk for severe weather on Sunday.

Voting:
Good = 48
Bad = 11

Tags:   nws  

The featured image displays a climatology of NWS issued tornado and severe thunderstorm warnings by month and by hour. In general, these warnings are issued during the spring and summer months in the late afternoon into the evening. The month of June sticks out as the most active month and the winter months the least.

Voting:
Good = 46
Bad = 12

Tags:   climate   nws  

The featured chart presents a quick analysis of near storm environment for the archive of NWS issued Tornado and Severe Thunderstorm warnings back to 1986 by month. The box and whisker plots help to visualize the range of values common during the issuance of the warning. In general, dew points are a bit higher for tornado warnings, which is expected as higher dew points indicate more energetic environments and perhaps higher relative humidities (lower cloud heights). For April, once the dew points reach the 50s severe weather appears to be more possible. Some severe weather occurred on Sunday over far southeast Iowa, where surface moisture was available (dew points much higher than the rest of the state). Higher dew points are expected to make their way north today with a warm front and our chances of severe weather will as well.

Voting:
Good = 18
Bad = 12

Tags:   climate   nws   dewpoint  

On Wednesday, a chart featured the slow start to the severe weather season in Iowa. Today's featured chart displays the period between the first and last Severe T'storm and Tornado Warning issued in Iowa. The year of 2008 really sticks out with period shown being nearly the entire year. The latest first warning was on 15 April of 2003, so we should see our first warning this year within the next few weeks.

Voting:
Good = 130
Bad = 22

Tags:   nws  

The featured chart displays the total number of tornado and severe thunderstorm warnings issued by the NWS for counties in Iowa. The top half of the graph shows the yearly totals up until 23 March and the bottom half displays the total for the year. 2010 has yet to see a warning issued in the state, but as other previous years show, a slow start does not mean the severe weather season will be a quiet one. The largest total on the chart for 2008 was after only having 4 warnings issued for the first part of the year.

Voting:
Good = 27
Bad = 4

Tags:   nws  

For most of the state, it has been a while since either a Tornado or Severe Thunderstorm Warning has been issued as is shown by the featured map. Please note that the map displays the warnings by county or forecast zone for simplicity. Generally, it has been over 6 months since Iowa has seen severe weather. March is the beginning of Iowa's severe weather season and strong storms are expected to stay just south of the state today.

Voting:
Good = 33
Bad = 5

Tags:   nws  

On April 1st, National Weather Service Offices in the Central Region will be changing its severe weather warning criteria for hail size to one inch and larger (previously was penny sized 0.75). Some details behind the reason for this change can be found with this NWS webinar. The featured pie charts attempt to look forward to one probable outcome of this change, a decrease in the number of warnings. Of the 2200 warnings issued in the past two years, 24% of them were verified by only hail stones that would no longer justify a warning. The lower pie chart details the initial justification for issuing the warning based on product text. Please note the warning may have included a justifcation of damaging wind as well as hail, but this analysis only worries about what hail size was included. Half of the warnings issued mentioned hail sizes that will no longer be severe, again there may have been damaging winds mentioned in those as well.

Voting:
Good = 36
Bad = 18

Tags:   hail   nws  

Looking at the current map of watches and warmings we see that we have got about everything form of weather shown. A flood warming (in blue), a thunderstorm warming (in red), a wind advisory (in brown), and a frost advisory (in green) are all working to make Friday a wonderful day. You may want to tie down those gnomes in the backyard before the wind carries them away or that angry neighbour takes the opportunity to carry them away and blame the wind!

Tags:   nws