Past IEM Features tagged: nws

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Watches

11 May 2017 12:02 AM
NWS Des Moines issued two watches yesterday, a Severe Thunderstorm Watch and a Flash Flood Watch. The eventual storms yesterday did yield a few Severe Thunderstorm Warnings over the watch area and even a few tornadoes further east within Davenport NWS's area, but no Flash Flood Warnings. The featured chart looks at some IEM computed metrics on the conversion of watches into warnings. The red bars represent the overall frequency of converting an individual watch event into a warning (meaning getting at least one warning anywhere within the watch). The blue bars represent the individual county/zone conversion of a watch into a warning. For example, there is a 59% chance of getting at least one warning within the Flash Flood Watch yesterday and 19% frequency of an individual county within the watch getting one warning. The moral of this story is that not all watches are created equal. The actual size of the event the watch is for creates large differences in the frequencies. One item to note is that this plot is for a straight conversion and does not consider the situation of the eventual warning being of a different type. For example, some Winter Storm Watches become Blizzard Warnings.

Voting:
Good: 5
Bad: 0

Tags:   nws   watch  


Tornado Watches into Warnings

30 Mar 2017 05:33 AM
Continuing with the theme of Severe Weather Awareness Week in Iowa this week, yesterday was the statewide tornado drill with a test watch and warning issued for the entire state. Today's featured chart looks at the conversion of counties under a tornado watch into a tornado warning. For example, if a tornado watch was issued for 60 counties and 30 of those counties received a tornado warning, that would count with the 50% bin for the plot. About 20% of all tornado watches receive no subsequent warning. Most watches have less than 30% of the counties included eventually get a warning. Thankfully, tornadoes typically are not widespread and limited to a couple of high end storms during a typical watch. Hopefully that is the case today with a significant tornado threat expected over the southern US.

Voting:
Good: 7
Bad: 0
Abstain: 1

Tags:   nws   spc   tornado   watch  


SPC Watch Issuance Confidence

29 Mar 2017 05:36 AM
Continuing with the theme of this week being Severe Weather Awareness Week in Iowa, today's featured plot looks at an analysis of convective watch issuance verification based on probabilities included in Mesoscale Convective Discussions (MCD) issued by the Storm Prediction Center (SPC). The SPC issues the MCD product to provide a short term update and analysis of the storm scale environment during active weather. Prior to most tornado or severe thunderstorm watch issuance, the SPC typically issues a MCD to highlight an area of interest and provides a confidence percentage on if a new watch may be soon issued. The association of an issued watch and the MCD product is not exact, so a simple algorithm was used with reasonable defaults to provide a verification of these percentages. The algorithm being the new watch needed to be issued with 2.5 hours and at least 50% of the watch polygon had to overlap with the MCD polygon. What is the moral of the story? Well, the high confidence MCDs typically get an issued watch, the lower confidence ones typically do not. The next time you read a MCD product from the SPC, check out the confidence noted and see if a subsequent watch is issued!

Voting:
Good: 6
Bad: 0

Tags:   nws   spc   mcd   watcfh  

NWS Local Storm Reports

28 Feb 2017 05:39 AM
The National Weather Service collects weather reports from their partners and the general public. These reports are then relayed to the world in the form of a text product named Local Storm Reports (LSR)s. The IEM processes and archives these reports to then make them available for download. The featured map displays the number of LSRs by NWS Forecast Office so far this year. Only reports of hail, tornadoes, and severe thunderstorm wind gusts and damage are shown. Significant amounts of severe weather are anticipated today and tomorrow, so areas in the central US that haven't seen much activity yet this year are expected to with this event.

Voting:
Good: 6
Bad: 1

Tags:   nws   lsr  

November Tornado Warnings Again

29 Nov 2016 05:33 AM
An ideal setup for tornadoes in Iowa during November materialized on Monday prompting the NWS Des Moines to issue seven tornado warnings. This makes for the second year in a row that November received the most number of warnings by the office. The featured chart displays the monthly warnings count for NWS Des Moines since 1986. Previous to last year, one has to go back to the famous tornadoes back in 2005 for issued warnings in November.

Voting:
Good: 7
Bad: 0

Tags:   nws  

Unique Alert Counts

10 May 2016 05:35 AM
Severe weather was abundant on Monday with damaging storms reported from Iowa to Texas. The NWS Des Moines issued a number of Tornado and Severe Thunderstorm Warnings along with a Severe Thunderstorm Watch. The featured top chart looks at the unique number of VTEC coded alerts that the office issues each year. The bottom chart explicitly lists out the unique codes used each year. Warning procedures have changed over the years, so some warning types are no longer used. This chart only covers those products that have Valid Time Extent Coding (which is most of the alert types), so the actual number of alert types is higher than shown.

Voting:
Good: 7
Bad: 3
Abstain: 1

Tags:   vtec   nws  

Local Storm Report Sources

22 Apr 2016 05:34 AM
The National Weather Service collects reports of weather from the public and disseminates these reports in the form of 'Local Storm Reports' (LSRs). These LSRs denote a generalized grouping of where the report came from (source). The featured chart looks at the yearly rank of the source code used in LSR reports by NWS Des Moines. The ranking is simply a measure of the number of reports received and then disseminated by that source over the year. The LSR program has changed over the years with some source groups being converted into other headings. The general PUBLIC remains a major and important source of reports for the NWS. So keep letting them know when you witness significant weather.

Voting:
Good: 13
Bad: 4
Abstain: 3

Tags:   nws   lsr  

Most Common NWS Alerts

05 Feb 2016 04:46 AM
After the Blizzard Watch and Warning issued this week by NWS Des Moines, one may wonder how common it is to see this alert issued. The featured chart presents the top 25 most issued alerts by NWS Des Moines as measured by the unique number of forecast zones or counties alerted. For example, a Severe Thunderstorm Warning for portions of Polk and Story counties would count as 2 in this analysis. The percentages shown are relative to the most frequent product, the Severe Thunderstorm Warning. So the Blizzard Warning comes in at #18 and Blizzard Watch at #25, which makes the Severe Thunderstorm Warning approximately 12 and 25 times more common respectively. Please note that the time duration difference in alerts is not considered for this analysis.

Voting:
Good: 47
Bad: 34
Abstain: 35

Tags:   nws   warnings  

2015 Tornado Watches

19 Aug 2015 04:12 AM
A Tornado Watch was issued Tuesday afternoon for portions of eastern Iowa. Thankfully, only a possible brief and weak tornado has been reported so far in Iowa. For some counties in northeastern Iowa, this was the first tornado watch of the season. The featured map displays the number of Tornado Watches issued by county so far this year. There are nine counties shown without a watch this year! The highest totals are found in southeastern Iowa with Scott and Des Moines county at seven. You can generate this map online for other warning types, other states, and even other WFOs in the country.

Voting:
Good: 12
Bad: 5
Abstain: 7

Tags:   warnings   nws  

Watch Duration

22 Jun 2015 05:43 AM
The NWS issued a Tornado Watch for much of southeastern Iowa on Saturday afternoon. Polk County (Des Moines) was initially included in the watch, but was removed after just 1 hour and 49 minutes as the storms fired east of the county. The featured chart looks at the frequency of tornado watch duration for the county. Two lines are plotted, the initial line represents the watch duration as initially issued, the final line presents what the final result for duration was (some watches/warnings can be extended in time / cancelled early). So having a 2 hour tornado watch is about a 1 in 10 frequency based on data back to 2005.

Voting:
Good: 16
Bad: 9
Abstain: 5

Tags:   warnings   nws  


Dense Fog Advisories

24 Oct 2014 05:44 AM
After the rain yesterday, skies cleared off and temperatures cooled creating dense fog this morning. The National Weather Service has much of the state under a Dense Fog Advisory this morning. The featured map displays the yearly average number of Dense Fog Advisories issued by forecast zone. The lines on the map are the edges of NWS forecast office zones. For Iowa, most of the state is in the 7-10 advisories per year range. The highest frequencies are near the Gulf of Mexico as the warm and humid air comes onshore and cools, creating fog. There is an interesting white patch over West Virginia. This is the Charleston, WV forecast office, which according to IEM databases has not issued a Dense Fog Advisory since at least 1 Jan 2006.

Voting:
Good: 28
Bad: 12
Abstain: 9

Tags:   nws   fog  


Watch into Warning

06 Jun 2014 05:42 AM
The Storm Prediction Center issued a tornado watch for portions of northwestern Iowa on Thursday evening. Thankfully the tornado threat did not materialize and no warnings were issued for Iowa after the watch came out. How common is it to have a tornado watch and then not a warning for a given county in the state. The featured map attempts to answer just that question with the percentage of events whereby a county under a tornado watch gets a tornado warning issued. While the percentages may appear low, they do roughly match the spatial confidence of a watch issuance. The overall map average is near 15%. Winneshiek County has the lowest percentage (0%: only 3 tornado warnings since 2005 and all not within a tornado watch) and Pocahontas County has the highest percentage (29%).

Voting:
Good: 47
Bad: 22
Abstain: 15

Tags:   tornado   nws   watch   warnings  


Svr/Tor Warning during Blizzard Warning

22 Feb 2014 09:22 AM
The recent blizzard event on Thursday featured a somewhat rare combination of a forecast zone being under a blizzard warning and also having a severe thunderstorm warning active at the same time! The featured chart presents an IEM computation of the number of times this has happened since late 2005. This combination happens approximately once each year for some lucky location in the country.

Voting:
Good: 31
Bad: 6
Abstain: 3

Tags:   nws   blizzard  


Blizzard Warnings

20 Feb 2014 05:16 AM
Blizzard warnings are active for today as a major winter storm strengthens over the plains. The featured map presents the number of NWS issued Blizzard Warnings since late 2005. You'll want to click the image for a better view. Blizzard conditions are not trivial to produce as the combination of near zero visibility needs to coincide with snow and very strong winds. The map shows the favored area for these conditions over very flat and mostly tree- less Upper Midwest. The gradient is clear in Minnesota as trees slow surface winds down and help keep blowing snow in check.

Voting:
Good: 47
Bad: 6
Abstain: 5

Tags:   nws   blizzard  


Winter Storm Warnings

04 Feb 2014 05:45 AM
The NWS has a Winter Storm Warning in effect for much of southern Iowa from today into Wednesday morning. The featured map presents the total number of Winter Storm Warnings issued by the NWS since late 2005. Northeastern Iowa has the largest totals in the state with the largest total (130) in the country for Flattops forecast zone in Colorado. Winter Storm Warnings are typically issued for significant snowfall accumulations over a twelve or more hour period. Our snowfalls so far this winter have not been excessive, so the number of warnings issued has been lower than recent average.

Voting:
Good: 61
Bad: 16
Abstain: 8

Tags:   nws   winter  


Daily Warning Frequencies

25 Jun 2013 05:13 AM
The featured chart displays two daily frequencies of tornado and severe thunderstorm warnings in the United States. The green bars are the percentage of years since 2002 that at least one tornado or severe thunderstorm warning occurred on. The blue bars are the simple average number of total warnings issued per day. So while the number of warnings peaks in mid June, just about every day during the summer season sees severe weather somewhere in the US. In fact, this chart shows there has not been a day in July without severe weather since 2002. More severe weather is possible today in Iowa along with continued heavy rainfall events.

Voting:
Good: 83
Bad: 17

Tags:   nws   severe  


Generate This Chart on Website
click image for a better view

The Last Tornado Warning

10 May 2013 05:15 AM
The Des Moines NWS Office released a statement yesterday noting that a tornado has not been reported in the state since 24 May 2012. Since 1950, this is the second longest streak on record for Iowa and only a few days away from the longest tornado-less streak back in 1955-1956. The featured map presents the number of days since the last tornado warning by NWS Office. For the Des Moines and Davenport offices, it has been over a year since the last tornado warning. Interestingly, this drought extends southwest to offices like Pleasant Hill (Kansas City) and Topeka, which are certainly in the traditional "Tornado Alley".

Voting:
Good: 172
Bad: 19

Tags:   nws  


Blizzard Warning Climatology

17 Mar 2013 03:55 PM
Blizzard Warnings are in effect over far northern Iowa for Monday thanks to a quick moving storm system that will dump snow and then blow it around a bunch. Are blizzard warnings common as we are mere days from spring? The featured chart attempts to provide a climatology of blizzard warnings in the continental US by week of the year. The unit is the combination of a single NWS weather forecast office issuing a blizzard warning for a given week of the year. For example, the peak value in late December represents, on average, seven NWS offices issuing at least one blizzard warning per year. The chart shows a remarkable drop during the heart of winter in January. The reason for this drop is that blizzard systems are driven by large temperature gradients and amble supplies of moisture, both of which are somewhat limited in January as most the country is at its coldest.

Voting:
Good: 7
Bad: 2

Tags:   blizzard   nws  


100% Chance

19 Feb 2013 05:41 AM
Those of you that fervently monitor the NWS forecast may have noticed something interesting this past Sunday. For some locations in the state, the NWS had a near 100% chance of precipitation forecasted for Thursday. Is it common to see a 100% chance forecast at four days out? The featured chart shows the frequency of this confident forecast based on the afternoon zone forecast for Polk County (Des Moines). Based on data back to 1 Jan 2009, this would appear to be the first time it has happened for the day 4 forecast for Polk County. The chart nicely shows that forecasters get more confident of predicting a 100% chance as the event gets closer. Today's forecast for Thursday remains at 100%!

Voting:
Good: 35
Bad: 2

Tags:   zfp   forecast   nws  


total of storm based warnings shown

Year to date warnings

07 Sep 2012 05:40 AM
Our recent weather has seen a stormy change when compared with the drought filled summer. One reflection of this is shown by the featured chart of accumulated severe thunderstorm and tornado warnings for each of the past five years. The top chart shows the nationwide accumulation while the bottom chart is for warnings issued by the NWS offices with Iowa coverage (includes some warnings slightly outside of Iowa). A logical correlation with drought conditions is the lack of heavy rainfall associated with severe thunderstorms. After being ahead of each year in late May for Iowa (purple line), the lack of many summertime storms now has 2012 as being the least of the past five years in early September.

Voting:
Good: 115
Bad: 17

Tags:   nws   2012  


2011: Plenty of Severe Weather

01 Jan 2012 02:14 PM
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  


Winter Storm Watch

08 Nov 2011 04:11 AM
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  


Tornadoes again

06 Sep 2011 05:55 AM
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  


R-squared for top chart is 0.6 and bottom is 0.4

Right Turners

07 Jul 2011 05:43 AM
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  


values shown are frequency (histogram)

Severe Storm Motions

06 Jul 2011 05:40 AM
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  


7 hit sites: EST, CIN, HNR, MCW, SHL, MIW, SUX

Blizzard Criteria

12 Dec 2010 12:32 PM
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  



Severe totals

21 May 2010 05:09 AM
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  


Warnings by Hour

09 Apr 2010 07:09 AM
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  


TO is Tornado Warning and SV is Severe Thunderstorm Warning

Dew Points and Severe Weather

05 Apr 2010 05:09 AM
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  


unofficial data computed by the IEM

The first and the last

26 Mar 2010 05:10 AM
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  


Unofficial totals computed by the IEM

Slow start for severe weather

24 Mar 2010 05:09 AM
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  


Click map for US view

Last Warning

10 Mar 2010 05:12 AM
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  


Based on warnings issued by Davenport, La Crosse, Sioux Falls, Omaha, and Des Moines not just for Iowa.

One Inch Hail

27 Mar 2009 05:59 AM
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  


8 AM watches/warnings on 1 Oct 2004.

A weather potpourri

01 Oct 2004 08:23 AM
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