Wednesday, December 18, 2013

Snow to Water Ratios, or How Much Water is in that Snow

If you are a CoCoRaHS observer in a part of the country that receives snow, then you no doubt have reviewed the snow measurement training  that's available on the CoCoRaHS web site, as well as the snow measurement animations on YouTube. You have reviewed these, right?

Taking a snow core
for a water equivalent measurement.
Snow measurement involves several more steps than just measuring what's in your gauge. One of the measurements we ask observers to make is the liquid water equivalent of the new snow measured from a snow core taken from the snow board or other flat surface. Some may ask "Why bother?  Doesn't one inch of water equal ten inches of snow?"   No, not really.

The adage "one inch of rain equals ten inches of snow" is a myth. It happens, of course, but only under certain circumstances. Observers should never "measure" snowfall by using the 10:1 ratio to convert the water in the gauge to snow depth. That's why we ask observers to measure the depth of new snow, and to measure the liquid water equivalent of that snow.

A stellar dendrite snow crystal
So what's with the 10:1 rule? It was first provided to weather observers by the U.S. Weather Bureau in 1875, although it later was qualified to be a rough approximation. It tends to occur or be close to 10:1 when the surface temperature is around 30°F during the snow.  However, the snow to water ratio, or snow density, is dependent on many complex factors. Among these are are the temperature and water vapor within the dendritic growth zone in the clouds (where the temperature is -12°C to -18°C), the depth of the dendritic zone. Dendrites are beautiful, ornate snow crystals that most people are familiar with. Additional factors affecting snow density include the shape of the snow crystals, vertical motion in the clouds, the amount of liquid water in the cloud, and the thermal profile of the layers from cloud level to the surface. Once the snow starts accumulating on the ground, compaction due to the weight of the snow begins to take effect. Wind will also affect the density of the snow. Strong winds break up the snow flakes/crystals into smaller pieces that allow the snow to compact further. Climatologically, the 10:1 ratio tends to occur only over a relatively limited area. A climatology of snow to water ratios was published in 2005 and showed that on average, a ratio of 13:1 was more typical.

30-year climatology of snow to water ratios.
Credit:  A Climatology of Snow-to-Liquid Ratio for the Contiguous United States Martin A. Baxter, Charles E. Graves, and James T. Moore, St. Louis University, October 2005. AMS Journal of Weather and Forecasting

Snow falling on Saturday
Snow to water ratios can change during a storm, and that can be a challenge for forecasters when trying to determine how much snow will result from the amount of liquid water expected. A case in point is the storm that hit my area this past weekend.  The snow started here on Friday afternoon. By Saturday morning at observation time (it was still snowing) there were 6.8 inches of snow that melted down to 0.67 inch of water. The snow to water ratio was 10.1:1. There was little wind during the storm and the snow came straight down, frosting trees, fences, and any horizontal surface, turning everything into a winter wonderland. It continued snowing until about noon Saturday at which point there was another 1.7 inches of snow. However, the lower levels of the atmosphere had warmed some so the snow was denser. That snow melted down to 0.28 inch of water for a snow to water ratio of 6.1:1. That's quite a difference.

Another 0.8 inch of snow on Sunday night melted to 0.06 inch water for a snow to water ratio of 13.3:1.  So over a period of a little over 48 hours I measured 9.3 inches and snow to water ratios ranging from 6:1 to 13:1.

In general, the colder it is, the less dense the snow will be. Snow falling at 20°F will tend to be dry and "fluffy" while snow at 32°F will tend to be wetter and denser, the good-packing snowball-making, snowman-making kind of snow.

Tuesday, December 3, 2013

Brrr-ace Yourself!!

We've had some cold weather in the last two weeks, but nothing like what will occur over the next several days. In the upper atmosphere a broad trough low pressure extends across the continental U.S.  and over the next several days that trough will deepen as a sharp ridge develops just off the west coast.

500 millibar map (~18,000 ft.) for 6:00 a.m.Tuesday, December 3

That will clear the way for cold air from the Arctic to plunge southward well into the U.S.  Snow cover is fairly extensive across Canada, but the southern extent of the snow is a little north of normal in the central and northern Plains. That will change over the next few days.

Snow on the ground for December 3, 2013.
While the upper air system gets its act together, a complex low pressure system will be organizing over the central Rockies, producing precipitation eastward along and behind the leading edge of the cold air. Winter Storm Warnings are in effect from western Nevada through the central Colorado Rockies, and from the eastern two thirds of Montana across the Dakotas and Minnesota to northwestern Wisconsin.

Watches, warnings and advisories in effect as of 7:40 p.m. CST.
Click this link for the latest version.

Four to eight inches of snow is likely from southern Utah to western Lake Superior, with amounts possibly reaching 12 inches or more in the Colorado Rockies and along the west side of Lake Superior.  On Thursday and Friday a mix of wintry precipitation including freezing rain is possible from southern Oklahoma through Arkansas into western Kentucky and Tennessee.

By Saturday the leading edge of this cold air mass will push off the east and Gulf coasts, but another reinforcing surge of cold air will push through the the central U.S. early next week as  the closed upper low settles in over the Great Lakes.

Forecast 500 millibar map for Tuesday, December 10.
Maximum temperatures are expected to be below zero in the northern Plains northern Great Lakes by Saturday. It will warm slightly and then the reinforcing surge of cold air will bring maximum temperatures back down by Tuesday.

Forecast maximu, temperatures for Saturday, December 7 (left) and Tuesday, December 10 (right).
On Saturday minimum temperatures are expected to be well below zero in the northern Plains and Rockies, and below freezing from the Appalachians to the Sierra Nevadas

Forecast minimum temperatures for Saturday, December 7.

The outbreaks of very cold air over this much of the country in early December do happen, but not very often. Severe cold air outbreaks have occurred in December 1977, 1983, 1989, and 2000. December 1983 still ranks as the coldest December on record for the Midwest, with December 2000 the second coldest and December 1989 the third. The unusually cold weather the first half of December 1977 was followed by a very mild period, and December 1977 ranks as the 27th coldest. However, just after Christmas that year it turned cold again, and the winter of 1977-1978 ended up going into the record books as one of the most severe winters on record for the eastern two-thirds of the country.

So, button up and batten down. Be sure to keep abreast of the latest weather developments at your local NWS office web site. It looks like winter weather will be holding on for much of the country, especially the central U.S., through the middle of the month.

Temperture outlook for the period December 11-17, 2013.
Credit: NOAA/NWS Climate Prediction Center

CoCoRaHS observers, it's time to review snow measurement techniques!  See the training animations at the CoCoRaHS YouTube channel. They are a great refresher and it will take only around 20 minutes to view them all.

Monday, December 2, 2013

Need Some CoCoRaHS Holiday Gift Ideas?

Snow stick
The holiday season is upon us, and if your searching for gift ideas for the CoCoRaHS observer in your life (or something to put on your own list) here are some suggestions. Some of these are of more interest to those in CoCoRaHS, but others will appeal to any weather enthusiast.

Now that winter is here, any CoCoRaHS observer that has to deal with snow will appreciate an extra outer cylinder. When it's snowing they just have to swap out cylinders and bring in the full cylinder to melt the contents. No waiting for the snow to stop if it's snowing at observation time. Even those who never see a snowflake during the year will appreciate the convenience of an extra outer cylinder when it's raining at observation time.

Staying with the snow theme, an aluminum snow stick will make the job of measuring snow depth a little easier. The snow stick is a heavy duty 30-inch aluminum ruler that is graduated in tenths of an inch.

A fan of snow or hail will appreciate a CoCoRaHS precipitation series t-shirt. They feature the symbol for the precipitation, a description, and the CoCoRaHS logo. They are available in adult sizes from small to XXL.

CoCoRaHS t-shirt, caps, and sweatshirts are also available.  There is also an "observer" polo shirt with the CoCoRaHS logo with the word "Observer" underneath. It's available in royal blue only.

The gauge parts and accessories and CoCoRaHS clothing can be found at

Fans of snow or photography will appreciate two books about snowflakes by Kenneth Libbrecht.  The Snowflake - Winter's Secret Beauty and The Art of the Snowflake: A Photographic Album feature breathtaking microphotography of snowflakes. The first book describes the many different types of snowflakes and other information about snow. The second book is more of a chronicle of snowflake photography from 1890 to the innovations in digital photography.

I'm a big fan of non-fiction books about significant weather events. Here are a few of my favorites. If you have any books to add to this list please feel free to mention them in the comments.

Isaac's Storm by Erik Larson.  This is the story of Isaac Cline, who was the chief meteorologist at the Galveston, TX office of the U.S. Weather Bureau during the Galveston Hurricane of 1900.

The Worst Hard Time: The Untold Story of Those Who Survived the Great American Dust Bowl by Timothy Egan. This book tells the story of those who suffered through the Dust Bowl in the Great Plains. It was the inspiration behind Ken Burns' dopcumentary The Dust Bowl.

The Children's Blizzard by David Laskin. This is a story of a Plains blizzard on January 12, 1888 that left 500 people dead on the prairie, many of them children,

White Hurricane - A Great Lakes November Gale and America's Deadliest maritime Disaster by David G. Brown.  This book is an account of the fierce Great Lakes storm in November 1913.  Here's my post about this storm.

 The Long Winter by Laura Ingalls Wilder. The Ingalls family deals with the brutal winter of 1880-1881 in the Dakota Territory.

Last but certainly not least, a great gift is a SAME encoded weather radio. Every home is required to have smoke detectors - every home should have a weather radio!  There are a number of models with a variety of features available.  You can learn more about NOAA Weather Radio and what's available in this post from last April.

Tuesday, November 26, 2013

Eastern Storm Messes with Holiday Travel

Surface map at 10:00 p.m. EST November 26
A low pressure system winding up in the southeast tonight has already caused problems for holiday
travel with heavy rain, freezing rain, and expected snow.

This surface low low is associated with the cutoff upper low that has been slowly crossing the southern U.S. this week. The cutoff low is in the process of phasing with the trough moving through the Great Lakes and it will be one large trough over the eastern U.S. on Wednesday.

500 millibar map analysis for 7:00 p.m. EST November 26

Rainfall totals in the Gulf Coast states exceeded 4 inches in Louisiana and Mississippi. Further to the northeast in the colder air up to a quarter inch of freezing glazed portions of North Carolina, northern Georgia, and Virginia.  Two to three inches of additional rain fell today in the mid-Atlantic states.

24 hour precipitation ending at 7:00 a.m. EST November 26
The surface low will move northeast along the coast on Wednesday, and is expected to be centered off the central New Jersey coast by early Wednesday afternoon.

Forecast surface map for 1:00 p.m. EST Wednesday, November 27.
A large band of snow and some freezing rain is expected west of the low track extending from the southern Appalachians to northern New England

Probability for the 24-hour period ending at 7:00 p.m. Wednesday, November 27

By Thanksgiving morning the low will be in eastern Quebec with light snow and flurries lingering over northern New England and lake effect snow showers from Michigan east through New York.

Monday, November 25, 2013

Big Chill Spreads Across Lower 48 States

It has been cold the past few days, but it was last week when the stage was set for an early appearance of winter weather across the country.  A broad upper level trough was located over the western U.S., and a surge of Arctic air was moving into the northern Rockies. On Thursday the trough became stronger, and by Friday a strong upper level low became disconnected from the westerlies (a "cutoff" low) while the northern portion of the trough continued east, driving cold air as far south as Texas and as far east as the Appalachians.

500 millibar analysis for last Wednesday though Friday
Surface weather map for Friday, November 22.
The surface low associated with the cutoff along with strong high pressure dropping south from Canada was responsible for high winds and heavy snow in the western U.S. from the coast to the Rockies and from Arizona to Idaho. Wind gusts reached 80 mph and resulted in power outages throughout the region.

A reinforcing surge of Arctic air pushed through the central U.S. on Saturday, and by Sunday morning the leading edge of the cold air mass had pushed well into the Gulf of Mexico and halfway through Florida.

Surface weather map for Sunday, November 24.
Meanwhile, the upper low continued to spin over the southwestern U.S. producing snow across the Four Corners region and in Texas. Freezing rain developed from New Mexico eastward across Oklahoma turning roads into skating rinks.

Snowfall for the 48 hours ending at 7:00 a.m. local time Monday, November 25.
In the eastern U.S. cold air flowing across the relatively warm waters of the Great Lakes produced snow from Ohio through New York.

As of this morning snow cover was extensive across Canada and the central Rockies.

How cold has it been? In the central two-thirds of the country temperatures the last week have been well below normal. Over the weekend minimum temperatures dropped into the single digits and teens as far south as the Ohio River and below freezing from central Texas eastward through central Mississippi and Alabama.

Minimum temperatures for 7:00 a.m. EST Sunday, November 24.

Thursday, November 21, 2013

Summary of Sunday's Tornado Outbreak

We will never know how many lives were saved or injuries avoided because of the timely and accurate warnings issued during Sunday's severe weather outbreak. It's easier to speculate on what could have been had there not been enough advance for people to take cover, and that picture isn't pretty.

The science of meteorology and the art and science of forecasting have come a long way in the last two decades. Technology has advanced, the forecasts are much better, there has been a greater effort at public education, and there are now multiple avenues to disseminate this information, including social media. Nevertheless, it takes the skill of the forecasters, the cooperation of the media, and the efforts of volunteer storm spotters for the system to work.  It certainly worked well on Sunday, November 17.  More importantly, those in the storm's path heeded warnings and took shelter.

The Storm Prediction Center had indicated the possibility of severe weather on Sunday as early as Thursday. As the forecast became clearer the outlook issued Saturday indicated a Moderate Risk for severe storms on Sunday.

The outlook issued early Sunday morning upgraded that to a High Risk for portions of Illinois and Indiana. SPC expanded that High Risk area in the next update issued at 8:00 a.m. CST through Indiana and into Ohio. High Risk outlooks for severe weather are pretty rare in November.

The SPC forecasters are very skilled at what they do, but often Mother Nature throws curveballs and forecasts aren't always perfect. On Sunday, however, they hit a home run out of the park (to maintain the baseball analogy). Here is map of all the storm reports received superimposed on the outlook map issued Sunday morning. Note that all but a few tornadoes occurred in the High Risk area, and all reports fell within the area outlined by the Slight Risk (yellow) - most were within the Moderate Risk area..

To go along with the above map, here is a summary map of all the watches and warnings issued between Saturday morning and Monday morning.

What made this day particularly challenging was the forward speed of the supercells and thunderstorms. Many were moving at 60 to 65 mph, so a delay in issuing a warning even for five minutes meant that a storm was five miles closer to a particular location.  Fortunately advances in radar technology allow forecasters to detect tornado potential before a tornado actually forms providing more lead time for a warning. Residents in Washington, IL were warned 15 minutes before the monster hit, no doubt a reason that more people weren't killed or injured.

Here is a summary of the strongest tornadoes that occurred on Sunday. There were a total of 24 tornadoes in Illinois and 24 in Indiana (preliminary numbers). Tornadoes also occurred in Kentucky, Missouri, Michigan, Ohio, and Tennessee. Seven tornadoes have been rated EF-3 and two have been rated EF-4 on the Enhanced Fujita Scale.  Of those nine EF3/EF4 tornadoes two tornadoes, including the EF-4 that hit Washington, Illinois, traveled over 40 miles and two others traveled more than 29 miles.

Tornado Est. Path Max
Location Intensity Max Wind  Length Width
Washington, IL (Tazewell/Woodford County) EF4 190 46.2 1/2 mile
New Minden, IL (Washington County) EF4 166 10.6 200 yds
Gifford, IL (Champaign/Vermilion) EF3 140 29.7 1/4 mile
Villa Grove (Douglas and Champaign) EF3 140 15.0 1/4 mile
Lafayette, IN EF3 NA 29.2 NA
Woodville, KY EF3 145 42.0 500 yds
Hopkins County, KY EF3 140 8.0 200 yds
Union/Henderson County. IL EF3 145 14.5 200 yds
Scott County, MO EF3 140 19.0 600 yds

Most of the damage surveys are complete, but the data presented here is still preliminary and subject to change as more storm reports come in. NWS offices are still updating information on their web pages. The NWS Chicago office and NWS Lincoln, IL office both have extensive information, maps, videos, and photographs on their web sites, as well as links to the other NWS offices with storm information on this outbreak.

Sunday, November 17, 2013

A Tragic November Day in the Midwest

Convective outlook issued at 8:00 a.m. CST, November 17, 2013
Residents of the Midwest aren't usually thinking about tornadoes in mid-November, especially less than a week since the first measurable snow of the season. However, they woke up this morning to an outlook for a High risk of severe weather from Illinois east through Indiana and into western Ohio and southern Michigan.

At 8:00 a.m. CST radars were clear of any storms in Illinois and showers and thunderstorms were exiting southeastern Indiana.  By 9:00 a.m. storms were beginning to fire in Illinois ahead of a strong cold front, and not long after the first severe weather warnings were being issued.

Surface map at 12:00 CST Sunday, November 17

Southwest winds were already gusting to 40-50 mph in the warm sector, and wind profiles through the atmosphere were favorable for large and long-track tornadoes. Sometime around 10:00 a.m. the first tornado warnings were being issued for areas west of Peoria, Illinois. Over the next six hours there were more than 65 tornado reports in Illinois, Indiana, and western Kentucky.  Forward motion of the individual storms was as high as 65 mph.

Storm reports for November 17
NWS Storm Prediction Center

Some of the worst tornado damage occurred in central Illinois as a supercell moved northeast spawning multiple tornadoes. One of the hardest-hit communities was Washington, IL, about 10 miles east of Peoria. Damage also occurred in many other communities through central and Illinois and the western half of Indiana, including Gifford, IL only 17 miles north of my home in Champaign County. One of our CoCoRaHS observers was able to photograph this tornado as it passed to the north of her location.

Gifford, IL tornado taken from approximately 3.5 miles south of Gifford at 12:51 p.m. CST.
Photo by Jessie Starkey

Kokomo, IN, north of Indianapolis, suffered major damage and a state of emergency was established until 6:00 a.m. Monday morning. Several other Indiana communities also suffered major damage. As of this post, there have been six confirmed fatalities from the storms, all in Illinois and dozens of people injured in all the states affected.

Damage assessment teams from the National Weather Service will be out over the next two days to confirm and rate the tornadoes. It seems likely that there will be at least one EF4 tornado.

How rare is a tornado outbreak for the Midwest in November?  Like spring, fall is a transition season from the warm season to winter, and there is a secondary peak in severe weather in the fall. However, tornado outbreaks like today are rare. The last time there was a high risk for severe weather in November was in 2002 and in 2005. These high risk areas were further south than the one issued today.

Here's an interesting fact from the NWS Chicago office.

"Since 1986 there have been 194 tornado warnings issued in Illinois during the month of November. Of those 194 warnings issued, 101 of them (52%) were issued today. Credit to meteorology professor Victor Gensini (Northern Illinois University and College of Dupage) for researching this stat."

The National Weather Service Chicago has a great summary of the November 17 severe weather outbreak in the Midwest. There are also links to storm information from other NWS offices who dealt with the storms

Thursday, November 14, 2013

A Rare Dry November Day Across the U.S.

This morning's CoCoRaHS precipitation map was very gray this morning. The lack of color on the map meant that there were few reports of measurable precipitation across the country. As of 8:00 p.m. CST today a total of 7,277 reports had been submitted to CoCoRaHS, but only 450, or 6 percent had measurable precipitation in the U.S. and Canada!  Compare that to November 1, when 56% of the 10,950 reports submitted had measurable precipitation.

The highest precipitation amount reported today was 1.26 inches at Hawaii station HI-HI-13, Volcano 4.3 SSE. The highest amount reported in the lower 48 was 0.95 inches at FL-MD-30, South Miami 0.5 N.

The main reason behind the dry weather was the massive high pressure system that covered the eastern two-thirds of the country yesterday and was the center of a cold, very dry air mass. The only precipitation to speak of in the continental U.S. was some light rain in the Pacific Northwest, some precipitation associated with the frontal system moving into the Rockies and High Plains, scattered snow showers in New York and Pennsylvania, and showers and thunderstorms in southern Florida.

Surface weather map for 4:00 p.m. EST November 13, 2013

November is typically a stormy month across the U.S. as it marks the end of the transition from fall to winter weather patterns. A dry day such as this in November over such a large portion of the country is a rare event. Below is the 30-year average precipitation for the U.S. for November 14. There is, on average, a relatively good chance for precipitation for most of the southeastern half of the country and the Pacific Northwest.  You won't see many precipitation maps like today's during the month of November.

Friday, November 8, 2013

The "White Hurricane" of 1913

One hundred years ago this week a massive storm battered the Great Lakes over a period of four days, November 7-11. The storm lingered over the region for four days, generating hurricane force winds (74 mph or greater) over the lakes, heavy snow and blizzard conditions, and 35 foot waves on the Great Lakes. A dozen ships were lost on the lakes, 33 others were damaged, and 250 lives were lost. This storm still remains as the worst U.S. maritime disaster.

The "White Hurricane" was the result of two separate weather systems that merged into one huge system that covered the entire eastern half of the country. In some respects the evolution of this system has some similarities to the evolution of Superstorm Sandy last year. On November 7-8 a strong low was moving across the northern U.S. trailing a strong Arctic cold front. This system primarily impacted Lakes Superior and Michigan, producing storm force winds, heavy snow, and high waves. Several large ships were damaged or run aground as this system moved across the lakes. In the meantime, weak low was moving northeast out of the Gulf states and over the southeastern United States on November 8.

Surface weather map for the morning of November 8, 1913 with notations showing the location of the cold front, the Arctic air, and the low in the southeast U.S.,

Over the next 24 hours the low over the Great Lakes phased, or merged, with the low in the southeast, resulting in explosive and rapid intensification, what we call a meteorological "bomb". On the morning of November 9, 1913 the intense storm was centered over Washington DC.

Surface weather map for the morning of November 9, 1913.

Instead of moving out to sea, the storm turned to the north northwest. The circulation of the storm was tapping the cold Arctic air on it's west side and feeding off abundant moisture from the Atlantic. By the morning of November 10 the center of the storm was located over Lake Ontario.

Winds associated with the storm were estimated to be at hurricane force (74 mph or greater) for a period from 10 to 20 hours over the Great Lakes. Survivors reported wind gusts to 90 mph on Lake Huron on November 9. Winds on Lake Erie were 50 to 70 mph with gust to 85 mph. The ferocious winds for a prolonged period of time over he lakes produced waves up to 35 feet occurring as often as every three minutes. Ships of the day were not capable of withstanding these extreme conditions, especially with whiteout conditions and buildup of ice on the ships . A total of 12 ships were lost and 33 others were damaged during the four-day period. Several ships lost during the storm were never located.

Location of ships lost during the White Hurricane of November 7-10, 1913.
Credit: National Weather Service Detroit, MI

Snowfall blanketed much of the Great Lakes region from this storm. Heavy snow fell across eastern Ohio, West Virginia, and western Pennsylvania with moderate snowfall extending west into Michigan. The Great Lakes were still relatively warm in early November, and the frigid Arctic air streaming over the warm waters resulted in heavy lake enhanced snow. Record snowfall occurred in Cleveland, OH  with 17.4 inches falling in a 24 hour period and a three day total of 22.2 inches.

It was November 11 before the storm finally began to pull away from the region. This storm had, as you might imagine, widespread impact. In some areas freezing rain glazed telephone, telegraph, and power lines prior to the snow and combined with the high winds caused extensive damage. The total damages from this storm were more than $117 million in today's dollars.

You can read more about this epic storm at the following web sites:

Centennial Anniversary Storm of 1913 - NOAA

The "White Hurricane" Storm of November 1913 - A Numerical Model Retrospective - NWS Detroit

November 9-11, 1913: Great Lakes Hurricane - Ohio Historical Society

Frozen Fury: The 1913 White Hurricane - Lake Superior Magazine

There has also been a book written about this storm.  "White Hurricane - A Great lakes November Gale and America's Deadliest Maritime Disaster" by David G. Brown

Thursday, October 31, 2013

Soggy Weather from Texas to the Great Lakes

A potent storm system traveling through the nation's midsection produced some of the heaviest rain in two months over a large swath of the central U.S. Much of this area has been dry the last two months so the rain is needed. In parts of Texas, however, it was too much of a good thing.

This storm came ashore in the Pacific Northwest on Sunday and intensified over Nevada High winds associated with this low pressure system buffeted California, Utah, Nevada, and Arizona early in the week. The winds were responsible for tipping over six tractor trailers on Interstate 580. The winds, sometimes gusting to 60 mph or higher, also damaged some buildings and caused areas of blowing dust. Blowing dust along Interstate 10 in Arizona resulted in a 19-vehicle crash that caused three deaths and a dozen injuries.

Surface weather map for Monday, October 28, 2013 at 5:00 a.m. PDT.
By Wednesday this system was moving out of the Rockies and into the Central Plains, with waves of
Surface weather map for Wednesday, October 30, 2013 at 7:00 a.m. CDT
low pressure moving along the frontal boundary. Strong southerly winds ahead of this system drew warm, moist air into the Plains and Midwest, leading to widespread showers and thunderstorms. Overnight Wednesday training thunderstorms dropped a foot of rain on parts of south-central Texas. A number of CoCoRaHS observers in Hays County, TX, just southwest of Austin, went out this morning to find out that their rain gauges had overflowed during the night. There were quite a few reports of rain around 11 inches, but some of these measurements are low because the rain gauge overflowed.  The highest amount reported was 12.45 inches in Wimberly, TX. The rain caused widespread flash flooding, as you might expect. The NWS office in Austin/San Antonio has compiled a description of this event including radar images and photos.

CoCoRaHS 24 hour rainfall amounts for Hays County, TX for the period ending the morning of October 31, 2013
Further north, two to three inches of rain fell from southeastern Kansas into northern Illinois as of Thursday morning, with three to four inch amounts in Chicago and the southern suburbs. Additional areas of showers and thunderstorms brought more rain to areas from central Illinois through Indiana during the day.

24-hour precipitation ending at 7:00 a.m. CDT Ocotber 31, 2013
Tornado watches were issued for southern Illinois, southeastern Missouri, southern Indiana, western Kentucky, and northern Tennessee Thursday night.  Two unconfirmed tornadoes were reported in southern Illinois this evening. Earlier today two tornadoes were confirmed in southwestern Louisiana.

Rain will fall in the eastern third of the U.S. the next 48 hours as the main low pressure system heads northeast into Canada and the trailing cold front sweeps to the the east coast.

48-hour Quantitative Precipitation Forecast for the period from 7:00 p.m. CDT Thursday to 7:00 p.m. CDT Saturday.