Wednesday, October 22, 2014

Changes to Severe Weather Outlooks

The Storm Prediction Center has instituted several changes to it's severe weather outlook products effective today.

If you are familiar with the convective outlooks that the SPC issues several times per day you know that the outlooks indicate where general thunderstorms are expected, where there is a low probability of severe weather (the "See Text" category), and where there is a Slight, Moderate, or High Risk of severe weather. Effective today there is one change and one additional category being used in the Day 1 through Day 3 outlooks.

There are real numbers behind the determination of each of the former and current risk areas. A 15 percent probability of a tornado (Moderate Risk category) may seem low, but the normal probability of a tornado, for example, during the peak of the season on May 13 in central Oklahoma is only about 1.5 percent. A Moderate Risk in  in this case would indicate about 10 times of the normal probability for a tornado.

The probability for a tornado changes by season.The probability of a tornado in central Oklahoma this week in October is normally about 0.15 percent, ten times lower than in mid-May.

Here are the new/revised categories and what they describe. The probabilities for Day 1 are more detailed than Days 2 and 3.

Day 1
General Thunderstorms
    10 percent or greater probability of non-severe or near severe thunderstorms

    - 2% tornado probability, or
    - 5% severe hail or severe wind probability.
    - 5% tornado probability, or
    - 15% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater.
Enhanced (the upper end of the former SLIGHT category)
    - 10% tornado probability WITH OR WITHOUT 10% or greater probability
      of an EF2 or greater tornado, or
    - 15% tornado probability, or
    - 30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater, or
    - 45% probability of severe hail or wind.
    - 15% tornado probability AND 10% or greater probability
      of an EF2 or greater tornado, or
    - 30% tornado probability, or
    - 45% severe wind probability AND 10% or greater
      probability of a wind gusts 75 mph or greater, or
    - 45% severe hail probability AND 10% or greater
      probability of hail 2 inches or greater in diameter, or
    - 60% severe wind probability, or
    - 60% severe hail probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter.
    - 30% tornado probability AND 10% or greater probability
      of an EF2 or greater tornado, or
    - 45% or greater tornado probability WITH OR WITHOUT 10% or greater probability
      of an EF2 or greater tornado, or
    - 60% severe wind probability AND a 10% or greater
      probability of a wind gust 75 mph or greater.
Days 2 and 3
General Thunderstorms
      - 10% or greater probability of non-severe or near severe thunderstorms.

      - 5% total severe probability.

      - 15% total severe probability WITH OR WITHOUT 10% or greater probability
        of significant severe.

      - 30% total severe probability WITH OR WITHOUT 10% or greater probability
        of significant severe, or
      - 45% total severe probability.

      - 45% total severe probability AND 10% or greater
        probability significant severe, or
      - 60% total severe probability (Day 2 only).

High (Day 2 only)
      - 60% total severe probability AND 10% or greater
        probability of an EF2 or greater tornado or a wind gust 75 mph or greater.

The SPC has a number of examples of the new vs. old  categories on its web site. Here is an example of the difference in outlooks for the day of the Southeast tornado outbreak on April 27, 2011.

The Day 1 convective outlook for April 27, 2011 in the new format (left) and old format (right)

The Storm Prediction Center is also considering changes to the Day 4 to 8 outlook product. This is still in experimental mode and more information can be found here.

Although much of the country is currently enjoying pleasant fall weather, we have entered a ramp-up period to a secondary peak in severe weather season. October through December is a period of increased frequency of severe weather and tornadoes from far eastern Texas through Alabama, an area dubbed "Dixie Alley", with a peak in the frequency of tornadoes in mid to late November.

Mean number of tornadoes for October through December in Dixie Alley.

If you would like to explore the climatology of severe weather the SPC has an interactive web page where you can view the probabilities for tornadoes, significant wind, and significant hail for 52 one-week periods. You can animate the maps to see how the severe weather shifts through the country and how the probabilities change from week to week.

Thursday, October 9, 2014

Super Typhoon Vongfong - An Impressive Storm

Not only has the eastern Pacific tropical season been above normal, but storms have been frequent in the central and western Pacific as well. Super Typhoon Vongfong has been in the news lately partly because of some of the spectacular photos and satellite images that have been collected as this typhoon has crossed the Pacific but also because it is the strongest storm of 2014 in any basin.

Vongfong was born as a weak depression just south of the Marshall Islands on September 30. By October 3 it had strengthened into a tropical storm, and 30 hours later a typhoon. It reached super-typhoon strength on the morning of October 7 with winds of 155 knots (178 mph).  Winds are currently down to 130 kts, still a strong,dangerous, and massive storm. Gale force winds (>34 knots/39 mph)associated with this storm cover an area of 340,00 square miles.

The track of Super Typhoon Vongfong.
While here in the U.S. we are familiar with the terms tropical storm and hurricane, terminology differs in other parts of the world. A typhoon is the same as a hurricane west of the International dateline. A "super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Center for typhoons that reach maximum sustained 1-minute surface winds of at least 130 kt/150 mph). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin.

One of the more interesting and spectacular images of Super Typhoon Vongfong was this visible image illuminated only by moonlight from NASA. Note the thunderstorms on the west and northwest outer bands of the storms identified by the bubble-like cloud tops.

This image is one hour earlier than the last position labeled on the chart above when the winds were 178 mph.

The Capital Weather Gang at the Washington Post assembled images of Super Typhoon Vongfong for their column today. Rather than reproducing them here, visit their web page "Twelve incredible images of most powerful storm of the year, Super Typhoon Vongfong".

As you can see on this image of the current and projected track, Vongfong is forecast to weaken as it moves north. It will reach Okinawa as a category 3 equivalent storm, and by late this weekend will be entering southern Japan as a category 1 equivalent storm with top winds of about 90 mph.

Current and projected track of Super Typhoon Vongfong.

Tuesday, October 7, 2014

Pacific Tropical System Injects More Moisture into U.S.

It has been an active tropical weather season in the eastern Pacific.  As of today there have been 18 named storms, with 13 of those reaching hurricane strength. The eastern Pacific season is above normal so far this year. At this point in the season the average number of named storms is 13 and hurricanes seven.

The increase in storms has been a "good news, bad news" situation for the southwestern U.S. It has been good because there has been significant improvement in (but not elimination of) drought conditions in much of New Mexico, the southern half of Arizona, and western Texas since early July. The bad aspect is the amount of flooding and flash flooding that has occurred as a result of heavy rains from the enhanced moisture.

As of noon Tuesday Simon was a minimal tropical storm located just off the central coast of the Baja California peninsula, and by evening Simon was downgraded to a tropical depression. Over the next few days moisture associated with Simon will cross through the Desert Southwest and then eastward across the country.

Quantitative precipitation forecast for the 72-hour period ending 7:00 p.m. CDT on Friday, October 10.
A large high pressure system over the upper Midwest will keep most of the precipitation associated with this moisture in the southern half of the country. A wave of low pressure moving across the frontal boundary dividing cool dry air to the north from the warmer, moist air to the south will provide the trigger for widespread and potentially heavy rain. Heavy rain is expected across Arizona - again - , and presenting a threat for flash flooding the next couple of days. The threat for heavy rain will then shift to the Central Plains and Midwest.

Surface weather map forecast for 7:00 p.m. CDT Wednesday, October 8.

Surface weather map forecast for 7:00 p.m. CDT Thursday, October9

Unfortunately very little, if any of that rain will reach parched California, where severe to exceptional drought encompasses 83 percent of the state.

Status of California drought as of September 30.

The rain through southern southern half of the U.S. will slow up fall harvest which has been progressing very nicely with the long stretch of dry weather at the end of September.

Monday, September 22, 2014

The 2014 Southwestern Monsoon in 8 Minutes - Spectacular Video

I've written a couple of posts this year and in the past about the southwestern monsoon season in the U.S. Even with maps, charts, and photos it's difficult for those who have never experienced that part of the country to imagine the scope of the weather that occurs during this important season. Today I came across a video that condenses the 2014 monsoon into eight minutes of boiling cumulus clouds, intense rain shafts, hail, haboobs, lightning, and desert scenery. Photographer Mike Olbinski spent the summer driving 14,000 miles to capture the images that make up this time lapse video.

Maps and charts really only tell the technical part of the story. A video such as this one easily explains why so many are fascinated with the weather.

Be sure to watch this full screen and turn up your speakers. You will want to view this more than once to catch all of the details.

By the way, if you do want a "technical" explanation of the southwestern monsoon see a previous post here.

Wednesday, September 17, 2014

Tropical Weather in the Desert Southwest - Odile is a Big Deal

A few days ago Odile was a Category Four Hurricane off the west coast of Mexico, and as of this morning was still a weak tropical storm. Odile fell apart over northwestern Mexico during the day, and is now just a shell of its former self.  However, the remnants of Odile will still have quite an impact on weather for the next few days, first across the southwestern U.S. and then perhaps across the extreme southern U.S.

Map shows the expected position of the remnants of Odile [D] at 11:00 p.m. PDT September 17.

The remnants of Odile were already feeding plenty of moisture into the Desert Southwest. Storms were bubbling up today across Arizona and New Mexico while some isolated storms popped up in southern California.

Yesterday there were some rare severe thunderstorms in southern California. Wind damage was reported in and around San Diego, where numerous trees and light poles were toppled and small planes were flipped over at Montgomery Field Airport. One inch hail was reported near Joshua Tree, CA in San Bernadino County.

San Diego radar image for 3:15 p.m. PDT September 16. Storms were moving to the west.
Conditions across southern Arizona and New Mexico today were decidedly tropical. It was quite humid with dewpoints generally in the mid to upper 60s. The dewpoint in Phoenix this afternoon reached 72°F, a level that is rarely seen there. Swamp coolers (evaporative air conditioners) don't work very well when the dewpoint is high. 

There are a combination of factors that will make the next few days messy, to say the least, in the Desert Southwest. CoCoRaHS observers in the region will be getting a lot of measurements in over the next several days.  Although we are the tail end of the summer monsoon season, there will be heavy showers and thunderstorms because of the plentiful moisture being fed into the region. Weak flow in the upper atmosphere will provide for little steering of the showers and thunderstorms, and those that develop could remain over a relatively small area for much of the storm's lifetime. That means potentially heavy rain for a long period of time.  That same weak upper flow also means that the leftovers from Odile will be slow to move out of the region.

500 millibar map for 5:00 a.m. PDT this morning. The circulation of Odile is caught in a ridge over the Southwest.

The NWS Weather Prediction Center is expecting as much as 4 inches of rain from southeastern Arizona through southern Arizona and into western Texas, with some locally higher amounts possible.

Rainfall forecast through 5:00 p.m. PDT Saturday, September 20.

Flood watches are in effect for large portions of Arizona and New Mexico.

Flood watches (dark green) and flood advisories (light green) in effect as of 5:00 p.m. PDT

Here is a broader picture of the rainfall expected in the southwest and across the country the next three days.

Quantitative Precipitation forecast for the 72-hour period ending 5:00 p.m. PDT Saturday, September 20.

The showers and thunderstorms are most numerous tonight south of the Arizona-Mexico border but are steadily feeding northward. The Tucson NWS office has a nice summary of daily and monthly rainfall records for a number of locations in southern Arizona on their web site. 

Tucson, AZ radar at 5:39 p.m. MST September 17

Monday, September 15, 2014

Snowtember Recap - the Snow and Cold Weather

Unless you weren't paying attention at all to the weather last week you have heard about the snow in the Rockies and northern Plains as well as the unseasonably cool weather that followed for much of the eastern three-quarters of the country. So how bad was it?

The snow forecasts were pretty much on target, though amounts in a few places (notably mountain areas) were a little higher than expected.  The CoCoRaHS observer at WY-SH-33, Sheridan 15.7 W on the east slopes of the Bighorn range measured a storm total of 19.0 inches of snow. Snow fell along the Front Range in Colorado as far south as Denver, with a half inch of accumulation in Boulder northwest of Denver. The accumulating snow reached as far east as Rapid City, SD with 3.5 inches, the earliest measurable snow on record. The old record for Rapid City was 2.8 inches on September 15, 1903. Six to 8 inches of snow fell in the Black Hills during this storm. North Platte, NE picked up a trace of snow, the earliest on record there.

72-hour snowfall accumulation for the period ending 7:00 a.m. local on September 12

From the National Weather Service:


CASTLE ROCK 5.2 SW                    1.0                   
HORSETOOTH MOUNTAIN 3.2 NNW           1.0                   
NEDERLAND 4.8 ENE                     0.9                   
ASPEN PARK 5.2 ESE                    0.7                   
BOULDER 1.6 S                         0.5                   

HEART BUTTE                           8.0                   
MELVILLE 4.7 W                        7.5                   
RED LODGE 4.2 W                       6.0                   
WYOLA 17.3 WSW                        6.0                   
ZORTMAN                               6.0                   
LIVINGSTON 6.6 ESE                    3.5                   


DOWNTOWN CUSTER                       8.0                   
MOUNT RUSHMORE                        7.0                   
HILL CITY 5 S                         6.0                   
LEAD 5.5 SSW                          6.0                   
RAPID CITY 6.9 W                      3.5 -- EARLIEST ON RECORD

LITTLE GOOSE                         18.0                   
SHELL CREEK                          14.0                   
STORY 0.8 W                          14.0                   
SHERIDAN 15.7 S                      13.0                   
BIG HORN                             12.0                   
BURGESS JUNCTION 4 NW                12.0                   
SOLDIER PARK                         11.0                   
BUFFALO 1 E                          10.0                   
BANNER                                7.0                   
CODY 5 ESE                            7.0                   
SUNDANCE 1 ENE                        6.0                   
DOUGLAS 6 S                           3.0  

Obviously that snow would not have been possible without some pretty cold air for this time of year. The cold air spilled south behind this storm system. On Friday, September 12 Cloud Peak, WY in the Bighorn Mountains recorded the nation's lowest temperature of 4°F.  By Saturday morning, September 13 the cold air had pushed as far south as central Texas and New Mexico.

Minimum temperatures for the period from 7:00 p.m. CDT September 12 to 7:00 a.m. September 13

Temperatures for September 12-14 were as much as 20°F below normal in the Central Plains, with much of the middle third of the country from 10°F to 15°F below normal.

In the northern tier of states the first freeze of the season was recorded from Wyoming east into northern Kansas and northwestern Wisconsin and northern lower Michigan.

Map of locations recording first 32°F freeze

Fortunately, a hard freeze (28°F) was limited to Wyoming, Colorado. the far western Dakotas, western Nebraska, and scattered areas in northern Minnesota.

Map of locations recording first 28°F freeze

For most areas the first occurrence of 32°F was about two to four weeks early.

Median date of first occurrence of 32°F in the fall.

The weather will gradually warm this week and it looks like we'll enjoy some "normal" September weather for the next week to ten days

Wednesday, September 10, 2014

It's Snowtember in the North

Snowy Calgary on September 8.
The forecast for much of the central and eastern U.S. has been calling for much cooler weather over the next few days. The cold air that will spread over the U.S. the rest of this week has already plunged into southern Canada and the northern U.S. Rockies. Calgary, Alberta has been hit hard by heavy, wet snow this week. On Monday Calgary picked up 11.8 cm (4.5 inches) of new snow, and on Tuesday an additional 1.3 cm (0.5 inch), and had 10 cm (4 inches) on the ground. This was all following a weekend with temperatures in the mid 70s Fahrenheit. The heavy, wet snow brought down trees that still were leafed out, and those in turn took down power lines. This morning an estimated 30,000 customers were without power. In addition to downed trees and power lines there were numerous traffic accidents, sort of typical for the first snow of the season, even in Canada. Some flights were delayed or cancelled at Calgary International Airport. Interestingly, schools remained open except for a few that had power outages.

This probably was a common sentiment
in Calgary. Photo by Deanna Allen
September snow is not that unusual for Calgary, with about 2 days of measurable snow on average. The snow Calgary has received this week is what they would normally receive for September and October combined. The snow continued today, and this morning Environment Canada issued a Snowfall Warning for the City of Calgary today with another 10 to 15 cm (4 to 6 inches) of snow expected. Except for damaged trees the snow will soon be a memory as a warmup is expected through the weekend and temperatures will be in the 70s Fahrenheit by early next week.

The cold air doesn't stop at the border, of course. Snow was falling in higher elevations in northern Montana today and Winter storm Warnings and Winter Weather Advisories are in effect until midday Thursday for areas on the east side of the Rockies as far south as northeastern Wyoming. Some snow also dusted the higher elevations in northern Colorado today.

View from Montana DOT camera at Two Medicine River  Bridge (elevation 4900 ft) near East Glacier at 6:19 p.m. MDT September 10
The higher mountains and passes could receive 6 to 12 inches of snow while the north-central plains pick up 1 to 3 inches. The southwestern valleys of Montana and lower elevations in Wyoming will see mostly a rain/snow mix.

Watches, warnings, and advisories in effect as of 6:27 p.m. MDT

As this cold air continues to spill south and east it will usher in an extended period of cooler than normal weather for the eastern two thirds of the country.

Minimum temperature forecast for Saturday, September 13

Maximum temperature forecast and departure from normal for Wednesday, September 17.

Tuesday, September 9, 2014

Soggy Times in the Desert Southwest

It's not often the rainfall in the Desert Southwest will make the news twice in less than a month's time, but this is one of those times. Late Sunday night into early Monday morning, September 8 thunderstorms feeding on a plume of moisture associated with what remained of Hurricane Norbert flushed out over the Phoenix metropolitan area. Record rainfall amounts occurred in a span of only 8 hours or so in an area not accustomed to multi-inch rainfalls.

Two clusters of storms merged. One moved from southwest to northeast west of Phoenix, and the second developed southeast of Phoenix and moved northwest, where it merged with the first cluster.

Reflectivity image for 11:47 p.m. MST September 7 (left) and 2:03 a.m. MST September 8 showing the two clusters of storms approaching the Phoenix area.

Water vapor satellite image for 3:45 a.m. MST. The complex of storms over southern Arizona can be readily seen.

Radar reflectivity image at 3:20 a.m. MST September 8.

The rain in the Phoenix area continued through mid-morning before tapering off. Rainfall amounts were unusually heavy and widespread.

Phoenix Sky Harbor Airport set new records for the highest daily rainfall for September 8 and the highest rainfall in a calendar day. The previous records had been in place 75 years or more.

Then, from late morning through early afternoon Monday heavy rain fell in the Tucson area. Tuscon received 1.87 inches of rain, a new record for September 8. The old record was 0.94 inch in 1919.  More information on the Phoenix rain can be found on this NWS Phoenix web page.

CoCoRaHS map for the Tucson area for the 24-hour period ending the morning of September 9.

About the same time the rain was tapering off in the Phoenix area thunderstorms erupted over southern Nevada northeast of Las Vegas in northeast Clark County. More than 4 inches of rain was recorded in some locations. Flooding damaged some homes in the small town of Moapa 50 miles northeast Las Vegas just off Interstate 15, forced the evacuation of an Indian reservation and sent torrents of water across Interstate 15. Much of this rain fell in only a two-hour period, pushing the Virgin River to near-flood stage.

Multi-sensor precipitation estimate for southern Nevada for the 24-hour period ending the morning of September 9.
Source:  NOAA Advanced Hydrologic Prediction Service

Here is a video showing some of the flooding on Interstate 15 on Monday.

The heavy rain story isn't over yet. Heavy rain is falling tonight along a stationary front draped across the Midwest, More than 3 inches of rain has already been reported in Iowa and northern Missouri, and flash flood warnings and watches are in effect for a large portion of the Midwest.

Watches, warnings, and advisories in effect as of 11:45 p.m. CDT September 8.

Thursday, September 4, 2014

Heat Bursts Over the High Plains

On early Thursday morning a somewhat rare and unusual phenomenon occurred in the High Plains. A series of heat bursts were observed in western Nebraska and central and southern South Dakota.

A heat burst is a downdraft of very warm and dry air, associated with decaying showers or thunderstorms that usually occurs in the evening or overnight. Heat bursts at the surface are characterized by dramatic rises is temperature, decreases in dew point, and strong gusty winds.

A heat burst develops when rain falls through a very dry layer in the middle levels of the atmosphere. As the rain falls through the layer of very dry air it evaporates. As the water evaporates, the air cools and accelerates toward the surface. Once all of the precipitation has evaporated the air can no longer be cooled by the evaporation process. This dense air continues to accelerate toward the surface but now rapidly warms as it compresses at a rate of 5.5°F per 1000 feet (called the dry adiabatic lapse rate). This downdraft hits the ground and spreads out in a similar fashion to a downburst from a severe thunderstorm.

This is the radar image from Aberdeen, SD shows the decaying showers and thunderstorms over southern South Dakota generally south of Huron (HON).

Last night's heat bursts were captured by a number of automated weather stations in the South Dakota Mesonet and NOAA's Climate Reference Network (CRN).

Temperature, humidity, and wind plots from White Lake, SD showing the heat burst. The temperature at White Lake rose from 73°F to 89°F and the humidity dropped from 92% to 24% all in the span of 25 minutes,
and the peak wind gust was 47 mph..

Temperature, humidity and wind plots for Oacoma, SD showing heat burst.

CRN temperature and humidity data showing heat burst near Harrison, NE.

I experienced the effects of a heat burst in July 1995 while in Scottsdale, AZ attending a meeting. It was in July and the high temperature for the day was 121°F. That night a number of us where sitting around the hotel pool around 11:00 p.m. when the winds picked up and the temperature went from about 102°F to 111°F in a matter of minutes. That heat burst came from a decaying thunderstorm over Phoenix. The effects were a little more dramatic closer to Phoenix and there was some wind damage as well. It was a very odd thing to experience, especially at those temperatures.

Wednesday, August 27, 2014

Musings on Heavy Rain

As I watch the rainfall reports coming in from around the country this month it seems like almost every day there is some place with excessive rainfall. Of course, the whole country hasn't experienced heavy rain this month. California, Oregon, Washington and other western states would give anything to have some of the rain that has occurred. I've written about a couple of these events, but I was curious to know on how many days there were reports of heavy rain, where, and how much.  So, I dug into the CoCoRaHS data and came up with a few interesting facts.

I defined my "heavy rain" amount as equal to or greater than 3.00 inches. That's a pretty decent amount almost anywhere for 24 hours, and if it falls fast enough it can cause problems.  I constructed a table showing the daily maximum values, the number of precipitation reports of 3 inches or more, and the states reports were received from.

Date Max Precip in. State #>=3in States/Provinces
8/1/2014 7.43 SC 75 FL LA NC PR SC TX
8/2/2014 4.96 NC 31 HI LA NC NM PR TX VA
8/3/2014 6.37 NC 24 FL GA NC PR
8/4/2014 6.95 FL 20 FL LA MS NC NY
8/5/2014 7.00 FL 17 FL IL ON
8/6/2014 5.65 ND 11 FL IA ND NE SD
8/7/2014 5.29 MO 40 IA KS MO
8/8/2014 7.96 HI 24 AL CO GA HI IL MO
8/9/2014 5.86 HI 23 AL AR GA HI KY NE NL SC TN
8/10/2014 6.96 SC 54 AK AR GA IL KS LA NC NE NM SC SD
8/11/2014 6.35 TN 28 AK AL GA IN MN MO MS NC SC TN
8/12/2014 8.03 MD 54 FL LA MD MI MS NC OH ON SC TN VA
8/13/2014 13.02 NY 137 AZ DE FL MD ME NC NJ NY PA SC VA
8/14/2014 6.22 ME 57 CT MA ME NH RI
8/15/2014 4.47 FL 4 FL SD 
8/16/2014 8.00 ND 31 CT FL IA MO ND
8/17/2014 4.50 ND 6 FL IL MO ND TX
8/18/2014 5.43 TN 20 AR MN MS NS TN
8/19/2014 8.10 NC 60 AL MI NC SC TN TX WI
8/20/2014 4.85 NC 5 MI NC OH TX
8/21/2014 4.20 MAN 4 HI MAN ND
8/22/2014 10.63 IN 133 IL IN NC NJ OH PA
8/23/2014 4.96 PR 25 IA IN KS KY MD MT NE PA PR WV
8/24/2014 5.86 MT 46 FL IL IN MT NC ND PR SC SD
8/25/2014 3.97 GA 6 GA MT  SASK TN WI
8/26/2014 4.93 NE 7 FL IA NE
8/27/2014 3.11 IL 2 IL MO

So far this month every day has had at least one report of 3 inches or more of rain. You can see when the larger (in terms of area) events occurred.  On August 12th there were 137 reports of 3 inches or more of rain. This was from the slow moving storm that moved up the east coast from Virginia to Maine that week. However, one contribution to this high number of reports is likely due to rain gauge (population) density. There were many more gauges sampling the storm that week in the east than, say, Wednesday 8/26 in Illinois. As of this posting there were only two reports of 3 inches of rain or more this morning, but there would have probably been more if we just would have had some gauges underneath those slow moving storms yesterday. According to the multi-sensor precipitation map for yesterday there were 3-inch plus amounts  in central Illinois, in northwest Missouri, the Oklahoma panhandle, and in central New Mexico.

However, there were no CoCoRaHS gauges capturing those 3 inch plus amounts in Oklahoma and New Mexico. NWS Cooperative observers in Atchison, MO received from 3.5 to 4.5 inches of rain in the 24-hour period ending this morning.

So far this month heavy rain has been reported in 42 states, 5 Canadian provinces, and Puerto Rico.

So how does this compare to the past several years? In 2011, 26 days had an observation of 3 inches or more, and in 2012 through 2014 each day from August 1-27 had at least one observation 3 inches or more. (I did not go back further since the number of active stations was much less in prior years.) Here is a chart showing the distribution of CoCoRaHS observations at different rainfall amounts for those years.

I don't think you can really draw any conclusions from all of this other than the fact that these heavy rain events occur frequently this time of year across much of the country and this year is not much different from the last few years. These are only the CoCoRaHS observations, and the raw numbers would look different (higher amounts on more of the days) if we included all of the NWS Coop observations. It's just an interesting way to look at the tens of thousands of CoCoRaHS observations our volunteers submit each month and an example of how important your observations are to describing the precipitation climatology.

Wednesday, August 20, 2014

Deluge in the Valley of the Sun

Last week exceptional rains occurred in the central and eastern U.S.  This week the weather eye turns to, of all places, the Desert Southwest. For most of the year "rain" and Arizona" are two words rarely heard in the same sentence.  However, the summer monsoon is in full swing, and on Tuesday the Phoenix area received some impressive rain amounts which caused major flooding on some creeks and rivers as well as flooding of many roads, including portions of Interstate 17.

The thunderstorms were set off by an approaching trough over the western U.S. With ample moisture available from the monsoonal flow and southwesterly winds forcing the air up over higher terrain, thunderstorms quickly developed and intensified.

The first round of storms occurred early Tuesday morning near Carefree and Cave Creek, located at the northeast corner of the metro area.
Radar reflectivity for 6:02 a.m. PST Tuesday, August 19. Storms are intensifying west and north of Phoenix.

By 7:00 a.m. very heavy rain was falling northeast of Phoenix over the higher terrain of Maricopa County.

Radar reflectivity for 7:02 a.m. PST Tuesday, August 19.  Heavy rain is falling east of I-17 and north of Phoenix.

The CoCoRaHS observer at Cave Creek 2.9 SSW (AZ-MR-17) reported 0.75 inch of rain in 15 minutes (that's a rate of 3 inches per hour) as did the observer at Carefree 2.1 E (AZ-MR-31). Both stations ended up with about 2 inches of rain. However, the heaviest rain was located north of our CoCoRaHS observers.

Initially floodwaters and debris covered the northbound lanes of Interstate 17, but by 10:00 am Interstate 17 four miles north of New River was totally under water and many cars were stranded, necessitating rescues.

Water covers the northbound lanes of I-17 Tuesday morning.
Credit: ABC15 Facebook page

A second round of heavy storms moved over the same area northeast of Phoenix during the early afternoon, and additional thunderstorms moved through the region though the afternoon and evening. However, most of the rain fell in these first two rounds.

Radar reflectivity at 12:02 p.m. PST on Tuesday, August 19.
A second round of heavy storms is moving through the Phoenix area.

Measured ainfall totals for this event reached 5.51 inches at Tower Mountain north of Phoenix, but there was a wide swath of  4-inch plus rainfall that extended to the southeast from there. The highest amount reported by a CoCoRaHS observer was 2.86 inches at Cave Creek 2.4 N (AZ-MR-163).

24-hour rainfall map for period ending the morning of August 20.
Area outlined in orange encloses rainfall amounts of 4 inches or more.
Base map credit: Flood Control District of Maricopa County

The National Weather Service Office in Phoenix has a web page with more information on this event, including photos of the flooding.