Tuesday, August 27, 2013

An Amazing View of the Tropics

No, I'm not talking about crystal blue waters and sandy beaches. 

The tropical storm season in the Atlantic seems to be progressing in fits and starts this year.  So far there have been six named storms and none have been particularly noteworthy.  Note that NOAA's updated hurricane forecast in mid-August maintained a high probability for an active tropical season. It may be just a matter of time before the first major storm forms and occupies our attention. In the meantime, there are a couple of neat web sites and pages you may want to visit to get in that "tropical mood".

Here is a very cool image of 170 years of tropical cyclone tracks.

Image of tropical cyclone frequency. Credit: NOAA

This image comes to us from NOAA's Environmental Visualization Laboratory. The National Climatic Data Center (NCDC) put together a database of 11.967 tropical cyclones from 1842 to 2012. Early reports, prior to the use of weather satellites, came from ship reports. These were generally reliable reports. However, if you look at the vast expanse of ocean and the preferred shipping lanes, no doubt some storms occurred and were never detected. Now satellites can detect even the smallest disturbance over the oceans.

The Environmental Visualization Laboratory created this image from the storm tracks in the database. On the left is the eastern hemisphere image of the Atlantic Basin and the eastern Pacific, while on the left is the western hemisphere. The brighter the color, the more cyclone tracks overlapped. Note the very bright spots in the western hemisphere in the Bay of Bengal (India) and western Pacific.

Contrast the frequency image with the image below, a depiction of the cyclone intensity. The colors represent the maximum sustained wind speed over the course of a storm's life.

Image of tropical cyclone intensity.  Credit: NOAA
There is a greater spread of strong cyclones over thee northwestern Atlantic. That makes sense since many tropical storms can originate off the coast of Africa and intensity as they move west across the Atlantic.In the western hemisphere the more intense storms appear to occur near the Philippines.

NOAA has a really slick tropical storm track viewer that allows you to view tracks in the database. You can search for all tracks that have affected a particular location, or you can search on storm names, or for an ocean basin.

Storm tracks within 65 nm of Cape Hatteras, NC, 1842-2012. Credit: NOAA

This image is a search of a storm tracks within 65 nautical miles of Cape Hatteras. Looks like hurricane convergence! Along with the image, there is a data viewer on the left side which lists each storm. You can also mouse over any of the tracks and that storm will be highlighted in the list. You can then view more information on the storm.

When you search on a storm name, you get a list of storms with that name and the year they occurred. This is an image of the track of Hurricane Andrew in 1992. Mouse over any of the points on the map and the storm information for that point is highlighted in the left data window.

Storm track for Hurricane Andrew in 1992 using the Historical Storm Track viewer
If you are a tropical storm fan you can kill a lot of time on this web site.

Finally, NOAA's Ocean Today web site has a nice video on the Hurricane Hunters. This is the first time I've seen this web site and it has a lot of excellent videos and information on the ocean environment. Be sure to check it out.

Tuesday, August 20, 2013

The Wet Get Wetter and the Dry Get Drier

The lack of rain in the central U.S. this month is becoming a bit of concern as soils continue to dry and crops, gardens, and lawns are thirsty for rain.  Abnormal Dryness as depicted on the U.S. Drought Monitor has returned to Iowa, Missouri, Wisconsin, and Illinois after being absent in those areas for much of the summer.

Meanwhile, the rain doesn't seem to want to stop in the Gulf States and the Southeast. Until recently a strong upper low rotating over southeastern Canada kept the jet stream pushed further south than usual for summer, bringing cool dry weather to the central U.S.

The 500 millibar map (~20,000 ft) on Sunday, August 18. A trough persists over the eastern half
of the country, though much weaker than earlier in the week
In the meantime, waves of low pressure moving along the stalled frontal boundary across the Gulf states provided a trigger for showers and thunderstorms from the Louisianna coast through the Carolinas.

Surface map for 7:00 CDT Sunday, August 18.

The persistence of this pattern is readily seen in an animation of the daily CoCoRaHS maps for the last seven days.

CoCoRaHS precipitation maps for 8/14 through 8/20/2013
A number of locations in the Southeast are on pace for the wettest year on record (information from the Southeast Regional Climate Center with totals updated through 8/19).

                       Precip through        All time wettest
Asheville, NC         58.62                 64.91  1973

Roanoke, VA          41.52               58.87   1948

Macon, GA            55.79                67.80 (1929)

For both Asheville and Macon it's really not a question of if the records will be broken, but when.  Roanoke's precipitation is currently 17.35 inches from the record. Normal precipitation for September through December is 13.12 inches, so higher than normal precipitation will be needed there to break the record. An interesting note about Asheville's rainfall -the  2013 rainfall total represents the largest departure from normal out of 180 cities reviewed in a recent study by the National Climatic Data Center (coincidentally located in Asheville).  The current total pf 58.62 inches is 31.41 inches above normal.

The are a number of locations in the western Carolinas with precipitation for the year-to-date in excess of 80 inches. Here are precipitation accumulations over 80 inches recorded by CoCoRaHS observers in the southeast. These are stations that had at least 95 percent of possible days reported from January 1-August 20.

SC-PC-4          Cleveland 8.0 WSW             89.15
NC-WT-10        Blowing Rock 2.2 NE           86.82
NC-AV-9           Linville 2.5 ENE                   83.00
NC-WT-1          Blowing Rock 22.8 ENE       81.31

More rain is on tap for the southeast for this week with an inch or two additional rain from northern Georgia through the western Carolinas and northward into West Virginia. Dry weather will continue across much of the central U.S., although a cold front will likely produce scattered showers and thunderstorms across the northern Midwest.

72-hour Quantitative Precipitation Forecast for the period ending 7:00 p.m. CDT Friday, August 23

Wednesday, August 14, 2013

Tropics Becoming Active

NOAA's outlook for the hurricane season this year was updated recently and continues to project a 70 percent chance of an above-normal season. In general things have been quiet so far with only four named storms to date (Andrea, Barry, Chantal, and Dorian) and none of them hurricanes.  Andrea affected the Gulf, southeast, and mid-Atlantic coasts in early June. Barry languished in the far southern Gulf. Chantal remained in the Caribbean, and Dorian made a long trip through the Atlantic before weakening and fizzling out in the waters east of Florida at the beginning of this month.

Today's tropical outlook for the Atlantic from the National Hurricane Center has identified two potential areas for tropical storm development in the next few days. The one of immediate concern is located in the northwest Caribbean. The second is a disturbance just off the African coast which became Tropical Depression #5 late today.

The disturbance in the Caribbean is expected move northwest and cross the Yucatan peninsula, then re-emerge over the western Gulf. It could develop into a tropical depression sometime Thursday. Development beyond Thursday is still a question mark. However, even without development into a tropical storm there will be plenty of moisture funneled from the Gulf into the Gulf coast and southeastern states and the rest of this week is likely to be wet.  The Quantitative Precipitation Forecast issued by NOAA's Weather Prediction Center is indicating that several inches of rain are possible in wide band from New Orleans through Georgia and into the southern Carolinas.

Quantitative Precipitation Forecast for the period from 8:00 p.m. EDT 8/14 to 8:00 p.m. EDT 8/17

This area could really use a break from the rain. Over the past 60 days rainfall has ranged from 150 to more than 300 percent of normal.

Map from the NWS Advanced Hydrologic Prediction Sercvice

Tropical Depression #5 southeast of the Cape Verde Islands will bear monitoring over the next few days. However, that is probably at least a week away from reaching the eastern Caribbean should it hold together.

Thursday, August 8, 2013

Bulls Eye on the Ozarks

For the last week the U.S. has been locked in a weather pattern more reminiscent of mid winter rather than mid summer. A strong upper low has been spinning over Hudson's Bay, keeping much of the northern and north-central U.S. cooler than normal and forcing the path of weather systems further south than usual.

500 millibar maps for August 2 (left) and August 7 (right), 2013

This has had the effect of establishing a quasi-stationary boundary from the east side of the Rockies to the east coast. Over the last week this front has oscillated from the central Midwest to the central Gulf Coast states and back as a series of minor waves have rotated around the upper low in Canada.

Surface map for 7:00 a.m. CDT 8/7/2013

These waves of energy interacting with the frontal boundary have generated prodigious rainfall amounts from south-central Kansas through southern Missouri and Tennessee. Unfortunately, the bulls eye of the heaviest rain is over the Missouri Ozarks. The Ozarks extend generally from northwestern Arkansas northeastward through Missouri to just south of St. Louis. The topography ranges from rolling to rugged making it a prime area for flash flooding. Rainfall this past week has exceeded 16 inches in parts of Missouri and 11 inches in northern Arkansas.  Here are some of the highest Missouri amounts:

The observer at Fort Leonard Wood 9.3 S (MO-TX-9) picked up 10.37 inches of the total 16.42 inches of rain so far this month in the 24-hour period ending Tuesday morning, August 6. Here's what the observer had to say in her observation notes that morning:

OH MY WORD!!! We emptied the gauge at midnight last night--there was 1.85 in the gauge. Between midnight and this morning I got the other 8.52!! I knew there might be a lot of rain in the gauge this morning, when I had to wade water on my front porch and slightly thru my living room to get outside!! AND, we live on a hill!! It had just rained so much that it had not drained off as yet! I am still stunned by the amount of rain--and it will still be going on for a few more days!! This could be quite the disaster--our communities all around are flooding and people being rescued! 

The radar loop from 1:00 a.m. CDT to 7:00 a.m. CDT shows how slowly the thunderstorms moved over the same area. Click here to see a 6-hour radar loop

The regional radar image at 2:03 a.m. CDT on August 6 shows the cluster
of thunderstorms already firing over south central Missouri.
It's no surprise that there was widespread flooding from the heavy rain.  A 14 mile stretch of Interstate 44 in Missouri was closed due to flooding. Many home were evacuated, and there are two confirmed fatalities from the flooding.

Five to more than 7 inches of rain in Middle Tennessee in the 24-hour period ending this morning caused widespread flash flooding there.

Unfortunately it doesn't look like there is going to be much of a break in the rain and a chance to dry out. The 7-day outlook for precipitation paints another several inches of rain over this same area.

Quantitative precipitation forecast for the 7-day period ending 7:00 p.m. CDT August 15.