Thursday, August 30, 2012

Midwest Braces for Isaac's "Leftovers"

Late this afternoon Tropical Depression Isaac was located near the Louisiana/Arkansas line. moving in a northerly direction at about 12 mph, twice the speed it was moving yesterday.  Isaac left up to 20 inches of rain behind in Mississippi and Louisiana, and those areas can expect more rain in the next two days. CoCoRaHS observers in Hancock County, MS near Kiln reported 24-hour rainfall totals of 14.18 and 13.89 inches. This was on top of 2.8 inches received the day before. In Tangipahoa Parish in Louisiana the CoCoRaHS observer at LA-TG-2 near Hammond reported 11.00 inches this morning. Observations in southern Louisiana were few and far between because of the flooding and power outages.
This is a map of the 24 hour precipitation accumulation produced by combining radar and rain gauge measurements. Source: NWS Advanced Hydrologic Prediction Service

Now the Midwest is bracing for Isaac's rain.  A flash flood watch is in effect for much of Arkansas, and will probably be extended through Missouri and Illinois tomorrow and Saturday. The forecast track of Isaac has varied a little over the last 24 hours, but the axis of heavy rain is still likely to be an area from Arkansas through most of Missouri, Illinois, Indiana, and Ohio. Generally 3 to 6 inches of rain can be expected, with some locations receiving more. Some of the squalls will be accompanied by strong gusty winds, and a few severe thunderstorms are not out of the question.

Forecast track for Isaac issued at 4:00 p.m. CDT
Quantitative precipitation forecast for August 30-September 4

Wednesday, August 29, 2012

Tropical Rain, and Lots of It

Deep tropical moisture along the southeast coast of the U.S. has been responsible for torrential rains in areas far removed from Hurricane Isaac.  For the 24 hour period ending on August 28th the heaviest rain was found along the central east coast of Florida.  CoCoRaHs observers in Indian River, St. Lucie, and Martin Counties reported seven to more than 10 inches of rain for the 24 hours ending Tuesday morning, with the CoCoRaHS observer at FL-IR-32 in Vero Beach reporting 10.50 inches of rain.

Rainfall for the 24 hour period ending Tuesday, August 28, 2012 in Florida

Yesterday the heavy rainfall fell along the South Carolina coast, with CoCoRaHS observers north of Charleston, SC reporting four to more than 6 inches of rain.   6.80 inches was reported this morning by the observer at SC-CR-46. A flash flood watch is in effect for the Charleston area through tonight with more rain expected.

Rainfall for the 24 hours ending the morning of Wednesday, August 29, 2012 in Charleston, SC

Meanwhile, Isaac continues his agonizingly slow crawl to the north and west through southern Louisiana. In the past 14 hours Isaac has moved only about 75 miles. As of 10:00 CDT this morning Isaac was still a minimal hurricane with gust over 80 mph measured in some areas. Isaac is pretty impressive on radar and exhibits a distinct but somewhat ragged eye. Radar estimated rainfall is almost 12 inches east of the storm center as of noon CDT.  With Isaac moving only at about 5 mph, much more rain can be expected the rest of today.

Radar estimated rainfall for the Gulf Coast at 12 noon CDT, August 29, 2012.

Hurricane Isaac as seen from the New Orleans, LA radar at 12 noon CDT, August 29, 2012

Tuesday, August 28, 2012

Isaac Pounding Gulf Coast

Hurricane Isaac has made landfall once already, moving over the Mississippi River delta shortly before 7:00 p.m. CDT August 28, but has since moved back out over the water. He is expected to make landfall again near Grand Isle, Louisiana later tonight.  Here is the radar image of Isaac as of 9:52 p.m. CDT tonight (August 28). 

Hurricane Isaac just off the coast of Louisiana, 9:52 p.m. CDT August 28  
 Isaac is "just" a Category 1 storm with top winds of 80 mph. However, it is producing dangerous storm surge from the Mississippi River Delta eastward.  Isaac's slow movement means that the Gulf coast will deal with prolonged high winds and very heavy rain. The NWS Hydrometeorological Prediction Center's latest 5-day quantitative precipitation forecast paints an impressive amount of rain from the Gulf Coast northward through the southern half of the Midwest as Isaac moves slowly northwest inland and then turns north and northeast.

CoCoRaHS observers across the central and southeastern U.S. will be getting a lot of practice measuring rainfall the next several days. In much of the Midwest there could be more rain the last week of August than fell in all of June and July.  In my post on July 31 I suggested that the central U.S. would love to see a tropical system bring it's rain to drought stricken areas, but that it was probably a long shot. Maybe I should buy a lottery ticket!

Thursday, August 23, 2012

A "Warm Up" for Isaac?

On Tuesday and Wednesday more than nine inches of rain fell in southern Pinellas County, Florida. On the morning of August 22 two CoCoRaHS observers in South Pasadena (west of St. Petersburg). Both reported more than seven inches of rain, and added another 1.50 inches by the morning of August 23. Station FL-PN-17 measured 7.95 inches on August 22 and 1.60 on August 23 for a two-day total of 9.55 inches.  Station FL-PN-6 measured 7.07 and 1.45 inches on th two days for a total of 8.52 inches. At station FL-PN-17 4.27 inches fell in the first 80 minutes of the storm! The thunderstorms were part of a complex of storms that moved from the Gulf east across central Florida.
Rainfall in Pinellas County, FL for the 24 hour period ending the morning of August 22, 2012
Buckets of rain could be in the cards for the Tampa area and western Florida early next week depending on the track and intensity of now Tropical Storm Isaac. The latest path projection takes Isaac into the western Gulf of Mexico. If that's the case the west coast of Florida will be in for some significant wind and rain.  In the meantime, the Tampa area will be generally dry the next couple of days as winds swing into the northeast cutting off the flow of moisture from the Gulf of Mexico.

Position and projected track of Tropical Storm Isaac as of 8:00 p.m. EDT August 23, 2012

Thursday, August 16, 2012

Record Heavy Rain in Alaska

While almost 62 percent of the lower 48 states deals with drought conditions, a series of storms systems have dumped excessive amounts of rain on northwestern Alaska so far this week. Amounts in excess of five inches have been recorded so far, with 5.0 inches at Red Dog Mine, 3.50 inches of rain at Kivalina and 2.50 inches a Kotzebue. Northwest Alaska is a relatively dry region, and the amounts received this week amount to about one-third of the average annual precipitation. In this region, a three inch rainfall in three days is a once in a hundred year event!

Image courtesy of NWS Alaska Region

The heavy rain has caused flooding and river levels are expected to remain high the rest of the week. Two more storm systems are forecast to hit the area in the next two to three days producing additional rainfall one one to three inches.

Weather map for Thursday, August 16.  Source: NWS Anchorage, AK
Forecast map for Saturday, August 18, 2012.  Source: NWS Anchorage, AK
Interestingly, it has been dry in central southern Alaska.  Anchorage has received only 0.61 inches of rain so far this month, with a normal August total 3.30 inches.  Fairbanks, with an average total of 1.88 inches in August, has only mustered 0.13 so far this month.

Most of our CoCoRaHS stations in Alaska are in and around Anchorage. A couple of the Anchorage CoCoRaHS stations have recorded a little over an inch for the month, but as is typical during the summer, precipitation can vary a lot over a small distance, even in Alaska.

Thanks to colleague Sam Shea for the "heads up" on this unusual event.

Monday, August 13, 2012

What Happened to Summer?

The recent spell of cool weather in the eastern half of the country has been a welcome respite from the brutal summer weather this year. Some may argue that most of the central and eastern U.S. has experienced two years worth of summer in two months this year.  Although August started out on the warm side, a significant change in the upper level wind patterns has brought an early taste of fall to the central U.S.

This all began at the end of the first week of August with the passage of a strong upper low through the middle of the country. As the trough developed over the eastern half of the U.S. the upper level ridge retreated westward. A second, even strong upper level low dropped into the Midwest this weekend, reinforcing the trough over the eastern U.S.

The 500 millibar map (~18,000-20,000 ft) for the morning of Friday, August 10
showing the strong upper low over the Great Lakes.

It's been a welcome turnaround. Temperatures the last week have averaged near to well below normal across much of the central U.S., while the western U.S. is experiencing the hot weather. In addition, there have been several rounds of much needed rain. It certainly hasn't broken the drought, but has slowed any worsening for the time being.

After piling up massive numbers of record high temperatures the last two months in the central U.S., record low minimum and record low maximum temperatures have been the rule the past few days. Saturday was a very cool day in Michigan with high temperatures topping out only in the low 60s, many of those records. Record low temperatures were also scattered through the upper Midwest this weekend with minimum temperature dropping into the low 40s.

Locations that set record low maximum temperatures on August 11.
A complete list of records can be viewed at the National Climatic Data Center Records Lookup page.

The surface low pressure system associated with the upper trough was of record proportions itself. The low generated gales across Lake Michigan and rare lake-effect thunderstorms dropped more than three inches of rain along the lake in northern Illinois. You can read an excellent discussion about this unusual storm system , including graphics and photos, at the National Weather Service Chicago web site.  Heavy rain also fell eastward through Michigan, with close to four inches in Bay County.

CoCoRaHS map for August 10. Note the high rainfall in northeastern Illinois and along the
south end of Saginaw Bay in Michigan.

CoCoRaHS map for August 10 showing high rainfall along the Lake Michigan shoreline in northern Illinois

So will this cool weather last? The 8-14 day outlook issued by the Climate Prediction Center on August 12th indicates a higher probability for cooler than normal weather for the central U.S.  I like that sound of that.