Monday, August 31, 2015

Cyclones Aplenty

If you're a tropical weather enthusiast the last week to ten days has been an interesting period. In the Atlantic Danny was briefly a hurricane, but fizzled out pretty quickly and fell apart last Monday in the eastern Caribbean producing heavy rain but not much else. In the wings was Tropical Storm Erika, which in the early stages appeared to be a threat to Florida. However, Erika never did make hurricane strength. It quickly fell apart as strong wind shear and the mountains of Hispaniola combined to weaken it, and the storm dissipated off the northeast coat of Cuba. Erika did bring heavy rain to the Caribbean last week and the remnant trough of low pressure continued to produce heavy rain across Florida and the southeast coast this weekend.

7-day accumulated rainfall for Puerto Rico ending the morning of 8/31/2015

24-hour precipitation for the southeastern U.S. ending the morning of 8/31/2015

A low pressure system that move off the west coast of Africa became Tropical Storm Fred Sunday,  Hurricane Fred early this morning, and was downgraded to Tropical Storm this evening. Fred is a small cyclone and likely will be moving into unfavorable upper air conditions in the next few days. Current forecasts call for it to be only a tropical depression by the end of the week.

Forecast track for Fred.

Conditions are even more interesting in the central Pacific, where three hurricanes are spinning.

NOAA GOES-West image of three hurricanes in the Pacific: Kilo (left), Ignacio (middle), and Jimena (right).

Hurricane Kilo is a Category 4 hurricane located several hundred miles east of Wake Island. Hurricane Ignacio is a Category 2 storm (it was a Category 4 storm on Sunday) located about 350 miles east of Honolulu. Hurricane Jimena is another Category 4 storm with peak sustained winds of 145 mph. Ignacio is not expected to directly threaten Hawaii, but it is producing dangerous surf conditions. This is the first time three major hurricanes (category 3 or higher) have been observed over the central Pacific at the same time.

The eye of Hurricane Jimena taken by astronaut Kimiya Yui on the International Space Station

The tropical cyclone activity in the Pacific is being supported by some very warm water. Below is an image of the Pacific sea surface temperature anomaly as of today.

Sea surface temperature anomaly on August 31. The water over which the hurricanes are located is 1.5°C to more than 2.5°C (2.7°F to 4.5°F) above normal.

Finally Tropical Depression 14-E developed in the eastern Pacific and late today was located about 700 miles south of the southern tip of Baja California. It is expected to strengthen to a tropical storm in the next 24 hours and turn to the north.

Wednesday, August 26, 2015

An Index of "Perfect Pleasant" Weather

I was reading through a recent post of Minnesota Weathertalk Blog by Mark Seeley, an Extension climatologist and meteorologist at the University of Minnesota when his mention of a "Camelot Climate Index" caught my attention.  There are a laundry list of indices used in meteorology and climatology of various types and purposes (Heat Index, Wind Chill Index, Southern Oscillation Index, lifted index, etc. etc.) but the "Camelot Climate Index" was a new one to me. Intrigued, I looked into it further.

The Camelot Climate Index attempts to identify locations with the "perfect pleasant climate", according to Jan Null, a Certified Consulting Meteorologist who developed the index. True to its name, the index was inspired by the lyrics to "Camelot" written by Alan Jay Lerner for the the 1960 musical. (Click image to see a larger version).

Null started thinking about such an index in the early 1990s, inspired by articles and rankings of the "best places to live". Most of these used the annual number of sunny days, which he felt fell short of truly defining a pleasant place to live. His idea of the ideal climate is sunny and relatively mild with few extremes in temperature, humidity or precipitation, and no snow. An interest in musical theater led to the connection with Camelot, and so he went on to develop the Camelot Climate Index using these variables.

The index is calculated using the 1981-2010 monthly average values of maximum and minimum temperature, average afternoon relative humidity, the number of days above 90 and below 32, precipitation, and sunshine data. He calculated weighting factors for the variables, and then subtracted the weighting values from 100 (100 represents "perfect" weather).

The resulting map of values indicates that the weather best meeting the criteria Null established extends from the Desert Southwest to and up the west coast. The highest index values (and thus the "perfect pleasant climate", as Null puts it) is along the California coast.

A complete description of the values and calculations used can be found on his Camelot Climate Index web page.

Null is the first to admit that the Camelot Climate Index is completely subjective because it's based on his perception of what makes the perfect climate - yours and mine may be different. Some time in the future he would like to construct a "variable" index that might be compiled for different perceptions of an "ideal climate".

We have been enjoying some beautiful (dare I say perfect?) late summer weather here in much of the Midwest this week. Skies have been mostly sunny, daytime highs in the 70s, overnight lows in the 50s, and no rain. While that's great weather for a week or three, to me at least, I don't think I could deal with it year round. I enjoy the four distinct seasons and the weather they produce. So while San Diego might be a great place to visit, I wouldn't want to live there. I have to have my snow and thunderstorms. Heck, even the song "Camelot" mentions snow.

Tuesday, August 18, 2015

Stirrings in the Subtropical Atlantic

It has been a quiet tropical storm season so far this year. As of today there have been only three named storms: T.S. Ana from May 8-11, T.S. Bill from June 16-20, and T.S. Claudette from July 13-14.  This morning Tropical Depression #4 was identified by the National Hurricane Center, and this afternoon it was upgraded to Tropical Storm Danny.

The first three named storms of 2015.

While this season may seem quiet, it is actually running close to normal this year so far. Normally by this date in August there are only three to perhaps four named storms, with one of those hurricane. None of the three storm this year attained hurricane strength.

Climatology of tropical cyclones in the Atlantic Basin. The green lines intersect at about today's date.

NOAA's outlook for the 2015 hurricane season, updated on August 6, maintained its earlier outlook for a below average tropical season. The outlook estimates a 90 percent probability for the following:

    6-10 named storms, which includes the three named storms to date
    1-4 hurricanes
    0-1 major hurricanes
The climatological average for a season is 12 named storms, 6 hurricanes, and 3 major hurricanes.

The outlook for a below-average season is the state of the current oceanic and atmospheric conditions and predicted conditions through the fall. These include the strengthening El NiƱo, which tends to produce strong vertical wind shear and enhanced sinking motion across the tropical Atlantic and Caribbean Sea, both kryptonite to tropical storm development, and cooler than average sea-surface temperatures in the tropical Atlantic.

There were only eight named storms in 2014, and then you have to back to 1997 to find a season with that few storms. The period from 1991 through 1994 were seasons with 8, 7, 8, and 7 storms respectively.  Residents of the Atlantic and Gulf coasts should not get complacent and drawn in to a false sense of security, however. It only takes one storm to cause devastation. Hurricane Andrew in August of 1992 was only one of seven storms that year, and was, at the time of its occurrence, the costliest hurricane in U.S. history. It is now second only to Katrina in 2005 (adjusted to 2010 dollars).

So what is the normal character of the season at this time? The season rapidly spins up to a peak about September 10, with a small secondary peak in mid October.

The typical origins and paths of tropical cyclones in August and September are shown below.

The origins and prevailing tracks of tropical cylones in the Atlantic Basin in August (top) and September (bottom).

Tropical cyclones can originate just about anywhere in the basin from August 21-31, with a band from the west coast of Africa through the subtropical Atlantic to the Caribbean most favored.

Locations of tropical cyclone formation for the period from August 21-31. Data for the Atlantic is from 1851-2009.
Source: National Hurricane Center

These maps and much more information can be found on the National Hurricane Center's Tropical Cyclone Climatology web page.

Tropical Storm Danny is currently expected to attain hurricane strength Thursday afternoon. It will several days before any threat to land can be determined. In the meantime you can follow the National Hurricane Center's outlooks and advisories on the NHC website.

Advisory on Tropical Storm Danny issued at 5:00 p.m. EDT August 18.

Tuesday, August 4, 2015

Tampa and West Central Florida Slosh Through Two Weeks of Heavy Rain

It has been a very quiet tropical season in the Atlantic Basin. There have been only three named systems in the Atlantic so far this season (Ana, Bill, Claudette), and none of these tropical storms have directly affected Florida. Typically the west-central coast of Florida gets about 60 percent of its annual average rainfall in the months of June through September, coinciding with the first half of the tropical storm season.

Tampa flooding on August 2.
Credit: John Kassel via Facebook
In the past two weeks west-central Florida has received from 10 to 24 inches of rain resulting in persistent flooding and a general mess for residents. The Tampa Bay areas has been been the bulls-eye for the heavy rain, with Pasco, Pinellas, and Hillsborough Counties recording the highest rainfall totals.

14-day accumulated precipitation for Florida.
Source: NWS Advanced Hydrologic Prediction Service

The rain has resulted in persistent and widespread flooding. The flooding has closed numerous streets and roads, overloaded wastewater pumping stations, and forced the suspension of trash collection. In Pasco County the Anclote River threatened 5,700 homes with flooding, with more than 320 homes evacuated.

Flooding on August 4 in the Seven Springs area of Pasco County.
Credit: Pasco County Sheriff

The river crested at 25.25 feet at 11:45 a.m. EDT this morning and is expected to steadily fall to below flood stage on Friday. The record crest for the Anclote is 27.7 feet set on August 8, 1945. The last time the river was this high was in June 2012 when it reached a crest of 26.81 feet.

The reason for the rain was a frontal system which stalled over central Florida and a series of low pressure waves along that front. The cold front pushed into northern Florida on July 24. It stalled over central Florida on July 25, and that's when the skies opened up. Two to seven inches of rain fell on the Tampa Bay area in the 24 hour period ending on July 25, and it has rained every day since.

Surface map for 8:00 p.m. EDT July 25, 2015

The chart below plots the last 14 days of rainfall for CoCoRaHS Stations FL-HB-55 (Tampa 5.0 NNE),FL-PS-4 (Port Richey 2.0 NNE), and FL-PN-41 (Tarpon Springs 5.6 E), the highest totals for the period in Hillborough, Pasco, and Pinellas Counties respectively. Though the rainfall totals for the stations for the two-week period are similar, the three stations generally had wide day-to-day differences in rainfall. Note that there is no observation available for FL-PN-41 for August 4 as of this post, but other CoCoRaHS observers in the vicinity had another two to three inches of rain this morning.

Although July 25 marked the start of the very heavy rain, rain has fallen in this area every day since the middle of July. As of today FL-HB-55 has had 21 consecutive days of measurable rain, FL-PN-41 21 days, and FL-PS-4 23 days. The average July precipitation for this area is about 7.90" and the average August precipitation 8.80".  Average annual precipitation for this area is about 52 inches.

The 11.84 inches measured in July at the Tampa International Airport was the 8th highest July total since records began in 1939. The record is 20.59 inches in 1960.   However, it was on the low end of totals reported by CoCoRaHS observers in the areas. A few Hillsborough County stations reported more than 15 inches for July. However in Pasco County FL-PS-4 tallied 20.90 inches for July, and in Pinellas County FL-PN-41 measured 26.90 inches.

The Tampa Bay area got a break today, as most rain was north and east of the area as the low pressure system lifted northeast. It was located on the South Carolina coast today. More than four inches of rain was reported  along the South Carolina and North Carolina coasts this morning.

24-hour precipitation ending the morning of August 4.
  The low is expected to continue into the Atlantic before merging with another weather system, but not before more rain and winds affect the coastal areas.