Quiet in the Atlantic; Two Disturbances for Hawaii to Watch

The tropics have fallen silent the past two days, with no named storms anywhere on the planet. It's not unusual to see a quiet period for tropical cyclones in July, which lies before the climatological peak months of August, September, and October in the Northern Hemisphere (a tropical cyclone is the generic term for all tropical depressions, tropical storms, and hurricanes/typhoons). The tropical cyclone-free period will likely be short lived, though, as Invest 90E, located in the Eastern Pacific about 1,050 miles southwest of the southern tip of Mexico's Baja Peninsula on Monday morning, may develop into a tropical depression by Wednesday. This system is something residents of Hawaii should keep an eye on. Satellite loops show the disturbance has good degree of spin, but heavy thunderstorm activity is limited due to dry air and moderate wind shear of 10 - 20 knots. The system is on a trajectory that will likely take it within 300 miles of Hawaii this weekend, but the latest SHIPS model forecast shows that late this week 90E will encounter higher wind shear, cooler ocean temperatures, and drier, more stable air. These conditions should cause significant weakening as 90E approaches Hawaii. In their 8 am EDT Tropical Weather Outlook, NHC gave 90E 2-day and 5-day odds of development of 40% and 70%, respectively. Hawaii should also watch a tropical wave located several hundred miles south of the southern tip of the Baja California peninsula. This wave was moving westwards, and should arrive in the vicinity of Hawaii by Tuesday next week. In their 8 am EDT Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day odds of development of 0% and 40%, respectively.


Figure 1. Tropical Depression Two in the Bay of Bengal as seen on Monday afternoon, July 27, 2015, from the MODIS instrument on NASA's Aqua satellite. The Indian Meteorological Department (IMD) gave top winds of 30 mph to the system, which was nearly stationary. Image credit: NASA.

In the Indian Ocean's Bay of Bengal, the Indian Meteorological Department (IMD) is issuing advisories on Tropical Depression 2, which is bringing heavy rains to portions of India and Bangladesh along the coast of the Bay of Bengal. Tropical depressions embedded within India's monsoon rarely grow into strong tropical storms, but can be prodigious rain makers. India's monsoon has been 12% below normal in rainfall as of July 22, so the country could use more rainfall--though perhaps not in the concentrated manner a monsoon tropical depression typically delivers, causing dangerous flooding rains. The Joint Typhoon Warning Center (JTWC) is not classifying this system as a tropical depression yet.

In the Western Pacific, all looks to be quiet until at least this Saturday, when both the European and GFS models predict a new tropical depression could form about 500 miles east of the Philippines.


Figure 2. Saharan Air Layer (SAL) analysis for 8 am EDT Monday, July 27, 2015, from the University of Wisconsin CIMSS shows plenty of dry air dominating the tropical Atlantic. Two tropical waves over the Eastern Atlantic were apparent, with no signs of development.

African tropical wave-watching season has begun
In the Atlantic, it's the time of year when we need to start watching the regular procession of tropical waves coming off the coast of Africa. About 85% of all major hurricanes in the Atlantic get their start as an African tropical wave, so these potential trouble-makers are important to track and monitor. We do have several solid tropical waves with decent spin and moisture that have pushed off the coast of Africa over the past few days, but these tropical waves face a rugged path ahead of them if they want to develop into tropical depressions. Wind shear off the coast of Africa is not prohibitive--a moderate 10 - 20 knots--but a massive area of dry, dusty air--a Saharan AIr Layer (SAL) outbreak common for this time of year--is dominating most of the tropical Atlantic, from the coast of Africa into the Central Caribbean. This dry air will make it difficult for any tropical waves to spin up into tropical depressions over the Eastern Atlantic. If something does manage to form, it will likely be short-lived, if it attempts to move very far west. High wind shear of 20 - 40 knots dominates the Caribbean, and is expected to stay strong for at least the next five days. The ensemble runs of the GFS and European models--done by running the models at lower resolution and varying the initial atmospheric conditions slightly to generate an "ensemble" of twenty potential weather situations (fifty for the European model)--do have a few of their 20 - 50 runs that develop a tropical depression from one of these African tropical waves later this week. However, none of these solutions have the storm that develops making it as far west as the Lesser Antilles Islands, and the operational high-resolution versions of our three top models for predicting genesis of tropical cyclones--the GFS, European, and UKMET models--do not show anything developing this week. NHC did not highlight any suspected areas of development over the next five days in their 8 am Monday Atlantic Tropical Weather Outlook.


Figure 3. Latest radar image out of Tampa, Florida.

Heavy rains in Florida from Gulf of Mexico low
A low pressure system has formed in the Gulf of Mexico off the coast of Florida, and will drift toward the Florida coast today, bringing 1 - 3" of rain with a few high spots of 3 - 5" over Central Florida over the next few days. The Tampa radar is estimating that this low has already brought 6+ inches of rain to the coast near Tampa, Fort Myers, and Naples. High wind shear of 20 - 30 knots is keeping this system from developing. While a number of members of the GFS and European ensemble model forecast do show this system developing, none of the operational versions of our reliable models for predicting genesis of tropical cyclones show development over the next five days. We should keep an eye on this system over the next few days, but I am not expecting it to develop.

Wunderblogger Steve Gregory will be providing updates by about 4 pm EDT on Monday, Wednesday, and Friday this week.

Jeff Masters