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000
AXNT20 KNHC 211805
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 PM EDT Fri Jul 21 2017

Tropical Weather Discussion for North America, Central America Gulf of Mexico, Caribbean Sea, northern sections of South America, and Atlantic Ocean to the African coast from the Equator to 32N. The following information is based on satellite imagery, weather observations, radar and meteorological analysis.

Based on 1200 UTC surface analysis and satellite imagery through 1745 UTC.


...TROPICAL WAVES...

A tropical wave over the eastern Atlantic extends from near 17N32W to 04N34W, moving westward near 20 kt. The wave crosses monsoonal southwesterly flow that exists off the coast of Africa, and where deepening moisture is evident. The GFS guidance shows a well- defined 700 mb trough with this wave. Scattered moderate convection is east of the wave within 120 nm north of the monsoon trough. METEOSAT imagery along with SSMI Total Precipitable Water (TPW) imagery show an extensive area of Saharan dry air and dust northwest through northeast of the wave.

A large amplitude tropical wave is over the central Atlantic with its axis extending from near 24N57W to 16N58W, and becomes ill- defined to 09N57W. It is moving westward at 15 kt. Total Precipitable Water imagery depicts a bulge of very deep moisture within 180 nm east of the wave from 17N-21N. This wave has been exhibiting a very well-defined inverted-v shape structure the past several days. Latest satellite imagery shows and increasing area of scattered moderate isolated strong convection from 19N- 22N between 55W-60W. Scattered showers and isolated thunderstorms are elsewhere within 180 nm either side of the wave. A portion of this wave is forecast to move across the eastern Caribbean during the upcoming weekend, bringing increasing moisture that will produce scattered showers and thunderstorms with gusty winds.

A pretty well-defined tropical wave, which in recent days was part of the large cyclonic envelop that encompassed the wave to its east described in above paragraph, is identified over the eastern Caribbean this morning with its axis from just east of Puerto Rico to 14N67W to inland Venezuela at 11N67W. It is moving westward at 20 kt. SSMI Total Precipitable Water imagery moisture to the west of the wave to near 71W, and within 120 nm east of the wave from 13N-17N. There is a well defined 700 mb trough associated with this wave. Scattered showers and isolated thunderstorms are seen west of the wave to 75W and north of 15N. Scattered showers are within 120 nm east of the wave north of 13N. Any of the shower and thunderstorm may produce strong gusty winds as the wave moves westward across the remainder of the eastern Caribbean this afternoon and evening, then across the central Caribbean on Saturday and Saturday night.

A tropical wave is over the western Caribbean with its axis near 83W south of 18N to the eastern Pacific waters near 04N84W. It is moving westward near 15 kt. This wave is the remnant of former Tropical Storm Don. Abundant low level moisture is to the south of 15N west of 80W, and has resulted in scattered moderate to isolated strong convection from 10.5N-13N west of 80W. Scattered showers and thunderstorms are elsewhere south of 14N west of 79W. The wave portion south of 15N will move inland Central America this afternoon, while the portion north of 15N will move inland the far southern Yucatan Peninsula and Belize on Saturday. Expect scattered to numerous showers and thunderstorms to affect much of the interior portions of Honduras and Nicaragua through Sunday and showers and thunderstorms to increase over the southern Yucatan Peninsula and Belize late Saturday through Sunday. This activity may contain gusty winds and produce locally heavy rainfall.


...ITCZ/MONSOON TROUGH...

The monsoon trough axis extends from 15N17W to 13N24W to 12N32W to 09N38W The ITCZ then extends from 09N38W to a weak low pressure of 1014 mb near 09N42W to 06N45W and to 10N59W. It resumes just west of the central Atlantic tropical wave to 10N62W. Aside from the convection associated with the tropical waves, scattered moderate isolated strong convection is just offshore the coast of W Africa from 11N-14N between 17W-20W. Scattered moderate isolated strong convection is within 240 nm north of the ITCZ axis between 42W-50W and also within 120 nm south of the ITCZ axis between 43W-48W. Scattered moderate convection is south of the ITCZ axis within 60 nm of 08N58W.


...DISCUSSION...

GULF OF MEXICO...

In the upper levels, a small upper level low is centered over Mexico near 23N100W. Another small upper level low is centered over the Yucatan Peninsula near 21N91W. A third and larger upper level low has dropped southwest from offshore the southeastern U.S. coast where it was yesterday to the present location of the NE Gulf. Plenty of moisture and instability is in place over much of the central and eastern Gulf. This has lead to the development of scattered to numerous showers and thunderstorms over these areas of the Gulf, with the added assistance of a weak surface trough over the north-central Gulf as well. The low over the NE Gulf will retrograde westward to the north-central Gulf by tonight. With weak pressure in place maintaining a relatively weak pressure pattern and forecast to continue through the next 48 hours, expect for the shower and thunderstorm to be the norm through the weekend. Some of this activity may contain frequent lightning and strong gusty winds.

A surface trough is over the central Bay of Campeche. Isolated showers and thunderstorms are within 180 nm west of the trough, and elsewhere over the Gulf. This activity will change little through Sunday.

CARIBBEAN SEA...

The main features in the basin are the two tropical waves moving through the basin. See the section above for discussions on these features. In between the tropical waves there ares of dry air aloft as noted in water vapor imagery. In addition, some Saharan dry air has advected in over portions of the Caribbean during the recent days. These conditions are allowing for mainly fair weather conditions outside the shower and thunderstorm activity related to the waves. They are forecast to persist through the weekend, with at most isolated showers quickly moving westward in the trade wind flow.


...HISPANIOLA...

A tropical wave moving quickly to the south of the island. An area of moisture to the northwest of the wave as noted in the SSMI Total Precipitable Water imagery is situated just to the south of Hispaniola, and is being enhanced by an upper level low located just east of the central Bahamas. Scattered showers and isolated thunderstorms within this area of moisture are just south of the central and eastern portions of the island as of early this afternoon. Scattered to broken low clouds are developing over much of the interior portions of the island as daytime heating combines with the presence of moisture and instability. Surface observations indicate that shower and thunderstorm activity may be developing from some of these clouds. Moisture from the aforementioned tropical wave is expected to come into play to allow for the formation of more showers and thunderstorms over much of the Hispaniola through early on Saturday, then diminish later in Saturday as the wave pulls off to the west of the Hispaniola regional waters.



ATLANTIC OCEAN...

Two tropical wave are moving through the far southern waters, an a tropical wave is over the far eastern Atlantic region. See the Tropical Waves section above for discussions on these features. details. In the upper levels, latest water vapor imagery shows a large upper level low just east of the central Bahamas, with a trough extending south-southwestward to Haiti and to across the central Caribbean Sea. Another large upper level low moving southeastward is noted over the north-central portion of the basin near 31N45W, with a trough stretching southwestward to 23N50W and to near 19N57W. At the surface, a 1023 mb high is north of the area near 34N39W with a ridge southwestward to a 1024 mb high at 29N54W, and a ridge extending from it to the central Bahamas. Aside from convection associated with the above described tropical waves, scattered to numerous showers and thunderstorms are observed over the far western part of the basin north of 24N and west 76W. Scattered showers and isolated thunderstorms associated with the upper level low near 31N45W exists from 25N- 31N between 40W-50W. Isolated showers and thunderstorms are seen elsewhere north of 20N west of 40W. The remainder of the basin is under very stable conditions as dry air aloft persists. The METEOSAT-9 imagery is depicting extensive plumes of Saharan dust overspreading the eastern portion of the basin, and much of the central portion outside moisture associated with the tropical waves. The most western extent of the dust has reached from the north-central and NW Caribbean Sea areas north to much of the Bahamas of early this afternoon.

For additional information please visit http://www.hurricanes.gov/marine

$$ Aguirre

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