007 AXNT20 KNHC 201452 AAA TWDAT Tropical Weather Discussion...Updated NWS National Hurricane Center Miami FL 1052 AM EDT Thu Sep 20 2018 Updated Caribbean section for SW Caribbean convection 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 0600 UTC surface analysis and satellite imagery through 1400 UTC. ...TROPICAL WAVES/TROPICAL TROUGHS... A far eastern Atlantic tropical wave has its axis along 21W/23W from 05N25N to 14N24W to 20N23W, moving westward at 10 to 15 kt. This wave is supported by a 700 mb inverted-trough from 14N to 20N. om 20N southward. This wave is associated with a distinct 700 mb trough from 13N to 20N. In addition, mid-level satellite winds show winds veering in direction from NE to SE across the wave axis. The total precipitable water (TPW) satellite animation shows a well-depicted plume of moisture trailing the wave to the coast of Africa. A far eastern Atlantic surface trough is analyzed from 10N31W to 19N30W, moving westward at about 10 kt. The trough continues to be reflected at 700 mb as a rather distinct inverted trough. 700 mb trough. No deep convection is seen with the trough, only isolated showers and thunderstorms within 60 nm either side of the A 1010 mb low, formerly a tropical wave, is analyzed near 10N48W, moving westward at 10 to 15 kt. The overall cloud pattern of this low appears more symmetric than 18 to 24 hours ago. Satellite imagery shows scattered moderate to strong convection within 120 nm of the low in the northern semicircle. Scattered moderate convection is within 90 nm of the low in the southern semicircle. Latest NHC Tropical Weather Outlook calls for some development of this system today before before upper-level winds become highly unfavorable for tropical cyclone formation starting tonight and continuing through the weekend. An eastern Caribbean Sea tropical wave has its axis along 69W south of 20N to inland Venezuela at 09N. It is moving westward at 10 to 15 kt. Satellite imagery shows a rather large area of scattered showers showers and thunderstorms east of the wave north of 15N and between the wave and 65W. The TPW animation concurs with this shower and thunderstorm activity in depicting low-level over the eastern Caribbean Sea. Satellite imagery indicates some Saharan dust in the wake of the wave over the far eastern Caribbean Sea and tropical Atlantic between the Lesser Antilles and 55W. The dust is forecast to continue to spread westward, across the eastern Caribbean Sea islands, bringing hazy skies and stable conditions through early Sat. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends off the coastal sections of Senegal near 13N17W to 12N21W. The ITCZ axis extends from 10N24W to 08N33W to 06N44W. Scattered moderate convection is noted from 04N to 10N between 11N and 20W, and within 180 nm south of the ITCZ between 28W and 31W. Scattered moderate convection is to the northwest of the ITCZ within 60 nm of line from 09N51W to 09N60W. ...DISCUSSION... GULF OF MEXICO... A cyclonic shear axis is observed on water vapor imagery to stretch from the NW Bahamas to S Florida, westward to near 26N85W, and northwestward to near 29N90W. Cyclonic flow covers the eastern gulf south of this shear axis, while strong easterly upper-level flow around the southern periphery of an anticylone anchored over far northwestern Georgia. Between 90W and 94W, the upper flow is anticyclonic from a weak anticyclone centered at 24N93W. W of 94W, the upper flow becomes broadly cyclconic in advance of an upper-level trough that is moving eastward across Texas and extends to the far eastern section of Mexico. The trough is moving through a very moist and unstable atmosphere. Scattered showers and isolated thunderstorms are over the far western gulf from 21N to 28N west of 95W. Mid and upper-level clouds associated with this activity are streaming northeastward west of 94W. A surface trough, remnants of former tropical cyclone Isaac, is located over the Yucatan Peninsula. No deep convection is noted. Earlier overnight scattered shower and thunderstorm activity observed over the eastern Bay of Campeche has weakened as the typical nocturnal Yucatan Peninsula trough has progressed westward across the eastern Bay of Campeche this morning. Isolated showers and thunderstorms are over the Bay of Campeche and waters adjacent to the Yucatan Peninsula. A weak surface trough is analyzed over the southeastern Gulf of Mexico from near Naples to near 22N88W. Isolated showers are near the trough from 22N to 24N bewteen 83W and 85W. Isolated showers and thunderstorms are over the remainder of the gulf waters. The forecast calls for high pressure to build across the eastern United States through tonight. The associated gradient will bring gentle to moderate east to southeast winds and 2 to 4 ft seas across the basin through Sun. The surface trough over the eastern Yucatan Peninsula will weaken as it gradually moves westward across the remainder of the Yucatan Peninsula through tonight. CARIBBEAN SEA...Updated A mid to upper-level central Atlantic trough axis stretches southwestward to a small cyclonic circulation over the Windward Passage, to 15N76W and to near 13N79W. Isolated showers and thunderstorms are just northeast of Jamaica from within 60 nm of 18N76W. Isolated showers and thunderstorms are seen from north of 16N and west of 79W. An eastern Caribbean Sea tropical wave, with axis along 69W, will move westward, across the central Caribbean Sea through Fri and into Central America on Sun. Scattered showers and thunderstorms east of the wave north of 15N and between the wave and 65W may be attendant by strong gusty winds and at times rough seas. This activity is underneath an upper anticyclone that should help sustain it through Fri even with the wave axis translating westward. The extreme northern edge of this activity is approaching the far southwest side of Puerto Rico and the Mona Passage. The gradient associated with high pressure building from the central Atlantic to the central Bahamas will support moderate to fresh trade winds and building seas across the entire basin from late on Wed through Fri. The eastern segment of Pacific monsoon trough protrudes into the far southwestern Caribbean Sea. With this feature there, and combined with the already low-level speed convergence in place there, this presently leading to increasing scattered moderate to strong convective clusters over the waters southwest of a line from near 13N83W to 12N80W to 11N76W. This activity is likely to remain active into this evening. Scattered showers and thunderstorms are approaching the lower Windward Islands over the waters just east of Trinidad and Tobago. ATLANTIC OCEAN... A narrow mid-upper trough is indicated by the models and observed in water vapor imagery extends from near 32N70W southwestward to 27N75W, where it transitions to a cyclonic shear axis to S Florida, and continues well into the southeastern Gulf of Mexico. To the east and southeast of this trough, a deep-layer trough extends from near 32N61W to 26N63W and to a small cyclonic circulation over the Windward Passage. A surface trough is analyzed from near 32N61W to near 30N63W. Also at the surface, a cold front extends from a 1004 mb low (gale center) located well north of the area at 33N68W, southwestward to 32N71W and to near 29N80W. Surface observations and satellite imagery suggest that a surface trough may be forming from near 27N68W to just east of the SE Bahamas. Isolated showers and thunderstorms are seen from 24N to 28N between 65W and 71W, while scattered showers and thunderstorms are along and within 60 nm south of the cold front between 70w and 79W. Similar convection is within 30 nm either side of a line from 23N69W to 25N65W to 27N61W, and also north of 29N between 58W and 63W, where upper dynamics are more robust. Over the eastern Atlantic, a broad upper-level low is observed near 28N37W, with a trough axis stretching southwestward to 29N48W and to a base near 13N57W. A cyclonic shear axis extends east-northeastward from the upper-low to 30N29W and continues northeastward as an upper trough to an ill-defined small upper- level low that is seen in water vapor imagery near 32N23W. A mid and upper-level trough is over the far eastern Atlantic along a position from near 24N22W to a small upper-level circulation at 20N23W and to base near 12N29W. Broad upper-level anticyclone is observed between this trough and the trough related to the aforementioned broad upper-level low. At the surface, the remnant low of former tropical cyclone Joyce is analyzed near 29N31W with a pressure of 1014 mb. Latest satellite imagery shows broken low and mid-level clouds with possible isolated showers within 90 nm of the low in the NE semicircle and 60 nm of the low in the SE semicircle. The recent trends in the satellite imagery indicate that this low is weakening further, and most likely will be a trough later today. A pair of 1021 mb high are analyzed. The first one is anchored near 30N49W, and the second one is just north of the area near 33N37W. The surface analysis depicts high pressure covering the area north of 23N from 40W to 60W. The aforementioned cold front is forecast to move southeastward to the waters north of 27N through Fri before it dissipates. The 1004 mb gale center north of the area is forecast to weaken as it moves southeastward to near 34N62.5W by early Fri. The attendant cold front will also weaken through Fri. The low will also weaken as it moves to near 31N67W by Fri evening. It is then forecast to drift westward and slowly weaken Sat through Sun. NE swell will move into the open waters east of the Bahamas Fri through early parts next week. For additional information please visit http://www.hurricanes.gov/marine $$ Aguirre