Note on Tropical Storm North of Puerto Rico on September 13, 1945
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THE BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY Entered as second class matter September 24, 1945, at the Post Office at Lancaster, Pennsylvania, under Act of Aug. 24, 1912. Acceptance for mailing at special rate of postage provided for in the Act of Feb. 28, 1925, embodied in paragraph 4, section 538, P. L. and R., authorized Sept. 24, 1945. Published monthly except July and August. PUBLICATION OFFICE. American Meteorological Society, Prince & Lemon Sts„ Lancaster, Pa. EDITOR. Robert G. Stone, P.O. Box 2061, South Station, Arlington, Va. SECRETARY. Charles F. Brooks, American Meteorological Society, Milton 86, Mass. TREASURER. Ralph W. Burhoe, Milton 86, Mass. Annual subscription, $3.50; single copies of this issue, 35^. Vol. 26 November, 1945 No. 9 Note on Tropical Storm North of Puerto Rico on September 13, 1945 HERBERT RIEHL Institute of Tropical Meteorology, University of Chicago, at the University of Puerto Rico, Rio Piedras, P. R. HE SEVERE HURRICANE which struck as observed at the Institute of Tropical T Florida on September 15-16, 1945, Meteorology, veered again to NNE and at passed a short distance north of times to NE during the late afternoon. A Puerto Rico on September 13. As early second, more remarkable shift to SW took as September 11, when observations from place shortly after midnight on September the Lesser Antilles first disclosed its pres- 13, when the center was still located nearly ence, there were indications that this storm 150 miles to the ENE of San Juan (FIGURE would not follow the northwesterly path of 1; see appended hourly sequences). A sim- its predecessor of only two days previous, ilar shift at Borinquen Field (014) fol- but travel much closer to the main island lowed two hours later. At Antigua (009) chain. Consequently, when the center, defi- and St. Kitts (010) such a sharp windshift nitely located by aircraft reconnaissance on line had not been in evidence. Nor was September 12, moved to the northeast of there any other synoptic system, from the northern Lesser Antilles on the after- which the shearline indicated in FIGURE 1 noon of that day, close watch was kept for could reasonably be derived. a recurvature westward which indeed ma- Thus one may conclude that this shear- terialized at nightfall. The track of the line developed in the eastern Caribbean and storm, as estimated by the writer, is con- moved northward against the circulation tained in FIGURE 1 and agrees closely with of the tropical storm. This circulation al- that charted at the U. S. Weather Bureau ready extended to at least 20,000 feet as office at San Juan. The absence of oceanic disclosed by the San Juan rawin at mid- data during the night of September 12-13 night with winds from N and NNE, about makes the exact path open to argument. 30 mph, to the top of the run. St. Croix However, the writer finds it very difficult (012) gives the first evidence of passage to reconcile the hourly observations, espe- of the shearline, even if the ENE wind at cially in the Virgin Islands and Puerto 2030, 60th Merid. time, may not be quite Rico, with an appreciably different chart- representative. If the more gradual turn- ing of the course. ing at St. Thomas (013) may also be The first hint at San Juan (015) con- ascribed mainly to passage of this shear- cerning the recurvature came when the di- line, then the latter moved northward at rection of movement of the low clouds which about 10 mph. With increasing distance had already backed to N and slightly W of from the center the shearline takes on al- N during the noon hours of September 12, most the character of a discontinuity in the Unauthenticated | Downloaded 09/29/21 03:28 PM UTC FIG. 1. Surface map, September 13, 1945, 0230, 60th Merid. time. The pressure changes are 24-hour changes. windfield. At San Juan and Borinquen Eico as shown in FIGURE 1. While this Field the windshifts were as impressive as again may be due to the lack of reports any synoptic shift at the surface noted by north of the island chain, it is more prob- the writer at Puerto Eico. able that the portion of the shearline north The 24-hour pressure falls continued to of Puerto Eico lost its identity in the cir- increase for several hours after passage of culation of the storm that was overtaking it. the shearline, especially at Borinquen Field. Arrival of the shearline at San Juan at The pressure at San Juan also continued to the surface was preceded by an increase of fall after passage, even after application squalliness and rain, and followed by a de- of a correction for diurnal variation, and crease of the shower activity. When the steadied about two hours later. The con- surface winds had already backed to SW, tinued pressure falls may be due to move- the low clouds were still drifting from ment of the low center along the shearline N-NNW and changed their course only rather than normal to it, and to deepening later. Thus there was evidence of a south- of the whole system. However, it proved ward slope of the shearline with height, most difficult to move the trough in the away from the wind, an unusual occurrence. pressure field on the maps northward past The precipitation on both sides of the shear- Puerto Eico in conformity with the shear- line came from clouds of shallow vertical line. Since this difficulty arose on several extent with a more stratiform than cumuli- successive hourly charts, the writer believes form appearance. Although moisture was that the pressure trough actually trailed high on the San Juan midnight raob at the shearline and passed the northern coast least to the 500-mb surface, there was no of Puerto Eico several hours later. solid overcast of middle and high clouds. The Borinquen winds returned to NW for Throughout the night the stars were visible a brief period at 0730, and on the 0830 between squalls, and an upper overcast de- maps one need no longer draw the peculiar veloped only with the approach of the main westward bulge of the isobars near Puerto upper trough well after sunrise on Septem- Unauthenticated | Downloaded 09/29/21 03:28 PM UTC HOURLY SEQUENCES Station Time (60th Merid.) DD Speed dp -24 hrs (mb) St. Thomas Sept. 13, 0130 NNW-NW 2 Beauf. (PAA and 0230 NW-WNW 4 Beauf. -8.1 U.S.W.B. obs.) 0330 WNW-W 6 Beauf. 0430 WSW 5 Beauf. 0530 WSW 7 Beauf. St. Croix Sept. 12, 1830 N 10 mph -2.3 2030 ENE 12 -3.1 2130 NW 2 -3.9 2230 NW 10 -4.3 Sept. 13, 0030 W 10 -5.6 0230 SW 15 -5.1 0430 s 20 -3.9 San Juan Sept. 12, 2030 N 11 mph -1.7 (NAS) 2230 N 12 -3.0 Sept. 13, 0030 N 15 -4.7 0230 WNW 10 -6.4 0430 SW 12 -7.0 0630 SW 15 -7.8 0830 SSW 7 -6.7 Borinquen Field Sept. 12, 2030 NE 8 mph -1.0 2230 NNE 16 -2.1 Sept. 13, 0030 NNE 12 -3.4 0230 ENE 22 -4.1 0330 ENE 10 -4.1 0430 calm 0530 SSW 5 -5.1 0630 SW 10 -5.4 0730 NW 11 -5.4 0830 WNW 8 -6.1 0930 WSW 8 1030 SW 16 ber 13. Tliese observations on the cloudi- Finally, there were some remarkable char- ness agree with computations carried out acteristics of the windfield that passed over at the Institute of Tropical Meteorology Puerto Eico on the south side of the storm. which indicated that above a shallow layer In the vicinity of the shearline itself, the of convergence near the ground, there was winds were fairly light though gusty (12-25 a deep layer of divergence in the middle mph). This is reasonable in view of the troposphere, followed again by convergence fairly flat pressure gradient and the large in the high troposphere. It should be men- cyclonic curvature of the isobars. There tioned, however, that the computation cov- was, as indicated above, at one time the sug- ered a 24-hour period. gestion that the surface winds blew from Unauthenticated | Downloaded 09/29/21 03:28 PM UTC 360 BULLETIN AMERICAN METEOROLOGICAL SOCIETY [Vol. 26 tlie southwest for several hours, when the range in the central portion of the island. pressure field still demanded a gradient In several other situations, however, when wind from the northeast. Later, however, there appeared to be much less occasion for when a field of straighter isobars with a strong winds, the mountain range did not stronger gradient moved over Puerto Rico seem to appreciably block the wind stream. the winds remained light and remarkably On latitude 18°, of course, gradient flow sub-gradient. Except for occasional squalls, is not to be expected. The amount of the the wind velocity at San Juan was 8 to 18 wind deficiency, however, is somewhat sur- mph although the calculated gradient wind prising. Presumably the lack of wind was exceeded 40 mph for some time. This is due to the deepening cent'er, which suggests in contrast to St. Thomas, which reported that considerable progress regarding the winds up to force 7 during the morning. understanding of the structure and devel- The fact that the highest winds along the opment of hurricanes could be obtained northern coast of Puerto Rico should have through further studies of the observed and occurred during daytime, when turbulence calculated fields of motion near tropical is greatest, did not help.