91L May Develop; Gert a Strengthening Tropical Storm

Dr. Jeff Masters ·

Above:  MODIS true-color satellite image of 91L on Monday morning, August 14, 2017. Image credit: NASA.

A tropical wave in the cardinal tropical Atlantic ( Invest 91L ) has the potential to develop into a tropical depression late this workweek as it moves westwards at about 15 miles per hour, but the calculate has a lot of doubt. Satellite images on Monday dawn showed that 91L was a complex, elongate system that stretched along a 700-mile long parcel of the central tropical Atlantic between 9°N and 14°N latitude. heavy thunderstorm bodily process had increased since Sunday, but remained disorganized, and only a modest degree of spin was apparent in the visible satellite imagination. Wind shear was mince, 10 – 20 knots, and sea coat temperatures were 27.5°C ( 82°F ) — about 0.5°C to 1°C above average for this clock of year, and adequate for growth. The Madden-Julian Oscillation ( MJO ), a blueprint of increase thunderstorm action near the Equator that moves around the earth in 30 – 60 days, is presently in a phase that is golden for supporting tropical cyclones in the Atlantic, as pointed out by tweets from The Weather Company ‘s Michael Ventrice .

Saharan Air Layer analysis for August 14
Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Monday, August 14, 2017, showed that dry Saharan air lay well to the north of tropical wave 91L, and should not be a major impediment to development early this week. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Forecast for 91L

Conditions appear friendly for development through at least Thursday. The atmosphere surrounding 91L will be damp ( mid-level relative humidity of 65 – 75 % ), and the dry air of the Saharan Air Layer ( SAL ) is far adequate north that it should not pose any contiguous issues. Wind shear along 91L ’ second way is predicted to be largely moderate, 10 – 20 knots, through Thursday, and sea coat temperatures will be 27.5°C ( 82°F ). All three of our top models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—predicted in their 0Z Monday runs that 91L would develop into a tropical depression this week. however, the calculate is a very complex one, since it is not obvious where the center of 91L will end up forming along its elongate axis. The 0Z UKMET model had 91L form along the western edge of this touch weather area, resulting in a tropical depression arriving in the Lesser Antilles Islands on Thursday. The european and GFS models had a more easterly formation placement, resulting in arrival of the storm near or just north of the Lesser Antilles on Friday. And we should not be convinced that 91L will develop at all this week, since the most recent 6Z Monday guide of the GFS model predicted no development of the storm. The 0Z Monday ensemble runs reflect the ongoing doubt. 91L becomes a tropical storm in about 75 % of the Euro corps de ballet members, but only about 20 % of the GFS members — and there are major differences among ensemble members in the versatile project tracks arsenic soon as 3 or 4 days from now. In its tropical weather expectation issued at 8:00 am EDT Monday, the National Hurricane Center gave 91L 2-day and 5-day odds of exploitation of 20 % and 60 %, respectively.

Figure 2. Tropical Storm Gert as seen by the GOES-16 satellite at 10:45 am EDT August 14, 2017. At the time, Gert had top sustained winds of 60 mph. Image credit: NOAA/CIRA/RAMMB. NOAA’s GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.

Tropical Storm Gert forms, but is not a threat to land

tropical Storm Gert formed on Sunday afternoon in the Atlantic waters between the Lesser Antilles Islands and Bermuda, becoming the seventh named storm of the season. Gert is not expected to be a terror to any nation areas. The seventh named storm of the Atlantic hurricane temper occurs, on average, on September 16, so we ’ ve seen a batch of early-season activity this class. The record earliest seventh Atlantic named storm was 2005 ’ mho version of Tropical Storm Gert, which got its list on July 24 of that class. Satellite images on Monday morning showed that Gert ’ s heavy thunderstorm activity had increased in intensity and administration, but was by and large confined to the southeast side of the center of circulation, thanks to potent upper level winds out of the north that were creating a moderate 15 knots of weave shear. Gert was embedded in an environment of dry breeze with a relative humidity at mid-levels of the air near 50 %, and the strong high-level winds were pushing this dry air into the northern share of the storm, limiting development of thunderstorms on that side. Sea surface temperatures of 29°C ( 84°F ) were golden for development.

Forecast for Gert

The 12Z Monday SHIPS model predicted that wind fleece over Gert would by and large be in the centrist range, 10 – 20 knots, through Wednesday good morning, which should allow at least humble intensification. The official NHC prognosis from 11 am EDT Monday calls for Gert to become a hurricane by Wednesday good morning, and so does our top volume model, the HWRF, although it is improbable that Gert will attain the major-hurricane condition predicted by HWRF for several runs. By Wednesday night, Gert will begin encountering very high gear levels of wind shear above 30 knots, which should convert the storm to a hurricane-strength extratropical storm on Thursday. The models are in excellent agreement on a track arcing to the north then northeast this week, taking Gert halfway between North Carolina and Bermuda, then out to sea well south of the canadian Maritime Provinces.

Yet another African tropical wave to watch later this week

Another tropical wave with the potential to develop into a tropical depressive disorder is predicted to emerge from the slide of Africa on Thursday. The 0Z Monday operational runs of both the European and UKMET models predicted development of this new wave by early future week, as did about 30 % of the 50 members of the European ensemble bode. This wave was predicted to take a more west-northwesterly track than 91L. Yes, the tropical Atlantic is heating up ! Bob Henson contributed to this position .

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