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Tropical Storm Chris Discussion Number 4

Wiki-Hurricanes Forecast Center

5:00 AM EDT Sun Jul 8 2018

An Air Force reconnaissance aircraft investigating the tropical depression found that the barometric pressure had decreased substantially, from 1014mb to 1009mb, over the past few hours. In addition, believable flight-level winds of 45kt were found in the deep convection located southeast of the center. This data supports upgrading the depression to Tropical Storm Chris with an initial intensity of 35kt.

Chris is currently located in a collapsed steering regime. It should drift southward over the next 24 to 36 hours as an area of high pressure builds to its north. Thereafter, an approaching shortwave trough is expected to accelerate the storm northeast. Most of the guidance is in good agreement, with the exception of the UKMET which sends Chris into Rhode Island and Massachusetts. This run has been discounted.

The system is currently experiencing moderate northerly wind shear from a nearby cold front. This front is expected to lift out later today, allowing anticyclonic flow to build aloft and upper-level winds to slacken. The broad trough across Canada and New England is expected to give way to a smaller cut-off low northeast of Chris, which should provide a ventilation channel to the east. In 48 to 96 hours, as the approaching trough sends Chris to the northeast, it is expected to be located in the southeastern quadrant of the jet, a notoriously favorable region for upper-level divergence and quick development of low-pressure systems. Water temperatures are more than adequate along the Gulf Stream, and although the surrounding environment is quite dry, the lack of vertical wind shear should keep dry air intrusions to a minimum. Intensity models vary a bit, with 70kt from LGEM representing the lower bound and 948mb from the UKMET representing the upper bound. Model consensus suggests a peak as a mid-grade Category 1 hurricane. However, given the factors mentioned above, I have opted to forecast a stronger storm at 85kt. This may be conservative if Chris makes the most of its interaction with the upper-level trough. Extratropical transition is expected by day 5 as the system moves over waters cooler than 20C.

FORECAST POSITIONS AND MAX WINDS

INIT 08/0900Z 35 KT 40 MPH

12H 08/1800Z 45 KT 50 MPH

24H 09/0600Z 55 KT 65 MPH

36H 09/1800Z 65 KT 75 MPH

48H 10/0600Z 75 KT 85 MPH

72H 11/0600Z 85 KT 100 MPH

96H 12/0600Z 75 KT 85 MPH...POST-TROP/EXTRATROP

120H 13/0600Z 60 KT 70 MPH...POST-TROP/EXTRATROP

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