Subtropical storm brewing near Bermuda; earthquakes and tornadoes for Oklahoma

An extratropical low pressure system that moved off the coast of South Carolina over the weekend is camped out over the Atlantic about 400 miles southwest of Bermuda. Satellite loops reveal that this low (98L) has developed a respectable amount of heavy thunderstorm activity near its center, and in a curved band to the north. Bermuda radar shows weak rain bands from 98L rippling across the island, with the strongest rain showers well to the island's southwest. Sustained winds at the Bermuda airport reached 30 mph, gusting to 44 mph this morning. Sustained winds near tropical storm force were occurring this morning at buoy 41048, about 300 miles west of Bermuda. Wind at the buoy were 38 mph, gusting to 47 mph at 6:50 am EST. Strong upper-level winds out of the west are creating 35 - 45 knots of wind shear over 98L, limiting development. Ocean temperatures are near 26.5°C (80°F), which is right at the boundary of being warm enough to support tropical storm formation. Since 98L is getting its start as an extratropical storm, it has cold air at its core aloft, and is surrounded by a large amount of dry air to its south and west. This dry air and wind shear suggests that 98L will initially be Subtropical Storm Sean if it develops, and not Tropical Storm Sean.


Figure 1. Morning satellite image of 98L.

Forecast for 98L
98L will drift slowly west or northwest today and Tuesday. The latest SHIPS model forecast predicts wind shear will fall to 20 - 30 knots on Tuesday, which should allow for some increased organization of 98L. The computer models show little or no development of 98L, with none of our reliable models predicting 98L will become a hurricane. NHC gave 98L a 40% chance of developing into a subtropical storm by Wednesday in their 7 am EST Tropical Weather Outlook. I'd put these odds higher, at 50%. Since 98L is probably already generating sustained winds in excess of 39 mph, it will likely be named immediately if it gains enough organization, and skip being classified as a subtropical depression. Bermuda is the only land area that need concern itself with 98L, as a trough of low pressure is expected to absorb 98L and lift it quickly to the northeast on Thursday. The center of 98L should remain more than 200 miles away from Bermuda during the week, but heavy rain squalls from the storm are likely to affect the island at times between now and Thursday. The remnants of 98L will likely bring heavy rain to Nova Scotia, Canada on Thursday night or Friday morning.


Figure 2. Severe weather outlook for today from NOAA's Storm Prediction Center.

Earthquakes and tornadoes for Oklahoma
Oklahoma has a chance for a rare multi-natural hazard day: simultaneous earthquakes and tornadoes. The damaging magnitude 5.6 earthquake that shook the state Saturday night spawned magnitude 3.3 and 3.4 aftershocks last night, and there is the potential for more aftershocks today. This afternoon, NOAA's Storm Prediction Center has placed much of Oklahoma and Central Texas in its "Slight Risk" area for severe weather, thanks to a strong low pressure system moving across the Plains. During the late afternoon, severe thunderstorms with high winds, large hail, and a few tornadoes are likely over this region. If you're in Oklahoma late this afternoon and feel a deep rumbling, it could be an approaching tornado OR an earthquake aftershock!

Jeff Masters