It has quietly been a somewhat active month for tornadoes between eastern Texas and southern Georgia. There have been five separate tornado “days”, with none of which gathering more attention than the December 23rd event that resulted in four fatalities in Columbia, Mississippi.
Friday and Saturday (Jan 2nd/3rd) will feature heavy rain, severe storms, and potentially more in the way of isolated tornadoes across the Deep South. This morning’s mid to upper level water vapor imagery captured a longwave trough with embedded shortwaves dipping into California and Nevada. As this trough slowly advances eastward over the next 2-3 days, it will provide enough upper level divergence and atmospheric lift to spawn low pressure at the surface. The first and fairly weak surface low will develop near the upper Texas and western Louisiana coasts on Friday, before a more dominant surface cyclone takes shape near Memphis by Saturday.
Despite the presence of low pressure, many more ingredients will have to come together in order for severe storms to develop. Most importantly, a warm, moist, tropical, and unstable airmass needs to be advected northward out of the Gulf of Mexico. With Friday’s coastal low being so weak and far south, it will be difficult for the “warm sector” to make it beyond shelf waters during the day. Below is a very coarse depiction of the position of the low and accompanying warm front based on GFS model guidance valid 6PM Friday.
Green colors on this graphic represent areas with Convective Available Potential Energy (CAPE) of 750j/kg or higher. Obviously, the severe threat will be limited to extreme southern coastal parishes of Louisiana and coastal counties of Mississippi until the warm front advances farther north. The onset of rainfall and isolated, non-severe thunderstorms will occur much sooner, however, as elevated instability will already be present along and even north of the frontal boundary. The Storm Prediction Center (SPC)is keenly aware of the setup, and their outlook for Friday includes a marginal risk in the same area highlighted above.
Their outlook is valid through Friday night, which is the reason why their marginal risk area extends as far north as Baton Rouge and almost McComb. The warm front will lift north throughout the night. By 6AM Saturday, the lowest surface pressures will be located over eastern Arkansas, which should allow the warm sector to make it well inland, and into south-central Mississippi by the early afternoon hours.
SPC has opted to not highlight any areas for severe weather on Saturday just yet. Their reasoning is due to uncertainty given large model differences, and the conditional nature of Saturday’s threat in general. Widespread severe storms may not materialize at all, for example, if the warm sector is not as potent as indicated by some guidance. With that said, there does appear to be enough data supportive of a few weak, isolated tornadoes and storms capable of damaging wind gusts. The two graphics below represent CAPE in excess of 750 j/kg across southern Mississippi and southwest Alabama, with moderate low level wind shear also present along the northern third of the unstable warm sector midday Saturday. Where these two parameters overlap is where the best bet for isolated tornadoes would be.
One negating factor is that by this time, the amount the change in wind direction with height will be lessening. With a trailing cold front quickly approaching, winds near the surface will be in the process of veering from southeast to southwest, which lines up better with the mid-level southwesterly flow already present with the mid-level trough swinging through the South. Therefore, dominant storm mode will likely evolve into a squall line, with the main threat being damaging winds with a non-zero risk of a few isolated weak, rain-wrapped, and embedded tornadoes. Needless to say, there are still a lot of question marks, but also a lot of reasons why you should be weather-ready along the Gulf Coast late Friday into Saturday.
One risk that also should not be overlooked is the threat of heavy rain. The Deep South has already encountered a few heavy rain events in recent weeks. With the ground already saturated, it wouldn’t take much heavy rain to cause a few pockets of flash flooding. Below is the five day accumulated rainfall forecast for the area.
Northern Indian Ocean Including Arabian Sea & Bay of Bengal: A broad low pressure area southeast of Chennai, India, has a low chance of development within the next 48 hours. The low is expected to parallel the east coast of the Indian peninsula as it gets picked up by a mid-level trough approaching from the west. The low is expected to weaken before nearing Bangladesh within the next four days.
Southern Indian Ocean: Tropical Cyclone Kate is expected to peak with maximum sustained winds near 115 knots. Kate is no threat to land as it recurves to the south, well west of Australia.
South Pacific/Australian Basin: The Bureau of Meteorology and US Joint Typhoon Warning Center are not expecting any tropical cyclone development in the short term. Long range model guidance suggests a moderate to high chance of Tropical Low and eventual Tropical Cyclone formation north of Western Australia within 5-7 days. Interests along the northern WA coast are advised to begin watching the tropics closely this week. Elsewhere, surface pressures may also begin to lower near the Top End and Gulf of Carpentaria within 7-11 days.
Northwest Pacific: Tropical Storm 23W (Jangmi) is passing through the southern Philippines. Jangmi may briefly intensify once entering the South China Sea. However, Jangmi is expected to peak as nothing more than a minimal tropical storm before being overtaken by increasing vertical shear southeast of Vietnam.
Central Pacific: No organized or modeled tropical disturbances.
Eastern Pacific: No organized or modeled topical disturbances.
Atlantic: No organized or modeled tropical disturbances.
Less than 13 months after historic Super Typhoon Haiyan ravaged the Philippines, Super Typhoon Hagupit is looking to do the same. Guidance is converging on a super typhoon landfall near, or just north of Tacloban within the next 48 hours. Unfortunately, Tacloban City was also considered to be the epicenter of the Haiyan aftermath. Within 48 hours, eastern and northern Samar could face the brunt of potentially yet another historic storm.
Checkout http://www.28storms.com/typhoon for in-depth coverage,