Small town USA guy. Politics nerd. Soccer fan. Interested in eyewalls, deformation zones, and hook echos.
By: TropicalAnalystwx13 , 3:12 AM GMT on June 07, 2013
The 2013 Atlantic hurricane season just started on June 1 but already we have our first named storm, Andrea. The system developed from a broad monsoonal gyre combined with the mid-level remnants of Hurricane Barbara from the eastern Pacific. Initially dubbed Invest 91L, the disturbance tracked slowly northward over the past few days, positioning itself north of the Yucatan Peninsula yesterday. Despite its ragged appearance on satellite imagery early in the morning, characterized by a broad low-level center with multiple mesovorticies within that and deep convection well removed from the low, a new low-level swirl developed underneath the shower and thunderstorm activity by midday, and it quickly tightened. A recon flight into the system revealed a closed center with plenty of tropical storm-force winds in the region of thunderstorms. Overnight, Andrea steadily strengthened despite forecasts of only slow intensification, and the system moved ashore the Big Bend of Florida earlier this evening, with winds of 65 mph. Current infrared images show Andrea has weakened since that time, likely a result of stronger wind shear from an approaching upper-level trough, drier air from the western Gulf of Mexico, and the fact that the storm is now inland. As of the latest advisory from the National Hurricane Center, the cyclone had sustained winds of 45 mph and a minimum barometric pressure of 993 millibars; Andrea is moving northeast at 15 mph. Tropical cyclone watches and warning remain in effect, listed below:
A Tropical Storm Warning is in effect for...
- The U.S. East Coast stretching from Flagler Beach, Florida to Cape Charles Light, Virginia
- The Pamlico and Albemarle sounds
- The lower Chesapeake Bay south of New Point Comfort
A Tropical Storm Warning indicates sustained winds of at least 39 mph are expected to occur across the warned area within 36 hours. In this case, within 24 hours.
Figure 1. Infrared imagery of Tropical Storm Andrea.
Forecast for Andrea & Impacts
Andrea is expected to continue its forward motion towards the northeast for much of the remainder of its life, ahead of the broad upper-level trough currently positioned across the Ohio River Valley. Much of the model guidance remains in agreement for a track across coastal portions of Georgia and South Carolina tonight, eastern North Carolina during the day tomorrow, out into the Atlantic southeast of Rhode Island early Saturday, over southern Newfoundland late Saturday, and ultimately out in the open north Atlantic by late Sunday. While doing so, Andrea is not expected to strengthen and should instead slowly weaken. The emphasis should be placed on slowly as the storm should become baroclinically supported tomorrow, meaning it will likely lose most of its tropical characteristics.
Regardless of strength, Tropical Storm Andrea is expected to impact the United States quite severely. Winds of tropical storm-force are likely to overspread Georgia and the Carolinas tonight and tomorrow; in conjunction with rainfall totals of 3 to 6 inches, these winds may be enough to uproot weakened trees. This may lead to sporadic power outages anywhere along the Eastern Seaboard. As far as storm surge is concerned, counter-clockwise winds onto the coastline of North Carolina and South Carolina during the day tomorrow are expected to "pile" the water onshore, leading to sea level rise of 1 to 2 feet as it occurs during high tide. Coastal regions may experience minor flooding. The main threat, at least through tomorrow, will be the threat for tornadoes. With southerly flow at 500 millibars and southwesterly flow at 850 millibars across North Carolina during the day tomorrow, wind shear may be sufficient to power low-topped supercells. Numerous EF0 and EF1s were already spawned across the state of Florida today. If a tornado warning is issued for your location you need to heed the warning. And be sure you have means to be notified of that warning...there are numerous apps for your phone, and if you have a NOAA Weather Radio, use it!
Invest 92L: the overlooked storm
As if tracking Andrea weren't enough for the first week of hurricane season, we also have a well-defined tropical disturbance in the central Atlantic. Yes, I did say central Atlantic. The National Hurricane Center has effectively dubbed this Invest 92L, though it has remained an organized entity since it left Africa on May 31. Though the system is being sheared now and its low-level circulation is exposed, 92L has had the classic appearance of a tropical cyclone over the past two days. Numerous ASCAT and OSCAT passes during this time have revealed a low-pressure area well separated from the Intertropical Convergence Zone with a closed, well-defined center and at least scattered deep convection atop the center. It has met the criteria of a tropical cyclone for two days now, yet the NHC just at 2pm EDT mentioned it in their 48-hour Tropical Weather Outlook, giving it a ~0% chance of tropical cyclone development (these chances were raised to 10% at 8pm EDT). The NHC does not like to upgrade "anomalies"...and a wave of this magnitude so early in the season is most certainly atypical...but a tropical cyclone is a tropical cyclone regardless of how rare it is. Hopefully this system is studied in-depth during post-season analysis and is reclassified.
Regardless of the past, 92L's time as a tropical cyclone has come and gone. The system is now feeling the effects of wind shear from a large upper-level trough that typically lies across the western Atlantic and central Atlantic during the early season, and the center of circulation is now well displaced from the deep convection. No further organization is expected as this system tracks west-northwest and continues in a high wind shear environment. The GFS and CMC models indicate this wave will slowly die as it tracks north of the Leeward Islands in a few days.
Development prospects in June
Now that Tropical Storm Andrea has moved ashore, it's time to start looking for what may be our second named storm of the 2013 Atlantic hurricane season, Barry. And even though we typically only see one June storm every two years, both the CMC and GFS...though through different means...say we may be dealing with two. The CMC shows a trough split in roughly 10 days off the Southeastern coastline, bringing ashore a well-defined area of low pressure near the Georgia-South Carolina border. The GFS indicates another monsoonal setup towards the end of June, tracking a weak tropical storm across the Yucatan and eventually into Texas. Though the MJO is expected to be in its downward pulse after this week, lessening the chances of tropical cyclone development as opposed to what they would be if the MJO were in its upward pulse, formation can never be ruled out; it has happened during the unfavorable mode of the MJO many times in the past.
Overall, I would put the prospects of another tropical cyclone in June at Medium, 30 to 50%.
Figure 2. The GFS' depiction of a tropical depression over the southern Gulf of Mexico in late June.
I'll have a new blog on Andrea tomorrow,
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