Monday, August 27, 2012

8/27 - 9:30pm - Update on Isaac's Expected Gulf Coast Landfall

Tropical Storm Isaac is teetering on the edge of becoming a hurricane this evening with winds of 70 mph as of this writing. The threshold for a Category 1 hurricane is 74 mph. The storm motion has slowed down over the past few hours, now moving northwest at only 10 mph. Issac continues to run into issues that prevent it from rapid intensification, which is certainly good news. The latest issue is a batch of dry air that is currently being pulled into the center of the storm. Without an uninhibited source of moist air the storm cannot form a defined eye or intensify very well. The dry air is why Isaac is lopsided right now with most of the intense winds on the south side.

How long will it be until this dry air is finished working through the storm? Probably a few more hours, possibly lasting into the early morning hours tomorrow if current model forecasts are correct. Speaking of models, the current track from the National Hurricane Center takes Isaac right into the Eastern Louisiana/Mississippi Coast areas late tomorrow night into early Wednesday morning as a weak Category 2 storm with 100 mph winds. With the lack of organization we're seeing right now it may only be a Category 1 at landfall. The latest computer model tracks, the spaghetti plots as you've probably heard about and seen over the past few days, have tightened up on a landfall location somewhere around New Orleans. It's worth noting that some of these models are now trending eastward after a massive amount of successive westward jogs in recent days. It doesn't appear that this landfall will take place further east than Biloxi, Mississippi given the current motion of the storm and model data, but the exact landfall location won't be the big story with this storm.

Storm surge exceedance probabilities
The big story here is how big and how slow this storm is. Sure, there will be some wind damage along the coast as the storm makes landfall, but the large area that Isaac covers means that quite an expansive area along the Gulf Coast will experience other effects from it. Storm surge at or above ten feet is possible in locations from New Orleans to Pascagoula, MS and rainfall amounts over a foot are forecast over much of Central and Southern Mississippi and Eastern Louisiana. Isaac will slow down even more than its current speed once it gets over land so heavy rain will be a good bet across much of the Gulf Coast states until we get into the weekend. Inland flooding will be a problem with the deluge that Isaac with unleash on the region as it crawls northward.

Rainfall through 7pm CDT Saturday
As far as local effects in North Mississippi and West Tennessee, Isaac will bring very heavy rain and winds with gusts in some cases near 40 mph. Starkville, MS may be looking at a six or more inch rainfall scenario through Saturday evening and areas in West Tennessee like Jackson will likely see more than three inches of rain. This highly depends on Isaac's exact track so some areas will see much more rain than others.

With just about any landfalling hurricane you can expect the threat of isolated tornadoes, and Isaac will be no different. The turning of winds with height in a hurricane or tropical storm create favorable wind shear for tornadoes to form in stronger parts of it. Due to this potential threat, the Storm Prediction Center has issued a Slight Risk for most of Mississippi and parts of Louisiana, Alabama, Georgia, and Florida.

So overall we're not looking at Isaac to be a major hurricane at landfall, but the effects from it will be widespread due to it's large size. It's a good thing that folks in a wide area are preparing for this storm since it could be a long haul given the slow motion that's expected from it.

Thursday, August 23, 2012

8/23 - 5pm - Tropics, Drought, Storms... Anything Else?

Computer model forecast tracks for Isaac
The big shakeup in the world of weather right now is the presence of Tropical Storm Isaac south of Puerto Rico. Much uncertainty still exists with where the storm will go but certainty of it making some sort of US landfall is growing. Current forecast data from multiple computer models has this storm going anywhere from the eastern Florida Peninsula to the Central Gulf over the coming days. The trend has been for these forecast tracks to shift westward over the past couple days and that means that those with interests along the Gulf Coast need to watch this storm extremely carefully. The fact that this storm may miss most of Hispanola and it's rough terrain means that we could have a stronger storm than previously expected by the time it reaches the Gulf. Once we get some better data tonight from a NOAA Gulfstream IV aircraft that is being dispatched to take measurements of the storm the model data and forecasts should improve a bit.

The 5pm EDT National Hurricane Center update on Isaac has again taken a more westerly track than before thanks to the latest model data. Their forecast has Isaac remaining a tropical storm until it clears the north side of Cuba on Monday and becomes a hurricane. This all depends on where the storm tracks because the less of Cuba and Hispanola it goes over, the more time over warm water it will have. Should the storm remain weak like it is now for a longer period, a more westerly track can be expected.

How 'bout the drought? Not much has changed from last week's drought monitor product for West Tennessee and North Mississippi. Much of West Tennessee is still suffering from a severe to exceptional drought, especially in areas near the Mississippi River and Kentucky border. The area of exceptional drought, the highest level possible, has contracted a bit since last week. Areas south of Benton County and east of Bolivar aren't faring as badly as the rest of West Tennessee, but it's worth noting that rain may be hard to come by over the next week. The worst of North Mississippi's drought is still confined to areas near Tunica and areas south of West Point don't have drought conditions at this time.

Speaking of rain, we're looking at just a small chance for some isolated showers and storms on Saturday afternoon in North Mississippi as an upper-level system moves toward us from the west. This will increase our moisture and instability a touch since winds will be turning more southerly at the surface. West Tennessee will be too far north for this storm chance unfortunately. Small storm chances will be in the forecast again as we head into the early part of the workweek, but uncertainly skyrockets after that because our weather will depend on where Isaac, by then likely a hurricane, will go. A more westerly landfall along the Gulf Coast would mean quite a bit of rain for the region while a more easterly landfall in Florida wouldn't affect us too much. This again is something we'll need to watch.

Sunday, August 19, 2012

8/19 - 6pm - Tranquil Weather for the Workweek

Mississippi State University
It's that time of year again when I switch over blog post content to cover Mississippi and West Tennessee in place of the Louisville area. I do this for two reasons: 1) It's difficult to write about the day-to-day goings on of weather back home in Louisville when I'm taking a full college course load at Mississippi State that requires me to keep up with and forecast weather for the Golden Triangle area here in Mississippi and 2) I'm convinced that it is better to forecast for the area which you are currently living in because you're always aware (sometimes painfully) of how accurate your forecast is. If anything significant happens back home in Louisville I'll be sure to include it in my posts here over the next few months, but for now I'll be covering weather down here.

While it was supposed to be a rainy day across the Golden Triangle today we actually saw quite a bit of sunshine and nice weather. A front that has become stationary over Central Mississippi put us in a much drier environment and this has cut off most of the needed moisture for showers and storms to fire up. There's still a chance that a few isolated showers and storms will get going around the region but that's a potential that will be highest during the early morning hours according to the latest short-range guidance.

The trough that's brought us the front and the cooler temperatures this weekend will continue to do so for most of the workweek as it slides slowly eastward. A lack of surface moisture and building surface high pressure will stave off rain chances and even clouds for the most part until we get close to the weekend. Temperatures this week will be gradually rising through the mid 80's for highs until we get to Thursday. By then stronger southerly flow will be bringing in warmer, moister air from the Gulf and get us back to around 90 for highs as we go through the late week and the weekend. Shower and storm chances will be returning by the weekend.

Click image to see a larger view
The tropics have been fairly quiet as of late near the US coastline but there's potential for that to change next week. A disturbed area of weather in the middle of the Atlantic Ocean is on a course that may take it toward the East Coast if the latest computer models are to be believed. It's way too early in the ballgame to discern whether or not this will be a threat to the US since it could curve back out to sea before hitting land or not strengthen into a tropical storm or hurricane. This will be something to keep a close eye on since it could become Tropical Storm Isaac.

Thursday, August 9, 2012

8/9 - 2:15pm - Severe Storm Threat Tonight

Small storms are popping up around the Louisville area after a fairly calm night that featured a few storms in Southern Indiana. The same trough of low pressure over the eastern half of the nation that's generating today's storm activity will see a potent burst of energy round the base of it late this evening into tomorrow morning. This piece of energy, which is really a small trough within the main trough over the east right now, is called a shortwave trough. This shortwave will have some fast winds aloft as it treks through Kentuckiana late this evening and that will along with a cold front moving through early tomorrow morning will trigger a threat of severe storms.

While scattered storms will be possible for most of today, the best timing for severe storms in Kentuckiana looks to be from 10pm to 4am. The main threats from the storms that go severe will be damaging winds and hail but any of these storms can have heavy rain and lightning. Since these storms will be hitting during the overnight period they won't have the punch that they could've had during the daytime. The tornado threat looks very low right now but as with any line of storms moving through there's a possibility that a kink in the line could form a small spin-up. A Slight Risk of severe weather has been issued for this evening through early tomorrow morning for the entirety of Kentuckiana. The good news with all of this is that much cooler air will be filtering in behind the cold front for Friday and the weekend!

Saturday, August 4, 2012

8/4 - 2:30pm - Waking Up to Storms Tomorrow

Scattered storms are firing between Evansville and Louisville right now and are slowly trekking eastward. These aren't expected to become severe but some downpours and lightning are certainly a possibility in Louisville as they continue to develop. The chance for storms in the area should remain with us in Kentuckiana through the early evening.

More storms forming in Illinois and Missouri will be our next focus as they move into Kentuckiana during the early morning hours tomorrow. Some of these could be strong to severe since a trough moving through to our north will be bringing faster winds aloft to the area. This means that damaging winds will be the primary threat from these storms should they hold together for us during the early morning. The 6z NAM model run (right) was pretty bullish on these morning storms but the 12z that just came in was more subdued with their intensity. In either case it looks likely that a lot of folks will wake up to storms early tomorrow morning.

Storms could fire up during the afternoon tomorrow if the morning's storms clear out in time for instability to recover during the afternoon. Another limiting factor for this will be the cold front that will be moving through during the day tomorrow. Storms will fire along and in front of that boundary so its speed will determine who gets storms during the afternoon. Right now it looks like Louisville and areas to the south and east will be in the zone for storms tomorrow afternoon and it's worth noting that their coverage won't be anywhere near this afternoon's and tomorrow morning's storms. Both Kentuckiana and West Tennessee are under a Slight Risk for severe weather tomorrow afternoon as these storms fire up in the heating of the day with the cold front in play. Strong winds will again be the main threat here.

After the front moves through on Sunday temperatures will be in the upper 80's to near 90 for the start of the workweek in Kentuckiana and storm chances will be nonexistent until Thursday.