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.

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.

7/7 - 10:30pm - Massive Heat Wave Ends with Storms

Today capped off the longest, hottest stretch of weather in Louisville that most people can remember. Today's 106° high at Louisville International Airport was the hottest it's been since July 14th, 1936 when it got to 107°. That temperature is Louisville's all-time record high and if we didn't have as many clouds as we did this afternoon in the city we would have met or broken it. This ends nine days of 100°+ weather, which is the third-longest stretch of triple digits ever recorded according to NWS Louisville.

Storms that will develop tomorrow will be ushering in cooler temperatures, which is welcome news! The bad part is that some of these storms may be severe due to the incredible amount of energy they'll have at their disposal from all the heat. Temperatures in the mid 90's tomorrow with sunshine and high humidity will power an awful lot of instability, or the tendency for air parcels to rise and create thunderstorms. A cold front sinking down from the north that will be stalling out once it passes just to our south will be the focus for storm chances starting tomorrow in the early afternoon in Louisville and lasting through Monday morning.

The main threats from these storms will be wind and hail, but there could also be a threat for some rotating storms too. While winds don't look favorable for any sort of big tornado problems tomorrow, a couple of these rotating storms may get just enough juice to put down a brief spin-up. EHI values in the adjacent image take into account both instability and helicity, which is a product of wind shear. The elevated values you see are powered mostly by instability and just a little bit of helicity, so again, winds aren't that favorable here. The Louisville area is no stranger to summertime tornadoes as you may remember a series of four of them that touched down in late June of last year.

Given the chances for some severe wind and hail tomorrow, the Storm Prediction Center has issued a 15% Slight Risk area for Kentucky. A better chance for severe weather (30%) exists closer to the East Coast where upper-level winds will be stronger as a trough digs down a bit into that area.

6/25 - 10:30am MDT - Great Faces. Great Places.

Today's blog post title is the South Dakota's state slogan. As you can probably gather by that, we're in South Dakota today! Southwesterly flow aloft at 500 mb (18,000 feet) will be present in the western half of South Dakota, northwest Nebraska, and Eastern Wyoming today as the general pattern we dealt with yesterday continues. While the winds aren't terribly strong, the RAP model is beginning to pick up on an area of enhanced speeds near 30-40 knots around parts of Western South Dakota and Northwestern Nebraska. If this enhanced area verifies there could be some better storms under it.

At the surface right now there are southeasterly winds across this area, but the more easterly of the wind vectors are favoring Western South Dakota right now. We'll see how this develops throughout the day as surface conditions continue to change. We'll want these more easterly wind vectors becuase they'll contribute to low-level wind shear that could possibly lead to rotating storms, should they form.

I say "should they form" because we're still dealing with some very warm temperatures aloft. These warm temperatures aloft, called "the cap", inhibit storm development because rising air parcels can't continue to rise buoyantly if the environmental air around them is warmer than they are. The good news here is that temperatures at the surface today will again be in the 90's to near or above 100 degrees. This will really heat up these surface air parcels so that they might shoot through the cap. All models I've looked at this morning have the cap breaking at some point in the early to mid afternoon, but where exactly this happens is still in question. The 14z RAP model has an interesting hole in the cap at around 4pm near Rapid City, but our current thinking is that the Black Hills near Rapid City will be a focus for storm development because they are a constant source of lift and thus a typical location for storm initiation.

To get some of the heating we'll need for storms to form today, we need to have a plentiful supply of sunshine. Right now we do have a layer of clouds over South Dakota that is inhibiting this right now, but western parts of the state are clearing out and it should hopefully leave us with blue skies later this morning. Storms should form sometime after 3pm this afternoon should things come together as forecast. The main threats will be wind and hail from any storm that forms, but we'll be on the lookout for some rotation in these storms as well. The Storm Prediction Center has a general thunderstorm risk for our target area today, but also mentions a 5% severe wind and hail risk.

Follow me on Twitter and Facebook for the latest updates this afternoon!

6/24 - 10:35am MDT - The Chase Begins!

This morning we're heading toward Cheyenne, Wyoming from Denver as we continue to look at weather data coming in. Today's setup still looks marginal by most accounts, but there are a few small features that could cause a few isolated storms to go severe. For one, surface winds out of the east in Wyoming, Northeast Colorado, and Nebraska will generate upslope flow, which is the lift mechanism we need to get storms going.

These winds will also create some shear because they will be interacting with southwesterly winds at 500 mb (18,000 feet). The issue with this is that the faster winds at this level will be further north toward Rapid City, South Dakota instead of where the more favorable surface winds will be just to the south. In any case, the wind direction at 500 mb will be good enough to usher in the wind shear and maybe some drying of the air at that level to increase instability.

When you put the expected helicity (caused by wind shear) and the instability together for today, you get the Energy Helicity Index (EHI). According to the 14z RAP model, the EHI may try to ramp up a little bit in Northeast Colorado and it even breaks out a little bit of precipitation just west of there too. Surface heating is going to be massive since temperatures will be in the 90's and 100's in some places, so we're hoping this also helps overcome some of the warm temperatures aloft (the cap) so that isolated storms can develop. We're grasping for small features today since there isn't any one thing jumping out to get our attention, but so far it looks like a storm or two could fire on the elevation near Cheyenne. We'll also be watching an area closer to Rapid City, SD to see how things develop up that way.

Follow my continuous chase updates on my Twitter and Facebook accounts today!

5/25 - 11:50am CDT - A Grand Finale

Ever since earlier this week we've had our eye on today's setup in Central Kansas. The moisture return, a low developing over Southwest Kansas, and upper-level wind support all kept us coming back to look at the models to see how it was shaping up. For a while it looked like the cap, or a layer of instability-inhibiting warm air aloft, was going to surpress storm formation but now we're seeing a situation where there will be just enough break in the cap to allow for a few isolated supercell storms to form. To get surface heating today to break that cap we need to clear out the clouds across Kansas first. A push of dry air at around 10,000 feet is currently moving from Southwest Kansas to the northeast and is eating these clouds for breakfast as you see in the satellite image to the right. We expect this to keep moving throughout the day and hopefully it will clear out the clouds in time to get some serious surface heating going.

The big story here today is the moisture return. For the first time this week we are seeing a situation where dew points will surge into the mid to upper 60's. Yes! This much-needed moisture will be the fuel for storms this afternoon and should give us a much better show than the moisture-deprived low-precipitation storms we've been seeing lately. While we don't have this moisture in the area quite yet, it's streaming in like crazy right now. Strong easterly and southeasterly winds are blowing this moisture into Kansas from Missouri and Oklahoma and will continue doing so throughout the day. Having mostly easterly winds in Kansas right now is a very good thing because this is creating just about optimal low-level shear for rotating supercell storms.

Speaking of wind shear, this will create quite a bit of helicity, or a corkscrew-like rotation of air in the atmosphere. When you combine helicity with instability in an equation you get the Energy Helicity Index (EHI). As I've said before I think that indicies and parameters can be distracting since human analysis is always better than a formula, but EHI seems to be fairly reliable since it is so basic and relies on two very basic ingredients. This afternoon's EHI values in Kansas will be the highest we've seen out here this season according to the Rapid Refresh model. That's some great news for storm chasers because higher EHI values typically correlate with enhanced tornado potential.

The Storm Prediction Center has a Slight Risk for severe storms today from Iowa to the Texas Panhandle and within that a 5% tornado risk in Kansas. This is quite the change from the previously non-severe "general thunderstorm" outlooks that they had been issuing for today because of the concern that strong capping may inhibit storm development. Today is our last chase day out here for Tour 3 and my last day out here until I come back for a week in June. If we could see some good storms today that'd be a great end to the first part of my trip! Tonight once we're finished chasing we'll be traveling back to Oklahoma City where I'll fly back to Louisville from tomorrow morning.

Follow up-to-the-minute reports on our chase on my Twitter and Facebook accounts!

5/22 - 9:45am MDT - Nebraska Here We Come!

Today's storm setup is tricky because we're dealing with, again, a situation where the moisture is not returning to the region as well as we need it to. The good news though is that the trough that is slowly making it's way eastward will finally be in play today, so upper-level wind support will be there for storms. Let's start with the moisture though. The higher dew points will be in the Dakotas today no doubt, but this is at the surface. With such a short time between being dry and having moisture moving into that region, this moist layer is likely very shallow and confined near the surface even though the short-range models this morning show it a little deeper than before (models have not been handling moisture well at all lately). This means that storms up that way would likely be low-precipitation supercells, maybe to the point where some of them are only putting down virga and not ground-falling rain. The better surface wind support near the low will also be in the Dakotas, but you won't get many, if any, tornadoes if the storms can't get any "meat" on them from the lack of moisture. Dew points in Northern Nebraska may be lower at surface, but we may have an overall deeper layer of moisture there.

The upper-level winds as I said before will be more favorable today than yesterday. The trough moving in will provide fast southwesterly winds in the Dakotas extending down to Northern Nebraska. This should bring in some drier air aloft and enhance wind shear for this afternoon's storms. Today we'll stay on the southern end of these winds aloft so that maybe we can get some deeper moisture coupled with the winds. Hopefully some supercell storms can form in this environment.

Because the models are having difficulties resolving moisture issues, today's CAPE (instability) forecast looks rather low in Nebraska with higher values north. This is something we'll be watching throughout the afternoon to see where it actually sets up. The potential lack of moisture and the best shear being removed from deep layer moisture will likely limit tornado potential today, but maybe we can see some high-based photogenic storms!

The Storm Prediction Center has a Slight Risk for severe storms today in the Dakotas extending toward Northern Nebraska. There's only a 2% tornado risk with this and I think this is a good call given the moisture issues here. If dew points were to be higher over a deeper layer in North Dakota I think you could have a pretty decent tornado risk from supercells given the forecast wind environment. In any case, our more southerly target today will put us in great position for tomorrow's chase, which will likely be in Nebraska.

Follow me on Twitter and Facebook for updates today!

5/10 - 11:20am MDT - A Change in Plans Today

Our original plan to stay in Southeast New Mexico today has been altered because of some new weather data that came in overnight. We did visit Calsbad Caverns this morning, but we're now on our way to the Del Rio, Texas area where storms are ongoing at the moment. This activity is definitely not making it easy for our forecast this morning because having clouds and storms in the hours before an afternoon severe weather setup is never a good thing. Nonetheless, we'll be looking for holes in this activity and/or for it to die off a bit this afternoon so that stronger storms can develop. The good thing is that wind shear today looks promising due to a cut-off low in Mexico creating strong southwesterly winds aloft that will contrast with the easterly winds at the surface.

The Storm Prediction Center has issued a Slight Risk for severe storms in the region, which includes a 5% tornado risk. Should this pan out today even with the ongoing storms this morning, this could be the best setup we've seen so far this week. Here's an interesting snippet from the SPC's 1630z discussion:

THE EWD PROGRESSION OF MID AND HIGH-LEVEL JET STREAKS ACROSS THE LOWER RIO GRANDE VALLEY WILL RESULT IN PRONOUNCED VERTICAL VEERING OF WINDS WITH HEIGHT WITH EFFECTIVE BULK SHEAR STEADILY STRENGTHENING TO 50-60 KT. AS SUCH...THE ENVIRONMENT WILL BECOME INCREASINGLY SUPPORTIVE OF INTENSE SUPERCELLS CAPABLE OF VERY LARGE HAIL...TORNADOES...AND DAMAGING WIND GUSTS.

Up-to-the-minute updates will continue today on Twitter and Facebook.

5/7 - 10am CDT - On Our Way to South Texas

The same cold front that we targeted yesterday in Kansas and Oklahoma is on the move southward and so are we. We're on our way to South Texas this morning to meet up with where the front, the dry line (boundary between moist air to the east and dry air to the west), surface heating, easterly winds, and moisture all meet up. Storms should fire along the front and/or dry line this afternoon as instability builds. You'll see in the image to the right that there's a sliver of high dew points that will develop in Southwest Texas this afternoon that will be our focus as the cold front moves south.

Due to the threat of severe storms this afternoon, the Storm Prediction Center has issued a Slight Risk area for portions of South Texas with a 2% tornado risk (pictured left). Some of these storms will have large hail and damaging winds. The tornado risk, like yesterday, looks fairly low due to weak winds aloft, but the directional wind shear will help out as well as high instability. There may just be enough ingredients to spawn one or two tornadoes today down there, so we'll cross our fingers and keep driving!

As usual, I'll be posting up-to-the-minute storm chasing updates on my Twitter and Facebook accounts today.