October 20, 2010

Is a Major Pattern Change On the Way?

Filed under: Forecasting,Mid-latitude cyclones,Tropical — Erin @ 10:32 pm

After weeks upon weeks of dry weather, we have had two rain events in the past couple of weeks as fronts moved through. The rain has not made a noticeable difference in the drought situation, though it should be noted that we are not under fire weather watches every day anymore.

However, the models are starting to suggest a major pattern shift in the long range. It’s important not to focus on specific details this far out, because quite frankly, some of the details are almost certainly utter rubbish. The idea is that we are shifting into a precipitation-bearing trough pattern at last. The GFS 4-5 days out is showing a fairly robust system of about 1000 mb that is expected to bring rain. Jackson’s NWS office is already talking about this system and has given it a 40% chance of producing thunderstorms on Monday. This is potentially an interesting situation as far as severe weather is concerned, and it seems pretty likely that this particular rain event will play out in some form over the Deep South.

In runs from the past several weeks, the GFS has been indicating, for the most part, a return to dry conditions for as far as the model shows. More recently, it’s stopped doing this. After the Monday event, the GFS shows a potentially more extreme event developing over the 28th—and a freeze.


This image shows the storm event. The freeze would follow.

This is where you start to raise an eyebrow at the output and take it with a grain of salt. This would be early in the year for a first frost, but it has happened before. We’ll need to keep an eye on this and see if it stays in the runs and if other models start to pick up on it.

After the proto-event #2, the GFS then kind of goes off, showing a winter storm situation unfolding. It’s exactly what I mean when I say that the details are not what counts here, but the suggestions of cooler air and more precipitation. This is at the end of the run, which is notoriously poor in accuracy. The blue “0” line represents the freezing point.

As I said, the freeze aspect of this is almost certainly complete foolishness. A third rain event, however, is not out of the question at some point in the two-week range, and such a cyclone could easily pose a winter storm problem for areas along the East Coast, as a nor’easter has already done this year.

We are reaching a time of year when the gates start to close for tropical activity. In part, this is because the jet stream is reforming at a more southerly location. The high wind shear generated by the jet stream is a death knell for tropical activity, but it is encouraging of frontal and cold-core cyclonic development.

Tropical Depression 19
However, tropical storms will continue to form as long as they have the necessary conditions, and as I type this, I see that a new depression has been classified. TD 19 is located in the Caribbean and is forecast to intensify a bit and move over the Yucatan Peninsula. The circulation center of this depression is sheared and somewhat displaced from the main convection, but it seems that the convection and low-level center are moving closer together as the source of the shear moves out. The system is expected to get into the western Gulf. It probably won’t affect the central Gulf states, but it should be noted that the HWRF model explodes this into a major hurricane and sends it into the west coast of Florida. This is currently an outlier, but that is a good model, so it should be watched closely. The Gulf is cooling, but there is definitely enough heat in it to sustain a serious storm. Because of high shear, the Gulf currently cannot host a tropical storm, but the shear is supposed to largely disappear by the time TD 19 emerges from the Yucatan.

It will be interesting to see what type of weather system we get if TD 19 does move farther east and north than expected and the Gulf states get significantly cooler weather. The output of the GFS that shows those rain events does not take into account the possibility of tropical energy entrainment, because it favors a southern path for the tropical depression.

Keep an eye out; things could get interesting.

October 11, 2010

Return of the Heat, Paula, and a Decent Chance for Rain

Filed under: Forecasting,Mid-latitude cyclones,Severe,Tropical — Erin @ 5:39 pm

The blog has been inactive recently because the weather in the Deep South has been profoundly, well, boring. After a brief cool spell brought on by a pair of cold fronts, we saw a return to the high 80s and further intensification of the drought. For Jackson, it was in fact the driest September on record and the maximum temperatures were among the highest (though the cool overnight lows resulted in a moderation of the overall daily averages). Incidentally, with the warmup came an increase in the number of red paper wasps that—apparently re-energized by the heat—wanted to use my house to overwinter, and one of them decided to light on an antique doorknob of virtually the same color as itself. If you have never been stung by this type of wasp before, believe me, you do not want to be.

Rain/Thunderstorm Event
However, at last there is a respectable possibility for some rain, and better yet, it is coupled with another cool-off. A low is moving in that is expected to bring rain on Tuesday. Jackson is even concerned about the threat of severe weather, and the SPC has put us under a Slight risk for Tuesday:

If cumulonimbi materialize, unplug your computers, modems, and printers. Trust me on this one.

Jackson’s forecast discussion speaks of air mass modification by the low pressure system. This is exactly what it sounds like; an air mass with very different characteristics moves in and alters the presiding air mass. A look at the water vapor map makes it very clear what is going on with this:

The curved form over Kansas is the upper level low. The leading edge of it is already approaching the area, and you can tell the difference between even the current humidity of the air and what is coming our way. Drier air has already been pushed out, as you may have noticed from the widespread cumulus cloud formation today.

Here is what Jackson has to say about the incoming system and the potential for severe weather:

THE AIR MASS WILL DESTABILIZE QUICKLY IN THE PATH OF THE DEEP UPPER
LOW AND MOISTURE WILL CONTINUE TO INCREASE AHEAD OF THE SYSTEM.
PRECIPITABLE WATER VALUES AHEAD OF THE SYSTEM ARE EXPECTED TO BE
AROUND 1.5 INCHES AND VERTICAL TOTALS ARE FORECAST IN THE UPPER 20S
FROM NE LA INTO WCNTRL AND NE MS BY 11/00Z. RADAR TRENDS ARE THAT
CONVECTION ASSOCIATED WITH THE OUTFLOW MOVING INTO EXTREME WRN ZONES
IS WEAKENING BUT FORCING REMAINS BETTER FARTHER UPSTREAM CLOSER TO
THE FRONT AND UPPER LOW. CONSIDERING THE EXTREME AIR MASS
MODIFICATION THAT OCCURRED JUST UPSTREAM EARLIER TODAY AND THE FACT
THAT IT WONT TAKE MUCH FOR STRONG CONVECTION TO DEVELOP IN AN AIR
MASS WITH THESE LAPSE RATES…WILL MAINTAIN MENTION OF STRONG
POSSIBLY SEVERE STORMS PER PREVIOUS FORECAST. LOW LEVEL MOISTURE
RETURN WILL BE HIGHEST OVER SWRN AREAS THIS EVENING AND…ALTHOUGH
LAPSE RATES WILL BE HIGHER OVER AREAS TO THEIR NORTH…SWRN ZONES
WILL SEE SUFFICIENTLY UNSTABLE CONDITIONS TO SUPPORT THE POTENTIAL
FOR STRONG…POSSIBLY SEVERE…CONVECTION ALSO. THE UPPER JET
REMAINS PARKED OVER CENTRAL/SRN ZONES THROUGH TUESDAY AND PERIODS OF
HEALTHY DIVERGENCE ALOFT WILL HELP SUPPORT CONVECTION THROUGH
TOMORROW.

VERTICAL TOTALS/LAPSE RATES WILL REMAIN HIGH THROUGH TUESDAY WHEN
THE UPPER LOW PIVOTS INTO THE AREA…AS WILL SYNOPTIC LIFT ACROSS
MUCH OF THE FORECAST AREA. ALTHOUGH LAPSE RATES WILL REMAIN HIGH IN
RESPONSE TO THE COOL UPPER LOW…AVAILABLE MOISTURE WILL BEGIN TO
DIMINISH FROM THE WEST EARLY TUESDAY. MODEL SOLUTIONS DISAGREE ON
THE EVOLUTION OF THE LOW LEVEL MOISTURE FIELD TOMORROW. THEY AGREE
THAT THE SURFACE BOUNDARY WILL BECOME MORE DIFFUSE AS IT MOVES
ACROSS THE FORECAST AREA BUT THE GFS REDUCES SURFACE DEWPOINTS
INTO THE MID 50S BY 13/00Z WHILE THE NAM/EURO MAINTAIN DEWPOINTS IN
THE LOW TO MID 60S. THE SREF LOWERS DEWPOINTS RELATIVELY QUICKLY BUT
HANGS ONTO PRECIP WELL INTO THE AFTERNOON IN THE WEST. FOLLOWED THE
HIGHER POPS FROM THE PREVIOUS FORECAST FOR TOMORROW OVER THE LATEST
MAV MOS IN CONSIDERATION OF THE SLOWER DRYING INDICATED BY SEVERAL
OF THE MODEL SOLUTIONS.

What all this means is that we have an interesting setup for mid-October unfolding and that if all the parameters come together, we could see some severe weather along with our rain. Probably not tornadic activity, but some significant thunderstorms could develop.

Tropical Storm Paula
Meanwhile, in the tropics, we’ve already seen Hurricane Otto come and go and are now on to Tropical Storm Paula. Otto was never a threat to anyone, but it was interesting to observe a classic subtropical-to-tropical transition take place. It also became a hurricane, which is uncommon with systems that begin as subtropical cyclones.

Paula has been a persistent disturbance for quite some time before finally getting its name (later than it should have, in my opinion) as a 60 mph system. It is not moving fast and is not expected to pick up much speed, but it will be moving over some warm water and should intensify into the season’s ninth hurricane. It is probably not going to be a problem for the central Gulf Coast, and is more likely to be yet another Florida event, but right now its forecast is uncertain because steering mechanisms are expected to essentially disappear in 3 days, leaving it drifting around in the Yucatan Channel as (probably) a hurricane. Most of the guidance then has it picked up and hauled northeast, and that is indeed the most climatologically likely track for systems of this kind at this time of year. However, storms in this general area in October are infamously hard to forecast; a particularly unfortunate example was Hurricane Mitch. Paula is likely to be an interesting system to watch and a troublesome one to forecast, and in that respect it is actually a welcome change of pace. Heretofore my synopsis of the 2010 hurricane season would be “a whole lot of normal.” Not normal numbers, of course; the season has obviously been above average in activity, but none of the systems except Hurricane Alex were particularly difficult to forecast or did anything unexpected. I like late season storms because they require a bit more of the human factor in forecasting.

September 30, 2010

Drought Conditions Develop

Filed under: Drought,Forecasting,Mid-latitude cyclones,Tropical — Erin @ 2:58 pm

There isn’t much to say about the weather in the Deep South, unfortunately. A cold front is set to move through the region this weekend, dropping temperatures down about 10 degrees again and making the air outside seem truly fall-like. However, it is going to be a dry cold front. Nothing has changed as far as rain is concerned; there is none predicted as far as the forecasts are made. The Weather Service in Jackson has begun issuing fire weather products for the state, and the office out of Birmingham has issued red flag warnings. Burn bans are also in effect for large areas, though it’s highly inadvisable to burn even if you aren’t officially under a ban. It’s hard to believe that within a half a day’s drive, the Carolinas are getting drenched with the rainfall of a trough and the remains of Tropical Storm Nicole entrained into the cyclone, while we in the Deep South are now officially under drought conditions.

With the state as parched as this, it’s reaching a point where we will look anywhere for something that might send us some moisture—or at least, that has not yet been eliminated as a contender for doing that. Out in the tropics, there is a new system in the Central Atlantic, 97L, probably one of the last to develop in places like that this year. However, while the track and development of this system are still very much up in the air, the 12Z GFS doesn’t seem particularly interested in the disturbance. It does, in the two-week range, show a big, wet cold-core cyclone coming through the Southeast and finally ending the drought. I am going to keep an eye on this model and see if it retains this feature; the models are beginning to show some skill in long-range forecasts. And besides, hope springs eternal.

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