Thursday, September 29, 2011

Cool Outflow Boundary Interactions Across Central Texas

The above image was just taken from the New Braunfels area doppler radar. It shows heavy shower and thunderstorm activity across portions of southcentral and southeast Texas.  If you look closely, you can see a few outflow boundaries.  These are like mini cool fronts that depict where rain cooled air is blowing out away from the thunderstorm activity.  I have highlighted them for you on the next image:

The Southern outflow boundary is moving toward the South, and the Northern one is moving toward the North.  Isolated thunderstorms are beginning to develop on the Northbound boundary, to the Northwest of Georgetown.

In a post earlier today, I pointed out that the HRRR model (an experimental, high resolution computer forecast model) was calling for widespread 1-2 inch rains with localized 3-4 inch amounts across much of this same region.  Below is the HRRR total rainfall forecast image from that post, valid 7pm CDT this evening:

On the same image below, I've zoomed-in on a section along the I-35 corridor from Austin to San Antonio.  Look at the forecast from the computer model for about 1 inch of rain within the purple circled area (which is over central and southern Hays County):

Now, lets take a look at the latest storm total rainfall estimate from the New Braunfels radar:

I have encircled approximately the same area of Hays County on the radar image, this time in white.  Note how remarkably accurate the HRRR was in estimating the location where the heavier rain would fall in such a small geographic area.  While it didn't do so hot on the exact amount of rain (the radar estimates about 3 inches, which is 3 times more than the HRRR forecast), it absolutely nailed the geographic area in which it took place.  I should point out that the computer model forecast was generated at 9am CDT this morning, some 6 hours before the event began.

As I often point out in situations such as this, it's more important to look for trends in the model data, instead of a precise answer.  This is a perfect example, where the model scores a 100% on identifying the area to watch, but a lower grade on the exact outcome of the event.  Still, a remarkable step forward in weather science from even just 1 or 2 years ago...

The HRRR rainfall forecast image above was valid through 7pm CDT this evening.  I'll make another post later this evening to show you how the final outcome of the event compared to the model forecast, rainfall wise...

***Updated 7:30pm CDT 9/29/11:

You have to use your imagination a little bit to fill in the blanks, but the above images are the storm total rainfall estimates from the New Braunfels, Houston and Del Rio radars, respectively, through shortly after 7pm CDT.

Overall, I'd say the HRRR model from earlier today (see images above from earlier post), did well with respect to the areal coverage of the rain.  It underestimated the intensity of the rain in some instances, but did tend to focus on the right areas that received heavy rainfall.

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Heavy Rainfall Instore for Some in South Texas Today...

A cold front is moving Southward into northwest Texas at midday (solid blue line on surface weather map above), while several weak outflow boundaries and sea breeze fronts (red dashed lines) are drifting West and Northwestward....

The clashing boundaries will result in widespread shower and thunderstorm activity across much of southcentral and southeast Texas later this afternoon and into this evening...

Not everyone will receive rain, but it appears as though a good many locations will have a shot at it.  The image below is from the latest run of the HRRR, an experimental, short range computer model, which shows widespread rains of 1-2 inches with localized amounts of 3-4 inches across portions of southcentral and southeast Texas by 7pm CDT this evening:

While this is certainly good news for some this afternoon and evening, it will be tempered somewhat by the fact that strong, gusty North winds behind the front will result in extreme fire danger across much of the central and eastern part of the state on Friday (just like what is happening across Western Oklahoma and the Texas panhandle today, with the strong, gusty North winds behind the front).

By the way, I alluded to the possibility for rain across this part of Texas during the last week of September in this post on September 13th.  Good job to the GFS model, which consistently forecast this event to take place...

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Wednesday, September 28, 2011

Much Needed Rain in Parts of Texas Hill Country Yesterday

The above image shows the radar's estimate of total rainfall across portions of the Texas Hill Country late yesterday afternoon and evening.  The highlighted red areas indicate where the radar estimates 4-5 inches of rain fell, mainly in rural parts of the indicated counties.

Some of the same activity eventually meandered into the Austin area before diminishing around sunset yesterday, however it had weakened considerably and only produced trace amounts of precipitation in most of that region.

There will be another opportunity for scattered shower and thunderstorm development across the region later this afternoon and evening, along a weak and stalled frontal boundary that roughly runs West to East through the region.    There are also several weak boundaries that were left behind by yesterday's activity that may also serve to focus some redevelopment later this afternoon as well.

The SPC in Norman, OK is forecasting that some of the thunderstorms may become severe on a more organized basis this afternoon or evening, particularly across far southeast Texas (including the Houston area) as indicated by the yellow shaded area on the image below:

The main threat with this activity will be large hail and damaging wind gusts.  Very heavy, beneficial, downpours of rain can also be expected to be concentrated in this region of Texas, which would certainly be welcome...

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Monday, September 26, 2011

Another Casualty of Irene: The Northeast Pumpkin Crop...

According to this article (just one of many circulating today), the pumpkin crop across the Northeast U.S. took a big hit from Hurricane Irene, and the heavy rains that closely followed from Tropical Storm Lee:

If you live in the Northeast U.S. and want to get a decent pumpkin this year, you'd better buy early...and be prepared to spend more than usual!

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Saturday, September 24, 2011

Tropical Weather Update: Another "Fish Storm" in the Atlantic...

A vigorous disturbance moving West off of the African coast has just been named Tropical Storm Philippe, the 16th named storm of the 2011 Atlantic season.  A visible satellite image recently taken just as the sun was setting over Africa shows the nice swirl pattern associated with the system:

The infrared satellite view of the system is below...

Philippe is likely to be what we call a "fish storm", meaning that only our friends of the aquatic persuasion are likely to be affected by the system (and any ships out at sea, of course).  Computer forecast models currently steer the system toward the Northwest and out into the open Atlantic in a relatively short period of time:

...and this trend is echoed by the official National Hurricane Center (NHC) forecast as shown below:

Meanwhile, several hundred miles back toward the West, Tropical Storm Ophelia remains rather disorganized, with the latest hurricane hunter aircraft pass having a difficult time locating the center of the system.  You can see Ophelia on the latest water vapor satellite wide shot of the tropics below (click to enlarge):

As alluded to in a post a couple of days ago, Ophelia's greatest threat may be to Bermuda (though still as a Tropical Storm rather than a hurricane) later in the coming week, but we still have plenty of time to monitor her progress in the meantime:

We've also seen a flare-up in thunderstorm activity over the Bahamas, about 175 miles East/Southeast of Cape Canaveral, Florida during the last 12-24 hours.  You can see the system on the satellite wide shot above, and here is a close-up visible and infrared look below:

The computer models are nearly unanimous in tracking the system Northward, with it passing just East of the outer banks during the day Monday.  The question is, will it organize into a tropical depression or storm by that time?

At this time, conditions in the middle and upper-levels of the atmosphere do not support rapid organization or intensification of the system, however it does bear watching, especially for the next 24 hours as it resides over the very warm waters of the northern Bahamas.

Way back on September 13th I made this post and remarked about how the GFS model had been persistent in showing a tropical system over Florida on or about 9-27-11 (the last graphic toward the very end of the post).

I think the disturbance currently over the Bahamas and just East of Florida has to be the one that the model was alluding to.  Fortunately, (for Florida anyway) it does not appear as though the system will be nearly as strong as the model had depicted (and it will also remain East of Florida and the southeast U.S. coast the way it looks now).  Still, not too shabby for a computer model prediction in the tropics nearly 2 weeks in advance.

As I've mentioned before, you have to classify such long range model forecasts as "information only", as the exact location and intensity of such a system is very difficult to predict, especially that far out into the future.  At least it gives you an idea as to where you should focus your attention and monitor for potential development in a given timeframe...

Speaking of long range model hints at tropical development, a natural question might be:  is there anything showing up in the long range right now?  The short answer right now is a simple "No", but I'll keep my eye on it for you...

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La Nina "Part II" - and What it Means for Fall and Winter 2011-2012...

In blog posts of the past, I've mentioned El Nino (which means "the boy" in Spanish) and its affects on the weather, but I don't believe I've ever really delved into La Nina (which means "the girl in Spanish) before.  Hence the reason for this post, which will describe the phenomenon and how it will have a dramatic affect on our fall and winter weather this year.

I apologize for the length of the post, but this is a very detail heavy subject.  I've been pulling it together for 3 days now, which also adds to the length, but hopefully you'll come away with a full understanding of whats going on...

Here in Texas, we consider La Nina the evil step sister of El Nino because of its direct role in the extreme drought that we're now suffering through (more on that later). On a more scientific basis, La Nina is the opposite of El Nino, in that it represents the presence of cooler than normal waters in the Equatorial Pacific Ocean.

The image below shows the average of the observed sea surface temperatures along the Equator (top half of image) and the departure from normal in the same area (bottom half of image).  Temperatures are in degrees Celsius, and I have added the locations of the Equator, Mexico and South America so that you can better orient yourself to the layout of the map:

As you can see, (and noted by the purple encircled region on the bottom half of the image), current sea surface temperatures (abbreviated SST) are below normal along that portion of the Equator along and to the West of South America.  This is the typical La Nina "signature" on the SST map.

So, what exactly does La Nina do to the weather pattern here in North America?  A graphic representing the "typical" impact for the fall and winter is shown below:

As you can see, the main branch of the jet stream (shown in purple) typically resides further to the North during a La Nina winter, particularly across the Western part of North America.  This generally results in warmer and drier than normal weather across the southern U.S. (bad news for the drought areas) and cooler and wetter than normal weather from the northern Plains and Great Lakes into the Northeast.

As alluded to in the title of this post, the 2011-2012 fall and winter La Nina will be an "encore" presentation (as La Nina was also present during the 2010-2011 season).  The jury is still out on just how strongly a "2nd year" La Nina affects the overall weather pattern.  In some cases, the 2nd year phase has been just as strong as the first, while in others it has been weaker (and has less of an affect, as a result).  This is why it's important, when making a longer range forecast, to not only apply the raw data to the forecast, but also apply the trend of the last several instances as well.  You then have to keep in mind that there could be some "swing" in either direction (higher or lower, greater or lesser impact, etc.) when looking at the overall trend.

The graph below shows the instances of La Nina in the "modern" era (since 1950 - click to enlarge):

With the above in mind, the series of maps below illustrate how we would expect temperature and precipitation trends to behave (with respect to normal) during the fall and winter months of a La Nina pattern.  These are called "analog" maps.

First, lets take a look at the likely temperature departures from normal for the period October through December (click to enlarge):

As indicated by the key on the lower left portion of the image, the red, orange and yellow shaded areas show a likely above normal temperature pattern, while the blues show regions where temperatures are more likely to fall below normal levels, on average.  The darker the shading of either color scale, the higher the likelihood of above normal (reds/oranges) or below normal (blues/purples) temperatures in a given region.

I have attempted to "connect the dots" on the same image below, with above normal temperatures likely within the red hatched area, and below normal temperatures within the blue hatched area:

Below are the temperature analog maps for the period November through January:

...and for the period December through February (I didn't bother to do a connect the dot map for this one, as much of the nation is overtaken by the likelihood of above normal temperatures, as you can see):

We can do the same thing with respect to precipitation.  First, the analog map for the period October through December:

On these maps, the brown and tan shaded areas show the likelihood of below normal precipitation, while the green shaded areas show the likelihood of above normal precipitation.  The darker the green shading, the higher the likelihood of above normal precipitation.  The darker the brown shading, the higher the likelihood of below normal precipitation in a given area.

When connecting the dots on this map (below), we can see a general trend for wetter than normal weather across much of the Northwest and northern Rockies, as well as the Great Lakes and Ohio Valley regions.  Unfortunately, we see a trend toward continued dry weather in drought stricken Texas and the Southwest:

Here are the precipitation analog maps for November through January:

...and for the period December through February:

As you can see, this type of pattern does not bode well for drought stricken Texas, hence the reason that the Climate Prediction Center is currently forecasting "persistence" with respect to the drought:

Note the pockets of "Drought Ongoing, Some Improvement" (the green hatched areas over the brown) in Oklahoma and Kansas and along the Colorado/New Mexico border areas.  The "improvement areas" correspond quite well with the precipitation analog maps for November through January (refer back to that image further up in the post), where precipitation is forecast to begin trending above normal during that period of time.

As I pointed out right before showing you the different analog maps, you have to keep the extremes in mind when looking at averages and/or departures from normal.  Lets take a look at the potential precipitation extremes in drought stricken Texas under the fall and winter La Nina pattern.

The first image shows the probability of different amounts of precipitation in various parts of Texas during the period October through December.  I have highlighted the Dallas/Ft. Worth area ("DFW" in red), the Austin area ("Austin" in red) and the Houston area ("Houston" in red). on both the small map and the data tables.  Click on the image to enlarge it:

As you can see for the Austin/San Antonio area, there is a 66% chance of less than 6 inches of rain falling during the entire 3 month period of October through December when you look at the raw data (pie chart to the left of the word "Austin").  When you look at the trend adjusted data (pie chart to the right of the word "Austin"), there is still a staggering 60% likelihood of less than 6 inches of rain during the 3 month period.  The normal for that time period would range from 6 to 9 inches (and keep in mind, we haven't seen a "normal" precipitation average in this region for nearly 12 months now).

If we look at the precipitation extremes for the Austin/San Antonio area under such a pattern (see image below), you can see that as little as 1 inch of rain has fallen in a La Nina pattern during that same time period (dark green shaded bar on the graph in the red circle).  When adjusted for the trend of the last several data sets, the minimum amount rises to about 3 inches (spotted green shaded bar on the same graph).

The picture doesn't get much better when we look at the period January through March:

The raw data shows a 57% chance of below normal rainfall (a 50% chance when adjusted for recent trends) for the Austin/San Antonio area during the 3 month period.   (Note that the precision, percentile wise, goes down as you progress out further in time).  When we look at the extremes for the pattern in the Austin/San Antonio area, the average on the low end is for as little as 1 inch (raw data) to 2 inches (trend adjusted data) of rain during the entire 3 month period (red circled area on the bar graph image below).

This information is obviously very detail oriented and data heavy.  For that reason I chose to focus on Texas in general (due to the drought) and the Austin/San Antonio area in particular.  You can go to this link and pull the data for your region.  It takes a little getting use to, but after playing around with the menu a little bit you should be able to pick it up pretty quickly...

Getting back to the bigger overall picture, when we take a look at the expected upper-air pattern and then pull all of the analogs, means and extremes together, a long range forecast is born.  The next two images kind of pull everything together, as far as my thoughts are concerned, as to the general trends that we're likely to see this fall and winter.

First, lets take a look at the likely scenario for temperatures across the country.  "Fall/Winter" describes the period October 1 through February 29 (yes, 2012 is a leap year), for purposes of this outlook...

The lighter blue and brown shaded regions indicate where I believe that there is a greater than 50% likelihood of below or above normal temperatures, respectively.  The darker blue and darker brown shaded regions indicate where there is a greater than 70% likelihood of below or above normal temperatures, respectively.

Lets take a look at the same outlook with respect to precipitation trends:

Based on the expected trends, there is a greater than 80% chance of below normal precipitation across the darker brown shaded area in Texas, and a greater than 70% chance of the same in the darker brown shaded region over portions of Florida.  There is a greater than 50% chance of below normal precipitation within the remainder of the lighter brown shaded regions.

I expect a greater than 50% chance of above normal precipitation within the lighter green colored regions.  There is a greater than 80% chance of above normal precipitation across portions of the Pacific Northwest (darker green shaded area), and a greater than 60% chance of above normal precipitation across the darker green shaded areas in the Great Lakes and Ohio Valley.

As we proceed into October we'll have a much better idea as to how strong the La Nina is becoming, which will allow us to focus even more on the potential for extremes - and which of them are more likely to take place later in the season.  There is another "wild card" in play, mainly during October, and that has to do with tropical systems that could affect Florida (and possibly Texas, though unfortunately that doesn't seem likely at this time).  A strong tropical system impacting Florida, particularly if at low speed, would obviously negate the below normal rainfall outlook for at least the 1st month of the period (which happens to be October) for that region.  Even in such a case, the overall pattern would likely revert back to a "below normal" rainfall situation for the remainder of the forecast period in that region.

Regardless of the particulars, there are a couple of headlines that are currently screaming from the likely pattern for this fall and winter:

(1).  Texas is not likely to see a break in the rainfall department - the drought will rage on (and it could very well continue into the spring of 2012 and beyond).

(2).  The "big cities" of the Midwest and Northeast should brace for some big snow days again this winter.  Several of the top 10 heaviest snows in both Chicago and New York City have taken place in La Nina years.

(3).  The higher elevations of the Pacific Northwest will have heavy snows this winter - and the lower elevations of the same region will have some big storms with high winds and heavy rain.

Stay tuned for updates as the season progresses!

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Thursday, September 22, 2011

Tropical Update - Ophelia and a Disturbance Near Puerto Rico

It's been a few days since I made a post regarding the tropics, and during that time one of the disturbances has been named Tropical Storm Ophelia (as shown on the latest satellite image above).  Another weak disturbance is currently moving over Puerto Rico, but has not been classified.

While maximum sustained winds in association with Ophelia have increased to 65 mph, she is not in a very favorable position to intensify much further at least for the next 48 hours or so, as stronger middle and upper-level winds are tearing at the Western side of the system a bit more.

Computer models generally track her toward the West/Northwest over the next few days before a more direct Northwestward turn late in the weekend:

In the longer term, some additional strengthening is possible as Ophelia is forecast to move out of the stronger shearing wind field as she approaches Bermuda from the South by the early to middle part of next week.  Folks in Bermuda will want to keep an eye on this system next week for sure.

At this time, it appears that steering winds next week will prevent the system from affecting the Eastern U.S.  By 8pm EDT next Friday, 9-30-11, the ECMWF Model is forecasting Ophelia to possibly clip the outer reaches of the Canadian Maritimes and possibly Newfoundland.  We'll have to keep an eye out for that possibility for late next week...

As far as the disturbance near Puerto Rico is concerned, it too is currently in a region that is not terribly favorable for additional development.  The GFS Model completely dissipates the system within 24-36 hours.  The ECMWF model tries to hold on to it (or an offspring of it), carrying it to the Yucatan Peninsula area in Mexico by Monday evening, the 26th (red circled area on image below):

At this time, I wouldn't be too concerned about this particular system, as the deck seems to be stacked against it...

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