Colder / Snowier Outlook Still on Track...

In a post last Friday, I remarked that winter weather lovers should take heart as a major change in the weather pattern would get underway later this month, bringing an end to unseasonably warm weather across much of the nation.  

I've continued to monitor the situation since the post was initially made, and I'm happy to report that the forecast is still on track... which is great news for those of us who are sick and tired of 70 and 80 degree weather in late November and early December.

To set the stage, brutally cold weather continues in Alaska and Canada, where temperatures are in the 30s, 40s and even 50s below zero (degrees F) have been observed for the last several days:

The -36 degree weather in Fairbanks on Monday wasn't cold enough to keep some college students from throwing a beach party:

...which gives new meaning to the word frostbite (please don't try this at home, folks).  

The atmosphere likes to balance itself out.  When it's really cold in one area, it generally tends to be really warm in another.  The "other" place recently has been much of the lower 48 U.S., where temperatures continue to average above normal.  

The image below shows the departure from normal temperatures for the period today through the 10th, showing the bullseye of "above normal" conditions shifting into the southeastern U.S. during the period:

All of that cold air up in Canada and Alaska is very heavy and dense, and will begin to force its way Southeastward into the lower 48 U.S. from mid through late December, as shown on the same departure from normal computer model forecasts below, valid for days 5-10 (December 10th-15th):

...and continuing through days 10-15 (December 15th-20th):

By the day 10-15 period (December 15th-20th), you've no doubt taken note of the brutally cold air (as noted by the purple departure from normal values) shifting South from Canada into the lower 48 U.S.  That is a trend that I believe will continue through late December and into January.

Increasing jet stream energy in the middle and upper levels of the atmosphere will also come into play during the month, which will develop and increase a large snow pack across much of the central and northern two-thirds of the nation.

One such disturbance will produce a band of locally heavy snow from southwest Kansas across southeast Iowa and the upper Midwest late this weekend and early next week:

This trend is forecast to continue into the middle and latter part of the month, as indicated by the longer range computer model depictions below.  I wouldn't necessarily pay attention to the amount of snow shown on these longer range images, but they should give you a good idea as to the general areas that are most likely to be impacted.

The first set covers the period December 15-20:

...and the second set leads us from December 20 through Christmas Day:

Each panel within the image shows the "opinion" of the 4 different computer model simulations that were generated for the given period.

The snow pack will not only serve to deliver that "winter feel" to a large part of the nation later this month, but will also tend to preserve cold air intrusions from the North.  As the cold air moves South, it tends to moderate as it encounters warmer ground and air temperatures.  With a heavy snow pack in place, the ground and air will be cooler, thereby reducing the amount of warming that takes place as the airmass moves Southward.  This should generally lead to gradually colder airmasses remaining "intact" as they move further south across the nation.

So, there you have it.  I am still confident that the middle and latter half of December, and likely into January, will be much more representative of winter than what we've seen so far.  Keep in mind, winter doesn't even "officially" begin until December 21, so we still have a long way to go here!

On the other side of the coin, folks in drought stricken portions of the South and into the Mississippi and Ohio Valley region are ready-set-go for any type of moisture that will come there way, regardless of its form (rain, sleet, ice or snow).  While cold air intrusions by themselves don't guarantee an increase in moisture, there are signs that a more active jet stream pattern will gradually increase the chance of measurable precipitation across the region.

The following images show cumulative precipitation totals from the GFS and the European computer forecast models, respectively.  They are valid for the period today through Thursday, December 13th:

While the forecast model's don't provide a specific precipitation accumulation forecast beyond the time ranges above, in general, they continue to suggest that an increasingly unsettled pattern will develop across portions of the southwest and southcentral U.S., which would generally equate to greater chances of precipitation across portions of Texas and Oklahoma.  

The general position of the jet stream is noted by the green arrow on the image below, and I've circled in yellow an area of increasingly unstable conditions that should translate into the southcentral U.S. during the week leading up to Christmas.  This would generally lead to an increased chance of rain and/or snow across this region.  The image itself is valid at 6am CST on December 21st:

For those who are interested, the brighter red shaded area extending across portions of the Pacific to Alaska indicates a "ridge" of high pressure pushing into the region, which should cause the jet stream to "buckle" and send additional shots of cold air Southeastward toward the continental U.S.  I've attempted to illustrate this on the identical image below:

The bottom line on all of the above is that a much more active weather pattern is instore for the lower 48 U.S. from next week, into the foreseeable future.  Stay tuned for details as this situation unfolds...

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Are You Dreaming of a White Christmas for 2012?

Thanks to a relatively warm and dry fall across a good part of the country, many are convinced that scenes like the one above will be hard to come by this winter...

To that I say, fear not, winter weather and snow lovers:  a major pattern change appears to be shaping up for mid through late December which will likely put down a snow field across a good chunk of the nation - perhaps just in time for many to have that much sought after White Christmas.

December will start out warmer than normal across a good portion of the nation, as indicated by the latest 6-10 day departure from normal temperature forecast from the CPC shown below (valid December 5th-9th):

Meanwhile, bitterly cold air will continue to build up across Alaska and Canada.  Temperatures across much of this region are currently sub-zero, with a large pocket of minus teens and minus 20s in western Canada and eastern Alaska:  

This bitterly cold air will continue to accumulate through the 2nd week of December, before the weight of the airmass combined with increasing jet stream energy from the West begins to break it loose and send it Southeastward into the U.S. in one or more waves.  This will likely result in below normal temperatures across much of the central and eastern two thirds of the country by the 2nd half of the month, similar to the area shown in blue and green on the departure from normal temperature forecast below:

As a result of the cold air intrusion(s) and increasing middle and upper-level energy, the chance of measurable snow will increase across a large portion of the country from North to South beginning around mid-December and continuing through the end of the month, including Christmas time.

The 4 panel image below shows the forecast snow depth from 4 different long range computer forecast models.  Each model forecast is valid for the 5 day period leading up to Christmas (December 20th-25th):

As you can see, 2 of the four computer model solutions suggest that measurable snow could reach as far South as northern Texas by that time period (bottom of the image).  The other 2 forecast the measurable snow to remain in the more "traditional" areas of the central and northern Plains and Midwest into the Northeast (top of the image).  

This is just meant to give an early "heads-up" for everybody, as we still have a lot of time to watch this and things can certainly change, but I don't want winter weather and snow lovers to give up on the prospects of a more active 2nd half of December (which is really when Winter officially starts anyway), which may also lead to a White Christmas for a good chunk of the nation.

So, without further ado, please cue Mr. Bing Crosby...

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NOAA's Winter Outlook Is In - Agrees with Theory that La Nina Will Dominate...

NOAA's National Weather Service issued their outlook for winter 2011-2012 yesterday, and it supports the theory that La Nina will continue to dominate the weather pattern across the U.S. this fall and winter.

Their outlook is eerily similar to mine (see maps at the bottom of the La Nina post that was linked in the 1st paragraph), with some variation in the percent likelihood of the various elements being forecast.  I am a bit perplexed by their forecasting a "nose" of drier than normal conditions from Oklahoma into Kansas.  That part of the forecast is not currently supported by the latest trends, and doesn't fit with the expected La Nina anomalies.

Unfortunately, they do concur with me in that the drought across Texas and adjacent areas will only worsen through at least the fall and winter season.

All in all, I see no reason to adjust my earlier forecast at this time, based on the trends we've witnessed over the last 15-30 days.  The real question in my mind at this point is how strong will La Nina ultimately become this winter, and how far will that influence stretch into 2012 (i.e., beyond winter)?  It's still too early to tell at this point...but we'll keep watching it for you...

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Tropics Still Don't Want to Give Up In Long Range...

In a few posts over the last 15-20 days, I've remarked that the medium to long range models continue to hint at the potential for significant tropical development along the southeast U.S. Coast and/or Florida in October.  The latest run of the GFS model, valid 2am EDT on Tuesday, October 11th, is no exception:

As you can see by the black encircled area on the above image, a tropical system (likely a storm) is indicated just off of the Eastern coast of Florida at that time.

The ECMWF Model has the same general idea of a "near Florida" storm, but it depicts the system just off of the West coast of the state at 8am EDT on Tuesday, October 11, as shown in the image below:

That particular model tracks the system on up into the interior Southeast by Wednesday ,October 12:

...while the GFS solution, originally on the Eastern side of Florida, tracks the system inland near the Georgia/South Carolina border at the same time on Wednesday morning, October 12th:

Either solution would result in heavy rainfall from Georgia on up Northward into the Appalachians - an area that certainly does not need any additional heavy rainfall after the repeat tropical and other heavy rain events of the last 30 days.

It's still obviously way too early to tell if either of the above scenarios will come to pass, but the trends continue to indicate that we need to keep an eye on Florida and the southeast U.S. coast as the official hurricane and tropical weather season slowly begins to wind down over the next 30 days...

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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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First Widespread Frost / Possible Freeze Far Northern Plains Next Week...

A surge of cold air is expected to drive South into much of the Northern Plains and Midwest by the middle to end of next week. Below is the latest GFS model forecast, valid 2am CDT on Thursday, 9-15-11:

I've drawn the front on the model image, which will bring the coldest air so far this season to places like the Dakotas, Minnesota and the Great Lakes region by the end of the week.

A widespread frost is likely and a freeze is possible within the blue outlined and hatched area on the top half of the image below (valid Thursday morning, 9-15-11):

The front will spread Southward into much of the adjacent Plains, Midwest and Ohio Valley through next weekend (GFS image below valid 7am CDT Friday, 9-16-11):

...which promises to bring cooler than normal temperatures across much of this region by that time (though not of the frost/freeze variety).  The images below show the expected "departure from normal" with respect to the average temperature for Friday (1st map) and Saturday (2nd map) of next week:

Greatest departure from normal, and therefore the coolest temperatures, can be expected in the darker blue shaded areas on the above images.

It does appear that this cool down will be relatively short lived, with temperatures forecast to rise back to near normal levels across much of the same region by the first of the following week (9-19-11):

Unfortunately, as you can see throughout all of the above, there is no major relief in site for Texas, with respect to either temperatures or precipitation.

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