The OLYMPEX field campaign

After being told enough that I should write a blog post sometime soon, I figured I could write up a blog post about why I've not been writing blog posts recently...

I'm a part of the OLYMPEX field campaign to observe precipitation processes in complex terrain.  "Why do we need to do that?" you may ask.  "Haven't we observed enough precipitation in mountains? Why do it again?"

The US (NASA) and Japan (JAXA) have cooperated to launch a new weather satellite called the "Global Precipitation Mission" (GPM).  This satellite has a whole suite of sophisticated instruments for observing precipitation from orbit, including both active and passive microwave sensors.  Think of it as a weather radar floating around in space.  We previously had another satellite carrying a radar called the "Tropical Rainfall Measuring Mission" (TRMM) which was in orbit since 1997 before finally coming to the end of its life earlier this year.  As the name implies, TRMM was mostly limited to the tropics for measuring precipitation (because of its orbit).  GPM will be able to sample precipitation in the mid-latitudes (e.g., the continental United States).  It's still a low-earth-orbiting satellite, so it only passes overhead maybe once or twice per day.  But it will be able to give us much better estimates of precipitation, particularly in areas where we don't have good radar coverage, like over the ocean or...in mountainous regions.

GPM was just launched earlier this year and we're still trying to calibrate it. Basically, we want to be sure that what the GPM sensors are telling us is actually what we're seeing in these clouds and on the ground.  That's why we need this field campaign--to observe lots of details about the precipitation over mountains and see if that matches what the satellite sensors are telling us.

To do this, we've chosen to focus on the Olympic Mountains of western Washington.  This is one of the wettest places in the country (particularly this time of year), so for the next two months we'll be doing an extensive observational study of every precipitation event that crosses the mountains.  There are many components to this observational study including:

Lots of ground-based instruments, including rain gauges, disdrometers (to measure raindrop size), vertically-pointing radars, and standard weather instruments.  We've installed these observations all over the Olympic Peninsula.  This was a bit of a challenge since most of the peninsula is wilderness or the national park.  A brave team of graduate students has spent the last few months hiking around installing these instruments (even having to use pack mules to carry in some of the equipment...).  We'll also have several supplemental radiosonde launching groups to give us observations of the atmospheric structure aloft.

We've got several supplemental radars set up around the peninsula, including the NASA NPOL radar (below, image from NASA), a "Doppler on Wheels", and an Environment Canada X-Band radar up on the southern end of Vancouver Island.  These radars will provide a continuous, 3D picture of what's going on during rain events.

 

And last, but not least, we'll have three research aircraft: a Citation jet, a DC8, and a NASA ER2, which is basically a U2-type spy plane converted for research purposes (seen below, image from NASA).  The DC8 and Citation will mostly fly in and around precipitating clouds to sample them directly.  The ER2 is equipped with the same type of radar instruments that are mounted on the GPM satellite.  As I mentioned before, the GPM satellite has an orbit that only brings it overhead twice a day at best.  That's not often enough for us to get a whole lot of samples in our two-month window.  However, the ER2 plane will fly at over 60,000 feet (the pilot actually wears a special "space" suit to fly at such high altitudes) and "simulate" what the satellite would see if it were flying overhead.  This will give us a lot of samples of what the GPM satellite instruments would see so we can make more rigorous evaluations.

This week we'll finally have the DC8 and the Citation jet around (with the ER2 arriving next week) so we'll finally start our sampling.  But how do we know when and where to fly these planes and point these radars?  That's where my team comes in--the forecasters.

I'm working as one of the lead forecasters to try and project when will be the best times to sample precipitation over the next 3-5 days.  We're interested in sampling a lot of different phenomena, from the warm, wet atmospheric rivers to isolated, post-cold-frontal showers that are common to this area.  For instance, the next four days look to be a gold mine of opportunity for the project, as we're looking to get a strong "atmospheric river" event pointing right at us, giving non-stop waves of precipitation.  Here's a 12-hour precipitation forecast for this Thursday from our latest local WRF run:

The higher elevations of the Olympics are forecast to receive between 20-50mm (0.75 to 2.0 inches) of rain in just that 12 hour period.  Here's the next 12 hours:

Another 75-125+mm (3-5 inches) in just that time alone!  Let's look at the next 12 hours!

Another 75-100mm (3-4 inches).  We're looking at over 10 inches of precipitation in the Olympics through the end of this week.  

You can see in the above maps how complicated this precipitation might be to observe.  There are steep gradients in the precipitation, with large amounts over the high elevations, but only light rain expected just north of Seattle.  We really want to see how well the satellite instruments might be able to catch these kinds of variations.

Another concern is snow.  How much of this precipitation will fall as snow as opposed to rain?  Where is the melting level actually going to be?  This model suggests we'll see some snow at the high elevations, upwards of 100mm (4 inches) at higher parts of the Olympics:

But how well can this model actually do at predicting precipitation type?  What kinds of small-scale structures lead to one mountain getting a lot of snow, while others getting barely any?  These are the kinds of questions we can try and answer over the next few months.

So that's why I've been busy...getting ready to support this campaign.  I've written a new website to view the output from our local WRF model (with more modern graphics) that's located at: http://www.atmos.washington.edu/~lmadaus/olympex

We're also heavily using ensemble-based products, including the SPC SREF, UW mesoscale ensemble, and the GEFS for longer-range forecasting.  It's quite the experience to take all of your collected meteorological observations and musings over the years and be forced to put it into practice every single day...

For more information on OLYMPEX, you can visit the main project website at:

http://olympex.atmos.washington.edu/

Of particular interest may be the "Science Summaries" tab, where every day some of our project scientists describe the kinds of weather that we observed and what we saw.

You can also follow updates on the project's Facebook page, or on Twitter by following @UW_OLYMPEX or #OLYMPEX.

We also made the front page of The Seattle Times today...
http://www.seattletimes.com/seattle-news/science/uw-nasa-deploy-scientific-blitz-to-study-rainy-olympic-peninsula/

The high desert gets soaked, but not as bad as Oklahoma

After days and days of high-precipitation thunderstorms with embedded tornadoes and large hail, Oklahoma is finally getting a bit of a break.  Just how wet was it there?

The Oklahoma Mesonet recently published rainfall totals for the past two weeks.  Here's what it looked like:

Huge rainfall totals across the state.  Over 12 inches in some locations in only 2 weeks.  The annual percentage of normal rainfall has spiked through the roof.  So far through the roof, in fact, that it's maxing out the colorbar on the Oklahoma Climate Survey plot, with most places greater than 180% of their total rainfall.

Needless to say, the latest monthly outlook from the Climate Prediction Center has drought conditions in Oklahoma and Texas "improving" over the next month...

But drought persists elsewhere---California is seeking no help and the persistent troughing has kept most of the heavy rain-makers to the south of the upper midwest, causing drought conditions to develop in Minnesota and the Dakotas.

Eastern Washington and Oregon have also been running a bit low on their precipitation and are in "drought" conditions as well.  A warmer-than-average winter has allowed the western side of the Cascades to see about normal precipitation, but without much snowfall accumulating in the mountains.  This is bad news for the eastern sides of the states, where they don't see as much precipitation, but rely on meltout from the snowpack to supply them with water for agriculture.

Here's a plot showing the snow water equivalent normals and what we've had this year at Stampede Pass, which is in the Cascades southeast of Seattle.

The light blue line is the normal Snow Water Equivalent, or the mass of water in the snowpack.  The dark blue is this year.  You can see that by this time of year we should be just dropping down from our peak snow in mid April, but this year we're already at zero snow.  Looking at a map of percentage of average snowpack as of last week, we can see pretty much everyone is basically melted out (or at least below 25% of where they should be).

But there is some hope for the eastern side of the mountains!  An upper-level low is churning its way through Oregon at this time.  It's moving slowly and a bit further south than usual, and this is allowing a fair amount of moisture to get to the eastern sides of the Cascades.  Here's the latest water vapor satellite image.

We can see the swirl of the low with its attendant moisture streaming up through Nevada and into Idaho.  That low is forecast to slowly lift north-northeastward over the next couple of days.   With cold air underneath that trough, we're expecting some destabilization and a few rounds of thunderstorms to develop.  All of this in an area that does not get much rain to begin with.  Eastern Washington and Oregon are basically deserts; here's a map (from the PRISM group) of annual average precipitation across Oregon.  Much of the high desert east of the Cascades sees 10-15 inches or less of rainfall per year.

Here's a similar map for Washington, with the same story away from the higher terrain:

Our middle- to long-range forecasts are calling for several inches of rain by the end of the week east of the Cascades.  This is an unusual setup.  Here's the total rainfall accumulation by Friday evening from this morning's local WRF model run.

Large swaths of area getting 1-3 inches of rainfall.  Great for farmers!  It may seem like these numbers are small in comparison to Oklahoma, but keep in mind that these are places that are lucky to get a tenth of an inch of rain with a good convective shower.  There are usually a few thunderstorms that occur throughout the year that will give a more thorough soaking; this will be one of those periods.

Cyclone Pam, Seattle rain records and the MJO

Last week the small island nation of Vanuatu was devastated by a major tropical cyclone: the Category-5-equivalent Cyclone Pam.  There has been terrible damage to the infrastructure on the islands, with estimates of up to 90% of the buildings in the capital of Port Vila.  Power was lost early on in the storm and is just now being restored in some places.  We can actually see this looking at the weather observations from Port Vila, here from Weather Underground:

You can see that before they lost power late on the 13th, the pressure was plummeting (down to 965 hPa) with wind speeds rapidly increasing to 45 mph.

The CIMSS Satellite blog had a post showing several satellite animations of the cyclone as it hit; I encourage you to take a look at it here.

The cyclone continued south and is giving a glancing blow to New Zealand today, where it has caused some power outages and a lot of rainfall.  Remember that in the southern hemisphere low pressure centers rotate clockwise.  Here's a satellite image from the New Zealand Met Service this morning, showing that the cyclone has basically been sheared apart, but there is still a lot of moisture with strong winds.

As The Weather Channel pointed out in an earlier blog post, Pam isn't the only cyclone we had in the western Pacific over the past week.  There were actually four active at once: Pam, which hit Vanuatu; Olwy, which is affecting the west coast of Australia; Nathan, which is hitting the York Peninsula of Austrlia, and Bavi, which is headed west towards the Philippines, but is expected to weaken before it arrives there.

Is there some uniting factor behind this burst of tropical cyclone formation?  It would appear so---the Madden-Julian Oscillation (MJO) is currently, by many measures, the strongest we have seen it in a very long time.

What is the Madden-Julian Oscillation?  The exact nature of it is still a somewhat poorly-understood topic in meteorology, despite intense research activity in recent years.  Basically, it's an area of enhanced convection (thunderstorm activity) that propagates around the equator every 30-90 days.  (See the Wikipedia article for a decent overview).  We can see this looking at a global satellite loop over the past week.  There's been a lot of cloud cover and convection moving out from Papua New Guinea and northern Australia and into the central Pacific.  You can see that as it moves east, it looks like there are tropical cyclones (Bavi and Pam) that are being "shed" from the convection.

It turns out that the basic structure of the MJO has an equatorial "heat source" being trailed by flanking "gyres" on either side of the equator.  If strong enough, these "gyres" can break away and form tropical cyclones, which is exactly what we saw last week.

How strong is the MJO right now?  Here's a diagram that tracks the strength and position of the MJO as it goes around the globe, called the Wheeler-Hendon diagram.  

You can see that there are different "zones" on the diagram, labeled with different geographical regions (the "Maritime Continent" (Indonesia, New Guinea, etc.) , "Western Pacific", "Western Hemisphere and Africa" and the "Indian Ocean".  So as the MJO moves around the globe, it's supposed to make a big circle around the diagram.  The magnitude of the MJO (how strong the convection associated with it is) is given by how far from the center you are.  The line on the plot shows where the MJO has been for the past 40 days.  You can see that for much of February (the purple part of the line, the MJO was very weak (close to the middle of the diagram) and not clearly being tracked around the globe.  But in March (the red part of the line), the MJO exploded in strength over the Maritime Continent and is now moving out over the western Pacific, getting even stronger.  This is WAY stronger than the MJO has been for a long time!  No wonder we are seeing powerful cyclones.

It turns out that an "active" MJO in the western Pacific can also contribute to heavy rain ("pineapple express") events on the west coast.  Here's a diagram from the MJO wikipedia article describing this connection:

So as the active area of convection on the equator associated with the MJO moves out over the Pacific, the moisture associated with that gets drawn northward to the west coast of North America, causing heavy precipitation.

Well, over the past two days we've had just that here in Seattle.  Here's an animated map of the total column water vapor over the past two days.  You can see the plume of moisture drawn northward from the tropics that brought heavy rain to Seattle.

So much rain fell (2.2") that Sunday was actually the second wettest March day on record in Seattle, and the single wettest day we've had since 2009 (courtesy of Scott Sistek).  It's amazing how a single weather anomaly like the MJO can contribute to both tremendous rain in Seattle and an extreme Cyclone in Vanuatu.

Heavy rain in the midwest, tropical storm down south

Repeated rounds of storms around the periphery of the still-persisting ridge in the southern US have contributed to rainfall totals that are well above climatology for the last few weeks in some locations.  In Chicago this is the wettest July on record with 9.75 inches so far this month.  Furthermore, this month is now the 9th wettest month of all months in Chicago since records began--pretty impressive.

There's a lot of water vapor to work with in the atmosphere over that region at the moment.  Here's the satellite water vapor image today:

GOES-E water vapor image from 1615Z, July 28, 2011.

The upper-level ridge across the south causes air to advect clockwise around its periphery.  As such, you can see a big plume of moisture stretching from Arizona and New Mexico through the central plains, into the midwest and out through the mid-Atlantic states.  This plume of moisture has been in place over the same area for several days now, and any storms that form in that area follow the same path.  Repeated rounds of storm after storm (MCC after MCC...) following the same path have contributed to the high rainfall totals.

The more moisture there is in the atmosphere, the greater the potential for heavy rainfall.  This seems like a very obvious statement.  To that end, one parameter that meteorologists use to evaluate the potential for heavy rainfall is called the precipitable water value (often abbreviated as PWAT or PWTR).  It's a pretty simple index to derive--basically, all precipitable water does is look at the profile of moisture content over a given location.  It calculates how deep the water on the ground would be if all of the water vapor over a given location were to immediately condense out and fall as rain.  It's saying, if all the water in the air over your head immediately condensed into rain and fell out, how much rain would fall.  Clearly this is never fully realized, as the atmosphere never dumps out all of its water vapor content at once.  But, in general, the higher the precipitable water values, the higher the likelihood for heavy rain.

Most soundings that you see will compute the precipitable water value for that sounding location.  The HOOT website soundings are no different.  Here's this morning's 12Z sounding from Davenport, Iowa, which is in the middle of that moisture plume.

KDVN sounding from 12Z, July 28, 2011.

First, let's look at this sounding for a moment.  Notice how the dewpoint trace (the green line) is rather close to the temperature trace (the red line) from the surface up to somewhere around 430mb.  That's a very deep layer of very moist air.  We'd therefore expect the precipitable water values to be rather high.  You can see the calculated preciptable water value under PWTR above the upper right corner of the sounding.  The black value is the value for the current sounding, and it shows a PWTR value of 2.29 inches.  So that's saying that, based on this sounding, if all the water vapor in the atmosphere over Davenport were to immediately condense out and fall as rain, 2.29 inches of rain would fall.  That's a lot.

But we've already established that this is a record-breaking month for rainfall in this region.  Just how anomalously moist is the atmosphere within this plume of moisture?  The Rapid City, SD forecast office of the National Weather Service has put together a really nice webpage with climatological values of precipitable water for each sounding site.  Here's their yearly graph of climatological precipitable water values for Davenport:

Monthly climatological precipitable water values for KDVN.

There are several curves on this graph, which are explained by a legend in the upper left corner.  Each curve represents a different percentile of the normal precipitable water values based on the averages from 1995 to 2010.  The mean value (the 50th percentile) is given by the red curve.  You can see that for the month of July, Davenport's mean PWTR value is about 1.2 inches.  This morning's value of 2.29 inches is well above that.  In fact, if you look at the range of values for July, you'll see that 2.29 inches isn't quite up to the maximum curve (the light green curve) but it is above the 99th percentile curve (the dark green curve).  That means this precipitable water is in the 99th percentile of all PWTR values seen during July over the past 15 years or so.  That's very high.

Furthermore, the dashed green line shows a value that is two standard deviations above normal.  This value is often used by forecasters to try and separate extreme values from more climatologically normal (but still high) values.  Any time you're more than two standard deviations above normal, it's considered to be a more "extreme" case with a high potential for heavy rainfall.  2.29 inches is definitely above the two-standard-deviations line.  All of this points to the potential for very heavy rain.

Unisys publishes an interpolated precipitable water map for the US after every sounding release time.  So, here's their map of precipitable water across the US this morning at 12Z:

Interpolated precipitable water at 12Z, July 28, 2011.

There's a swath of elevated precipitable water values from eastern Nebraska east through southern Michigan.  This is the corridor where rounds of storms are able to produce very heavy rainfall and bring us these extreme events.

Also notable is the large area of very high precipitable water values right on the Gulf Coast.  They cite 2.4 inches at a point near Lake Charles, Louisiana.  There, too, precipitable water values are in the 99th percentile and thunderstorms with heavy rain are forecast for the next few days.

Of course, a very potent rainmaker looks to be bearing down on the Texas gulf coast in the next day or two.  Tropical Storm Don has formed in the Gulf of Mexico and is moving northwestward.

NHC predictied 5-day path cone of TS Don as of 10AM CDT, July 28, 2011.

It's not forecast to strengthen beyond tropical storm strength, which is good news for the gulf coast in terms of lessening the potential for damaging winds.  What's even better news is the amount of rain this storm could bring to central Texas, which is currently suffering from "exceptional" drought, as seen on the map below.  In contrast, with the extreme rainfall over Chicago recently, I'm pretty sure the little "A" for "abnormally dry" over northern Illinois is going to be disappearing soon...

US Drought Monitor from the Climate Prediction Center/UNL, as of July 19, 2011.

Heavy Precip in California

The big weather story for the past few days and right on into today has been the heavy rain and snow falling throughout much of central and southern California.  As noted by Patrick Marsh's (and many other peoples') blog posts and forecast discussions over the past few days, extreme precipitation amounts are being forecast for California.

A check of the most recent hemispheric analysis shows a rather deep 500 mb trough and surface low off of the northwest coast.

Fig 1 -- 12Z analysis of 500mb geopotential height (shaded) and mean sea-level pressure (contoured) on Dec. 18, 2010. From the HOOT website.

The 500 mb trough extends all the way down through northern California and well out into the Pacific.  The height gradient there (as well as the temperature gradient since temperature corresponds to thickness which roughly corresponds to height) hints at a jet stream oriented so as to bring lots of moisture up from the central Pacific and into the California coast.  We can see this on the water vapor imagery.

Fig 2 -- 2330Z GOES-W water vapor image on Dec. 18, 2010. From the HOOT website.

Of course, this image is from nearly 12 hours after the above analysis, but it still shows a strong current of moisture being pulled out of the tropics and into the California coast.  Note the enhancement of the water vapor right after the stream hits the coast--this shows where air is being lifted over the Coast and Sierra Nevada mountains. Since the water vapor channel on the GOES satellites tends to only pick up water vapor in the upper levels of the atmosphere, low-level moisture is often not as visible on these images.  However, when moist air is forced to rise over the mountains, all that very moist air in the lower levels (since, after all, most of that moisture is coming from the ocean surface which is at the lowest possible level...) is lifted up to heights where it becomes more detectable by the GOES imager.  Thus we see an enhancement of water vapor right as the air ascends over the mountains.

A quick look at the radar mosaic confirms large areas of moderately heavy precipitation falling across much of central and southern California.

Fig 3 -- Base reflectivity radar mosaic at 2358Z on Dec. 18, 2010.  From weather.gov.

No wonder the Los Angeles forecast office has flood watches and advisories out for most of their forecast area--southern California doesn't usually get this much rain.  But the ski buffs in the Sierra Nevadas must be loving all this new snow...

Of course, many people (at least, if they were the Seattle media) would blame this large amount of rain on the fact that we're currently in a La Nina pattern with cooler-than-normal sea surface temperatures across much of the equatorial central Pacific.  It's true that the west coast is expecting a wetter-than-average season right now.  The latest images from the Climate Prediction Center show how the sea-surface temperature anomalies have been and are remaining cold over the past few months--a pattern that defines a La Nina event.

Fig 4 -- SST anomalies for several weeks ending the week of Dec. 8, 2010.  Click on the image to see the animation.  From the Climate Prediction Center.

So this current deluge just reinforces the connection between a wetter-than-normal west coast and La Nina conditions over the Pacific.  Is it proof of such a connection in and of itself?  No.  But we'll just say that because we're seeing La Nina conditions, this heavy rain isn't as unexpected as it would have been otherwise.

Heavy Rain in the East

Today finds some very wet weather on the eastern coast of the US.  Here's the national radar mosaic from early this afternoon:

Fig 1 -- Base reflectivity radar mosaic for 1946Z, Nov 30, 2010. From the College of DuPage website.

 

There's a large area of precipitation extending all the way from the middle Atlantic states down to the Gulf Coast.  There are even tornado watches for parts of the southeast.  Pretty potent system.  It seems even more so when you look at the surface map from 12Z this morning:

Fig 2 -- Sea-level pressure (contoured) and temperature (shaded) from the RUC analysis at 12Z, Nov 30, 2010.  From the HOOT website.

There's a very deep surface low that's pretty obvious over western Lake Superior at this point.  By this analysis, the minimum closed contour of pressure is 998 mb--fairly strong.  We can also see the sharp cold front associated with this low in three ways on the map above:

1. Since one of the "adages" I mentioned in an earlier post was that pressure tends to fall as a cold front approaches and then rise again in its wake, we can conclude that a cold front tends to lie in a local pressure trough.  We can see an elongation on the contours around the low pressure center along a line stretching from near Chicago, though Indiana and down into northern Alabama.  We would suspect that some sort of boundary would like in this pressure trough.
2. There is a shift in the winds along that same line.  To the west, winds are out of the west and become more northwesterly to northerly the further south you go.  To the east, winds are generally southerly.  This implies an area of convergence in the winds along that line. Convergence like that is typically associated with a front.  But, in connection with our first observation above, if there is a low pressure trough along that same boundary, since air tends to flow from areas of high pressure to low pressure, we would expect winds to head toward our trough.  Thus, this convergence makes even more sense.
3. There is a strong temperature gradient across the front, particularly to the south.  Since the technical definition of a front is a strong gradient in potential temperature, this is the surest sign we have a front there.

However, one thing I would point out is how weak the temperature gradient gets once we get close to the low.  In fact, you've got similar temperatures surrounding the low itself--the temperature gradient doesn't really sharpen up until you get into Illinois and points south.  This make me think that the low is beginning to get occluded (which often occurs as a cyclone becomes stacked in the vertical).  To see our vertical support, let's look at the winds aloft:

Fig 3 -- 300 mb wind and height analysis from 12Z, Nov 30, 2010.  From the HOOT website.

Once again, a nice jet streak overhead, though this is just behind the front.  We are all the way up at 300 mb, though, so the thermal gradients may tilt with height causing the jet to be back over Illinois instead of directly over the front.  Our surface low is still located over the "exit" region of this rather cyclonically curved jet (it's even over the left exit region, which is better for divergence).  So we can still infer that the surface low is still being supported by the winds aloft.  However, note how the real core of the jet streak--where the strongest winds are--is down over Missouri, Arkansas, Oklahoma and Texas.  It's not coincidental that our strongest temperature gradients are further south.  But we also saw in our surface analysis that temperature gradients seemed to be weakening around the surface low.  This might explain why the jet seems so much weaker and less organized further north.

So what can we conclude about this cyclone, then?  Considering the weakening temperature gradients around the low over western Lake Superior, stronger temperature gradients further south, and consequently the better upper air support further south, I might suspect that the northern low is going to slowly weaken in favor of a stronger low somewhere further south.

Now this was at 12Z--what has happened since then?  Here's a look at the pressure falls as of 20Z this afternoon:

Fig 4 -- 3-hour pressure falls and wind vectors as of 20Z, Nov 30, 2010.  From the College of DuPage website.

The problem with using the pressure falls map this far north is that there are very sparse observations over northern Ontario and northwestern Quebec.  Therefore it's difficult to tell what the low is doing as it moves into that part of Canada.  However, we can see a general area of pressure rises over Minnesota and Wisconsin associated with the low passing by them earlier.  What's interesting is the elongated area of pressure falls along the eastern slopes of the Appalachians--somewhat further south than we'd expect given the typical storm track.  This is probably pressure falls in association with the cold front approaching (remember the front lies in a pressure trough).  Yet, in the absence of a strong signal for where the  surface low is going to move--could this also indicate some cyclogenesis further south?  Perhaps.  We'd have to wait and see.

Of course, one thing that this cyclone is definitely doing is bringing a lot of rain. However, it's not just along the cold front as we can see in the radar image back in figure 1.  So, what is providing the lift in that broad region?  The answer is isentropic lift.  When air moves, it tends to want to do so without gaining or losing any energy, or rather it moves isentropically--keeping the same entropy.  This means that the air may rise or sink depending on whatever path satisfies this condition.  We measure the entropy by using potential temperature which accounts for the energy in both the actual temperature and the pressure.  Two parcels of air at the same potential temperature have the same entropy.  Therefore, a map of a constant potential temperature surface is an isentropic map.  Since parcels want to conserve their entropy, if a parcel starts out at a certain potential temperaure, it wants to stay at that potential temperature as it moves around. Therefore, we can start inferring how air is going to move based on the structure of isentropic surfaces.

Below is the 300 Kelvin Isentropic surface from 12Z this morning.

Fig 5 -- 300K Isentropic Surface with heights and winds from 12Z, Nov 30, 2010.  From the HOOT website.

This shows the contours of height (in terms of presure level) of the 300K isentropic surface.  Remember, if air starts on this surface, it wants to stay on this surface.  Green shading indicates moisture at this level, and the wind barbs are wind along this level.

Take a look at what's happening in the southeast and on the east coast.  Note now the pressure contours decrease as you go further north.  Since pressure decreases with height in the atmosphere, this implies that the isentropic surface is higher off the ground to the north and closer to the ground to the south.  Also note the winds in this area.  They are all blowing from south to north in a region with lots of moisture.  We can conclude that there is very moist air on this surface being advected from south to north.  But as it moves north, this air must follow the surface.  Since the surface is getting higher as we move north, the air must be rising too.  This is a phenomenon known as isentropic lift--and there's a lot of it going on in the eastern US.  That's what's causing the precipitation to form over such a large area and to be so heavy--lots of isentropic lift.

Of course, once air becomes saturated it doesn't follow potential temperature surfaces anymore, but once the air is saturated--we're getting condensation and rain.  So it still gives a good solid reasoning behind all the rain in the eastern US...