Poor Convective Timing and Cold Pools

Many people I know were discussing the moderate risk of severe weather that the Storm Prediction Center had issued last night for today in western Oklahoma:

Fig 1 -- SPC Day 1 convective outlook as of 12Z, May 11, 2011. From the SPC.

However, by mid-morning, that moderate risk had gone away (and the slight risk had expanded considerably further north)...

Fig 2 -- SPC Day 1 convective outlook as of 1630Z, May 11, 2011.  From the SPC.

So what happened?  Why did the severe potential decrease over that area from what we were expecting?

The answer lies in the timing of convection with this surface low--and what convection in the early morning does for the next day.

Take a look at Norman, Oklahoma's sounding from 12Z this morning:

Fig 3 -- KOUN sounding from 12Z, May 11, 2011. From the SPC.

This is a pretty good sounding in terms of its thermodynamics if we're looking for severe weather.  There's high CAPE values and a capping inversion is present (the winds are another story, however).  Remember that the capping inversion is what keeps a "lid" on the convection during the day.  We see that capping inversion as the abrupt warming with height shown in the red temperature line right at 850mb in the above diagram.  Now, often we talk about the capping inversion being "bad" for convective development because it prevents storms from forming.  However, to get severe storms, particularly supercells, you want there to be some sort of capping inversion in place.  This allows two things:

1. By having a capping inversion, it ensures that storms can't just fire off everywhere--only in unique places where there is enough lift to overcome the capping inversion can storms form.
2. Having the capping inversion in place allows the surface to heat up (and possible more moisture to advect in) throughout the day, creating an even more unstable environment so that if that instability is released in the late afternoon, the convective development is particularly explosive.  With no capping inversion in place, as soon as the surface started heating at the beginning of the day things would quickly become unstable and storms would fire off without taking advanatage of a full-day's worth of heating.

So, for severe, supercellular storms, we do want there to be a capping inversion, particularly in the morning to mid-afternoon hours.

Also, in the above sounding, notice that I drew a yellow circle around the layer right at and above the capping inversion.  Notice how dry the air is right here--the dewpoint temperature (shown by the green line) is much less than the actual air temperature, indicating dry air here.  Now, this can be a good thing for severe thunderstorm formation as dry air aloft tends to contribute to stronger downdrafts and more damaging winds at the surface (but that's for another blog post).  But, for now, just keep this feature in mind.

This morning, as the sun came up, the visible satellite image showed this:

Fig 4 -- GOES-E visible satellite image from 1316Z, May 11, 2011.  From the HOOT website.

You can see the nice, comma-shaped curl of the low-pressure cyclone over eastern Colorado.  But notice what we have in western Kansas and western Oklahoma--lots of clouds!  It turns out that there was enough lift and enough low-level moisture early this morning to cause showers to form over western Oklahoma and western Kansas.  So what does this mean?

Mid-level winds this morning were generally out of the southwest, as shown in this GFS analysis from 12Z this morning:

Fig 5 -- GFS analysis of 700mb winds (colors) and height (contours) for 12Z, May 11, 2011.

 So what would these winds do?  They brought the clouds (and their moisture) northeastward and into the moderate-risk corridor that the SPC had outlined.  How significant was this moisture?  Here's the special sounding launced at Norman five hours later at 17Z:

Fig 6 -- KOUN sounding from 17Z, May 11, 2011.  From the SPC.

Notice how much the dewpoint has increased in the area I had circled-- the air is no longer very dry there--it's nearly saturated (i.e., the air temperature and the dewpoint temperature are about the same).  So what happened when those clouds moved in?  Remember that at 12Z this layer of air was rather dry over Norman.  So, as clouds and their liquid water droplets moved in, the liquid water in those clouds started evaporating into the much drier air.  In one of my previous blog posts, I talked about how condensation (going from water vapor to liquid water) releases a lot of energy, which in turn warms the air.  When the opposite happens--when liquid water evaporates into water vapor--it consumes a lot of energy, which in turn cools the air.  This is a phenomenon called evaporative cooling.

You can see in the sounding above that that's exactly what happened--not only has that circled layer become moister, but the temperatures have cooled down considerably.  In fact, they've cooled so much that the capping inversion (which was a layer where temperatures warmed considerably with height) is no longer visible--the cooling due to the cloud water has removed the cap!

Now remember what I said before--to get supercellular types of storms, you want the capping inversion to linger throughout the day, at least into the middle of the afternoon, to allow instability to build and to prevent storms from firing up everywhere.  But now it's late morning and the cap seems to have completely disappeared over Norman due to this cloud intrusion.  The result?  Here's the radar two hours later at 1938Z:

Fig 7 -- NEXRAD base reflectivity radar mosaic for 1938Z, May 11, 2011.  From the NWS.

Storms basically just fired up everywhere.  As a result, though there was and still is a risk of severe weather, the potential for isolated supercells and strong tornadoes has gone away.  The cap just eroded too early.  The wind shear also wasn't that great yet anyhow--we needed a strengthening of the winds aloft forecast for later in the day to get the kind of shear necessary to support strong rotation.  So, things were just timed poorly.

One interesting facet of these storms is how they have fired and propagated ahead of the convergence along the main dryline/front.  Here's the surface analysis from the SPC around the time of the radar image above:

Fig 8 -- SPC surface analysis of dewpoint temperature (color shadings), temperature (red contours), mean sea-level pressure (black contours) and wind (barbs) for 19Z, May 11, 2011.

Some features stand out.  Note the surface low (pressure is in the black contours) is analyzed in extreme southeastern Colorado near the Oklahoma panhandle.  The dryline is clearly visible as the strong gradient of moisture extending along a north-south line across far western Oklahoma.  It's the area where the dewpoint temperature (the colored shadings) drop from greens down through blues to nothing--indicating dewpoints to the west of the dryline are below 56 degrees Fahrehnheit while to the east they are in the upper 60s.  Also notice in the wind barbs how there is convergence along the dryline--winds to the west of the dryline are out of the west whereas winds to the east of the dryline are out of the south-southeast.

However, the storms at that time were more in west-central Oklahoma--slightly ahead of the dryline.  Look at the the temperature contours (the red lines) in western Oklahoma.  See how underneath the storms it's much colder than it is elsewhere?  In fact, while temperatures are in the mid-80s in eastern Oklahoma, underneath the storms it gets down into the mid-60s.  This is a result of all that cold, downdraft air falling with the rain underneath the storms.  This zone of colder temperatures underneath the storms is often referred to as the "cold pool". 

Here's a map from the Oklahoma Mesonet from one hour later.  The storms have moved further east, and you can see that there's a wide swath of colder temperatures underneath them:

Fig 9 -- Air temperature at 2m from the Oklahoma Mesonet at 5:25PM CDT, May 11, 2011.  From the Oklahoma Mesonet.

Notice that both in front of and behind the storms the temperatures are in the 70s, whereas underneath the storms, the temperatures drop to the low 60s.  This is a very well-defined cold pool.

One interesting thing about cold pools is that they can provide a lifting mechanism for storms to propagate into areas even when there is a capping inversion present.  Think about the leading edge of the storms and their cold pool--it's like a miniature cold front.  Downdraft air from the thunderstorms can blast out in front of the storms and provide convergence and lift as it runs along.  This allows storms to keep going as they track along with the leading edge of their cold pool. So, if a bunch of storms can establish a decent cold pool, they don't need to have a front or other convergence around to provide lift--the leading edge of their cold pool can provide them with their own lift.  It's a lot like the squall-line and bow-echo dynamics I discussed in a previous blog post--convergence along the leading edge of their cold pools are what keep them going.


So...even though the moderate risk got cancelled, there still have been a lot of severe storms today, including in the convergent zone along a warm frontal boundary in the upper midwest.  This slow-moving cyclone will continue to track across the country over the next day or two, bringing even more chances of severe weather.

Diagnosing a Day-Three Moderate Severe Risk

I shouldn't necessarily say "upcoming"--there are some ongoing severe storms in northern Oklahoma and eastern Kansas even late this Friday evening:

Fig 1 -- KICT 0.5 degree base reflectivity from 0345Z, April 9, 2011.

Several reports of large hail were associated with these storms as they moved through the area.  No tornadoes reported with these storms.  There were possible tornado reports with another group of storms in the Kentucky/West Virginia area on Friday afternoon, however.

But let's get right down to it.  The SPC has another slight risk of severe weather tomorrow for two regions--one in the upper midwest and the other in the Ohio Valley down through the Carolinas.  The big buzz as of late, though, is for the day 3 moderate risk issued on Friday for Sunday.

Fig 2 -- SPC day 3 convective outlook issued 07346Z, Friday, April 8, 2011.

As Patrick Marsh noted on his blog, day three moderate risks are rather rare--in the past ten years there have been only eight day-three moderate risks issued by the SPC.  To issue such a significant risk from three days out means that the SPC has high confidence that there will be severe weather in that particular region during that time period.  Let's see what the latest models have been saying.

To get severe weather (in general) there are five basic ingredients (at least, this is how I was always taught to think about this:

1. Shear
2. Instability
3. Lift
4. Moisture
5. "Exhaust"

Why do I put them in that order?  Because when you make an acronym out of them, you get "SMILE".  You can tell I was taught by veteran storm chasers...

So let's start looking at these particular parameters, though not in that order.  The "Exhaust" parameter is supposed to refer to upper-air features that can evacuate air out at the top of a thunderstorm from fast-rising convective plumes.  I prefer to think of this ingredient as "upper-level support" for a more wide-scale convective situation--i.e., I'm looking for divergence aloft.  Here's the current synoptic pattern at 300 mb (the upper troposphere):

Fig 3 -- 00Z analysis of 300 mb winds (colors) and geopotential height (contours) for April 9, 2011.

There's a deep, positively-tilted trough over Nevada and California on Friday evening.  The eastern US has a generally zonal pattern with a weak ridge across the central US.

So what's going to happen aloft?  The models are in agreement that the deep trough will be slowly moving across the country over the next two days.  What's remarkable about this trough right now is that it's staying very deep but is still advancing eastward fairly steadily.  Often when we get really deep troughs they have a tendency to "cut-off" from the main flow and just spin around over one area for a while without really moving. But this one seems to be on the move.  Here's the NAM model 300 mb wind forecast for Saturday evening:

Fig 4 -- NAM 24 hour forecast of 300 mb winds (colors) and geopotential height (contours) for 00Z, Sunday (Saturday night), April 10, 2011.

The 300 mb trough has moved over the Rocky Mountains and a strong jet streak is forecast to build on its eastern side.  I'm not too impressed with the flow to the east of that, though.  There is a weak(er) east-west jet streak across the northern Great Lakes, but otherwise there's generally weak flow coming down across the Ohio Valley and the Carolinas.  This makes me think that the severe threat tomorrow for both those areas is going to be somewhat conditional--there doesn't look to be widespread upper-air support in the models.  However, that strong jet streak on the leading edge of the trough is definitely going to start driving things down below.  Here's the NAM surface forecast for Saturday evening:

Fig 5 -- NAM 24 hour forecast of surface temperature (barbs), mean sea-level pressure (contours) and winds (barbs) for 00Z, Sunday (Saturday night), April 10, 2011.

As we would expect, under the divergent exit region of this jet streak (which has a slight cyclonic curvature to it) we see lower pressure developing.  Pressure is going to be falling all day Saturday in the central plains as that jet streak approaches from the west.  With falling pressure, winds are going to pick up trying to fill the ever-increasing pressure difference.  As such, you can see that strong southerly winds are being forecast across a broad swath of the south-central part of the country.  Warm (and moist, as we'll see in a minute...) air is being brought up north with these winds.  The relatively weak upper-level pattern across the eastern US today and tomorrow has left a relatively "unforced" flow at the surface, which is easily distorted by the lowering pressure in the central plains.  This is allowing for that impressively large warm sector to develop.  We can see some boundaries forming--there does appear to be a cold front shaping up in this forecast across northern and western Nebraska.  The shift of winds from southwesterly in west Texas to more southerly across Oklahoma and eastern Texas hints that here may be a dryline forming there.  We can check the dewpoint forecast for that:

Fig 6 -- NAM 24 hour forecast of surface dewpoint temperature (colors) and winds (barbs) for 00Z, Sunday (Saturday night), April 10, 2011.

There is indeed a dryline forecast to be present at that location.  It isn't the sharpest dryline I've seen, but there's a clear division between moister and drier air forecast to set up from central Kansas down through western Oklahoma and into Texas.  The amount of moisture being brought northward is very rich to the east of the dryline.  Note the wind barbs--the flow across the entire central part of the country is forecast to be coming right out of the Gulf of Mexico.  This is a lot of moisture--with dewpoints in the 70s being forecast in northern Missouri, southern Iowa and southern Illinois.

The structure of this low-pressure center is also kind of unique in these plots.  If you go based on the structure of the moisture field, the main low-pressure center seems to be somewhere in northeastern Nebraska at this time.  Futhermore, it looks like the dryline extends all the way to the low-pressure center.  The cold front that we saw on the surface temperature forecast can also be seen in the moisture field as the pronounced wind shift between northerly to southerly winds stretching through western Nebraska and into northern Colorado.  But look at the location of the cold front on the dewpoint forecast map--the cold front is advancing into the dry air behind the dryline.  This means that, though the cold front would provide lift, there isn't much moisture in the air it would be lifting.  As such, the cold front isn't going to be much of a player on Saturday.

That upper-level trough is forecast to move slowly--by Sunday afternoon the NAM forecast has it positively tilted out over the great plains:

Fig 7 -- NAM 45 hour forecast of 300mb winds (colors) and geopotential height (contours) for 2100Z, Sunday, April 10, 2011.

This is still forecast to be a very deep trough--it extends all the way from Canada down to Mexico.  Furthermore, the jet streak on its leading edge is very well-organized in this forecast, with 140 knot winds in a rather compact area.  With such a strong, straight jet, we'd have to assume that the cold front has begun to assert itself and has moved down into the central plains.  Also, based on their being divergence in the left exit region of a straight jet streak, we'd expect to see the surface low somewhere over northwestern Iowa or southern Minnesota.  What does the NAM say?

Fig 8 --  NAM 45-hour forecast of surface temperature (colors), mean sea-level pressure (contours) and winds (barbs) for 21Z, Sunday, April 10, 2011.

Well--the low-pressure center is not as organized as I would have thought.  The NAM seems to have some difficulty placing the low-pressure center, but based on the swirl in the wind field our initial guess of northwestern Iowa does seem reasonable.  The cold front is right about where we expected it to be with that jet streak aloft, however.  But by Sunday afternoon, it's clear that the warm sector has reached at least all the way into southern Wisconsin, with a cold front roughly from Omaha down through central Kansas.  It's difficult to figure where the dryline would be and where the cold front really is once we get further south from there.  The dewpoint forecast should help with that:

Fig 9 --  NAM 45-hour forecast of dewpoint temperature (colors) and winds (barbs) for 21Z, Sunday, April 10, 2011

The moisture field shows a dryline actually located a bit further east than I would have guessed--located from the Kansas City area down through central Oklahoma and into central Texas.  Once again, the cold front still seems to be advancing into the dry air behind the dryline--so the cold front still looks to be playing a minor role in forcing lift for convection.  Yet there is a moderate risk out for eastern Iowa, northern Missouri, northwestern Illinois, southwestern Wisconsin and southeastern Minnesota for this time.  Where's the lift?

Let's re-evaluate our conditions for severe weather in that area with what we know so far (out of order):

1.  Moisture--This one seems to be very well-established.  Dewpoints at the surface are forecast to be in the 60s or greater all the way up to southern Wisconsin in the forecasts.  That's definitely the kind of moisture we would be looking for to get severe weather.  Moisture is not an issue.
2. Exhaust/Upper-level support--The 300mb winds for Sunday afternoon do have the broad left-exit region of a jet streak over the moderate risk area on Sunday afternoon.  That should provide enough divergence aloft to support thunderstorm growth below.
3. Wind shear--Haven't really touched on this one much yet, but we can check it quickly.  Note that the surface winds in the warm, moist sector at this point are forecast to be rather strong--20 knots out of the south-southwest in the moderate risk area. The winds are somewhat lighter further north, but I don't really trust the NAM's winds in that "area of ambiguity" surrounding the low-pressure center.  Let's compare these winds with the forecast 850mb winds (a little ways off the ground):

Fig 10 --  NAM 45-hour forecast of 850 mb winds (colors) and geopotential height (contours) for 21Z, Sunday, April 10, 2011.

Note that the winds are more southwesterly just a bit off the surface over the moderate risk region.  This points to directional wind shear, at least.  Winds are also somewhat faster than at the surface--30-40 knots across the moderate risk region.  So there's good speed shear as well.  However, remember in one of my recent blog posts I talked about how the low-level winds (but not the surface winds) tend to pick up right after sunset.  Here's the forecast 850 mb winds for 6 hours later at 03Z (around 10 PM CDT).

Fig 11 --  NAM 51-hour forecast of 850 mb winds (colors) and geopotential height (contours) for 03Z, Monday, April 10, 2011

Look at how much the wind speeds increased as soon as night fell!  Now we're looking at west-southwesterly winds at 50-60 knots in the low levels of the atmosphere right above the surface in southern Wisconsin.  That's a whole lot of low-level wind shear.  So, I definitely think that wind shear will be sufficient to produce severe storms in the moderate risk region.

But what about the last two--lift and instability?  Some would argue that these are the most important ingredients to look for.  Instability seems to be a given.  Notice how warm the air was at the surface in the forecasts above.  Some people in southern Wisconsin and northern Illinois are forecasting highs in the mid 80s on Sunday.  Combine this with dewpoints in the 60s and it becomes very hard to consider the situation "stable".  Let's check the forecast NAM soundings from Earl's Skew-T page to see what we're looking at.  Here's the forecast sounding for Davenport, IA (roughly in the middle of the moderate risk region) for 21Z Sunday (Sunday afternoon):

Fig 12 -- NAM 45 hour forecast souding for KDVN, valid 21Z, Sunday, April 10, 2011.

I apologize for the small type and the cluttered diagram--if you click on it you can see the full-sized version and actually read things.  Note the CAPE value given of 2702 J/kg.  A very large amount of convective available potential energy.  But can this instability be tapped into?  Notice that there is still a capping inversion present at around 800-850 mb.  Remember that this capping inversion is a layer where the temperature of the environment actually warms with height instead of cooling like normal.  We can see that abrupt bump in the temperature curve right at that 800-850 mb layer.  So we do have a capping inversion to deal with that will keep a lid on tapping into that instability.  However, let's look at the forecast sounding for three hours later at 00Z (around 7 PM CDT):

Fig 13 -- NAM 48 hour forecast souding for KDVN, valid 00Z, Monday (Sunday evening), April 10, 2011.

The cap is no longer present!  So this forecast sounding has an uncapped profile with 2871 J/kg of CAPE.  Very unstable.  Very dangerous.  Particularly with all that wind shear...

Just for comparison, let's look at a point further south like Saint Louis:

Fig 14 -- NAM 48 hour forecast souding for KSTL, valid 00Z, Monday (Sunday evening), April 10, 2011.

Not as impressive.  Note that the temperatures don't cool as rapidly with height as they did up in Davenport.  There seems to be warmer air in the mid- to low-levels of the atmosphere that is not present further north.  There's still CAPE listed--at some 1532 J/kg it's pretty significant, too.  But the level of free convection (the level you'd have to lift surface up to before it could tap into that instability) is up at around 625 mb.  That's an awful long distance to have to lift air before it freely convects.  Particularly without a cold front in play to do the lifting for you.  The difference in the atmosphere further south partially explains why the moderate risk area was placed further north.

So what about that lift?  I believe that's the most conditional part of this setup, actually.  We've seen that the model says there will be upper-level support, wind shear, moisture, and instability.  All it takes is a little convergence at the surface to provide the lift necessary to start thunderstorm development.  This is particularly true with a very unstable and uncapped profile like we see in the forecast for Davenport.  Since some thunderstorms are expected in the region on Saturday, residual outflow boundaries and other perturbations in the wind field from the Saturday storms may linger on into Sunday and provide focal points for new convective development then.  With so many factors favoring thunderstorm development, it's virtually certain that storms will form.  In fact, with wide-spread vertical motion favored by the divergence aloft, we could see storms firing up all over the place.  It will be interesting to see how this event is organized.

Without an organized forcing mechanism like a cold front, the storms that form will have a greater tendency to remain surface-based, drawing warm moist air directly from the surface layer.  Combine this with the strong low-level wind shear and you have the ingredients for tornadic thunderstorms.  This also justifies the heighted "moderate" severe risk that the SPC has put out there.

More updates will come as this event draws closer.

A Look at Lake Effect Snow

Cold air advection in the wake of yesterday's cyclone passage across Lake Erie is causing a very well-defined, wide band of lake effect snow south of Buffalo this afternoon:

Fig 1 -- 0.5 degree base reflectivity image from KBUF at 2113Z, Dec 1, 2010

Heavy snows are being forecast for the area as per the latest Lake Effect Snow Warning for the Buffalo area:

SNOW ACCUMULATIONS: 14 TO 20 INCHES IN THE MOST PERSISTENT
  BANDS. SNOWFALL RATES OF 1 TO 2 INCHES PER HOUR LIKELY.

So what goes into a lake effect snow event?  What conditions have to be met to see extraordinary snow bands like this one?

There are at least five main qualifications that usually must be met to get a good lake effect snow event.  This list is adapted from a presentation in Dr. Fred Carr's mesoscale meteorology class at the University of Oklahoma (though I believe the actual presentation was given by Todd Kluber).  But here are the five:

1. Lake-air temperature difference --> Generally, the lake surface water temperature should be at least 13 degrees Celsius warmer than the 850mb temperature.
2. Height of the capping inversion --> To get any real snow at all, the capping inversion needs to be at least 1.9 km above the ground.  For heavier snows, the capping inversion should be more than 2.3 km above the ground.
3. Wind direction over lake fetch length --> The winds must be blowing in a direction such that the length of open water over which they pass is at least 80 km for light flurries and at least 160 km for vigorous banded snow.
4. Vertical wind shear --> The best banding occurs with little to no directional shear of the wind underneath the capping inversion.  If there is more than 30 degrees of directional shear, the snow band location becomes difficult to pin down and with more than 60 degrees of directional shear bands are unlikely.
5. Lake-land temperature difference --> In some cases, "lake breeze" convergence zones can set up if you get offshore (or onshore) flow in association with a lake-land temperature difference (like the sea breeze near the ocean).  This convergence can enhance bands of lake effect snow.

So...let's see if these conditions are being met in our Buffalo case today.

First, the lake surface water temperature  should be at least 13 degrees Celsius warmer than the 850mb temperature.  This should be easy enough to check.  NOAA's Great Lakes CoastWatch division publishes daily estimates of lake surface water temperature.  Here's yesterday's image (today's isn't out yet, but lake temperatures don't change as fast as air temperatures...):

Fig 2 -- Analysis of Great Lake surface water temperature for Nov 30, 2010.  From NOAA CoastWatch and the Great Lakes Environmental Research Laboratory.

Now, I am not a fan of their color scale on here at all.  How are you supposed to tell the difference between those different kinds of green on that color bar? But after careful, close-up analysis I concluded that those dark and light greens in Lake Erie (the lake west of Buffalo) average out to a lake temperature of about 10 degrees Celsius.  Well above freezing.  So now we need our 850 mb temperature. We could try using this morning's 12Z sounding from Buffalo:

Fig 3 -- 12 Z sounding from Buffalo, NY on Dec 1, 2010.  From the HOOT website.

Well. If we grab our 850mb temperature from this sounding we get right about 0 degrees Celsius.  That's a lake-850mb temperature difference of only 10 degrees Celsius.  Rather large, but not the 13 degrees Celsius we wanted it to be.  Of course, this sounding was taken much earlier this morning.  Note that the inversion (and a corresponding wind shift) is also right around 850mb.  The wind direction at this level shifts from westerly to southerly with height--a backing of the winds.  If we went back to those thermal wind arguments, we might suspect that there was cold air advection going on in that narrow layer.  So perhaps our 850mb temperature has since cooled a bit. Let's see what a model sounding for this afternoon says:

Fig 4 -- RUC 21Z Analysis sounding for Buffalo on Dec 1, 2010.  From the twisterdata.com page.

This forecast sounding is actually based on the analysis for the 21Z RUC model run, so it's probably the best we are going to get for a sounding at the time the above radar snapshot was taken.  Sure enough, our 850mb temperature is forecast to have cooled to around -7 degrees Celsius.  This would make our lake-850mb temperature difference 17 degrees Celsius.  So our first condition most likely checks out.

Next, we need the height of the capping inversion to be at least 1.9 km above the ground and over 2.3 km above the ground for very heavy snow.  Fortunately in the forecast sounding in figure 4, the capping inversion starts at right around 700mb and on the left they've conveniently labeled the height of the 700mb level at 2838 meters.  That's about 2.8 km, which is greater than 2.3 km--enough for heavy snow.  Since lake effect snow is essentially convection driven by cold air over a much warmer lake, like all convection in ther atmosphere it is going to be inhibited by any inversions aloft (unless the lake is much, much, much warmer than the air).  As such, the capping inversion essentially represents the maximum height the lake effect clouds will be able to reach.  Deeper clouds tend to produce more snowfall.  This is why we need such a high capping inversion for heavy snow.

For our third condition, we need to have winds beneath the capping inversion blowing across at least 160km of open lake for heavy snow.  Well, according to the forecast sounding above, the winds beneath the capping inversion are generally out of the west southwest.  So, let's draw a line from just south of Buffalo (the middle of the snow band) to the west-southwest (actually, if you go into the numerical values for the sounding, it's more west by south, or around 250 degrees) and see how much open water that covers.

Fig 5 -- Fetch length from a point south of Buffalo west by south across Lake Erie.  The fetch distance is about 174 kilometers.  Image from Google Earth.

We see in this direction that the winds are travelling over 174 kilometers of open water--more than the 160 km we needed.  The longer the air spends over the lake, the more water vapor it can accumulate and consequently the more snow can fall once it reaches land.  The band of snow as seen on the radar above stretches well out over the lake, too, so we know we're actually reaching saturation well before we even hit land.  Also note in the image above that if the winds become even slightly more westerly, the amount of open water the air will be traveling over will increase dramatically.  Therefore wind direction (and lake geometry) can play a huge role in determining how much snow we'll get.


For the fourth condition, we want as little wind directional wind shear as possible beneath our capping inversion.  Once again, in our forecast sounding above we see almost no directional wind shear beneath the capping inversion--all the winds are generally out of the west-southwest.  Strong directional wind shear would limit the ability for convection to occur over the long fetch of the lake as winds at different levels would be advected over different lengths of warm water.  This could create spurious inversions and other inhomogeneities that would disrupted the banded structure of the snow and perhaps inhibit it all together.  To get a strong band of snow, you need uniform wind directions through the cloud layer and we have that.

The fifth condition is a bit more tricky to apply, as it only describes enhancement of precipitation due to lake breeze convergence.  It's possible to look into this more, but we've already more than satisfied our other lake effect snow conditions.  So, there may be some enhancement due to convergence of winds over the lake, but it's very difficult to quantify that.

And there you have it.  Lake effect snow in Buffalo that should be and is happening, with all of our forecasting rule-of-thumb guidelines met.  Note how a lot of this analysis was simply based on looking at a sounding (or rather, a forecast sounding) over Buffalo--and that's it.  This is why the forecasters at the National Weather Service--Buffalo forecast office developed the now widely-used forecast sounding analysis tool called BufKit.  It was originally designed to predict lake effect snow, but has now been expanded to all sorts of uses.  I encourage you to look into the software if you want to experiment with some fun sounding analysis tools.  And best of all, it's free...