Ramblings of a Graduate Student

One consequence of a moderate-to-strong El Nino is a decrease in snowfall for the central and northern Rockies, including northern Colorado.  On the flip side, the southern Rocky Mountains receives above normal snow fall, such has been the case throughout New Mexico.

Tonight northern Colorado, including the cities of Denver and Boulder, is in the midst of a significant winter storm.  Anecdotal evidence indicates that 6 inches of snow has accumulated in parts of Denver in less than 5 hours (1.2 inches per hour).  In fact, a friend of mine living near Boulder, CO summarized the storm with a FaceBook posting stating, “This snow storm has gone from zero to holy cow in about 60 minutes.”

Examining regional radar images leads me to believe that the snow should continue to accumulate quickly for the next several hours.

There are many disappointed snow lovers in Norman, OK tonight after the big snow storm has (so far) been a bust.  There is a chance that snow developing in far southeast Oklahoma could work its way back into the Norman vicinity, but it most likely will be light and is far from certain.

I’ve been asked a lot tonight, “What went wrong?” and, in my opinion, the lack of rich, deep moisture for this storm to work was ultimately it’s undoing (here in central Oklahoma).  This was not a complete surprise; I even mentioned it here in the blog (check out the last paragraph of Day 76 and the last paragraph of Day 77) as well as in the Hazardous Weather Testbed Map Discussion on Friday.  With this said, it doesn’t mean I’m not disappointed.  I really wanted a big snowfall.

So how did all this play out?  Well, during the morning to early afternoon hours (in the wake of the 850-700mb front), a mid-t0-upper level dry slot overspread much of south-central Oklahoma.  This act to shut off the precipitation throughout this region.  (The dry slow is the portion of the storm where dry air wraps into the storm.  It is responsible for causing the storm to take a comma shape [ , ] instead of a backslash [ \ ]).  In fact, you can see evidence of this drying aloft in tonight’s 00 UTC (7 PM CDT) sounding from Norman, OK (shown above, annotated below).

This drying aloft occurred very close to what is known as the “Dendritic Growth Zone” (DGZ), which is the layer of the atmosphere that promotes large ice crystal development.  These large ice crystals ultimately turn into large snowflakes should they make it all the way to the ground.  Unfortunately, when drying aloft occurs very near the DGZ, and strong lift is present, the large ice crystals struggle to develop.  As such, the snow that fell in Norman was being generated lower in the atmosphere where smaller ice crystals develop.  This manifested itself in the form of  ”snow grains” that seemed to fall all day at my house, not the large snowflakes needed for rapid snow accumulation.

If more robust moisture had been available to this storm, the dry slot might not have pushed as far north and east as it did, resulting in large snowflakes falling instead of the snow grains.

This isn’t to say that large snowflakes weren’t present anywhere today.  In fact, just north of the strong mid-level jet streak (and associated “new” dry slot), a new area of precipitation is curently developing.  Reports from underneath this developing area of precipitation indicate that “half dollar” sized flakes are falling and accumulating quite quickly.

This new area of precipitation is developing in a region of strong lift on the nose of the jet streak, and is quite robust as a result of deeper moisture beginning to work its way into the storm (green arrow below).  Once again, had the deeper moisture worked its way into the storm earlier, the heavier precipitation would have developed a bit further earlier in the day, closer to central Oklahoma.

As the precipitation in eastern Oklahoma continues to develop, expand, and intensify it will begin to “use up” most of the moisture that has been feeding into central Oklahoma, which has been necessary for snow production there.  As such, one would expect the snow to decrease in central Oklahoma, and current radars indicate this to be the case.  This trend should continue for the next several hours until all that is left is flurries.

With this said, the storm is just now beginning to intensify (meteorology students should think latent heat release) and as it does so, the precipitation in eastern Oklahoma should begin to wrap around the mid-level low located along the Red River.  If the mid-level low moves slowly eastward, there is a chance that the “wrap around precipitation” would make it all the way into central Oklahoma.  This is a long shot, but essentially the only shot left at seeing measurable snow in this part of the state.

And for good measure, I thought I’d show the same kind of plot used on Day  77 (linked above).  Notice how the deeper moisture is now working its way into the storm via the Gulf of Mexico.  I guess I’m left asking myself, “What could have been?”

I’ve dreaded having to write this post for most of  the day.  When I woke up this morning, numerical guidance from overnight and this morning almost unanimously indicated the potential for a significant winter storm in central Oklahoma.  Deep down I knew that forecast and that kind of agreement wouldn’t last.  I was fairly sure that by the time I sat down to write tonight’s post, I would be even more uncertain about tomorrow’s weather events than when I woke up.

I was correct.

Instead of walking through various predictions, which are all over the place, I thought I’d attempt to explain what tonight’s 00 UTC (7 PM CDT) North American Model (NAM) appears to be doing.  The idea is that instead of giving pretending this model is correct, I’ll attempt to explain why it is producing the forecast it is producing.  Someone could then watch the evolution of these features overnight into tomorrow to have a better idea of where and when the heaviest snow will fall.  Here we go…

The next three images are 30, 36, and 42 hour forecasts, valid Sunday morning at 1 AM CDT, Sunday morning at 7 AM CDT, and Sunday afternoon at 1 PM CDT  respectively.  The color fill is the model’s predicted 6 hour accumulated precipitation for the previous six hours.  As a first guess, anywhere underneath blue dashed lines could expect to see snow or sleet, whereas places in the red dashed lines would expect to see rain.

Notice in the image above the light green color fill extending from northwest Texas, northeast to an area of heavier precipitation along the Arkansas, Oklahoma border.  This would be precipitation resulting from the mid-level low circulation centered around 700 mb.  It is often referred to as “wrap around” snow or “comma-head” snow.  The latter is more of a descriptive term since when this portion of a storm is at it’s most potent, it looks like the top portion of a comma.  At this time, the precipitation in the comma head is fairly light.

However, if we step forward in time 6 hours, we begin to see darker green shading develop in the comma head.  At the same time, the darkest green color begins to shift westward – opposite to the large scale flow.

Stepping forward yet another 6 hours, we see the trend continuing.  This would seem to indicate that the heaviest snow in the comma head would actually occur overnight Saturday night into early Sunday afternoon.  The key to being able to identify where and when the heaviest snow will occur with this storm is to identify what processes are occurring in the model that result in an increase in the comma head precipitation.  If we can identify these processes, we can then watch for them tomorrow, and adjust our forecasts accordingly.

Without going into why I knew to look here, let’s take a look at the 300 mb level, home to the jet stream for this storm.  The next three images are all of the 300 mb level and are 30, 36, and 42 hour forecasts just like the previous images.  The color fill in these set of images represents the forecast wind speed, with darker colors indicating stronger wind speeds.  The wind direction can be determined by the “barbs” (look like a stick and with more sticks and flags hanging off it).  The portion of the barb without anything hanging off of it is the direction the wind is blowing to. So, in southern California in the first image below, the wind is blowing from the northwest to the south east.

The key feature in the image above is the patch of darker shading located near the Big Bend of Texas.  This is known as an upper-level jet streak.  Notice how it is still to the west of the axis of symmetry in the upper-level trough.  Watch what happens to this jet streak as we step forward in time.

Notice how the jet streak has become longer and is now on the east side of the upper-level trough.  Again, we’ll step forward 6 more hours.

Now the jet streak is even larger and almost entirely on the east side of the upper-level trough.  This coincides with when the heaviest precipitation develops in the comma head (shown above).  So the question is, how are these two phenomena related?

The answer is somewhat subtle.  If you look closely at the middle image of the precipitation plots, you’ll notice that a “L” appears in central Arkansas.  The appearance of the “L” on that map indicates the development of a surface low pressure.  This surface low pressure rapidly strengthens as the left-exit region (northwest portion) of the jet streak spreads over the surface low.  As the surface low continues to develop and become stronger, the winds on the north side of the low also strengthen.  A consequence of all this strengthening is the rapid advection of moisture into the comma head.  An over simplification of what happens is that because there is more moisture available in the comma head, more precipitation can fall.

So, the key to tomorrow is knowing where and when the upper-level jet streak will overspread the surface low pressure.  When this occurs, the entire storm will strengthen and the precipitation rates (should be mainly snow and sleet) will increase in the comma head.  However, one thing to remember is that a slower upper-level jet streak would indicate that the mid-level cold air (necessary for snow) will be slower to arrive than previously thought.   This would indicate a slower transition from rain to snow, and result in a wetter snow that is easier to melt on a warm ground.  It will be interesting to see how this evolves…

Note:  The “left-exit” region of a jet streak is not always on the northwest side of the jet streak. Ultimately, the “left-exit” region is dependent on the orientation of the jet streak.

Note:  For completeness, the surface low pressure system isn’t the only part of this storm that strengthens as the upper-level jet streak begins to overspread the surface low.  The entire storm strengthens.  Examine the plots below. Notice that the number of black contours increase in the middle of the “low”. Also notice how the numbers associated with those contours (black lines) also become smaller. These are signs of a strengthening storm.

The next three images are the evolution of the 500 mb low at the same time steps as above.

The next three images are the evolution of the 700 mb low at the same time steps as above.

The next three images are the evolution of the 850 mb low at the same time steps as above.

It has been a very hectic day, and the numerical weather prediction models for this weekend have helped make it even more hectic than it would have been otherwise.  Last night, I mentioned how almost all the numerical guidance (at least those available to me when I blogged) indicated that the upcoming southern plains storm would be relatively minor, with most of the precipitation being generated in the quickly passing frontal zone.   This was mainly the result of the numerical guidance keeping the mid-level (short) wave (trough) “open” and preventing the development of what is known as a closed low.  Beginning with today’s 12 UTC (7 AM CDT) numerical models, the forecast of an “open wave” during the weekend has dramatically changed.  However, before I jump into all of this, let’s first take a look at what I mentioned a couple of days ago - the chance of severe thunderstorms on Friday.

There are several things working against the chance of a widespread severe weather outbreak tomorrow.  However, the biggest one is the lack of deep, rich atmospheric moisture.  (Moisture is an essential ingredient in the development of any thunderstorm.)  The image above depicts the amount of moisture currently throughout the troposphere.  Notice how the Gulf of Mexico has relatively low moisture values.  This is because a storm that moved through the Gulf of Mexico during the last 36 to 48 hours has “cleared out” all the available moisture and shoved it south and east out of the Gulf.  This means that it will take some time for the moisture to come back into the southern plains – most likely longer than we have.

Another ingredient necessary for severe thunderstorm development is a source of lift (or rising motion).  The image above is from this evening’s 00 UTC (7 PM CDT) North American Model (NAM) and depicts a forecast of rising motion (lift) at 00 UTC (7 PM CDT) tomorrow evening.  What is important about this forecast is that the rising motion is found in a skinny band located along the cold front and not in a widespread area ahead of the cold front.  This leads me to believe that the warm sector will most likely be void of enough lift to develop thunderstorms, so we’ll be forced to rely on the cold front to generate enough lift for severe thunderstorms.  This means there will really only be one shot for severe thunderstorms – not repeated chances – however thunderstorms will certainly be possible.

Lastly, severe thunderstorms need to have a lot of instability.  One measure of instability is known as Convective Available Potential Energy (CAPE).  The image above depicts the amount of CAPE at the same time as the previous image’s vertical velocity (lift) depiction.  Notice how there is hardly anything there!  This means that the entire line of thunderstorms that might develop along the cold front would have to share what little instability is depicted above.  This will also act to limit the severe threat.

And, lastly, the snow potential.  It’s a bit late, so I won’t go into all the details that I had previously planned to cover.  The image above from the National Weather Service Forecast Office in Norman, OK, pretty much says it all.  A good portion of Oklahoma will see the potential for a significant snowfall over the weekend.  Keep in mind that these are “accumulation” forecasts, or in other words, the amount of snow you’ll see on the ground.  This is impressive when you consider that a lot of the area will see temperatures in the upper 60s to lower 70s tomorrow.  Ground temperatures will be incredibly warm at the start of the event, meaning that a lot of snow will melt initially.  A lot of snow would have to fall in order to get accumulations forecast above.  Actually, if we took some of the models at face value, portions of Oklahoma could see 1-2 FEET of snow…

With all this said, referring back to the first image, I have concerns about moisture into this storm.  This could be one potential failure mode.  This certainly bears watching…

Friday night I discussed the possibility of setting up another southern snow storm.  All current indications are the are suggesting another Texas snow storm late Monday night into early Wednesday.  This will be the second snow storm for portions of central and northern Texas in less than two weeks.  (Remember, 11-12 February portions of northern Texas – including the Dallas-Fort Worth metropolitan area – received over a foot of snow.)

The image above is from the National Weather Service’s Hydrometeorological Prediction Center.  It is the probability of a given point receiving over 8″ of snow between 00Z Tuesday (6 PM CST Monday) and 00Z Wednesday (6 PM CST Tuesday).  Notice how areas south of the Dallas-Fort Worth metropolitan area have a 10% chance of receiving 8″ or more of snow – including the Waco, TX area.  Per the NWS office in Fort Worth, the last time Waco, TX received 4″ of snow was 1982.  This has been one of the greatest winters ever!

As many of you may already know, I’m currently attempting to take “a snow shot of America” tomorrow.  For those who don’t know what that is, you may be wondering what on earth is “a snow shot of America”.  Simply put, it is a collage of pictures taken of the snow on the ground from every state.  Why tomorrow?  There is a chance that there will be snow on the ground in all 50 states at the same time.  It is unclear if this has ever happened in recorded history.

Above is this morning’s National Snow Analysis; it excludes Alaska and Hawaii, but I’ll get to those states in a minute.  As you can see by looking at the analysis, snow exists in same form or fashion in every one of the contiguous states, with the exception of Louisiana and Florida.  However, a storm moving across the southern United States is poised to change that map drastically by tomorrow.  Local National Weather Service offices in the Deep South are currently forecasting up to an 1″ of snow as far south as the beaches of far western Florida Panhandle.  2-4″ may be possible in areas just north of Pensacola, FL.

Below is the current National Watch/Warning graphic.  The hot pink color is Winter Storm Warnings.  It isn’t very often you see these extended all the way to the Gulf Coast.  Nor do you see it over such a large area this far south.  Here’s to hoping for Florida snow tomorrow!

Now, about Alaska and Hawaii…Alaska has a lot of snow in the state, so I’ve never worried about them.  I was led to believe based on a The Weather Channel tweet that there was snow in the mountains of Hawaii.  This seemed perfectly logical to me, so I didn’t question it too much.  Old webcam images showed snow, and a blizzard in the past few weeks blanketed the mountains.  However, further investigation has led doubt to finding snow on the mountains.  It certainly seems possible that there is snow in a sheltered patch, however, finding the snow will be tough.  I keep reminding myself that “absence of evidence is not necessarily evidence of absence”.  In other words, just because we can’t find it, doesn’t mean it isn’t there.  Here’s to trying!

In any event, Florida snow is more rare than Hawaiian snow, so getting 49/50 and the 48 contiguous states is still an impressive feat!