Ramblings of a Graduate Student

Tonight I continue discussing the potential weekend storm, in particular the possibility of winter weather.  The image above is the Hydrometeorological Prediction Center’s probability of a point exceeding 4″ of snow accumulation between 7 PM CDT Friday and 7 PM CDT Saturday.  Notice how a large potion of Oklahoma is contoured within  the 10% threshold and areas from Oklahoma City northeast along I44 and west back to I35 are contoured in the 40% threshold with areas in northeast Oklahoma along the Kansas border contoured in the 70% threshold!

Below is the same kind of graphic, for the same time period, only instead of the probability of a point exceeding 4″ of snow accumulation, this is for 8″ of snow.  Again, northeast Oklahoma is the area targeted.

This time the plot below is for 12″ snow accumulation.  While the probabilities are low, areas are still being highlighted.

Now, without going into all the details of the the data I’ve examined up to now, I am beginning to think this storm won’t be as big of a deal as I was thinking 2 days ago.  The reason for this is that the storm is moving relatively quickly, moisture isn’t all that great, and most of the precipitation in the last few model runs has been tied to the frontal zone – which moves fairly quickly through Oklahoma.  Based on this, I think the graphics posted above are a bit optimistic regarding Oklahoma’s snow chances.  However, the event is still 2-3 days into the future, so a lot can change between now and then.

As I mentioned last night, big changes are looming on the horizon for people in the southern plains of the United States.  Tonight I’ll examine two different forecast fields for two different forecast times as simulated by the European Center for Medium-Range Weather Forecasting (ECMWF) numerical model, run at 12 UTC this morning (Tuesday).  The first image is valid 12 UTC Saturday (7 AM CDT) and the second image is valid 12 UTC (7 AM CDT) Sunday.

One of the things that should have been conveyed in last night’s post is that there exists a potential for snow across portions of the southern plains between Friday night and Sunday morning.  One of the things that a meteorologist will examine when forecasting for snow is the temperature of the 850 mb pressure level (roughly 1.5 km above ground level over the ocean).  If this level is above 0C, or 32F, then snow is extremely difficult to get at the surface, however other winter precipitation types (sleet or freezing rain) would be possible if the surface temperature is below 0C, or 32F.  If this level is below 0C, or 32F, then snow is possible, assuming precipitation did fall.

The image above is the forecast 850 mb temperatures for 12 UTC Saturday, followed by the 12 UTC Sunday 850 mb temperatures.  As one can see, temperatures across a good potion of Kansas, and the western 2/3 of Oklahoma, are below 0C (32F) by 12 UTC Saturday (7 AM CDT).  This would mean that if precipitation were to occur, snow would be one of the possible precipitation types about which to worry.  By 12 UTC Sunday (7 AM CDT), the 850 mb temperature (shown below) has cooled even further across an even larger area – including much of Arkansas.

This, in and of itself, would certainly suggest that meteorologists would need to be concerned about the potential for snow, should precipitation occur.  However, I haven’t said anything about whether precipitation (and, in particular, snow) would be falling at the two times discussed above.  The plot immediately above and below attempt to address the, “Will is precipitate?” question.  These two images are the 700 mb relative humidity fields – which are being used as a proxy for precipitation fields, since those are not freely available.

Most snowflakes (the big, pretty ones anyways) tend to develop around 700 mb in what is known as the dendritic growth zone, if sufficient moisture can be found at this level.  Based on this, if the relative humidity is high at the 700 mb layer, we can (to a first approximation) assume that snowflakes are certainly possible in the cloud.  If we couple that with the temperatures at 850 mb being below 0C (32F) we can make a first guess forecast as to whether snowflakes are possible to reach the surface.

If we notice the 12 UTC (7 AM CDT) Saturday forecast, we see that the relative humidity is extremely low at 700 mb across central Oklahoma.  This is known as the dry slot, and is a good indicator that precipitation would not be occurring at the surface.  However, notice the high relative humidity values to the south and west of central Oklahoma at this time.  By 12 UTC (7 AM CDT) Sunday, both of these features have moved east of Oklahoma.  Since I do not have forecast images from 12 UTC Saturday and 12 UTC Sunday, we are left to assume that the positions of the Saturday morning features have translated to their new locations Sunday morning.  (This far into the future, that is a relatively safe approximation.)  If we do this, we can assume that the moist air at 700 mb will traverse the state of Oklahoma, with 850 mb temperatures (first two plots above) below 0C (32F), during the day Saturday.  This would indicate that snow is a possibility (but not a certainty) during the day Saturday throughout central Oklahoma.  It will certainly be interesting to watch this unfold during the next few days…

Not much time to blog tonight as today got away from me.  (I spent about an hour and a half with reporters from South Korea doing stories on meteorology in the United States).  However, I thought a nice view of the western hemisphere from the Geostationary Operational Environmental Satellites (GOES) 12.  This image is valid 8:45 PM CST 2 March 2010.  The white (clouds) along the east coast of the United States are associated with the storm that brought snow to northern Georgia and portions of the Carolina’s today.  The good news for the northeast United States is that this storm should remain south and east of them, giving them a reprieve from all the snow of late!  (The storm that brought all the snow to the northeast last weekend is located to the northeast of the east coast cyclone.  It is the white speckled comma looking shape that extends westward toward the east coast cyclone.

This week continues to be a busy one for me, but hopefully in the coming days I’ll find the time in the next few days to discuss the possibility of late weekend or early next week thunderstorms in the central plains.  It is still a ways off, but severe weather cannot be entirely ruled out.  (I should also add that is cannot be entirely ruled “in” either!)

Above is this morning (12 UTC; 6 AM CST) surface pressure chart, as analyzed the by the Storm Prediction Center’s surface objective analysis dataset. (Essentially, what this means is that this is a model generated field that is “tugged” back toward truth through the use of observations.)  A couple of things stand out 1) the very strong low pressure centered over the New York City, NY area and 2) the strong high pressure located over the central Rocky Mountains.  The brown dashed lines indicate “troughs” or low pressure, or areas of lower pressure that are not completely closed off.  You’ll notice a trough to the northwest of the strong northeast surface low (this is indicative of strong warm aid advection aloft).  There is also a trough in the southern plains.  This trough is associated with a small, but potent, mid-level shortwave trough / closed low that brought rain (and some snow) to central Oklahoma over night.

After several blog posts about Texas snow, it’s now Oklahoma’s turn.  The image below is from the National Weather Service’s Hydrometeorological Prediction Center (HPC) and displays the probability of a point receiving at least 4″ of snow between 00 UTC Friday 26 February (6 PM CST Thursday 25 February) and 00 UTC Saturday 27 February (6 PM CST Friday 26 February). Notice how a significant swath of Oklahoma has a chance of seeing 4″ of snow Thursday into Friday!

And if you don’t trust the HPC, here is the National Weather Service Forecast Office in Norman, OK’s take on the matter.

I will be holding a map discussion tomorrow for scientists in the National Weather Center (particularly National Severe Storms Laboratory and Storm Prediction Center) so I’ll hopefully have more to say on this tomorrow.  As for tonight, my attention was focused on the USA-Switzerland and Russia-Canada hockey games and so I didn’t have much time to prepare an in-depth discussion!

Today, areas south of the Dallas-Fort Worth metropolitan area received a significant snowfall by their standards.  As of this writing (9 PM CST), the National Weather Service Forecast Office in Fort Worth, TX has a graphic indicating that Waco, TX had received 3″ of snow (light snow was still occurring).  I thought tonight’s blog post would focus on what was the source of the lift associated with this precipitation.

As my former students should remember, when precipitation develops north of the surface warm front, typically one of two (or a combination of these two) mechanisms are responsible for the sustained lift necessary for precipitation to develop.  These two mechanisms are isentropic lift or frontogenetical forcing associated in the 850-700 hPa layer.  It just so happens that the heaviest precipitation across central Texas today coincided with the 850-700 hPa frontal zone.  This is illustrated in the sequence of images below.

In the images below, the Petterssen Frontogenetical equation values are contoured in purple (essentially, the larger the number the stronger the front in the layer) as analyzed by the Rapid Update Cycle (RUC) numerical model.  The corresponding radar reflectivity is underlayed in blue, green, and yellow.  Notice how the precipitation and the 850-700 hPa frontal zone are juxtaposed very nicely throughout the day.

1200 UTC 23 February 2010 (6 AM CST23 Feburary 2010)

1500 UTC 23 February 2010 (9 AM CST23 Feburary 2010)

1800 UTC 23 February 2010 (12 PM CST23 Feburary 2010)

21 UTC 23 February 2010 (3 PM CST23 Feburary 2010)

00 UTC 24 February 2010 (6 PM CST 23 Feburary 2010)

Above is the precipitation forecast for the next 5 days from the National Weather Service’s Hydrometeorological Prediction Center.  Several areas of the United States will experience a wet 5 days.

1. The northeast will experience a very strong storm system during the next 5 days.  A strong cyclone will park itself over New England and continue to draw in warm, moist air into a cold airmass at the surface.  Although along the coast will see a lot of rain, a lot of snow will fall farther inland.  This could be the first real big snow for upstate New York this winter.
2. Central Texas and areas along the Gulf Coast will experience several mid-level shortwave troughs moving through the area.  Each of these troughs will result in a new round of precipitation – some of it falling as snow tomorrow!
3. Oklahoma and Arkansas will experience precipitation late week as more mid-level shortwave troughs move through the southern branch of the jet stream.  It is too early to tell, but this may fall as snow across the northern portions of the states.
4. The west coast will continue to be battered by numerous storms over the next five days.  Each one will bring more precipitation to central and northern California.

With all of this, my attention will be focused on the northeast for the next few days.  The storm that is forecast in this area could be one for the record books.  The potential exists for a lot of snow in the inland areas, a lot of coastal rain, and beach erosion along the coast of Maine.