Name_________________________
Spring 2015
SO441 Lab 3: NWP and Extratropical Cyclone Development
Objectives:
- Interpretation of upper-level vorticity, both relative and absolute, and its relationship to
surface pressure
- Analysis of vorticity and vorticity advection at 500 mb
- Analysis of temperature advection at 850 mb
- Analysis of sea level pressure WRT vorticity and temperature advection
- Analysis of resulting precipitation
General tasks / introduction.
In this Lab, you will be using supplied charts downloaded from www.twisterdata.com and that
document the predicted environment of the snow event in Annapolis on Tuesday, 17 February
2015 and the event that produced Thunder snow in New England on Saturday, 14 February 2015.
The 17 February data will be used to explore the evolution of GFS predicted environment in
Annapolis. The 14 February data will be used to explore vorticity and the effects of vorticity and
temperature advection associated with the passage of an upper-level trough through New
England.
To examine the evolution of the 17 February weather event, we will look at the predicted 0600Z
17 February 2015 from 10 February 2015 0600Z 168hr MSLP prediction through the 17
February 2015 0600Z 00hr MSLP “prediction” (you can consider this an observation). All of
these are provided in your Lab 3 slide deck.
To examine the evolution of this 14 February weather event, we will look at the 500 mb height
and wind chart, the 500mb relative vorticity chart, the 850 mb temperature and wind chart, the
sea level pressure chart, and the composite radar reflectivity (from
http://vortex.plymouth.edu/upcalc-u.html. All of these are provided in your Lab 3 slide deck).
For this lab, you will use charts from the Lab 3 slide deck to observe and describe the evolution
of the GFS forecast for 06Z on 17 February 2015, and annotate and describe the evolution of the
system that produced the thunder snow in Boston, MA on 14 February 2015. You will then
write a 1 to 1.5-page summary for each portion of the lab (NWP and Extratropical Cyclone
Development). In the NWP case you will discuss the evolution of the surface pressure field and
500mb trough, and how long it took the model to predict the system that brought us snow on 16
and 17 February. In the 14 February extratropical cyclone case you will discuss the evolution of
the upper-level trough and its associated vorticity and temperature advection. You will also use
sea level pressure and composite radar charts to explain the changes in weather patterns at the
surface in context of the upper levels (where is the low with respect to the temperature and
vorticity).
Specific tasks.
1. Open the slide deck provided and review the slides to become familiar with the content.
2. NWP
A. Discussion
1) The slides associated with the NWP portion of the lab are in the front half of the slide
deck (slides 1-19). As mentioned above the charts provided are GFS forecasts of 500mb
heights/winds and surface temperatures/pressure/winds. All charts are valid at the same
time (0600Z 17 FEB 15) with the longest time frame prediction at the beginning and
finishing with the 0 hr prediction (can be considered an observation).
B. Action
1) Quickly cycle through the 500mb wind and height forecast to observe the evolution of
the upper level prediction.
2) Quickly cycle through the surface forecasts to observe the evolution of the surface
features across the US and specifically the east coast.
C. Deliverable
1) In a 1 – 1.5 page (max) write up (using a word processing application and double
spaced is fine) discuss the following. (It is recommended that you annotate the charts in
the following section during your lab period before attempting your write up)
a) Thoroughly discuss the evolution of the upper level charts. Where is the ridge,
where is the trough, where is the jet streak (remember your geography counts)?
If the chart features change significantly with time address this in your write-up.
b) Thoroughly discuss the evolution of the surface charts. Do the locations of
surface features (highs, lows, and fronts) change significantly with time, etc?
c) Given the 0hr “observation” as a reference, when does the “forecast” become
reasonably accurate?
d) Any other observation you find interesting.
3. Extratropical Cyclone Development
A. Discussion
1) The slides associated with this portion of the lab are in the back front half of the slide
deck (slides 20 – 45). As mentioned above the charts provided are GFS 0hr forecasts
(observations) from 00Z on the 14th through 12Z on the 15th. This time period covers the
system that produced the thunder snow in Boston that made Jim Cantore freak out.
B. Action (this portion requires a fair amount of annotation)
1) Locate the surface charts and place a large red “L” (32 font) at the location of the
surface Low on each chart.
a) Copy the L to the 850mb and 500mb charts that correspond to the same
timeframe.
-You should be able to simply copy (right click or control-C) from the
surface chart and it will be in the correct location
b) Copy the location of the surface Low to the composite radar chart (times will
be 15 minutes off on the radar chart, but this is close enough).
-This will not be a direct copy and paste since the data is from different
sites. You must observe the location on the surface chart and put the low
in the correct location on the radar chart.
2) Move to the 850mb chart and annotate the warm and cold air advection. You can
locate the temperature advection by observing where winds cross isotherms at as close to
90 degree angles as possible (Remember temperature advection is u  T  u T cos ,
where
u is the wind velocity, T is the temperature gradient,  is the angle between the wind
and the temperature gradient, and the negative sign is there for convention such that warm-air
advection is positive). We use the 850mb chart so we can assume that we are above the
frictional effects of the surface.
a) Using the tools in ppt, place a red circle/oval around the warm air advection
(Use red line and red fill (70% opacity)), and place a blue circle/oval around the
cold air advection (blue line and blue fill as before).
b) Copy the circles to the 500mb and surface charts (no need to add to radar) at
the appropriate timeframes.
3) Move to the 500mb relative vorticity chart and annotated the trough axis and vorticity
advection. Note that this is relative vorticity (absolute vorticity minus the Coriolis term),
but it will not affect the location of the vorticity advection. Also note that winds are not
plotted and you will need to reference the 500mb wind chart if you need it.
a) Using the freehand line tool annotate the trough axis as accurately as possible.
Make the line dashed and 4 pt.
b) Using a black box with no fill annotate the vorticity advection. Similar to
temperature advection, this is where winds (which you know to be geostrophic)
cross lines of constant vorticity at angles, with more vorticity advection occurring
when the winds cross the vorticity lines at angles closer to 90 degrees. (Remember
the magnitude of vorticity advection is u    u  cos . Like before, u is the
wind velocity, but this time,  is the vorticity gradient and  is the angle between the
wind and the vorticity gradient.)
d) Copy the trough axis and vorticity advection box to the other 500mb, the 850,
the surface chart, and the radar chart
C. Deliverable
1) Soft Copy of your annotated charts via email.
2) 1-2 page (max) write up (double spaced is fine) discussing the evolution of the
extratropical cyclone development including the following
a) The spatial relationship of the trough axis, cold air advection, surface low,
vorticity advection, and precipitation at each time period
b) Location of the warm air advection with respect to the low on the same period
and to the location of the low on the next time step (how does the 12Z location of
the Low relate to the prior 00Z warm air advection, etc.)
c) Is there any “digging” of the trough?
d) Does the magnitude of the vorticity (and therefore advection) change with
time, if yes, why?
e) Does the Low deepen with time, and how does this relate to vorticity?
f) Any other information you find interesting and pertinent to the discussion.