Slantwise Convection Forecast Lab
2002-2-26
Case:
Region:
2000-12-31
Southern Interior BC (West of Coast/Cascade Mountains) south of
latitude 53
Model Data: from 2000-12-31-0000Z
Time period: 3118000Z to 010000Z
Examine ETA model forecasts from 0-24 hours starting with an area covering BC, AB,
SK, WA, MT, ID, OR and the eastern Pacific. Focus on the period between 18Z and 00Z.
1.
In a GARP window, display 500 mb height and surface isobars. Make sure to
select the model data from 20001213/0000. Add to this 300 mb isotachs in
knots. Before displaying click the more box and turn off contour line and select
color fill with a minimum of 60 and an interval of 20.
Note any significant features or processes which might produce precipitation east of the
coast ranges.
2.
In another GARP window, display the 500 mb height field along with the absolute
vorticity (Avorobs).
3.
Examine a few model soundings looking specifically at the stability, moisture and
wind characteristics. Ignore anything below about 800 mb as this portion of the
column is not significant yet. For simplicity use the 18 hour forecast from
310000Z and choose several stations such as yyf, eat, ywl, ycg and yxc. Use
others or lat/long points if you wish. Format example for lat/long is 49.5;-120
4.
In a third GARP window, display the 700 mb height field. Add the 700 mb RH
but click more and turn off the contour line and select color fill, minimum 80,
interval 10.
5.
Add to the 500 mb height/vorticity window the 6 hour precip in mm. Again, click
more and turn off the contour line and select color fill, minimum 0.5 and interval
0.5.
6.
Finally, open another window and display the 700 mb height and temperature
fields. Set the temperature interval to 1 degree if it does not default to that. At
18Z, compare the regions of strong temperature gradient to the regions of strong
vorticity advection.
Slantwise Convection factors:
In the interests of time. Consider two point locations, Cranbrook (YXC) and Penticton
(YYF). For each one, assess
Factor
Penticton
(Y/N)
Cranbrook
(Y/N)
S or SW flow, little directional shear,
windspeed increasing with height
weak gravitational stability
weak inertial stability
at or near saturation
Strong thermal gradient
7.
Assuming that slantwise convection appears possible, it will be necessary to a
cross section of Mg and e perpendicular to the thermal wind. First, add thermal
wind vectors to the 700mb height and temperature GARP window. In order to
remove terrain effects, select the vector thermal wind and input Level 1 of 500 mb
and Level 2 of 700 mb.
8.
To start, choose a cross section from say a point on Vancouver Island to a point in
western Alberta. Campbell River (YBL) to Lethbridge (YQL) might be a good
start. Select Level 1 of 300 mb and Level 2 of 1000 mb. Select ThetaE_K and
select more and set the contour interval to 1. Add the M_surface and set that
contour interval to 5. Focus on the 18Z and 00Z time period. Are there any
regions where the M surfaces slope less than the e surfaces?
9.
Try a few other cross-sections, perhaps zooming in a little on the threat area.
West to east cross sections might be easiest to manipulate quickly. Perhaps start
with 48;-124>48,-116 and then move north in 1 degree latitude steps.
10.
Repeat a few cross-sections with EPV although this is not quite as easy. Define
the cross section as earlier. Set Level 1 to 300, level 2 to 1000. Then select
PotVort_Ks-1P-1. In the box immediately above the Refresh button, change
THTA to THTE. Click on More and set the contour interval to 5. Also activate
color fill and enter interval 5, maximum 0. This will shade the negative EPV
differently. The GARP colors may be hideous – sorry about that.
11.
Given all the information viewed thus far, think about what type of snowfall
forecast might be issued to cover this case. Also, given that the valleys, below
800 mb are generally saturated, is there any reason to expect that high elevations
(850 mb) will experience different snowfall than valley bottoms (950 mb)?