Synoptic scale storm systems producing greater than one inch of
rainfall across southern New England are quite common from early fall through late spring.
These systems often have favored areas of orographically induced heavier rainfall. This
paper will identify the mechanisms favorable for orographically induced rainfall
associated with these in southern New England. A decision tree will be formulated to allow
the forecaster to identify these areas and to more accurately adjust rainfall guidance
from numerical models to account for these orographic effects.
Eleven synoptic scale rainfall events were examined, from
October 1995 through May 1996. Numerical Forecast Guidance from the 48 km Step Mountain
Coordinate Model (Eta), for at least two model runs prior to the onset of precipitation,
was collected for each event. This model was chosen because of its greater resolution of
topography and its performance in forecasting precipitation.
As stated by Junker (1992), the development of heavy
precipitation is dependent on adequate moisture and upward motion. When factoring in the
orographic component, one must also closely examine the boundary layer inflow. Basic model
parameters that were examined included low- and mid-level flow, vertical motion, warm air
advection, and moisture availability and advection through the use of theta-e analyses.
Low- and mid-level inflow was examined through an analysis
of the boundary layer wind fields from 1000 mb through 700 mb. The magnitude and track of
the vertical motion fields were examined for 850 mb and 700 mb, as well as mean omega in
the layer 850-500 mb. Temperature and dew point advection were examined at 850 mb and 700
mb, as well as the Laplacian of Thickness Advection at 700 mb. Layer differential positive
vorticity advection and Q-vector forcing were also considered, along with moisture
availability and advection, using Theta-E and precipitable water.
All but two of the events studied had some of the typical
synoptic forcing mechanisms common to synoptic flash floods (Chappell, 1986), including a
slow moving 500 mb trough or closed low, a slow moving surface frontal boundary, mid-level
flow parallel to the boundary, precipitable water that was much above the climatological
normal, and the presence of a strong right rear quadrant of an upper level jet. The heavy
precipitation of the remaining two events were associated with cyclogenesis along the east
coast and the associated left front quadrant of a strengthening jet on the east side of a
500 mb closed low. All eleven events also featured a well defined theta-e ridge which
traversed the forecast area.
Three types of rainfall patterns were identified. Type A
produced heavy rain over the Connecticut River Valley. Type B produced an orographically
induced rainfall maximum across the east facing slope region of the Berkshires, stretching
from western Massachusetts through northwest Connecticut, with a marked shadowing of the
Connecticut River Valley. A second maximum was positioned over the higher elevations of
northeast Connecticut, northwest Rhode Island, and central Massachusetts. This region is
the dividing zone between the coastal plain and the start of the interior highlands. Type
C featured heavy rainfall across the northeast portion of Massachusetts and extreme
southeast New Hampshire.
3.1 Type A Pattern
This pattern produced the heaviest rainfall in the
Connecticut River Valley, with amounts diminishing to the west and east. ETA Model
guidance for these events was characterized by southerly flow at the surface, averaging
25-30 kts for at least 18h prior to the onset of rain. Flow at both 700 mb and 850 mb was
southwesterly, averaging 45 kts at 850 mb and 65 kts at 700 mb. Significant moisture
advection (dew points of at least 10EC) was present. Also, the 1000-500 mb thickness
exceeded the Preferred Thickness, as reviewed by Junker (1992), by at least 100 meters.
With regard to the vertical motion fields, Type A events were significantly weaker than
Type B or Type C events, possessing generally 6 Mbar/sec at 850 mb and 7.5 Mbar/sec at 700
mb. Actual rainfall maxima for these events was 2 times the maximum rainfall forecasted by
the ETA Model.
3.2 Type B Pattern
Type B events produced the heaviest rainfall over the east
slopes of the Berkshires, with a second maximum over the interior highlands. The most
notable feature was the pronounced shadowing of the Connecticut River Valley, where
rainfall was generally one half to two thirds of that which occurred over higher terrain.
These events were characterized by southeasterly flow of
35-45 kts at 1000 mb. At 850 mb and 700 mb, the inflow was southerly, but with a
noticeable variation in speeds, ranging from as little as 30 kts to as much as 70 kts.
This range of speeds at 850-700 mb played an important role in the magnitude of the
observed rainfall maxima. Even with significant theta-E and dew point advection, when the
850-700 mb inflow exceeded 50 kts, the observed rainfall maxima were much less than those
associated with inflow speeds at 850-700 mb below 50 kts.
||Rainfall maxima exceeded ETA forecasts for all
cases. For those events where the flow at 850-700 mb was above 50 kts, but theta-E and dew
point advection were weak, the observed rainfall was 1.25 times the maxima forecasted by
the model. For those events where the flow at 850-700 mb was above 50 kts, and the theta-E
and dew point advection were strong, the observed rainfall maxima were 1.75 times greater
than that of the model.
For those events where the
low at 850-700 mb was below 50 kts, and theta-E and dew point advection were weak, the
observed rainfall maxima was 1.5 times greater than the model forecast. For those events
where the flow at 850- 700 mb was below 50 kts, but the theta-E and dew point advection
were strong, the observed rainfall maxima was up to three times the maximum forecasted by
3.3 Type C Pattern
Type C pattern featured the heaviest rainfall along the
leading edge of the interior highlands of northeast Massachsuetts and southeast New
Hampshire. These events were characterized by east-northeasterly flow at 1000 mb. These
events possessed the greatest magnitude of the 2-D Laplacian of Thickness Advection and
the greatest amount of theta-E advection at 850 mb. The location of the maximum
precipitation corresponded with the greatest theta-E advection at 850 mb.
There was a marked variation in rainfall amounts between
events of this type. This appeared to be associated with temperature advection at 700 mb.
Where warm air advection was present, the observed rainfall maxima were as much as three
times the model forecast. When 700 mb advection was either neutral or cold, the rainfall
maxima were only 1.2 to 1.4 times greater than the model forecast.
4. THE DECISION PROCESS
The decision process for determining the favored regions
for enhanced rainfall and the amount of modification to the forecast should start with an
analysis of the 1000 mb flow. The inflow will identify the favored regions, as presented
in Section 3. The adjustment to make to the rainfall forecast will be a factor increase
above the Eta model forecast, the magnitude of which is determined by a closer examination
of flow at 850 mb, theta-E advection, and 700 mb temperature and dew point advection.
4.1 Identifying Favored Areas
The first step in this process is to identify the direction
of flow, based on the 1000 mb winds. The breakdown is as follows:
|1000 mb Direction
4.2 Modification to the Model
Type A is the most straight forward of the three event
types. For this case, model rainfall forecasts should be increased by a factor of 2.
For Type B events, the factor of adjustment is related to
the speed of the 850 mb and 700 mb wind fields. If the wind field is greater than 55 kts
at these levels, rainfall in the favored areas should only be increased by a factor of
1.25 to 1.5. When the wind field is less than 55 kts, rainfall amounts should be increased
by a factor of 1.8 to 2.0.
For Type C events, the factor of adjustment is most
dependent upon the presence of warm air advection at 700 mb. If warm air advection is
present, rainfall amounts should be increased by at least a factor of 2-2.5. If cold air
advection is indicated, the rainfall should be increased by only a factor of 1.3.
Synoptic scale heavy rainfall events across southern New
England possess unique characteristics in relation to the locations of the heaviest
orographically induced rainfall, that are influenced by the direction of the low level
flow. The magnitude of the rainfall amounts in these favored regions were influenced by
the availability and advection of moisture (i.e., the magnitude of theta-E advection at
850 and 700 mb and the dew point advection at 700 mb.
The decision process developed through this research
provides yet another tool for the forecaster to utilize in the preparation of Quantitative
Precipitation Forecasts (QPF), and to more accurately position the maxima of QPF. This
will result in more timely and accurate river flood and flash flood watches and warnings.
Chappell, C. F., 1986: Mesoscale Meteorology and
Forecasting, American Meteorological Society.
Junker, N. W., 1992: Heavy Rain Forecasting Manual,
National Weather Service Training Center. 91 pp.