**May 28^th^ Bear Creek Flash Flood Meteorological Analysis**

**By: Mike Evans**

Flash flooding occurred in Bear Creek township in southeast Luzerne
county late in the afternoon on the 28^th^ as a mesoscale convective
system developed across the area. New cells continually developed on the
southwest flank of the system, then trained to the east across the flood
area. The following figures show approximately 3 hours of radar imagery
from the event. Note how storms continually formed on the southwest
flank of the system, then moved east.

![](media/image1.jpeg){width="5.995138888888889in"
height="4.873611111111111in"}

*Radar imagery at 1847 UTC, May 28, 2002*.

![](media/image2.jpeg){width="5.997222222222222in"
height="4.845833333333333in"}

*Radar imagery at 1949 UTC, May 28, 2002.*

![](media/image3.jpeg){width="5.997222222222222in"
height="4.928472222222222in"}

*Radar imagery at 2049 UTC, May 28, 2002.*

![](media/image4.jpeg){width="5.993055555555555in" height="4.55625in"}

*Radar imagery at 2144 UTC, May 28, 2002.*

Radar estimates of total rainfall exceeded 9 inches in just a few hours
before the storms finally moved south of the area.

![](media/image5.jpeg){width="5.995138888888889in"
height="4.873611111111111in"}

*Storm total radar estimated rainfall through 2159 UTC, May 28, 2002.*

This event can be classified as a "frontal" type of flood event based on
a classification scheme developed by Maddox in the late 1970s. In a
"frontal" type of flood event, thunderstorms continually develop along a
west to east oriented stationary or slow-moving warm front. The
following figure shows a schematic of the environment characterized by
"frontal" type flash flooding, as defined by Maddox.

![](media/image6.png){width="6.376388888888889in"
height="5.022222222222222in"}

A schematic of the environment associated with a Maddox "frontal" type
flood. Note the low-level jet oriented perpendicular to the surface
frontal boundary.

The thunderstorms associated with this event could also be considered to
be "backward propagating" in the sense that new storms continually
developed southwest of the main convective system, then trained to the
east, causing the flooding.

The following is a list of characteristics of the environment associated
with a Maddox "frontal" type flood environment, accompanied by backward
propagating thunderstorms.

1)  The area is typically in the right entrance region of an upper level
    jet.

2)  An east-west surface boundary is typically present.

3)  Low-level jet: Normal to the upper level jet, normal to the surface
    boundary, and just upstream from the flooding MCS. The speed of the
    low-level jet is typically greater than or equal to 30 knots. A
    horizontally broad low-level jet is preferable to a narrow low-level
    jet.

4)  A weak surface low-pressure area is typically located upstream of
    the flood.

5)  A weak short wave is located upstream of the heavy rain.

6)  Weak mid to upper level wind shear (weak 850-300 mb winds) is
    present.

7)  Thickness diffluence is present.

8)  Thickness saturation: climatological location of heavy precipitation
    based on the following table from Funk 1991: Use PW of the
    in-flowing air, 1000-500 mb thickness at the location of interest.

> 1000-500 thickness = 552 dm, PW=0.70
>
> 1000-500 thickness = 558 dm, PW=0.80
>
> 1000-500 thickness = 561 dm, PW=0.90
>
> 1000-500 thickness = 564 dm, PW=1.05
>
> 1000-500 thickness = 567 dm, PW=1.15
>
> 1000-500 thickness = 570 dm, PW=1.25
>
> 1000-500 thickness = 573 dm, PW=1.40
>
> 1000-500 thickness = 576 dm, PW=1.55

1000-500 thickness = 579 dm, PW=1.70

1000-500 thickness = 580 dm, PW=1.90

9)  Maximum of CAPE is near or upstream from the MCS. Values of greater
    than 2000 J/kg are favorable.

10) Theta-e ridge along and upstream from the MCS.

11) The strongest low-level moisture convergence is located upstream
    from the MCS.

12) Warm cloud depth (T greater than 0C) is 3 km or greater.

13) Deep saturation; ie no significant dry layers in the soundings near
    the thunderstorms.

14) Long, narrow CAPE profiles are preferable to short, fat CAPE
    profiles.

15) Small MBE vectors, or MBE vectors parallel to the low-level boundary
    (along with the absence of dry air). The MBE vector is the vector
    difference between the mid-level flow, which steers the individual
    thunderstorm cells, and the opposite of the low level flow, which is
    an indication of where new cells will develop based on how the
    low-level flow interacts with the old cell's cold pool. Small MBE
    vectors would indicate the potential for stationary areas of high
    reflectivity, while MBE vectors parallel to the low-level boundary
    would indicate the potential for training storms.

Note: Individual cell movement should not be confused with system
movement. Often-times, individual cells within a large convective system
will move in one direction, while the system of cells moves in another
direction. MBE vectors predict the movement of the system, not
individual cells.

Note: In uni-directional flow, the mid-level "system advection" vector
will oppose the low-level "system propagation" vector, which can result
in small MBE vectors and could imply slow cell movement. However, when
dry air is injested into a storm, in a uni-directional flow regime, the
movement of the individual cells and the MCS will often be very fast, in
the direction of the flow, not in the direction of the smaller MBE
vector. This is because in that case, the cold pool can become very cold
relative to the surrounding air, and will tend to propagate quickly east
in the direction of the mid-level steering flow. The best inflow
relative to the cold pool (gust front) in that case will then be on the
downwind side of the cold pool, causing the propagation vector to point
in the same direction as the advection vector. A warmer cold pool favors
a slower moving cold pool, in which case the best storm relative inflow
toward the cold pool will be on the upwind side of the cold pool. The
result is that the propagation vector would point upwind, opposing the
advection vector, and resulting in a slower moving or upwind moving
storm.

The rest of this case study will examine data from this event (primarily
Eta 6-12 hour forecast gridded data and soundings), and compare the data
to the rules listed above.

1)  The region is typically in the right entrance region of an upper
    jet. This is a favorable area for upper-level divergence. The
    left-exit region is also a favorable area for upper divergence,
    however the best low-level moisture is typically located underneath
    areas south of the main jet, making the right entrance region most
    favorable for convective heavy rains during the warm season. In our
    case, northeast Pennsylvania was in the right entrance of an upper
    jet located east of Maine. It could also be argued that northeast
    Pennsylvania was in the left exit region of a jet over the southern
    Great Lakes area.

![](media/image7.png){width="5.99375in" height="2.7618055555555556in"}

*Eta 6 hr forecast 250 mb isotachs verifying at 5/28 18z.*

2)  An east-west surface boundary is typically present. The next two
    figures indicate that a weak west to east surface boundary was
    located across northern Pennsylvania during the afternoon on the
    28^th^. This boundary may have been a combination of a synoptic
    scale front, and an outflow boundary left-over from convection that
    moved across southern New York state earlier in the morning.

![](media/image8.png){width="6.750694444444444in" height="3.69375in"}

*Surface plotted winds, temps and dew pts 5/28 19z.*

![](media/image9.png){width="5.99375in" height="2.7618055555555556in"}

*Eta 3 hr forecast pmsl verifying 5/28 15z.*

3)  A low-level jet oriented normal to the upper level jet, and normal
    to the surface boundary. The speed of the low-level jet is typically
    30 knots or greater. A broad low-level jet is preferable to a narrow
    jet. The role of the southerly low-level jet is to transfer moisture
    and instability over the frontal zone. In addition, a southerly
    low-level jet will act to continuously develop new convective cells
    at the nose of the jet, as the jet interacts with the cold pools
    generated by previous convection. A broad low-level jet is
    preferable because it would be able to feed new cells across a
    relatively large area. In our case, we had a weak southerly jet
    extending up through 850 mb across eastern and central Pennsylvania.
    The forecast speed of the low-level jet was only 20 knots. It may be
    that the 30 knot rule was derived mostly in the Plains, where the
    low-level jet is often stronger than farther east. Also, keep in
    mind that a southerly flow across eastern Pennsylvania has a
    significant upslope component into the higher elevations of the
    Pocono mountains, where this flood occurred. Therefore, a lower
    speed may have been required than what would be needed over the
    Plains. The width of the low-level jet appears to have been
    approximately 150 miles or so, covering much of central and eastern
    Pennsylvania.

![](media/image10.png){width="6.245138888888889in" height="3.1875in"}

Eta forecast 850 mb isotachs verifying at 21z 5/28. Note the south to
north maximum of around 20 knots indicated over eastern and central
Pennsylvania.

![](media/image11.png){width="7.122222222222222in" height="4.25in"}

*Eta forecast cross section of wind speed (kts) taken from Lancaster Pa,
to Binghamton NY.*

4)  A weak surface low pressure system is typically located to the west
    of the flood. Our case had a weak low pressure system to the west,
    located over northern Ohio at 21z on the 28^th^.

![](media/image9.png){width="5.99375in" height="2.7618055555555556in"}

*Eta 09 hr forecast pmsl verifying 5/28/02 21z*.

5)  A weak short-wave at 500 mb is typically located upstream of the
    flood. Our case had this feature, located over central Pennsylvania
    at 18z.

![](media/image12.png){width="5.997222222222222in" height="3.05625in"}

*Eta 06 hr forecast 500 mb height and vorticity verifying 5/28/02 18z*

6)  Weak mid to upper tropospheric winds with weak shear. In our case,
    the winds from 700 mb to 400 mb were from the southwest, gradually
    increasing with height from less than 10 knots at 700 mb to around
    30 kts at 400 mb. The winds also increased with decreasing height
    below 700 mb, reaching around 15 to 20 knots with the low-level jet
    at 850 mb.

![](media/image13.png){width="6.503472222222222in"
height="4.582638888888889in"}

# Eta bufkit forecast sounding at avp verifying 5/28/02 17z

7)  Thickness diffluence. The following figure shows weak thickness
    diffluence across Pennsylvania at 21z on the 28^th^.

![](media/image14.png){width="5.997222222222222in" height="3.05625in"}

Eta forecast 1000-500 mb thickness verifying 21z 5/28.

8)  Thickness saturation. Look for precipitable water values in the
    inflow region of at least the saturation value corresponding to the
    1000-500 mb thickness in the area of interest. In this case, the
    thickness across northeast Pennsylvania was around 563 dm. Based on
    table shown earlier in this document, the saturation precipitable
    water for a thickness of 563 dm would be around 1.00 inches. The
    following figure shows the Eta forecast precipitable water for our
    case, indicating that downstream "inflow" values of around 1.3
    inches. Based on the data from the table, this would be sufficient
    for a heavy rain event.

.

![](media/image15.png){width="5.995138888888889in"
height="4.678472222222222in"}

6 hr eta forecast precipitable water values verifying 5/28 18z.

9)  Maximum CAPE is near or upstream of the flooding MCS with values of
    2000 J/kg or greater. In our case, the Eta forecast CAPE values were
    around 1800 J/kg over central Pennsylvania, which would have been
    just upstream of the flood. Some heating in the warm sector just
    after this time could have resulted in CAPES of near or just over
    2000 J/kg.

![](media/image16.png){width="5.993055555555555in"
height="4.082638888888889in"}

Eta 3 hr forecast CAPE verifying 15z 5/28.

10) A theta-e ridge along and upstream from the flooding MCS. This is
    obviously favorable since it would provide a source of maximum
    moisture for the MCS. In our case, there was a pronounced theta-e
    ridge at 850 mb, with its axis just to the west of the flooding MCS.

![](media/image17.png){width="5.995138888888889in"
height="4.678472222222222in"}

Eta 6 hr forecast 850 mb theta-e verifying 5/28 18z.

11) Maximum low-level moisture convergence along and just upstream from
    the flooding MCS. In our case, Eta forecasts did not appear to
    support a concentrated area of moisture convergence near or just
    upstream from the MCS. The relatively weak low-level jet in our case
    appears to support only weak, spotty areas of low-level moisture
    convergence on the Eta forecast. In reality, moisture certainly must
    have converged in the area where the low-level jet intersected the
    MCS across northeast Pennsylvania. Since the Eta did not forecast
    this convection accurately, it missed this area of moisture
    convergence.

![](media/image18.png){width="6.627083333333333in"
height="3.3819444444444446in"}

*Eta forecast 850 mb theta-e convergence verifying 5/28 18z.*

12) A warm cloud depth of 3 km or greater. This is favorable, since warm
    cloud rain producing processes tend to be most efficient at
    producing heavy rain. In our case, the warm cloud depth appeared to
    be only around 2 km.

![](media/image13.png){width="6.870138888888889in"
height="5.096527777777778in"}

Eta bufkit forecast sounding at AVP verifying 5/28/02 17z.

13) Deep saturation; ie no significant dry layers in the forecast
    soundings. None of the soundings in our case appear to exhibit any
    significant dry layers.

14) Profiles with long, narrow CAPE are more favorable for heavy rain
    and flooding then profiles with short, fat CAPE. A deep layer of
    CAPE will produce a very tall thunderstorm, which is best for
    precipitation efficiency. Short, fat profiles of CAPE will produce
    very strong updrafts, but the storms will not be as tall. This would
    be more favorable for severe weather than for heavy rain. In our
    case, all of the forecast soundings along and south of the front
    exhibited narrow, deep CAPE profiles.

15\) Small MBE vectors, in the absence of dry air, are favorable for
individual cells to remain nearly stationary, or sometimes to even track
just upstream from their initial development points. MBE vectors
parallel to the low-level boundary can result in cells continually
training along the boundary. Either situation can be favorable for heavy
rain. Recall that an MBE vector is the vector difference between the
mid-level flow vector, which steers the individual cells, and the
opposite of the low-level flow vector, which dictates where new cell
development can be expected based on how the low level flow will
interact with the old cell's cold pool. In our case, the MBE vectors
were not that small, and pointed toward the southeast (the mid-level
flow vector pointed toward the east, and the opposite of the low-level
flow vector pointed toward the south. The resultant MBE vector pointed
southeast). This would imply that the individual cells in our case
should move southeast. However, the key to this event, was that while
the individual cells tended to move east, the entire system remained
anchored as new cells continually reformed upstream from the old cells.
This was most likely the result of new cells forming in response to the
low-level jet interacting with the nearly stationary frontal boundary,
plus the effect of the low-level jet being oriented in a direction that
was "upslope" relative to the rising terrain in the southern Pocono
mountains.

![](media/image19.png){width="5.988888888888889in"
height="4.454861111111111in"}

Eta Bufkit AVP forecast sounding verifying 5/28 18z. Note the MBE vector
pointing toward the southeast (blue arrow).

SUMMARY:

The flash flood event of 5/28/02 in northeast Pennsylvania exhibited
many of the characteristics of a classic Maddox "frontal" type flash
flood event. A few aspects from this case that were less than classic
was the relatively weak low-level jet, forecast at only around 20 kts at
850 mb. The weak low-level jet forecast by the Eta model also resulted
in a weak low-level moisture convergence signal. Aspects of this event
that were typical of "frontal" flash flood type environments included
the presence of an east-west surface frontal boundary under the right
entrance of an upper jet, sufficient instability and moisture on the
warm side of the boundary, and a topographic feature that helped to
anchor the convection.