HVYRAIN: Excessive Rainfall Discussion

Msg # 1154 of 2431 on Fidonet Weather Echo
Time: Tuesday, 6-02-20, 8:34
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FOUS30 KWBC 020834
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Tue Jun 02 2020
Day 1
Valid 12Z Tue Jun 02 2020 - 12Z Wed Jun 03 2020
...Midwest / Great Lakes...
Quasi-zonal flow across the northern U.S. will see a low amplitude
shortwave tracking from the Northern High Plains into the Upper
Great Lakes Tuesday afternoon into the overnight hours. At the
surface a cold front will drop slowly southward across MN/WI,
followed by a surface wave advancing along the boundary toward the
middle Missouri River Valley by evening. Thunderstorms are
expected to blossom near the surface low in the vicinity of the
eastern Dakotas and southwest Minnesota around 21Z, increasing in
coverage through the evening.
Initial cells in the vicinity of the tri-state region of SD/NE/IA
(and possibly up into southwest Minnesota) could show brief
training and mergers as 850 mb winds begin to increase ahead of
the low. Passage of the cold front, however, should keep this
threat limited to a few hours near peak heating. Downstream,
convection near the slow moving front extending across southern
MN/WI into MI will be accompanied by 30-40 kt of 850 mb flow,
favoring repeating cells within the unidirectional cloud-bearing
layer, beneath the right entrance region of a 80-100 kt upper
level jet max. On the other hand, instability and downdraft CAPE
are such as to promote cold pool development and gradual southward
advancement of the heavier echoes. Thus, the hi-res model signal
for event totals in this regime is relatively tame. Three-hour
flash flood guidance (FFG) values of 1.5 to 2.5 inches are common
across much of the Marginal Risk area, extending from the Missouri
Valley into Lower Michigan. Most hi-res model solutions are
hovering around producing local two inch amounts over three hours,
which in most cases will only approach, but not exceed FFG.
There is also some question as to how far east the threat extends.
The NAM forecasts strong CAPE values to extend toward eastern
lower Michigan and the Detroit area. Only the NAM CONUS Nest,
though, indicates much development of robust convection there
overnight. Certainly the upstream forcing will be arriving, and
should promote thunderstorm development, but the Nest also
indicates a good deal of propagation, limiting rainfall totals.
For now we have left eastern lower Michigan out of the risk area,
but the risk could expand depending on trends.
Day 2
Valid 12Z Wed Jun 03 2020 - 12Z Thu Jun 04 2020
...Mid MS Valley into the Ohio Valley...
Strong instability and deepening moisture along a slow moving
frontal boundary extending from the Mid MS Valley across the OH
Valley into the northern Mid Atlantic states could support
convection capable of producing storms producing heavy to locally
excessive rainfall during Day 2, mainly during the late afternoon
and evening hours.
A cold front extending from Quebec across the Upper Great Lakes to
the northern Mid MS Valley at the start of Day 2 drops slowly
south during the period, as short wave energy crosses the Great
Lakes into southern New England.  Lift ahead of the short wave is
expected to supply synoptic scale ascent along the front, as
1500/2500 J/KG of MUCAPE extending the boundary during the
afternoon and evening hours, peaking near 04/00z. The resulting
storms are expected to track slowly east southeast ahead of the
front, aligned roughly with the mid level flow.
Organization of the convection is expected in the right entrance
region of a 100 knot jet streak moving from the Great Lakes into
northern New England, and convective complexes are expected along
the front from northern MO across the OH Valley into western and
central PA. Due to the amount of instability present, the storms
are expected to be mostly outflow driven. However, as a low level
southwest flow transports 1.50 inch precipitable water air along
the front, the column moistens, which could support more efficient
rainfall producers.
As the propagation vectors becomes better aligned with the mid
level flow, an environment fostering training develops,
particularly across OH into western and possibly central PA.
Training in the deeper moisture could support hourly rainfall
rates nearing 1.25 inches anywhere along the front from central IL
into western PA (which is seen in much of the 00z high resolution
guidance), peaking between 03/20z and 04/04z. Three hour flash
flood guidance is as low as 1.00/1.50 inches over OH into western
PA, and these values are attainable where training occurs. Based
on this, a Marginal Risk was placed over the OH Valley into the
western third of PA to over the low end flash flood threat.
Further west across northern MO, the flash flood threat remains
more nebulous. Storms that form in the evening are expected to
develop southwestward along the strong instability gradient (where
2500/3500 J/KG of MUCAPE is expected to reside). Storm motions
slow here, which increases the threat of short term training and
possible cell mergers, most of which occurs in the 04/03z to
04/08z time frame. Three hour flash flood guidance values here are
as low as 1.50/2.00 inches, as seven day rainfall has been 300+
percent above normal, suggesting wet antecedent conditions. While
training or cell mergers would be necessary to initiate flash
flooding, there is enough of a model signal for rainfall amounts
to support extending the Marginal Risk over northern MO for Day 2.
A developing long wave trough extending from the Lower MS Valley
into the central Gulf of Mexico move slowly eastward during Day 2,
possibly closing off over LA/MS by the end of the period. Ahead of
the mid level system, deep moisture and instability will combine
to support convection developing along the Gulf Coast from MS/AL
to the FL west coast. From this distance, it appears as though the
strongest convection (and the heaviest rainfall) remains just
offshore, possibly affecting the FL west coast the most.
Though there is a signal for heavy rain with this feature, the
threat remains nebulous, and for that reason, no excessive area
was assigned. Should the model signal for heavy rainfall increase
for the FL Panhandle down along the FL west coast, a Marginal Risk
could be needed in later forecasts.
Day 3
Valid 12Z Thu Jun 04 2020 - 12Z Fri Jun 05 2020
...Southern Appalachians into the northern Mid Atlantic...
A weakening frontal boundary extending from the Southern
Appalachians into the northern Mid Atlantic states becomes the
focus for deep moisture and instability supporting convection
capable of producing heavy to locally excessive rainfall during
Day 3.
There is some model spread concerning where the weakening front
becomes nearly stationary during the afternoon and evening hours,
and the latitudinal differences have important implications for
the placement of the axes of heavy rainfall. Based on a frontal
boundary extending from the southern Appalachians into the
northern Mid Atlantic, an axis of 1500/2500 J/KG of MUCAPE is
expected to form as the cloudiness from morning convection to the
west begins to thin, allowing any CIN to be eroded. As the storms
form, they will tap into 1.50/1.75 inch precipitable water air,
making them capable of producing hourly rainfall rates in excess
of 1.25 inches, especially across portions of western WV into
northern MD.
As the boundary becomes more or less parallel to the mid level
flow, some training of storms is possible here. Where training
occurs, rainfall amounts for Day 3 could excess 2.00 inches, but
given the spread in the position of the front on Day 3 across
VA/MD/DE, is it difficult to determine where this might occur. The
ingredients for flash flooding are expected to be in place from VA
across MD/DE, but from this perspective, the details remain
somewhat nebulous. Based on the above, and after collaborating
with WFOs PHI/LWX, a Marginal Risk was placed in these areas for
Day 3.
Further south across the Southern Appalachians, the moisture is
not quite as plentiful, but there should be sufficient instability
to support at least scattered storms, focused mainly on the
terrain. Propagation vectors here are weak (less than 10 knots)
and oppose the mid level flow in spots, which would support an
environment of short term training and backbuilding. Three hour
flash flood guidance values are as low as 1.00/1.50 inches across
portions of western VA into far eastern TN, where 7/14 day
rainfall amounts have been 300+ percent of normal.
The areal extent of the convection could be a limiting factor for
an enhanced flash flood threat in this area, but like further
north, the ingredients are expected to be in place, especially
into the early evening. Based on this, the Marginal Risk was
extended from the northern Mid Atlantic states down along the
Southern Appalachians into eastern TN.
Tropical moisture ahead of a mid level trough across the central
Gulf of Mexico becomes focused on the FL Panhandle into the Big
Bend region during Day 2. A low level convergent southwest flow
transports 2.00/2.25 inch precipitable water air (which is between
two and three standard deviations above the mean) across this
area. Convection developing in the south southwest flow could
train or backbuild, as the propagation vectors drop below 10
Where the convection train in the deep moisture plume, hourly
rainfall rates could excess 2.00 inches, and extended training or
backbuilding may result in local rainfall amounts in excess of
4.00 inches (which is shown in the 00z NAM/UKMET). The limiting
factor here could be instability. All of the 00z model (to some
degree) place the instability gradient along the shore, which
often happens within deep moisture plumes near FL. It is possible
that the best convection remains just offshore in the better
instability, rather than track across the Panhandle and Big Bend
areas (which both the 00z GFS/ECMWF appear to imply).
Three hour flash flood guidance values are generally above 3.00
inches, and should the convection train over land, these values
are certainly attainable. However, given the model spread
concerning the inland extent of the instability and varying QPF
amounts in the deep moisture plume, no excessive area was assigned
for this forecast. However, if there is better model agreement
with respect to more robust instability over land or better
agreement placing the QPF maximum, a Marginal Risk could be needed
in later forecasts.
Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt
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