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FOUS30 KWBC 170902
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
502 AM EDT Wed Jul 17 2019

Day 1
Valid 12Z Wed Jul 17 2019 - 12Z Thu Jul 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MS VALLEY, THE NORTHERN MID ATLANTIC INTO SOUTHERN NEW
ENGLAND, AND NORTHERN ALABAMA...

...TN and OH Valley into the northern Mid Atlantic and southern
New England...
Post-Tropical cyclone Barry will continue moving east northeast
across the OH Valley today and into PA by tonight. Plenty of
moisture still associated with this system, with an expanse of PWs
greater than 2" along and to its east. The best coverage of
convection today will likely be along and northeast of Barry from
portions of OH, into PA, southern NY and east into southern New
England. Will have several factors combining to produce at least a
localized flash flood threat across this region. Will have
enhanced southwesterly 850mb flow ahead of Barry, a front dropping
south helping enhance low level convergence, and an upper jet over
northern New England placing the area in the favorable right
entrance region for increased upper level divergence. It would
appear that we have enough ingredients in place to support the
potential for some organized west to east training of convection
near the tightening lower level convergence axis. Cells will be
very efficient rainfall producers given the high PWs, thus any
training would likely result in a flash flood risk. Still some
uncertainty with regards to exactly how organized convection is
able to get, and whether cells will take on enough of a southward
prorogation with time to limit the duration of heaviest rainfall
rates. Thus it is possible that the flash flood threat remains
localized and restricted to sensitive urban areas...but
nonetheless enough ingredients in place to warrant a Slight
risk...with at least some potential for more organized clusters of
flash flooding.

South of the Slight risk the expectation is that convection will
be more isolated in nature and quick moving, both which suggest
the flash flood risk is isolated and highly localized in
nature...with a Marginal risk sufficing. The exception is across
northern AL, where we will carry a small Slight risk. Note some
tightening of the low level convergence over this area this
morning, with building instability upstream advecting into the
region. Thus at least some potential for a period of backbuilding
cells through the morning hours, which could locally drop 3" or
more. This setup does not appear anywhere near as good as what
happened over southwest AR last night, but the basic setup is
similar, just to a much lesser extent.

...Upper MS Valley...
Convection is racing eastward across the Northern Plains this
morning, and the environment will remain conducive for additional
activity today into tonight. The biggest threat for flash flooding
appears to be during the evening into the overnight hours with the
nocturnal increase of the low level jet. The convection this
morning will likely leave a nice low level boundary in its
wake...which will be reinforced into a developing warm front as
forcing approaches from the west this evening. With the increasing
low level jet into and over this front...expect elevated
convection to expand in coverage. Wind fields support backbuilding
convection and should have plenty of upstream instability
available to support continued convective growth on the southwest
flank of developing convection. The magnitude and persistence of
850mb moisture transport, and the presence of an upper jet to the
north enhancing upper level divergence, all point to the potential
for a widespread, and locally significant, flash flood threat.
Some potential that a Moderate risk may need to be considered
later today. At this time there is too much spread on the favored
axis and exact evolution of convection, that think a Slight risk
is best for now. The global models are much further north than the
high res...and with this forecast package went with a weighted
compromise towards the high res guidance. Although, at times the
high res can be too far south when there is strong forcing to the
north, as is the case here...so was hesitant to completely buy
into some of furthest south solutions.

Chenard


Day 2
Valid 12Z Thu Jul 18 2019 - 12Z Fri Jul 19 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN GREAT LAKES REGION...

...Northern Wisconsin/Northern Michigan...
At the start of the forecast period, amplifying shortwave at the
nose of unseasonably strong 250mb jet that is flat and stretches
well back into the Northeast Pacific will be lifting out of the
area into Ontario, with a strong surface low pressing the
attendant frontal zone across N MN/Lake Superior.  However, given
the orientation of the deeper flow out of the southwest veering
slight west, the frontal zone flattens out across the UP/N WI into
south central MN.  The 850mb LLJ will be strong across the region
at over 40kts and with solid moisture convergence at the
inflection where the front flattens will support total PWats AOA
2".   The LLJ will continue to be strong throughout day with
slight backing supporting moisture flux anomalies over 4 Std.Dev
from normal.

However, there will be a ongoing convective complex across the
Upper Mississippi River Valley and there is growing uncertainty in
its evolution on D1.  Hi-Res CAMs suggest with high instability
and strong LLJ, propagation vectors will have a stronger southerly
component potentially across NW IA/N IL into NW IND.  Global
guidance is quicker to kill off the MCS and favor stronger
ascent/forcing near the front and therefore supports stronger
convection across N WI into the UP and Northern Portions of the LP
of MI.  The former would disrupt warm advection of necessary
moisture/instability across Lake Michigan for the build up for a
complex Thursday evening into Friday morning.

Given the uncertainty in the placement, but the very strong
synoptic environment for heavy rainfall clearly capable of
producing heavy rainfall rates and short-term totals capable of
scattered flash flooding conditions warranting a Slight Risk.  At
this time, given the stronger larger scale signals will favor the
global guidance slightly in the placement of the Slight Risk,
however, will broaden the Marginal Risk of excessive Rainfall
further south and west to account for the backbuilding system.


...New Jersey/NYC...
By the start of the forecast period, the deep moisture conveyor
belt associated with Post-Tropical Cyclone Barry has pressed
eastward offshore into the Northwest Atlantic.  However, its
remaining sheared shortwave that supported a new surface
circulation out of the Mid-Atlantic lee surface trof on Wednesday
(not PTC Barry) will be in the vicinity of the Mid-Delaware River
Valley, maintaining some near surface moisture.  This will result
in a tightening deep moisture gradient with 1.75-2" remaining
through the total depth by midday.  Given a slight trend for
eastward press of the moisture, remaining high clouds are likely
to eastward as well allowing for increased insolation and
instability to build by early afternoon across New Jersey.  Hi-Res
CAMs and even global models suggest SBCAPEs to exceed 1000 J/kg
perhaps reaching as high as 2000 J/kg.  As such there is a solid
clustering of Hi-Res CAMs suggesting 1-3" totals are possible with
slower moving/merging convective cells in the afternoon.  As such
have maintained a Marginal Risk of Excessive Rainfall for NJ and
the low FFG areas near the NYC metro, but have pulled the Risk
area out of southern New England where lingering
clouds/precipitation is more likely to limit sufficient
instability to build back (additionally higher FFG values).

Gallina


Day 3
Valid 12Z Fri Jul 19 2019 - 12Z Sat Jul 20 2019

...The Probability of rainfall exceeding flash flood guidance is
less than 5 percent...

...Upper Mississippi Valley into Northern Great Lakes...
Exceedance of flash flooding is highly contingent on the placement
and evolution of multiple convective complexes expected on day 1
and day 2 lowering Flash Flood Guidance values across of
convection.

A frontal zone will remain stationary through day 2 into day 3,
pooling moisture and therefore instability throughout these few
days.  By day 3, moisture flux convergence will reach a general
lull given the LLJ will start to orient more parallel to the
boundary.  While there will be sufficient moisture (up to 2" total
PWats across S MN into N WI), the unseasonably strong polar jet
will be fairly flat oriented across the region, reducing upper
level shortwave/PVA support. However, convection that does develop
will have the tendency to move east along the frontal zone
reducing flash flood guidance for any further upstream development
that would have the tendency to repeat/train.  Currently, the
global guidance suggests the best corridor would be from SE MN to
the northern LP of MI, but given higher probabilities of
convection further south and/or west on D1 and D2 it is hard to
support placing a precise Marginal Risk at this time, but will
likely change as hydrologic/FFG situation becomes better realized
in the coming day or so.

Gallina




Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.t...
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.t...
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.t...


$$




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