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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0100Z Jul 20, 2018)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Thu Jul 19 2018

Day 1
Valid 01Z Fri Jul 20 2018 - 12Z Fri Jul 20 2018

...A SLIGHT RISK OF EXCESSIVE RAINFALL CONTINUES OVER EASTERN
PORTIONS OF THE MID-UPPER MISSISSIPPI VALLEY...

...Mid-Upper Mississippi Valley...
0100 UTC Update...Fairly robust (unseasonable) left-exit region
upper jet forcing (broad-scale upper level difluence and
divergence) along with the uptick in low-level southerly inflow
ahead of the mid-upper level trough will maintain an enhanced
(slight) risk overnight.

...Central to Eastern Gulf Coast into the coastal Southeast...
A surface frontal boundary is expected to remain stationary this
period from the Lower Mississippi Valley eastward into the
Southeast.  Unstable air along and to the south of this
front..mu-cape values 1000-2000 j/kg will persist Thursday into
Thursday evening across the Central to Eastern Gulf coastal region
into the coastal Southeast.  This will support widespread
scattered convection in the axis of much above average pw
values...2 to 2.5 standard deviations above the mean.  Heavy to
isolated excessive rainfall amounts possible with slow moving
cells across these areas...with hourly rainfall rates of 2"+ per
hour possible.  Changes to the previous outlook were to extend the
marginal risk area farther westward into the central Gulf coastal
region from southern Georgia...the Florida Panhandle..southern
Alabama into far southern Mississippi.

...Southwest...
No significant changes made to the previous marginal risk area
extending across portions of the Southwest.  With the mid to upper
level ridge remaining in place this period across this area and
continued much above average pw values...additional widespread
scattered convection likely Thursday afternoon into Thursday
evening...especially along the high terrain from west-central New
Mexico westward along the Mogollon Rim of Arizona...into southern
Utah and southern Nevada.

Hurley/Oravec



Day 2
Valid 12Z Fri Jul 20 2018 - 12Z Sat Jul 21 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE OH/TN
VALLEY...

...Lower Great Lakes / Ohio Valley / Tennessee Valley...
Showers and thunderstorms are likely on Friday into Friday night
from portions of the Great Lakes south through the TN Valley. A
developing deep layer low will slowly push east into the Great
Lakes, with a cold front extending to its south. Moderate, to even
high, instability is forecast ahead of this front, with PWATs
above 1.5". This region will also be well positioned in the left
exit region of the 250 mb jet, increasing upper level divergence.
Thus seems likely that scattered to widespread convective activity
will develop through the day Friday.

The most intense activity is likely to focus across portions of
KY/TN into northern MS/AL. It is here where the best overlap of
instability and deep layer wind shear exists. Thus would
anticipate to see one or more organized convective clusters across
this region. This activity may very well forward propagate off to
the south into the instability pool...with these fast storm
motions potentially limiting the flash flood risk. However, will
maintain the Slight Risk for now, as the combination of favorable
forcing, high PWATs and high instability supports the potential
for short duration heavy rates...and some of this area had been
anomalously wet of late. Also note several of the 12z HREF members
depicting localized 3"+ amounts.

Further north into OH/IN note less shear and lower (although still
decent) instability. This suggests that activity here may not be
quite as organized/intense...however also favors slightly slower
cell motions. Also the closer proximity to the deep layer low
suggests that multiple rounds of storms are possible. This
combined with with several 12z HREF members depicting 3"+ across
this area warrants an extension north of the Slight Risk. Further
north into MI/WI instability will be lower. Although cool mid
level temperatures underneath the low, and weak mean flow suggests
slow moving cells producing brief period of heavy rainfall are
possible. Thus will extend a Marginal Risk into this area.

...Coastal GA/SC/NC...
A shortwave along the coast combined with increasing mid/upper
level forcing with the approach of the longwave trough...supports
the potential development of an area of low pressure off the
southeast coast during the day Friday. Plenty of moisture will be
in place along the coast...with PWATs greater than 2". Instability
will thus be the main question with regards to the heavy rainfall
and flash flood potential. This will be dependent on exactly where
the low forms, which will help impact the location of the coastal
front. The 12z GFS and a majority of the 12z HREF members bring
the coastal front inland enough to bring heavy rainfall ashore. In
fact, several HREF members show the potential for some 5"+ amounts
along the coast. On the other hand, the ECMWF and UKMET have been
further east...generally keeping the instability and heaviest
rainfall amounts offshore. Confidence in this forecast is below
average and will thus need to closely monitor. For now will stay
close to continuity and favor the heaviest rains staying just
offshore...and thus maintaining just a Marginal Risk. However
trends overnight into early tomorrow should make this forecast
more clear...and could certainly see a more organized flash flood
risk develop should the low become organized enough to push the
coastal front inland.

...Northern FL...
Will maintain a marginal Risk for portions of northern FL. With
the area of low pressure potentially developing off the southeast
coast...a trailing front will extend southwest across the FL
peninsula. Sometimes the tail of the fronts in these situations
can act as a focus for heavy rainfall...with a bit of enhanced
flow interacting with the tropical PWAT plume. Note several of the
12z HREF members hit this area with 3"+ amounts as well. Thus see
no reason to change from the going Marginal Risk area.

...Southwest U.S....
Diurnal instability should led to monsoonal thunderstorms are
again expected on Friday. Model forecast lapse rates of 7-8 deg
c/km combine with areas of terrain aided lift to produce
showers/storms.  The models do not show particularly heavy rain,
but several burn scars exist in the area. Cells that develop over
the burn scars will be capable of producing flash flooding. Deep
layer moisture of 1 to 2 standard deviations above climatology
will expand northward out of AZ into UT as southerly flow deepens
around the west side of the ridge center. Therefore the Marginal
Risk area extends from AZ/western NM northward into Utah. 

Chenard

Day 3
Valid 12Z Sat Jul 21 2018 - 12Z Sun Jul 22 2018

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EAST...

...Great Lakes into the Southeast...
A broad Marginal Risk extends from the Great Lakes into portions
of the Southeast. This area is along/ahead of the longwave trough
axis and attendant cold front. Plentiful instability and moisture
is expected along this corridor. The best overlap of instability
and shear again ends up on the southern extent of this area. Thus
the most organized activity will likely be here, but again some
questions as to storm motions potentially limiting the flash flood
threat. Some chance that 850 mb flow becomes a bit more favorable
for some backbuilding of cells b this time, which could locally
increase the flash flood risk. Thus some chance we may need to
embed a Slight Risk within the Marginal at some point, but for now
will stick with the Marginal.

Instability is less further north, suggesting cell intensity will
be less. However still plenty of moisture, and as you go north the
front becomes nearly stationary. This persistent low level
convergence axis may focus enough cells to pose a localized flash
flood risk...even if the overall storm intensity as not as high as
further south.

...Long Island, Northern New Jersey and southern New England...
The potential low off the coast could bring a heavy rainfall
threat into portions of NY into southern new England by Saturday
night. Still quite a bit of uncertainty with the track and
intensity of this potential low. The ECMWF remains east of the
GFS/NAM solutions. For now generally favored a compromise
solution...which is pretty close to the 12z GEFS mean. Low/mid
level WAA and PWATS will be plentiful for heavy rainfall ahead of
this low. Main limiting factor will be instability, although could
see just enough move ashore to produce some locally intense rates,
dependent on the low track. Given the uncertainty with both the
intensity/track of the low, and the degree of instability present,
will just maintain a Marginal Risk for now. Will need to monitor
over the coming days, as could envision the need for a Slight Risk
depending on how things trend.

Chenard


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