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FOUS30 KWBC 201557
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sat Jul 20 2019

Day 1
Valid 16Z Sat Jul 20 2019 - 12Z Sun Jul 21 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS INTO THE MIDDLE TO UPPER MISSISSIPPI VALLEY...

...16Z update...

Only minor adjustments were made to the previous outlook based on
observational trends and 12Z model guidance.

Across the Upper Tennessee Valley into the Central Gulf Coast, 12Z
RAOBs indicated PWATs between 2.0 and 2.1 inches from BMX, LIX and
JAN. Disorganized but slow moving convection appears likely to
pose a threat through the afternoon/evening in the vicinity of a
slow moving 700-500 mb low/trough centered from the western FL
Panhandle northward into AL and GA. The Marginal Risk was expanded
slightly north and west to account for developing convection this
morning across east-central TN and to account for well above
average rainfall from the past week due to T.S. Barry where soils
may be more sensitive to flash flooding into MS and far
southeastern LA.

Elsewhere from the Central Rockies to New York and Pennsylvania,
minor expansions of the Slight and Marginal Risks were drawn to
account for the potential for heavy rain rates in the 1 to 2 hour
time frame within standardized precipitable water anomalies of 2+,
to overcome 1 hr FFG/ARIs as convection matures later this
afternoon and evening, shown with the 12Z hi-res guidance.

Otto

...previous discussion follows...

...Lee of the Central Rockies across the Central Plains...Mid to
Upper Mississippi Valley into the Lower Great Lakes...
The surface frontal boundary stretching from the lee of the
Central Rockies...east northeastward across the Central Plains and
into the Mid to Upper Mississippi Valley and Lower Great Lakes
will be a focus for potentially active convection this period.
This frontal boundary will remain along the southern edge of the
westerlies where upper difluence maxima will accentuate lift in an
axis of widespread instability along this front...li values -4 top
-12...mu-cape 2000-4000+ j/kg.   Strong frontal convergence
expected along the length of the front with inflow of pw values  1
to 2.5+ standard deviations above the mean expected through the
period.  Given the above...there will likely be widespread area of
heavy to locally excessive precipitation this period in the
vicinity of this front.  The slight risk area was drawn to cover
the axis of higher href neighborhood probabilities...40 to 60%+
of 2"+ along this front.   With respect to the previous outlook
for this period...the slight risk was expanded farther to the west
southwest across portions of the Central Plains into the lee of
the Central Rockies.

...Mid to Upper Tennessee Valley into the Central Gulf Coast...
Model consensus is for additional heavy rain potential this period
stretching from the Central Gulf coast...northeastward into the
Middle to Upper TN Valley/Southern Appalachians in a weak mid to
upper level shear axis.  PW values expected to remain 1 to 1.5+
standard deviations above the mean and instability is forecast to
build to 1000-1500 j/kg by Saturday afternoon. Expect convection
to blossom quickly Saturday afternoon and dissipate shortly after
sunset.  FFG values are relatively high across this region...but
there is the likelihood of additional heavy rains falling across
areas that received heavy rains in the past 24 hours.  This may
result in isolated runoff issues.

Oravec



Day 2
Valid 12Z Sun Jul 21 2019 - 12Z Mon Jul 22 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
PLAINS EAST INTO THE OHIO VALLEY...

...Central Plains, Mid-MS Valley, OH Valley...
A front will continue to slowly move into the Central Plains,
Mid-MS Valley into the OH Valley Sunday into early Monday. Models
are coming into slightly better agreement on the orientation and
timing of the front, especially across the OH Valley.  The GFS and
now CMC are showing signs, however, of more organized convection
as mid-level energy moves atop the aforementioned front.  The rest
of the model guidance remains weaker with the impulses and the
position of the jet axis a bit farther removed to the north and
east.  Thus the GFS and CMC have higher QPF amounts across eastern
KS into northern MO as an organized complex moves from SE NE into
MO Sunday evening/night. Given the GFS has shown run to run
continuity, the CMC is now showing similar features and GEFS
probabilities support the higher QPF amounts, felt it should be
incorporated into the model blend.

With respect to the ingredients in place, moisture and instability
will continues to pool right along and south of the boundary, with
precipitable water around 2 inches and MUCAPE values over 3500
J/kg. As previously mentioned, there continues to be uncertainty
with respect to the interaction between the mid-level energy and
the surface features, thus resulting in questionable organized
convection in the central CONUS.  At this time, forecasting weak
to modest mid-level impulse atop this surface boundary which
should promote convection along the front from the Central
Plains/Mid-MS Valley across the OH/TN Valley.

Areal average QPF generally ranges between 1-2+ inches with
locally higher amounts associated with convection. The two main
areas of concern are across east KS/northern MO with the potential
for organized convection and portions of the OH Valley where FFG
are lower. Rain rates could climb over 1.5 inches/hour. The
propagation of the convection along the front should limit the
flash flood potential as it sinks south. However, if a complex
forms as depicted by the GFS, some training could occur. There is
enough signal to support isolated to potentially more widespread
flash flooding concerns, therefore a Slight Risk was introduced to
encompass these regions.

...Southern Rockies...
As a front continues to dive south through the Northern Plains,
settling into the Central Plains, Mid-MS Valley and OH Valley, a
trailing boundary along the Front Range will act as a catalyst for
convection during the afternoon into the overnight hours Sunday
into Monday.  Mid-level high pressure will build overhead with
impulses rounding this feature which too will promote vertical
motion. At the surface, as the Canadian high builds south into the
Northern Plains, easterly flow will enhance upslope flow during
the afternoon. Moisture that pooled south of the aforementioned
front will bank up against the high terrain with instability of
over 2000 J/kg available during the afternoon/evening hours.
These factors should promote convective initiation along the
windward slopes of the Rockies in CO and NM.  Given fairly weak
flow, convective cells should have fairly slow storm motion along
the slopes with more erratic propagation as they grow upscale.
Regardless, expect heavy rainfall is given decent moisture
transport across this region. Hourly rain rates may approach 1
inch/hour which is just shy of the one hour FFG (1.5 inches)
across this region.  Again, given the uncertainty with respect to
the evolution and storm motion, feel there could be some locations
that see 2+ inches when all is said and done.

Models have trended down with QPF for the 00Z forecast suite and
thus have trimmed amounts slightly.  Though ensemble
probabilities, albeit somewhat low, still highlight this region as
exceeding 2 inches.  Models tend to do poorly within this flow
regime/pattern and thus will maintain the Marginal Risk across
this area with some refinements made based on WPC QPF.

Pagano

Day 3
Valid 12Z Mon Jul 22 2019 - 12Z Tue Jul 23 2019

...THERE IS A SLIGHT RISK FROM THE TN VALLEY AND SOUTHERN
APPALACHIANS NORTH INTO PORTIONS OF NEW ENGLAND...

...TN Valley/Appalachians/Northeast...
A front will slowly approach the TN Valley, Appalachians Mountains
moving through the Northeast Monday into Monday night as a deep
trough digs south across the Great Lakes.  Moisture and
instability will pool along and just south of this boundary.
Precipitable water values will climb to over 2 inches across
portions of the TN Valley, banking up against the western slopes
of the Appalachians and into the mid-Atlantic region Monday
afternoon/evening. This is over 2 standard deviations above the
mean. Pockets of instability over 2000 J/kg was noted within the
model soundings, especially across the mid-Atlantic region.
Within the mid-level, impulses will continues to round the
aforementioned trough.  Upper level jet axis will pivot to the
northeast from Ontario into Quebec positioning the right entrance
region across the central Appalachians into the Mid-Atlantic and
Northeast by later in the period.  All this to say mid/upper level
support is plentiful and with such a strong front, this would only
suggest widespread and potentially multiple sounds of
shower/thunderstorm activity, especially Monday afternoon through
the evening hours.

The main area of concern with respect to heavy precipitation will
be from the expected pre-frontal convection which will impact
portions of the TN Valley into the central/northern Appalachians.
Areal average precipitation in this area is around 1-2 inches with
higher values possible with training as the mean wind becomes
better aligned with the corfidi vectors.  With sensitive soils
across much of this region as evident by FFG as low as 0.75
inches, felt it advantageous to introduce a Slight Risk from the
TN Valley/southern Appalachians north into New England.  Given
such a strong synoptic pattern, especially for this time of year,
and the multi-model signal for very heavy precipitation
(widespread 2+ inches), feel a Moderate Risk may be warranted if
QPF increases and model guidance comes into better agreement on
placement.  A Marginal Risk extends as far south as the Lower MS
Valley who have seen quite a bit of rain with Barry and are still
rebounding from flash flooding.

Pagano



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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