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FOUS30 KWBC 202034
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 PM EDT Sat Jul 20 2019

Day 1
Valid 1852Z 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...
19z update...the Marginal Risk was extended across portions of
eastern TN into much of western and central NC, as slow moving
convection tracks westward in an axis of 2.00 inch precipitable
water air. The combination of deep moisture and slow cell motions
under the flat ridge position could foster an environment for
hourly rainfall rates of 2.00/2.50 inches over this area.

Three hour flash flood guidance values are generally above 2.50
inches over this area, so training would be needed to initiate
flash flooding. While the overall threat for flash flooding is
fairly low, local issues are possible where training results in
local 3.00/4.00 inch rainfall amounts into the late afternoon.

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/Hayes



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

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN ROCKIES...CENTRAL PLAINS...AND OHIO VALLEY...

...Central Plains, Mid-MS Valley, OH Valley...
A cold front will slowly south over the Central Plains and Midwest
Sunday through Sunday night ahead of an amplifying trough that
pushes from the northern plains to the western Great Lakes.
Surface low pressure over the south-central Plains will slow the
cold frontal progression down the eastern portion of the central
Plains...KS/MO. Two inch PWATs ahead of the front are 2 to 2.5
standard deviations above normal and high instability will make
for a notable flash flood threat near the KS/MO border. However,
this area has been totally dry over the past week with ongoing
flooding limited to main stem rivers. Therefore, the Slight Risk
was maintained over this area.

Farther east across the OH Valley is an expansive area of heat and
humidity that will be overtaken by the cold front and prefrontal
thunderstorms on Day 2. Pooling moisture with PWATs of 2 to 2.25
inches (2 standard deviations above normal) and high instability
and light westerly mean flow will allow multiple boundaries to
trigger convective thunderstorms, potentially repeating over
areas. Given increasing flow immediately ahead of the cold front,
the Slight Risk for excessive rain was expanded east well into WV.
The rest of the Slight Risk was maintained with low flash flood
guidance across this area. Given the aforementioned ingredients,
areas of Moderate Risk may be able to be determined in subsequent
forecasts.


...Central and Southern Rockies...
A cold front continues to dive south through the central High
Plains Sunday with the Canadian high building south into the
Northern Plains, providing easterly flow that will enhance upslope
flow during the afternoon. Moisture that pooled south of the cold
front will bank up against the eastern slopes of the Rockies with
instability of over 2000 J/kg available into the evening hours.
These factors should promote convective initiation along the
eastern 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 if it continues could approach the 3 hour FFG (1.5
to 2.5 inches) across this region. Given this threat, coordinated
a Slight Risk along the NM/CO border with WFOs PUB and ABQ.


Pagano/Jackson

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 ACROSS THE MID-ATLANTIC AND ACROSS MUCH OF NEW
ENGLAND...

...TN Valley/Appalachians/Mid-Atlantic/Northeast...
A cold front will slowly cross the TN Valley and Appalachians
Mountains Monday/Monday night while pivoting and slowly sagging
southeast through the Mid-Atlantic and Northeast as a deep trough
digs south across the Great Lakes. Moisture and instability will
pool along and south of this boundary with PWATs 2 to 2.25 inches
across portions of the TN Valley, banking up against the western
slopes of the Appalachians and across the Mid-Atlantic region
Monday afternoon/evening. This is 2 to 2.5 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.  An 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. Plentiful mid/upper level
support with such a strong baroclinic gradient suggests widespread
and potentially multiple rounds of thunderstorms.

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 with 3hr FFG 1.5 to 2.5 inches,
continued the Slight Risk. Given such a strong synoptic pattern,
especially for this time of year, and the multi-model signal for
very heavy precipitation (widespread 2+ inches), 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.


...Southern Rockies...
Easterly flow behind the cold front pushing south into Mexico will
direct moist air into the southern Rockies topography. PWATs 2
standard deviations above normal and some Monday afternoon
instability warrants a Marginal Risk for eastern slopes of the NM
Rockies.

Jackson



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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