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FOUS30 KWBC 210058
QPFERD

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

Day 1
Valid 01Z Sun Jul 21 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...

...Lee of the Central Rockies across the Central Plains/Mid to
Upper Mississippi Valley...
A low level northeast flow transports 1.00/1.25 inch precipitable
water air (which is about two standard deviations above the mean)
across portions of eastern CO into southeast WY in the wake of a
surface cold front. Convection forms and expands in coverage
across portions of east central and southeast CO in an axis of
2500/3500 J/KG of MLCAPE, as the mid level cap in place begins to
break. Initially, due to dry air in place, the storms are expected
to remain primarily outflow driven.

However, as convective coverage increases, weak mid level flow
will foster an environment where short term training and cell
mergers, particularly over southeast CO. The HRRR over the past
four runs has shown an axis of 2.00/4.00 inches of rain from near
KLAA northeast to the CO/KS border, aligned well with the
instability axis. Other high resolution guidance showed the axis
as well, though the magnitudes were not as high. It is possible
that these amounts could be overdone, considering how dry the
column is before convection expands in coverage.

Moisture and instability snake along and north of the frontal
boundary extending from southeast CO across
northwest KS/southeast NE to the IA/MO border by 21/12z. An
initial area of storms moving across southern IA into far
northeast ME could wet the soils during the evening hours, but
these storms are expected to remain progressive through the
evening.

After this area of convection moves southeast, additional storms
follow the axes of moisture and instability, and possibly expand
in coverage ahead of a weak short wave (possible MCS) tracking out
of CO into western NE into IA and MO, mainly after 21/06z. While
high resolution guidance has been consistent with the scenario,,
is has not been consistent with the placement of the highest
rainfall amounts. Recent HRRR runs have backed off on its QPF
amounts here, showing local 2.00/3.00 inches rainfall extending
from central NE into portions of far southern IA into northern MO.

By contrast, the 12z ARW WRF and most recent experimental HRRR
showed 3.00/4.00 inches of rainfall on the IA/MO border through
21/12z. Those solution indicated a better training signature on
the simulated radar product, as propagation vectors drop to near
zero between 21/09z and 21/12z. Three hour flash flood guidance
values here are fairly high, as conditions have been relatively
dry over the past seven days. The Slight Risk was left in place as
conditions across southern IA/northern MO have been fairly dry
into the lat week. However, the evening storms here could make
this region more vulnerable to flash flooding as the convection
over the Central Plains moves east along the front. A Moderate
Risk was briefly considered, but antecedent conditions, as well as
some spread in the placement of the highest rainfall amounts this
evening and again late tonight, did not support an upgrade at this
time.


...Mid to Upper Tennessee Valley...
Activity following a moisture plume across TN/KY this evening is
moving with the mid level flow north of a weak broad closed mid
level low over the southeast states. Though the loss of daytime
heating could allow the activity to slowly weaken through 21/04z,
any organization in the axis of deeper moisture could pose a low
end flash flood threat with local hourly rainfall rates near 2.00
inches, particularly over urban areas. The most recent HRRR
indicated that the convection could survive until the early
morning hours, after which time the remaining instability is
consumed or becomes elevated, allowing the storms to become less
focused.

Flash flood guidance values are generally above 2.50 inches, so
training would be required to present a more organized flash flood
threat across this area. Based on this, the Marginal Risk was left
in place.


...Northern New England...
Convection developing in an axis of 2000/3000 J/KG of MUCAPE over
southwest Quebec  tracks ahead of a short wave crossing northern
New England between 21/06z and 21/12z. As the storms move
eastward, they will encounter instability becoming more elevated
over northern NY state, northern VT/NH into northern ME, and start
to slowly weaken. However, the storms are embedded in an axis of
1.50/1.75 inch precipitable water air (which is between two and
three standard deviations above the mean) which extends to the
rooftop of ME.

There is a decent high resolution guidance signal (led by the most
recent HRRR) for local 2.00 inch rainfall amounts approaching
21/12z, especially across northern ME, where training is possible
as the mid level flow parallels a frontal boundary across Quebec.
Three hour flash flood guidance is as low as 1.50/2.00 inches
here, so there is a low probability flash flood threat. Because of
this, the Marginal Risk was extended to cover much of northern New
England.

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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