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FOUS30 KWBC 210823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Jul 21 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 Mon Jul 22 2019 - 12Z Tue Jul 23 2019

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS A
PORTION OF THE MID-ATLANTIC...

...TN Valley/Appalachians/Mid-Atlantic/Northeast...

A cold front will slowly cross the TN Valley and Appalachians
Mountains Monday/Monday night while slowly sagging southeast
through the Mid-Atlantic and Northeast as a deep trough digs
across the Great Lakes. Moisture and instability will pool along
and south of this boundary with PWATs 2 to 2.25 inches (which is
2-2.5 standard deviations above the mean) across portions of the
TN Valley, banking up against the western slopes of the
Appalachians and across the Mid-Atlantic region Monday
afternoon/evening. Pockets of instability over 2000 J/kg was noted
within the model soundings, especially across the Mid-Atlantic
region. The instability gradient seems to bisect PA/NJ which will
likely play a role in convective developement and evolution.  Some
concern also remains in terms of destabilization from cloud cover
during the morning into early afternoon across portions of PA.
Within respect to the mid-level, impulses will continue to round
the aforementioned trough helping to promote lift ahead of the
surface front.  An upper level jet axis will pivot south into the
OH Valley 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 biggest discrepancies among the
models is the timing of the frontal passage.  Regardless, there is
clearly a multi-model signal for excessive rainfall that stretches
from the TN Valley northeast into New England.

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 thanks to training convection as the
mean wind becomes better aligned with the corfidi vectors. Given
the sensitive soils, especially across portions of the
Mid-Atlantic, and better model signal/ingridents for heavy
rainfall across portions of MD/PA/NJ, introduced a Moderate Risk
to highlight the potential for more widespread flooding concerns.
This upgrade was closely collaborated with local offices,
RLX/PBZ/CTP/LWX/PHI. Hourly rain rates could exceed 2+ inches and
with 3 hourly FFG as low as 1.5 inches in some locations.  Another
area of concern is across portions of the TN Valley where heavy
rainfall is forecast to occur on Day 1 which may reduce FFG values
even more. Have a Slight Risk across this area, extending
northeast. A Marginal Risk stretches as far south as the Lower MS
Valley where they have received quite a bit of rain associated
with Barry but are quick rebounding.

...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
and southern CO Rockies.

Jackson/Pagano


Day 3
Valid 12Z Tue Jul 23 2019 - 12Z Wed Jul 24 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST...

...Virginia/Carolinas/Georgia...

A front will continue to move south and east Tuesday shifting off
the NC coast by early Wednesday as a longwave trough pivots
through the OH/TN Valley.  Moisture and instability will pool
along and just south/east of the front allowing for pre-frontal
convection to likely develop Tuesday afternoon/evening across
eastern portions of VA/Carolinas and Georgia. There is strong
evidence for heavy rainfall as seen by the multi-model signal for
high QPF totals, especially across the piedmont/sand-hills of
VA/NC/SC.  The timing of the front will certainly play a role in
terms of convective initiation and the evolution through late
afternoon/evening.  With precipitable water values climbing above
2 inches and MUCAPE values around 3500 J/kg, the ingredients point
to storms being heavy rainmakers.  Mid-level energy moving atop
the surface front combined with the right entrance region of the
upper level 125kt jet will only act to enhance lift across this
region.  Given this, fully expect a round or two of convection
with the potential for training as noted by the mean flow becoming
better aligned with the corfidi vectors.

Areal average precipitation of 1-2+ inches with locally higher
values can be expected.  Hourly rain rates may also exceed 1.5
inches. However, given very high flash flood guidance across this
region, felt flash flooding could be somewhat limited. Therefore,
introduced a Marginal Risk for excessive rainfall.  If QPF totals
increase and there is better model agreement on timing of the
front and the overall placement of the highest QPF, a Slight Risk
may be warranted, especially across portions of eastern VA/NC,
where soil saturation is slightly higher per the National Water
Model.

...Southwest/Southern Rockies...

With the cold front well south of the area and high pressure
continuing to build in from the north, easterly flow will enhance
upslope convection across the higher terrain.  Low to mid-level
moisture will still be present as noted by precipitable water
values of 1-1.25. Instability will climb to over 1500 J/kg MUCAPE.
 And with slow storm motion just after convective initiation, this
will exacerbate the potential for flash flooding across this
region.  Therefore a Marginal Risk was introduced across portions
of the higher terrain in AZ/NM/CO.

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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