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FOUS30 KWBC 220049
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
848 PM EDT Sun Jul 21 2019

Day 1
Valid 01Z Mon Jul 22 2019 - 12Z Mon Jul 22 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS INTO THE LOWER MISSOURI VALLEY...IN THE LEE OF
THE CENTRAL AND SOUTHERN ROCKIES...AND ACROSS OHIO AND TENNESSEE
VALLEYS...

...Central Plains into the lower Missouri Valley...
The focus for the outlook period will be across east KS into
central and southern MO, as there is a multi model signal for
local 3.00/5.00 inch rainfall amounts. Convection is developing
rapidly in a 2500/3500 J/KG axis of MLCAPE this evening. Synoptic
scale lift associated with a weakening MCV (seen in the GOES-16
visible imagery as a swirl over northeast KS) is expected to allow
the convection to become better organized across eastern KS this
evening.

The mid level northwest flow is expected to result in the
convection dropping southeast across eastern KS into southwest MO
between 22/03z and 22/09z, as a short wave in the flow crosses the
region. The storms will have access to a plume of 2.00+ inch
precipitable water air (which is between two and three standard
deviations above the mean) along the front, and the combination of
moisture and instability is expected to support hourly rainfall
rates near 2.00 inches during this time frame. While there could
be some repeat cells in short term training, especially across
eastern KS, the system is expected to be mainly progressive as it
drops toward the MO/AR border after 22/06z.

As mentioned earlier, there is a strong model signal for 3.00+
inch rainfall amounts with the storms, especially over eastern KS,
where the best threat of short term training is expected. There
are some members of the 12z/18z guidance package (including the
12z WRF ARW/WRF NMMB) that showed the potential for 6.00+ inch
amounts over eastern KS, though in different locations. At this
point, the convection appears as though it may be moving too
quickly to support rainfall amounts this high, though those values
could server as a ceiling for where training occurs.

Three hour flash flood guidance values are generally above 3.00
inches, as the area has been relatively dry over the past week.
This could be the limiting factor for an enhanced flash flood
threat here. A Moderate Risk was considered for eastern KS into
southwest MO, but the speed of the convection, as well as spread
in the guidance with respect to the location of the highest
rainfall amounts, precluded this possibility,


...Lee of the Central to Southern Rockies...
A low level east northeast flow continues to feed 1.00/1.25 inch
air into southeast WY/eastern CO into eastern NM in the wake of a
cold front boundary over the TX Panhandle. The airmass has become
uncapped along the spine of the terrain here, and being under the
mid level ridge, the activity has been fairly slow moving
(generally under 10 knots) as it trundles southeast. The mid
levels are still dry, and the storms initially have been more
outflow/hail dominated.

During the evening hours, the convection peaks in a ribbon of
1500/2500 J/KG of MLCAPE, which is expected to maintain itself
through 22/04z (per the most recent RAP). The convection is
expected to remain close to the terrain over CO into northern NM,
and propagation vectors suggest that the activity drops south
southeast. This motion could support cell mergers and short term
training in a fairly high moisture airmass, particularly over
eastern CO. Hourly rainfall rates of 1.00/1.50 inches are possible
over the Front Range (which is supported by the most recent HRRR),
and appears to be plausible where training occurs.

Synoptic scale forcing is generally weak, so the extent of
convective coverage is in doubt. In any case, the convection is
expected weaken after 22/06z, as the instability becomes consumed
or becomes elevated. The Excessive Rainfall Outlook was left
mainly intact to cover the threat, though it was contracted along
the eastern edge to better fit convective trends.


...Ohio Valley/Interior Mid Atlantic....
Deep moisture and instability focused along and south of a frontal
boundary will feed convection that spreads out along the front
across northern OH into western PA. The best convection is
expected before 22/06z here, as it follows an axis of 1500/250
J/KG of MLCAPE (which become more elevated with time). Along and
just south of the front, precipitable water values top out over
2.00 inches (which is between two and three standard deviations
above the mean). As the propagation vectors become better aligned
with the mid level flow, training in a high moisture environment
could support hourly rainfall rates near 2.00 inches over
OH/northern Kentucky into far western WV.

Three hour flash flood guidance values here are as low as 1.50
inches (not including the effects of heavy rainfall earlier
today), so the Slight Risk was left in place. There is a multi
model signal that the instability becomes consumed over elevated
between 22/06z and 22/12z, which should result in a decreased
coverage of storms, and a general lessening of the flash flood
threat.

Across portions of VA into PA, instability and moisture in place
could support convection developing on outflows from storms over
the Ohio Valley. and the most recent HRRR indicated that this
could continue through much of the overnights hours. As was the
case of the OH Valley, hourly rainfall rates on any convection
that is regenerated on boundaries could produce hourly rainfall
rates near 2.00 inches. As is the case further west, three hour
flash flood guidance values are as low as 1.50 inches
(particularly over WV). The Slight Risk was extended a bit further
east to capture this potential, though it is not clear how
extensive the threat will be.

Hayes



Day 2
Valid 12Z Mon Jul 22 2019 - 12Z Tue Jul 23 2019

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN MID ATLANTIC AND NORTHEAST...

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

2000 UTC Update -- Have expanded the Moderate Risk area slightly,
to encompass more of the Urban Corridor (DC-Balt area), along with
more of northern NJ into far southern NY (including NYC), western
LI, and far southwest CT. This was based largely on the antecedent
soil moisture and/or relatively low short term FFG. The models
continue to advertise a heavy rainfall signal within this area,
with the outlook period now fully captured by the 12Z high-res
windows. Isolated-scattered 3-5+ inch totals were noted per the
guidance, especially the CAMS, ahead of a slow moving frontal
boundary where for a period of time the 850 mb flow is expected to
become nearly parallel and of similar magnitude of the mean
850-300 mb wind. This will lead to an enhanced risk of training
convection, with again the metropolitan areas along with other
locations with wetter soils/lower FFGs most susceptible. For more
on the meteorological setup, please refer to the discussion below.
-Hurley

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 development 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/ingredients 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.

Pagano


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC INTO THE SOUTHEAST...

...Virginia/Carolinas/Georgia...
2000 UTC Update -- Upgraded a large portion of the Marginal Risk
area to a Slight -- based on the 12Z model trends in depicting a
rather large heavy rainfall footprint ahead of the advancing upper
trough and associated surface cold front. Nudged the northern edge
of the Slight Risk toward the southern edge of the day 2 Slight,
in order to prevent a gap between both days (i.e. same
event/similar meteorological setup straddling 12Z Tue).
Areal-average QPF within the Slight risk areas is considerably
higher compared to previous forecast cycles -- based on a
multi-model blend centered around the NBM and WPC bias-corrected
ensemble QPF. These ensembles indicated a fairly widespread swath
of 1.5-2.5+ inches of rain, which is quite noteworthy considering
the absence of high-res guidance during the period. -Hurley

Previous discussion..

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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