Excessive Rainfall Discussion


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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
906 PM EDT Sun Jul 22 2018

Day 1
Valid 01Z Mon Jul 23 2018 - 12Z Mon Jul 23 2018

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL TODAY AND THIS
EVENING ACROSS THE MID-ATLANTIC...

...Mid Atlantic...
Will maintain a Slight Risk stretching from eastern NC north into
portions of central PA. The best signal for the heaviest
additional rains through 12z is across eastern NC. Note a strong
signal in the 18z HREF guidance...with neighborhood probabilities
of exceeding 5" over 60%. Recent HRRR and RAP runs also hit this
same general area. Appears to be associated with some enhanced
upper level divergence as the region moves into the favorable
right entrance region of an upper jet rounding the base of the
trough. Also note a bit of an uptick in 850 mb moisture transport,
with PWATs also increasing. Thus seems to be a pretty good signal
that locally heavy rains and at least some flash flood threat will
move into coastal NC overnight. Main uncertainty is with regards
to how far west these heavier rains can make it...and will
continue to monitor trends.

Further north along the Slight Risk corridor do note a gradual
increase in upper level divergence and PWAT values as we head
through the night. Unclear if any feature will be present to help
organize activity across this region...but at the least would
expect tropical showers and embedded thunderstorms to push
northward across the Slight Risk corridor. Given the high moisture
content and warm rain processes in play...these showers will
likely remain quite efficient rainfall producers. Thus while
activity may remain disorganized...the efficient nature of
showers...and the expectation that coverage should increase by
later tonight...warrants the continuation of the Slight Risk. It
also warrants the continuation of the Moderate Risk across
portions of northeast VA, MD and far southern PA. This area has
been hit hardest over the past few days...with much of the area
having saturated soil conditions. Also note that recent HRRR/RAP
runs do want to concentrate an area of heavier rains overnight
somewhere across this same general area.

Isolated flash flood issues are possible over the next several
hours across northern NC into southwest VA...with merging
convection near an area of low pressure. Also, locally heavy bands
of rain may move into portions of southern New England overnight.
Recent runs of the RAP/HRRR have backed off from the magnitudes
earlier runs this evening had shown. And recent satellite trends
do suggest the more tempered amounts are probably more likely.
With that said, will still need to monitor this area through the
night...with the tropical moisture supportive of very efficient
rainfall producers.

...Northwest Florida...
An axis of very heavy rainfall has occurred this afternoon/evening
across portions of northern FL. The activity does appear to be
more progressive off to the south now that the shortwave has
caught up to the convective complex. Thus think storms will
generally tend to remain more progressive off to the south through
the night. However, still anticipate heavy rainfall rates...and at
least some potential for cells continuing to build into the the
western extent of the complex...resulting in training. Thus while
the more progressive nature of things probably means the flooding
threat is lower now than it was earlier in the
afternoon/evening...at least a localized threat likely remains
given the efficient nature of cells and the extreme instability
advecting into the west coast of FL.

...Northern Sierra into the Great Basin...
Slow moving cells in an environment characterized by near record
PWATs and anomalous instability for this region...will continue to
pose a flash flood risk in any more susceptible areas for the next
several hours...with activity eventually dissipating with the loss
of daytime heating.

...Southeast CO into western KS...
An area of enhanced low level convergence has resulted in an
expanding area of convection cross southeast CO this evening.
Chaotic storm motions has and will result in a cell merger
potential for a few more hours. Where cells do merge localized
flash flooding is probable.   

Chenard


Day 2
Valid 12Z Mon Jul 23 2018 - 12Z Tue Jul 24 2018

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID-ATLANTIC STATES...

...Eastern U.S...
A multi-day atmospheric river event is essentially expected to
impact portions of the Eastern Seaboard, and especially the
Mid-Atlantic region, as the synoptic scale pattern features an
upper trough/closed low digging across the interior of the
Southeast on Monday and becoming nearly stationary by Tuesday.
Meanwhile, there will be a notable westward expansion of the
subtropical ridge offshore the East Coast and into especially
areas of the Northeast. This will allow for some retrogressive
flow across the Mid-Atlantic and New England that will afford a
westward expansion of the deep layer tropical moisture plume
currently off the East Coast. Still think that diurnal
heating/instability, orographic ascent, modest right-entrance
region upper level jet dynamics and interaction with smaller scale
mesoscale boundaries in a very moist airmass will result in a
rather widespread area of heavy showers and thunderstorms.
Rainfall rates are likely to be impressively high, 2 to 3
inches/hr in some cases, given very efficient rainfall processes.
The latest 12Z suite of guidance supports the idea of localized
rainfall totals reaching 3 to 4+ inches, and this could even be a
little conservative given the high moisture profiles, multitude of
at least modest forcing mechanisms, and deep layer unidirectional
flow that will support training convective bands. The expectation
is that the heaviest rains will set up across areas of northwest
NC and stretching north up across the Blue Ridge from southwest VA
northeast up across the WV/MD panhandles and eastern PA. The
previous Slight Risk area has been adjusted a bit to account for
the latest QPF trends, however, we have now introduced a Moderate
Risk to account for the increasingly saturated/compromised soil
conditions. The Moderate Risk area will encompass the Blue Ridge
from north-central MD up across south-central PA, and this area
will need to be monitored for significant flash flooding concerns.
Model QPF also still strongly supports the idea of heavy to
excessive rainfall across portions of eastern NC in a region of
deep layered, onshore flow with high precipitable water values
(well in excess of 2 inches) in place, and concerns for some
training convective bands across the area. A Slight Risk area is
maintained across this area.

...Central/Southern Rockies...
A plume of deep moisture, characterized by precipitable water
values of nearly 1.5 inches, will extend to the northwest of the
Gulf of Mexico towards the eastern High Plains of CO/NM, including
the Front Range. The moisture will be focusing along a frontal
zone draping down across the region and becoming anchored up over
the adjacent high terrain. The precipitable water values will be
as much as 2 standard deviations above normal and this will lead
to more efficient convective cells for heavier rainfall rates.
Convergence near the front and modest shortwave energy rounding
the Southwest U.S. ridge axis will work in tandem with the
moisture, diurnal heating and orographics for rather focused areas
of heavy showers and thunderstorms. Some expansion of the Slight
Risk area was done for this update, with areas of central and
eastern NM included given some model signal for locally heavier
amounts. As much as 1 to 2+ inches of rain can be expected this
period. The main threat area will tend to be down the CO Front
Range and across the Sangre De Cristo range of northern NM. Will
need to also monitor the San Juan mountains of southwest CO, but
for now they are maintained in a Marginal Risk. The more sensitive
areas to flash flooding will again be to any burn scar areas.

...Sierra-Nevada into central/southern Nevada...
Monsoonal moisture will again tend to be focused to some degree up
across the higher terrain of the central Sierra-Nevada and
adjacent areas of central/southern NV. Slow cell motions and heavy
rainfall rates will favor some additional concerns for isolated
flash flooding. The local burn scar areas will again be a more
sensitive area to monitor. A Marginal Risk remains in place across
this region.

Orrison



Day 3
Valid 12Z Tue Jul 24 2018 - 12Z Wed Jul 25 2018

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC STATES...

...Eastern U.S...
Overall, very little change as compared to the day 2 excessive
rainfall outlook period as atmospheric river conditions are
expected to continue along the Eastern Seaboard with a focus on
the Mid-Atlantic region. A broad mid to upper level trough will
become elongated over the eastern part of North America, and this
coupled with a strong subtropical ridge offshore of the East Coast
will leave the area from the Southeast U.S. northward to New York
in a particularly moist environment with deep layer south to
southeast flow coming in off the western Atlantic Ocean. Weak
embedded shortwave energy within the deep layer flow coupled with
diurnal heating/instability, orographics/upslope flow over areas
of higher terrain, and smaller scale mesoscale boundaries will
again foster areas of particularly focused convection. There are
differences among the models with the exact placement of heaviest
rainfall, but the most likely areas to see higher totals will be
across the Blue Ridge and some adjacent Piedmont areas along an
axis from west-central VA to central/eastern PA. Locally as much
as 3 to 5+ inches of rain will be possible, and especially where
more organized training convective elements set up considering the
unidirectional flow through the vertical column. Given the high
multi-day rainfall totals that are anticipated, there has been an
expansion of the Slight Risk area to include a large area of the
Mid-Atlantic region, and also the inclusion of a Moderate Risk
area for the aforementioned areas where the heaviest totals are
most likely to occur. The highly saturated soil conditions in
conjunction with the additional forecast rainfall amounts may lead
to areas of significant and high-impact flash flooding.

...Central/Southern Rockies...
Deep-layered moisture will remain in place from the Central
Rockies with low level flow feeding in additional moisture from
the Gulf of Mexico. The precipitable water values will be as much
as 2 standard deviations above normal and this will lead to more
efficient convective cells for heavier rainfall rates. Convergence
along and south of the front in addition to weak shortwave energy
rounding the Southwest U.S. ridge axis will work in tandem with
the moisture, diurnal heating and orographics for yet another
round of rather focused areas of heavy showers and thunderstorms.
As much as 1 to 2 inches of rain with locally heavier amounts can
be expected. The main threat for flash flooding will remain across
the CO Front Range, the San Juan mountains of southwest CO, and
across the Sangre De Cristo range of northern NM with the more
sensitive areas including any burn scars. A Slight Risk area is
currently highlighted across this region.

...Sierra-Nevada into central/southern Nevada...
Monsoonal moisture will again tend to be focused to some degree up
across the higher terrain of the central Sierra-Nevada and
adjacent areas of central/southern NV. Slow cell motions and heavy
rainfall rates will favor some additional concerns for isolated
flash flooding.

Orrison



Day 1 threat area: http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt






Last Updated: 906 PM EDT SUN JUL 22 2018