Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1206 PM EDT Mon Jun 17 2019
Day 1
Valid 16Z Mon Jun 17 2019 - 12Z Tue Jun 18 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC/OHIO VALLEY AND OVER PORTIONS OF THE WESTERN
PLAINS...
16z Update: Overall the forecast looks in good shape. Convection
is beginning to develop along the stationary front draped across
the OH Valley. Given recent rainfall resulting in saturated ground
conditions and higher stream flows...some flash flooding is likely
this afternoon from southern IL into southern IN, northern KY and
central/southern OH. Overall this convection should not be all
that organized, and should tend to be of pulse to multi cell
variety. Not seeing much to suggest a more prolonged organized
threat...and thus thinking localized to widely scattered totals of
2-3" are most likely...with amounts higher than that pretty
isolated in nature. Thus while some flash flooding is likely,
current thinking is that rainfall magnitudes will fall below what
would warrant a Moderate Risk at this time.
Added a Slight Risk to portions of northeast CO into western NE.
Cells moving into the foothills and High Plains may tend to
exhibit slow/chaotic cell motions resulting in some merging
potential over eastern CO. This activity should also merge with a
southward dropping front over NE, which may very well focus a
corridor of enhanced rainfall magnitudes over central/western NE
by this evening. There appears to be enough of a threat to exceed
flash flood guidance over both these area to warrant a Slight Risk.
Chenard
...Earlier Discussion...
...Mid-MS Valley/OH Valley/TN Valley/Mid-Atlantic...
Deep confluent mid to upper level flow exists across the Ohio
Valley, Great Lakes into the Northeast at the tail end of strong
mid-level trof in Southeast Canada. The slow moving southern
stream shortwave will continue to march northeastward from NE OK
into Southern MO and the edge of the confluent flow, continuing to
shed weak shortwaves enhanced by daily convection. Currently one
is moving out of Ohio into PA/NY. As such a stationary front
remains draped across Central PA, Central OH back to north-central
MO. Southern stream moisture continues to surge northeast and
interface with the surface to 850 boundary, with 1.75-2" total
PWats measured across the MS River into the Lower Ohio Valley.
Instability is likely to redevelop/near along the boundary across
the MS and Ohio Valley with narrow skinny profiles still
supporting up to 2000 J/kg across SE MO to SW OH. The main
shortwave in OK will shed shortwave energy northeast later this
morning and help support convective redevelopment likely
along/south of the frontal boundary. Convergence is likely to be
more oblique as the warm conveyor belt veers across the Ohio
Valley, though this aligns nicely with unidirectional steering
flow aloft, so new convection that does develop is likely to
train, perhaps over significantly reduced very low FFG values
across SE IL, S IND, into SW OH. However, with weaker
instability/convergence the confidence is precise latitude of best
training may not overlap with this area and guidance is scattered
enough to not support a increased risk category across this area
at this time in coordination with local forecast offices. Though a
flash flood watch is in effect for this potential. So a broad
Slight Risk is depicted from the down-shear inflection of the
shortwave in MO through the Ohio Valley into the Mid-Atlantic.
Across the Mid-Atlantic, recent drier conditions have been in
place relative to upstream but there is also increased confidence
in heavy rainfall placement, especially along the frontal zone
where prolonged moderate rainfall near the moisture convergence
should remain fairly stationary today allowing for multiple bouts
of heavy rainfall. South across MD/VA...trailing edge of
shortwave/outflow boundary from this morning's convection will
press across the Appalachians. With early morning/early afternoon
sun, increased instability should strong convective updrafts. The
uncertainty remains in the orientation of the convective
development, if it will be more-north/south coming off the terrain
or be angled a bit more NE to SW along the outflow
boundary/shortwave trof axis. The latter would support greater
potential for training/repeat tracks resulting in greater rainfall
totals. The former, would result in strong rainfall rates
(1.5-2"/hr) but not support totals to exceed higher FFG values.
As such have encompassed the urban centers from DC to NYC in the
Slight Risk with Southward and eastward exurbs within the
confluence/moisture axis for more scattered/isolated flash
flooding occurrences.
...Mid to lower MS Valley/TX Gulf Coast...
A Slight Risk extends south from the confluence of the MS/OH/TN
Rivers into N MS/NE AR. This was to account for multiple bands of
thunderstorms with potential training early in the morning as well
as a secondary round after 06z as the Gulf stream tightens across
the MS River and the old MCV from the MCS currently along the TX
Gulf Coast starts to lift northward with enhanced moisture flux.
Deep SSW to NNE steering flow supports training of convection
along these bands. Best training/confluence is further north
toward the shortwave/forcing throughout the day and so have
included W TN/NE AR/N MS into the Slight Risk, while extending a
Marginal Risk into Upper Texas coast with the dying MCS.
...Central High Plains...
Strong closed low over N MN, will have leave a draped trof
extending back SW across the Sand Hills, that has pooled modest
moisture along it with total PWats around 1.25". With daytime
heating, instability should rise to around 2000 J/kg and with
sufficient low level convergence should spark convection into the
afternoon hours. Increasing low level flow from return moisture
through the Central High Plains will intersect with this area
supporting southwest propagation with time. Concurrently,
typical diurnal upslope flow regime will increase moisture along
the WY/CO High Plains through morning, eventually with peak
day-time heating, thunderstorms will develop, cluster and advance
with solid forward motions through evening hours. Toward 00z, LLJ
will maximize with 20kt of 850mb flow that is also confluent as
Central Plains flow responds to the emerging shortwave out of the
Central Rockies. The convection/forcing will merge across the
Sand Hills and surge in intensity given increased moisture (up to
1.4"), the resultant complex will propagate slowly given weak
overall flow, but still be supported aloft with fair outflow. As
such, rain-rates of 1.5"/hr will compound totals resulting an area
of 2-4" across the southern Sand Hills. Typically this would
result in flash flooding, but excellent draining should limit the
threat. Still, the I-80 corridor and locales along the edge of
the Sand Hills are not as fortunate but the complex should be
propagating a bit faster and along with reduced instability from
loss of daytime heating should result in weaker updrafts. So
overall, greatest rainfall should fall in the best soil conditions
but lower FFG areas should have less rainfall, as such a Marginal
Risk should suffice unless timing results in complex moving out of
the Sand Hills.
Elsewhere south across NM/CO/KS, without merging convective
complexes and reduced orthogonal intersection with southerly
inflow, convective complexes developing out of terrain should
propagate east a bit faster reducing rainfall totals in one given
spot. Still lower 1hr FFG values (1.5-2") are still in the realm
of being exceeded given strong updrafts leading to isolated spots
for flash flooding. As such the Marginal was extended across the
High Plains into NE NM.
...Florida...
Unstable, narrow skinny profiles supporting 2.25" total PWats
within mildly confluent southerly low level flow that continues
throughout the day around 15-20kts is likely to support off/near
shore convection this morning along Southeast FL leading to
outflow that will enhance convection across the peninsula with
peak heating eventually merging with Gulf sea breeze by afternoon.
Upper level slightly arched/anticyclonic curvature to the 25-30kt
3H outflow will evacuate thunderstorms well enough for some
increased organization (less pulse) particularly across SW coastal
counties from TPA to the Everglades with 3"+/hr rain rates
possible. While this region can withstand a large amount of
precipitation, the area is well above normal over the last week or
so (per AHPS) including some 6" reports yesterday. National Water
Model supports saturated soils across a bulk of the area in the
.8-.9 range for saturation ratios with a few streams running above
normal. As such there is a Marginal Risk painted across these
saturated areas, along the I-75 and I-95 urban corridors as well
as Miami-Dade urban areas.
...Sierra Nevada Mountains...
Weak shortwave continues to drop south into Southern CA today
containing to support modestly steep mid-level lapse rates for
modest rebuild of instability by mid-morning/afternoon. Modest
moisture remains within the central CA valley with total PWats to
1" initially but some mid-level moisture advected from the north
should help increase deeper moisture through peak heating as well.
Weak upslope flow responding to mountain heating will feed this
moisture up to convection that develops on the range. Instability
up to 1500 J/kg per multi-model signal will support solid
updrafts. Proximity to left exit region of strengthening jet
streak sliding east should support some increased shear for some
organized convection. Counter inflow/outflow may even anchor
convection along the eastern slopes long enough to support
scattered heavy rain rates capable of flash flooding. As such
have introduced a Marginal Risk of Excessive Rainfall on this
possibility.
...Eastern Great Basin/Utah...
Inner core vorticity center of larger scale western trof, will
meander northeast across UT today, this should sharpen confluent
southwesterly flow across the central CO River Basin and eastern
Great Basin converging across terrain of S UT initial, sliding
north with time. Moisture is quite limited with .75" TPW values
expected, yet 15-20kts of near cloud base inflow and modestly
steep lapse rates supporting 750-1000 J/kg of instability may
support solid rain rates over 1"/hr. This may pose isolated areas
of flash flooding to occur across E NV into Southern and Central
UT this afternoon. As such have included a Marginal Risk of
Excessive Rainfall across areas matching instability south of the
shortwave in the increased confluent flow.
Gallina
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Mon Jun 17 2019
Day 2
Valid 12Z Tue Jun 18 2019 - 12Z Wed Jun 19 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND ACROSS PORTIONS OF THE CENTRAL PLAINS...
...Ohio Valley/Appalachians/Mid-Atlantic...
A mid/upper level trough will help undercut the ridging across the
southeast coast. With mid-level impulses riding the trough, they
will interact with a quasi-stationary front draped from the
Mid-Atlantic westward into the Mid-MS Valley to start the period,
Tuesday morning. Anticipate the boundary to slowly sag southward
and continue to act as a focus for convection through Wednesday
morning. The best moisture (as been from moisture flux out of the
Gulf and Atlantic), lift (height falls and mid-level energy) and
instability (surface base CAPE >2000 J/kg) will pool south of the
boundary over parts of the Ohio Valley, central Appalachians and
Mid-Atlantic. The position of the front/convective outflow
boundaries and the shortwave locations/trajectories will all
impact what is observed on Day 2. The model signal for a large
swatch of 1-3+ inches from KY to Long Island has diminished since
the previous forecast. The latest model guidance suggests a few
very localized areas of convection that will be largely influenced
by surface convergence, but mostly the mid-level energy. It
appears the signal for the heaviest precipitation will reside
moving across the southern Appalachians given the mid-level trough
appears to become a bit more amplified. Hourly rain rates in this
region will be 1-1.5 inches/hour with QPF totals 1-2 with locally
higher amounts expected. Therefore, the Slight Risk was adjusted
for this region. While KY may not receive as much precipitation,
the soils are very saturated so the threshold for flash flooding
will be lower. In an effort to reduce the overall coverage of the
Slight Risk area and with the understanding that a large amount of
uncertainty remains where convection may occur, a Marginal Risk
was maintained, though a Slight Risk may need to be redefined for
northern KY/southern OH into WV.
...Central Plains...
Increasing southerly winds will draw moisture rich air into a
deepening surface low as it migrates eastward into the Central
Plains. Strong moisture advection will overspread the frontal
zone, pooling east of the trailing front over the Central/Southern
Plains. Moderate instability on the order of 1000 to 3000 J/kg
will be present from AR to NE. Strong lift was also noted as the
strengthening southerly jet and juxtaposition of the northern jet
provides forcing for ascent aloft and also within the mid-levels
noted by a negatively tilted trough axis. Given these
ingredients, the environment will be conducive for widespread
organized convection that will be capable of producing very
efficient rain rates. Convection coming off of the Front Range
will likely develop into a mesoscale convective system across
southern NE with a cold pool diving south along an instability
gradient. Multiple models are signaling the highest QPF to occur
over southeastern NE into eastern KS, southwestern IA and
northeastern MO associated with the aforementioned MCS and where
surface/boundary layer convergence resides; with a possible
secondary max axis set up over central OK with a trailing front.
WPC QPF over much of this region is 1-2+ inches with locally
higher amounts possible and largely influenced by convective
development. These amounts may reach or exceed local FFG in a
short period of time. The Slight Risk for excessive rainfall was
refined per the latest QPF forecast. The Marginal Risk spans from
NE across IA southward to northern TX.
Campbell/Pagano
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 PM EDT Mon Jun 17 2019
Day 3
Valid 12Z Wed Jun 19 2019 - 12Z Thu Jun 20 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND OHIO VALLEY...
...Southern Plains/Mid-MS Valley/Tennessee and Ohio
Valleys/Mid-Atlantic...
The confidence remains low for the Day 3 forecast given the highly
variable QPF location/axis/amounts and timing amongst the
deterministic and ensemble guidance. Antecedent rainfall and
well-saturated soils across this sector of the country will keep
the threat for flash flooding elevated during this period.
Deep moisture advection (as seen from precipitable water of 2+
inches) into the TN and OH Valleys, along with moderate/strong
instability of 2000 to 3000+ J/kg, will favor widespread
convection across this location and southwest into the Southern
Plains. Additionally, large scale forcing for ascent is noted from
the strong 250mb jet and mid-level impulses. While model spread
is quite high for Day 3, key ingredients seem to favor portions of
the Mid-MS Valley into the OH/TN Valley. Areal average 1-2+
inches of precipitation will be possible in this region, though
much depends on the convection observed on Day 2 with lingering
outflow boundaries. Though the flow associated with this system is
quite pronounced, models are struggling to depict the frontal
placement associated with this system likely due to difference
among the mid-level energy and upper level trough axis.
Nonetheless, attempted to place the heaviest rainfall axis across
the best overlapped ingredients for heavy rainfall and efficient
rain rates.
Given the heavies QPF is forecast to fall across a portion of the
OH Valley, this will be problematic given they have received
300-600% of normal precipitation over the past week alone.
Therefore, this region will be super sensitive to additional
rainfall which led to a Slight Risk area being introduced at this
update. Given uncertainty remains in terms of convection
evolution, the Marginal Risk accounts for the expected placement
uncertainty.
...Northern Plains...
A closed upper level low will continue to drop across the Canadian
Rockies resulting in mid-level energy ejecting from the trough
axis across MT. With plenty of atmospheric forcing for ascent
aloft, moisture surging north across the Dakotas (as noted by
precipitable water values approaching 1.25 inches) and weak
instability in place, this will be enough to develop a surface low
and potential convective complex moving east out of the Rockies.
Given flash flood guidance is a bit lower across this region, felt
a Marginal Risk was needed to account for isolated flash flooding
concerns.
Campbell/Pagano
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
