Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Tue Aug 14 2018
Day 1
Valid 01Z Wed Aug 15 2018 - 12Z Wed Aug 15 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE NORTHEAST
AND THE CENTRAL U.S...
...Southwest U.S...
The convection has been expanding in coverage through the late
afternoon across the broader Southwest U.S. as strong diurnal
heating has contributed to MLCAPE exceeding 2000 j/kg over central
and southern AZ, and with more modest instability farther north
over central and southern NV. The convection has been focusing
within an axis of PWATs that are locally as much as 1.5 standard
deviations above normal and there has been plenty of orographic
support as well. Going through the evening hours, the convection
over southwest CO, southern UT, NV and adjacent areas of the
Sierra-Nevada should begin to weaken as instability becomes
increasingly exhausted. However, there will still remain some
localized concerns for flash flooding for a few more hours. A
somewhat more organized convective threat is noted through the
evening down across areas of central and southern AZ as convection
advances southwest down off the Mogollon Rim. A consensus of the
12Z hires CAM guidance and recent HRRR runs show some locally
organized convection that will tend to be cold pool driven through
the evening and in a favorable instability environment given the
aforementioned MLCAPE values. While the dominant convective threat
will be through 06Z, there will be some convection that could make
down across southwest AZ going well past the time frame. The
overall flash flood threat will be localized, but certainly a
concern as there may locally be 1 to 2+ inches of rain that falls
in a short amount of time. As such, the Marginal Risk is
maintained at this time.
...Northeast...
The deep low over the Northeast will begin to pull off to the
east-northeast overnight and will be weakening. Much of the
instability over the region has been exhausted or is in the
process of being exhausted as lingering clusters of showers and
thunderstorms linger and rotate around the periphery of the closed
low. This activity should weaken going through 06Z as instability
and overall forcing weakens across the region. Given the low FFG
values across the region, and recent heavy rainfall from today
alone, the threat of flash flooding will continue for a few more
hours. As supported by the latest hires guidance, conditions
should improve with time overnight. The Slight Risk has now been
removed, but a Marginal Risk is maintained to address the
short-term threat and especially for areas of southern NY and
northeast PA.
...Plains and Midwest...
An upper low lifting from Kansas toward Iowa will support a more
organized threat of flash flooding overnight within the moist
plume arching up through OK, AR and MO, and even to some extent
with the deep layer cyclonic flow involving the eastern side of
the circulation across eastern KS, southeast NE and southern IA.
The latest hires model consensus though, including the 12Z HREF,
and consecutive runs of the HRRR support the heaviest convective
rainfall and organization focusing along a highly confluent and
moist/unstable boundary layer across areas west-central to
northeast OK, far southeast KS, northwest AR and southwest MO.
Enhancing surface convergence and moisture/instability transport
via an increasing southwest low level jet is expected to encourage
a band of potentially training and backbuilding convection in a
WSW/ENE fashion with enhanced rainfall rates that may exceed 2
inches/hr. By later in the night, after 06Z, the convection will
likely have sufficient organization to begin bowing a bit more
quickly off to the east which after impacting central/northeast
OK, and possibly southeast KS, will put it in a position to get
into areas of northwest AR and southwest MO going toward 12Z.
Rainfall amounts of as much as 3 to 5+ inches can be expected with
this event across these areas, and thus a Slight Risk is maintain
here. Lesser totals are expected farther north over the Midwest
involving southeast NE and southern IA, but slow cell motions here
could promote locally heavy totals of a few inches and thus some
flash flood concerns.
...Northern/Central High Plains...
A small closed circulation is likely to form in the mid levels
overnight within the tail end of a trough dragging across the
northern Rockies and Plains. This along with post-frontal upslope
conditions should prove favorable for organized thunderstorms over
Wyoming and western South Dakota, and perhaps far southeast
Montana. Deep layer wind speeds are sufficiently strong to promote
at least 10 to 15 knot motion for mature storms, but locally heavy
rain rates and the either mergers or repeat cells could lead to
rainfall exceeding FFG values. A Marginal Risk of excessive
rainfall is maintained here, and at least for the next few hours
has been placed farther south down across the Front Range where
slow-moving thunderstorms developing off the terrain is seen still
moving very slowly east over the High Plains. Some short-term
flash flooding concerns will continue to here through about 06Z
before dissipating.
Orrison
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Tue Aug 14 2018
Day 2
Valid 12Z Wed Aug 15 2018 - 12Z Thu Aug 16 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE OZARKS TO
THE MIDWEST...
...Southern Plains to Midwest...
A mid-level cyclonic shear axis accompanied by a plume of enhanced
subtropical moisture shifts northeastward in advance of a slow
moving baroclinic zone Wednesday. Two inch PWs are expected with
850-mb moisture flux anomalies exceeding 4 standard deviations
above normal in some locations along the boundary. While the
better forcing for ascent will sit farther north, generally near
the northeastward migrating surface low center, the remainder of
the boundary should be plenty buoyant to support active
convection. This overall combination of ample moisture, sufficient
forcing, and diffluence aloft support the Slight Risk area
extending from the vicinity of Chicago southwest across the AR
Ozarks. Ongoing rain is lowering flash flood guidance values over
AR/OK and short-term rainfall rates could produce flash flooding
conditions, particularly in urbanized locales. The Slight Risk was
expanded over the Ozarks due to increased confidence of heavy QPF
there and the terrain.
...Arizona...
A Marginal Risk was maintained across the southern half of
Arizona. The guidance shows the 700-500 mb ridge axis shifting
slowly toward the east which allows ample subtropical moisture to
work its way northward along the western flank of the anticyclonic
circulation. 700-mb winds are quite weak during the period
suggesting some of the convection may struggle to exit the higher
terrain. Thus, the biggest flash flooding threats will be across
areas of topography, particularly into the Mogollon Rim. The 12Z
GFS has its usual high bias with QPF, so a blend of the 12Z
GFS/ECMWF/UKMET/CMC was used.
...Northern Great Plains...
An upper low sweeping southeast across South Dakota Wednesday will
drive periods of organized convection. While the better tongue of
moisture sits downstream ahead of the leading height falls, a
separate axis of higher precipitable water air works its way into
central/eastern South Dakota. While the 12Z consensus of global
models and CAMs support heavy amounts (max 2-4 inches), the region
has been dry the past week. An additional limiting factor is only
modest instability profiles. Consequently, will maintain the
Marginal Risk with continued southeastward extensions based timing
of 12Z CAMs.
...Maine...
A cold front associated with a low over southeastern Canada moving
toward Newfoundland will cross northern New England Wednesday
afternoon. PW of 1.5 inches ahead of the cold front is a
standardized anomaly of 1.5 over northern Maine. The peak diurnal
timing of the frontal passage should maximize instability which
will be in the 1000-1500 J/Kg range and allow heavy thunderstorm
development. Sub 2" flash flood guidance in northern Maine
warrants a Marginal Risk.
Jackson
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 PM EDT Tue Aug 14 2018
Day 3
Valid 12Z Thu Aug 16 2018 - 12Z Fri Aug 17 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE EASTERN
GREAT LAKES...
...Eastern Great Lakes to Tennessee...
12Z models have become more progressive with a modest low moving
across the Lower Peninsula of Michigan Wednesday. Ample lift from
the mid-level shortwave sits underneath ample right entrance
forcing aloft. In advance of the frontal zone, precipitable water
value anomalies jump into the 2 to 2.5 standard deviation range
accompanied by a significant uptick in low-level jet speeds (30 to
40 knots). The Slight Risk was shifted east (essentially
surrounding Lake Erie) based on the 12Z global model consensus.
...Arizona into New Mexico...
High precipitable water continues to lift north out of the Gulf of
California/Sea of Cortez in a monsoonal pattern. Diurnal heating
over the region should force convection over the higher terrain of
AZ/NM initially although the weak 700-mb winds suggest the
activity may struggle to push off the mountains into the lower
elevations. Expanded the Marginal Risk into western NM on account
of the 12Z consensus.
...Central and Southern Great Plains...
A broad Marginal Risk area was maintained which extends over
sections of the Central to Southern Plains. The Central Plains
portion is a continuation of activity congregating underneath the
upper low ejecting out of South Dakota. Steep lapse rates
underneath the low center should force scattered to locally
organized convection as it drops south and east. An uncertain
amount of related convection will develop farther south of this
wave where ample instability will be present over KS/OK. The 12Z
GFS/EMCWF are farther south and warranted maintenance of the
Marginal Risk into eastern OK.
Jackson
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
