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FOUS30 KWBC 060050
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
849 PM EDT Mon Aug 05 2019

Day 1
Valid 01Z Tue Aug 06 2019 - 12Z Tue Aug 06 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST...UPPER MIDWEST...& COASTAL SOUTH CAROLINA...

...Southwest...
Scattered convection is most concentrated in and around the
topography of AZ and southwest NM at this time.  With precipitable
water values 1-2 standard deviations above the mean across the
Southwest and potential for continued cell mergers for a few more
hours...locally heavy precipitation and isolated runoff issues are
possible within terrain.  Temperatures at 700 hPa around 15C
should keep issues out of the lower elevations/deserts across the
Southwest due to mid-level capping.

...Upper Midwest...
A cold front moving southeast across the Upper Mississippi Valley
and into the Upper Great Lakes is acting as the focus for
thunderstorms with heavy rainfall.  Well defined upper difluence
ahead of the associated southeastward moving shortwaves are
enhancing lift in the axis of above average precipitable water
values along and ahead of this front.  The thunderstorm area is
beginning to narrow and increase its forward propagation which is
slowly reducing hourly rain amounts.  The heavy rain threat across
the region is expected to wane by 06z due to the narrowing line
and increasing forward movement.

...Coastal SC...
The mid to upper level trof axis is pushing into the Southeast.
Thunderstorms are concentrating along the SC and southeast NC
coastal plains forced by outflow boundaries from earlier
convection in GA and NC.  This region should receive another 2-3
hours of heavy rains and did receive some locally heavy rains over
the past 24 hours.

Roth/Hurley/Oravec


Day 2
Valid 12Z Tue Aug 06 2019 - 12Z Wed Aug 07 2019

...MARGINAL RISK OF EXCESSIVE RAINFALL FOR
SOUTHWEST...CENTRAL...AND NORTHEAST PORTIONS OF THE
COUNTRY...ALONG WITH SOUTH FLORIDA...

...Arizona to Colorado and New Mexico...
Monsoonal moisture with precipitable water values 1 to 1.5
standard deviations above normal will remain in place across
portions of the Southwest states, circulating around a sprawling
upper high anchored over New Mexico. Another push of deeper
moisture is slated to arrive across southern Arizona Tuesday
night, maintaining moisture 1.5 standard deviations above normal.
CAM and global guidance favor convective QPF over western NM to
southern CO where instability is greatest, so the Marginal was
maintained there. Little QPF is in AZ where instability is
suppressed. Left the Marginal Risk for AZ for now given the light
southerly mean flow which may trigger isolated thunderstorms/flash
flooding.


...Central Plains...
Weak mid-level energy in northwest flow will allow surface low
pressure to develop on an existing front in south-central NE
Tuesday night. Convergence of 1.75 inch PW air (about one standard
deviation above normal) on southwest 850mb flow of 15 to 20 kt
with around 1500 J/Kg MLCAPE will promote thunderstorms to develop
and potentially move along the axis of the new front. This
potential for repeating cells warrants introduction of a Marginal
Risk for excessive rain.


...Ohio Valley to Lake Erie...
Expanded the Marginal Risk area over much of the Ohio Valley ahead
of an approaching cold front. Precipitable water values of 1.5 to
1.75 inches are expected which is up to one standard deviation
above normal. Warm cloud depths of 3 to 3.5 km are still expected
during the afternoon/evening hours. With sufficient instability
and lift in place, efficient rain producing thunderstorms are
likely with some probabilities of exceeding 1 inch in 1 hour in
places. Despite drier than normal conditions over the last 7-14
days for much of this area a localized flash flood threat is
present.


...Northern Mid-Atlantic...
Precipitable water of 1.5 inches ahead over the northern
Mid-Atlantic/NYC Tuesday evening is about one standard deviation
above normal. The combination of 1500 J/Kg MUCAPE with southwest
mean flow and low propagation vectors in a similar direction
warrants an isolated flash flood threat from southern NY to NYC.
Therefore a Marginal Risk was raised.


...South Florida...
Elevated moisture persists over southeast FL through Tuesday. The
presence of westerly low level flow along with typical instability
leads to an isolated flash flood threat over a similar area to
recent days. Therefore a Marginal Risk was raised for a portion of
the southeast FL peninsula.

Jackson


Day 3
Valid 12Z Wed Aug 07 2019 - 12Z Thu Aug 08 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE CENTRAL PLAINS...

...Central Plains...
The approach of a cold front over the central Plains in persistent
northwest flow with existing boundaries brings a flash flood
threat Wednesday night. 12Z global guidance has converged a bit in
focus for Wednesday night activity over northeastern
KS/southeastern NE into western MO. Moisture increases to around 2
inches (1.5 to 2 standard deviations above normal) on a
southwesterly low level jet. Ample instability will be in place
from Wednesday afternoon, so ingredients for heavy thunderstorms
are coming together. Remnant boundaries ahead of the cold front
will focus precipitations. Given consensus QPF and wet antecedent
conditions in portions of eastern KS, adjusted the Slight Risk a
bit to the southwest.

...Northeast U.S....
Subtropical moisture will be drawn over the Northeast U.S. during
the period by persistent west to southwesterly flow that taps
Precipitable Water values at or over 1.5 inches through the
period.  Difluence in the upper level flow develops as an
anomalously deep trough over Ontario approaches...which aids
precipitation development due to broad vertical velocities ahead
of a shortwave embedded within the synoptic scale trough. Minor
differences are noted in the 12Z global guidance suite, so the
large risk area is still warranted. Focused Marginal Risk north
from the Baltimore metro based on 12Z consensus.

...Southwest U.S....
Monsoon moisture will still be present over the Southwest U.S.
during the period, although the overall threat looks to be waning
late in the period as heights and thickness values increase in
response to a deepening system off the west coast. Precipitable
Water values still look to be in the 1.25 to 1.5 inch range (with
highest values close to the international border at the time of
maximum daytime heating).  That should be more than sufficient for
convection to produce localized intense rainfall rates...
especially in the late and and early evening.

Jackson



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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