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FOUS30 KWBC 052036
QPFERD

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

Day 1
Valid 16Z Mon Aug 05 2019 - 12Z Tue Aug 06 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST...UPPER MISSISSIPPI VALLEY INTO THE UPPER GREAT
LAKES..SOUTHERN APPALACHIANS...COASTAL CAROLINAS AND COASTAL
SOUTHEAST FLORIDA...

...Southwest...
Another day of widespread scattered convection possible in the
vicinity of the closed upper high forecast to remain centered near
the four corners this period.  With pw values forecast to remain 1
to 2 standard deviations above the mean across the Southwest and
potential for slow moving cells...locally heavy precipitation and
isolated runoff issues are possible.  The previous marginal risk
area was decreased in size and concentrated across the Southwest,
not extending northeastward into the Rockies.

...Upper Mississippi Valley into the Upper Great Lakes...
A surface cold front will be pressing southeast Monday across the
Upper Mississippi Valley and into the Upper Great Lakes.  Well
defined upper difluence ahead of the associated southeastward
moving shortwaves will enhance lift in the axis of above average
pw values along and ahead of this front.  Model consensus is for a
fairly broad area of moderate to locally heavy rains across these
areas. The previous marginal risk area across this area was
expanded northeastward to cover the areas where HREF neighborhood
probabilities were high...60-90%...for 1"+ rains.  The greatest
heavy rain threat across the northern portion of the marginal risk
area appears to be in the 1800 utc Monday to 0000 utc Tuesday time
frame...with the southern portion of the risk area having the
greatest heavy rain threat in the 0000 to 0600 utc Tuesday time
frame

...Southern Appalachians and Coastal Carolinas...
The mid to upper level trof axis will be pushing eastward from the
Central to Southern Appalachians and into the Southeast this
period.  Scattered convection likely ahead of this trof with two
marginal risk areas depicted.  One is across the Southern
Appalachians  from far northern Georgia into western North
Carolina where HREF neighborhood probabilities are high for 1 and
2"+ amounts.  With ffg values across this region relatively
low...there may be isolated runoff issues.

The other marginal risk area is along the coastal sections of
South Carolina and North Carolina.  This region is being
highlighted by the latest guidance for locally heavy rains and did
receive some locally heavy rains over the past 24 hours.

..Coastal Southeast Florida...
A marginal risk area was added across coastal Southeast Florida
for isolated urban runoff issues this afternoon.  Hi res models
are showing potential for heavy rains across coastal southeast
Florida...with this reflected in the HREF neighborhood
probabilities which are very high for 1 and 2"+ amounts...90%+ and
50-70%+ for 3"+ amounts. Per collaboration with WFO MFL, have
expanded the Marginal Risk to include much of South FL, while
adding a Slight Risk over central and eastern portions. South FL
will continue to reside along the subtropical jet during the
period, within a weakly-difluent upper flow pattern as the
southern periphery of the upper trough tracks slowly eastward
across the eastern Gulf. 12Z KMFL RAOB depicted a PW of 2.26" and
wet bulb zero heights near 15kft -- a highly favorable profile for
optimal rainfall efficiency, especially considering mixed-layer
CAPES between 2000-3000 j/kg. The latest HREF neighborhood
probabilities of 1 hourly QPF exceeding 2" tops out between 30-50+
percent over portions of the outlook area between 19 and 23Z, with
6 hourly probs of >3.00" between 50-90% across eastern areas
toward the Gold and Treasure Coasts.

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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