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FOUS30 KWBC 050828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 05 2019

Day 1
Valid 12Z 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.

Oravec


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

...MARGINAL RISK OF FLASH FLOODING ACROSS THE SOUTHWEST U.S. AND
LOWER GREAT LAKES REGIONS...

...Arizona to Colorado and New Mexico...
Monsoonal moisture 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 portions of southern Arizona and
potentially into far southern Nevada Tuesday night/Wednesday
(particularly shown by the latest GFS). However, the rest of the
deterministic models hold the deeper moisture back into Arizona.
Saw little reason to make signficant changes to yesterday's Day 3
Excessive Rainfall Outlook as it becomes today's Day 2 ERO in the
Nevada/Arizona portion of the forecast. Elsewhere, only minor
changes were made where PWs 1.5 to 2.0 SD above the climatological
mean are present. With the favored upslope flow initiating and
sustaining convection, locally intense rainfall rates will be
possible.

...Lower Great Lakes Region...
Maintained the Marginal Risk area over the Lower Great Lakes
region which will be positioned along and ahead of an approaching
cold front.  Higher moisture is expected to pool across portions
of northern Indiana, northern Ohio, and western Pennsylvania.
Precipitable water values of 1.5 to 1.75 inches are expected,
which generally are at or above the 90th percentile for the area
this time of the year. Additionally, the latest model guidance
still indicates warm cloud depths of 3 to 3.5 km will be present
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. The 00Z HREF probabilities also indicate isolated areas
where the probability of exceeding an inch per hour rainfall rates
peak over 20 percent in the Day 2 period.  Given the spread of QPF
solutions and uncertainty with the timing of the cold front, still
think a Marginal Risk is appropriate.  This is reinforced by the
idea that conditions have been drier than normal conditions have
been observed the last 7-14 days.

Bann


Day 3

The Day 3 outlook will be updated by 0830Z.


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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