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FOUS30 KWBC 060829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue Aug 06 2019

Day 1
Valid 12Z Tue Aug 06 2019 - 12Z Wed Aug 07 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST/SOUTHERN GREAT BASIN...NORTHERN TO CENTRAL
PLAINS...THE OHIO VALLEY INTO THE NORTHERN MID ATLANTIC AND OVER
COASTAL SOUTHEAST FLORIDA...

...Southwest into the Southern Great Basin...
There is not expected to be any significant changes to the large
scale flow across the Southwest this period with the upper high
forecast to remain centered near the four corners..  The nam and
gfs both show shortwave energy rotating northward on the western
side of this upper ridge from northwest Mexico into the Southwest
and southern Great Basin.  The axis of above average pw
values...initially across southern Arizona into southern New
Mexico...is expected to expand northward with time across the
Southwest and into the southern Great Basin as the shortwave
energy pushes north.  The marginal risk area was expanded farther
northward into the southern Great Basin from the previous issuance
to account for the northward push of higher pw values.  Widespread
scattered convection again likely across this area.  With pw
values increasing and potential for slow moving cells...locally
heavy rains and isolated runoff issues are possible.

...Portions of the Northern to Central Plains...
A marginal risk area was maintained over the Northern Plains for
convection that may form Tuesday afternoon into Tuesday night in
the mid to upper level northwest flow across this region.  The
previous risk area was expanded farther northward into northeast
Nebraska and eastern South Dakota to cover the range in latitude
of the potential narrow axes of heavy rains.

...Ohio Valley into the Northern Mid-Atlantic...
A cold front is forecast to move southeast from the Upper
Mississippi Valley/Great Lakes region into the Ohio Valley this
period.  Fairly well defined upper difluence expected ahead of the
associated amplifying upper trof over the Ohio
Valley...accentuating large scale lift in the axis of 1.75"+ pw
values in the vicinity of this front.  Model consensus is for the
potential for moderate to locally heavy precip totals from the
Ohio Valley in the Mid-Atlantic...although details vary with
respect to the location of the max amounts. The convection is
expected to be fairly progressive...reflected in the HREF
probabilities that are high for 1"+ amounts but drop off
significantly for higher totals.   The previous marginal risk area
was expanded southward to cover the range of model solutions with
respect to the timing of precip moving south and location of
heaviest totals.

...Coastal Southeast Florida...
A marginal risk area was continued over coastal southeast Florida
for isolate urban runoff issues again Tuesday afternoon.  Hi res
guidance shows potential for another round of active convection
over south Florida...maximizing along the southeast coast late
Tuesday afternoon.  HREF neighborhood probabilities are high for 1
and 2"+ amounts...90%+ and 60-80% respectively and 45-60% for 3"+
totals.

Oravec


Day 2
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 AND THE NORTHEAST...

...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. The 06/00Z global guidance appears to have
converged a bit more  in focus for Wednesday night activity over
the Central Plains but arrive at their solutions in a somewhat
different fashion.  Still think that northeastern KS/southeastern
NE into western MO is the general area of concern, but the
guidance in this set of model runs has shifted the location of
maximum precipitation southward. Precipitable Water values are
forecast to 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 rainfall from thunderstorms are coming
together. Remnant boundaries ahead of the cold front will focus
precipitations. Think the potential for flash flooding decreases
west of this area given warm temperatures aloft which dampen the
instability.

...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. Model to
model differences and run to run differences in the 06/00Z model
runs are fairly minor, suggesting that the large Marginal Risk
area is still warranted with only minor adjustments.  However, the
model signal was better for some heavy rainfall over parts of
Pennsylvania into New York state which prompted the inclusion of
small Slight Risk.

...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.

Bann



Day 3
Valid 12Z Thu Aug 08 2019 - 12Z Fri Aug 09 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS...

..Far Eastern Kansas to Western Missouri...
The threat for some heavy to potentially excessive rainfall will
continue over portions of Missouri and perhaps far eastern Kansas
given southwest low level flow continues to direct unstable air
towards a low level boundary.  Confidence is low on placement of
heaviest rainfall at this point because any shortwave energy in
the northwest flow aloft will be weak and difficult to time and
placement of any low level boundary will depend on earlier
convection. WPC manual QPF went on the wetter side of guidance in
deference to the southwesterly synoptic scale flow drawing in an
airmass with Precipitable Water values around 2 inches...although
warm temperatures aloft may be a factor in limiting CAPE and
associated updraft strength.  Will maintain a Slight Risk given
the uncertainty at this point...displaced a bit south and west of
the Day 2 Slight Risk position.

...Wyoming and Southwest South Dakota into parts of Colorado and
Northwest Kansas...
There is some concern for heavy to possibly excessive rainfall
from the Black Hills and Pine Ridge to the Plains of eastern
Colorado and northwest Kansas as moisture gets drawn northward
over the Plains and Precipitable Water values increase to values
at or above 1.5 inches.  PW values of 1.25 inches make their way
into southeast Wyoming which would be roughly 2.5 standardized
anomalies greater than climatology and in the 95th percentile .
This occurs at the time of max heating, so thinking is that any
convection which ignites will be able to tap the deep moisture and
produce intense rainfall rates.  The concern is that temperatures
aloft will initially be warm given a mid/upper level ridge axis in
the vicinity...but the ridge begins to flatten Thursday afternoon.


...Northeast U.S....
The chance for heavy to possibly excessive rainfall will linger
over portion of New England early on Thursday before the deepest
low level moisture and upper support move off-shore.  The 00Z
numerical guidance shows that Precipitable Water values of 1.5
inches or greater should be confined to New England at the start
of the period and then begin to decrease during the afternoon as
low level winds shift to the west and drier air advects into the
region.  Given model guidance showing the potential for 1+ inch of
QPF, will maintain a Marginal Risk downstream of the area
highlighted in the Day 2 period.

Bann



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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