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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0820Z Jul 23, 2019)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Tue Jul 23 2019

Day 1
Valid 01Z Tue Jul 23 2019 - 12Z Tue Jul 23 2019


...Lower Mississippi Valley through the Tennessee Valley into
Central Appalachians, and portions of the Mid Atlantic and
The first batch of excessive rainfall continues across portions of
southern New England early in the period, as the storms follow the
deepest moisture and edge of the instability gradient offshore
this evening. Based on extrapolation, the flash flood threat
across southern New England should wane as storms move offshore,
along with the best instability and moisture.

Further south across NJ/PA into the southern Appalachians, There
is a lull in the activity as the airmass across this region has
been overturned by earlier storms. However, the airmass change
lags the frontal boundary, and the moisture and instability remain
in place through 23/05z along and east of the Interstate 95
corridor. It remains unclear how much the airmass can recover
after the previous storms, but in deference to the most recent
HRRR runs, the Slight Risk remains in place through the overnight
hours, though it is possible that the greatest threat could end
before 23/06z. Based on input from WFO PHI, the Moderate Risk was
left in place over a portion of eastern PA into northwest NJ, as
this area was hit hard by earlier flash flooding.

Finally, across KY/TN, the airmass remains relatively unaffected
by the earlier convection passing to the north, as convection
follows the best instability axis into eastern TN/eastern KY
before 23/03z. As the storms track eastward, they will tap in to
the 2.00+ inch precipitable water air just ahead of the front. As
the mid level flow parallels to the slowly sinking frontal
boundary, training remains the primary flash flood threat, as
hourly rainfall rates could top out near 2.00 inches (which is
supported by the most recent HRRR runs). High resolution guidance
suggests that local 3.00/4.00 inch rainfall amounts are possible
before the instability is shunted southward with the front. Based
on this, the Moderate Risk was left in place for the evening

...Portions of the Southwest into the Southern High Plains...
Monsoonal moisture across portions of the Southwest into the
southern High Plains will fuel heavy to excessive rainfall, mainly
during the evening into the early overnight hours

There are three areas of concern for this outlook period. The
first is southeast NM, where a low level easterly flow continues
to feed deepening moisture (as precipitable water air values top
out near 1.50 inches) and 2000/3000 J/KG of MUCAPE across west TX
into southeast NM. An MCV dropping south from northeast NM will
provide sufficient lift to support convection along the
instability axis. Storms or small convective clusters could train
along the TX/southeast NM border during the evening hours,
resulting in hourly rainfall rates near 1.50 inches.

The MCV drops southeast to the far western TX Panhandle before
23/06z, as the instability either slowly weakens or becomes more
elevated. Some of the high resolution guidance (including the most
recent HRRR runs) have been slow to pick up in the development,
even though the most recent RAP runs have depicted the placement
and movement of the MCV well. Thus, the flash flood threat could
continue into the early morning hours.

The second area of concern is northwest NM, where convection
forming in the differential heating over the terrain in northwest
NM. The convection is expected to be more scattered in nature than
southeast NM, as the mid level forcing under the ridge here is not
nearly as impressive. However, slow cell motions to the south
could result in cell mergers and short term training, which could
support hourly rainfall rates over an inch. Three hour flash flood
guidance values here are as low as 1.50 inches, so a flash flood
threat will continue until the activity until the instability in
place becomes exhausted, which could occur as later as 23/06z.  

The third area of concern is a small portion of south central AZ,
mainly for the evening hours. There is a multi model signal for
convection developing over nearby Mexico to track over south
central AZ this evening. The convection follows a monsoonal
moisture plume (with precipitable water values nearing 1.75
inches, which is near two standard deviations above the mean) on
the broad southeast flow under the mid level ridge in place over
the Southwest. Being just east of the mid level ridge position,
slow northwest cell motions are expected (generally under 10
knots) into south central AZ. The slow cell motions could foster
an environment for cell mergers and short term training, which in
turn could result in hourly rainfall rates approaching 1.50 inches
(which is supported by members of the 12z model suite, as well as
the last few runs of the HRRR).

Instability over this area is impressive, with the most recent
HRRR/RAP soundings showing local 2000/3000 J/KG of MUCAPE in place
through 23/06z. Model soundings also indicate that the cap remains
in place for much of the evening into the early overnight, which
could limit the areal extent of the convective coverage. However,
with the ingredients of flash flooding in place, a Slight Risk was
left in place here.


Day 2
Valid 12Z Wed Jul 24 2019 - 12Z Thu Jul 25 2019


...Northern FL/Southeastern GA...
A cold front will become stalled across the region and off the
southeast coast by early Wednesday. With a decent moisture and
instability gradient present combined with mid-level impulses
riding atop the aforementioned surface boundary, convection will
likely develop with efficient rain makers. Precipitable water
values of 2+ inches will be transported by 15-25 knot
southwesterly winds. Marginal MUCAPE values of 1000+ J/kg are
expected across this region Wednesday afternoon. Within the upper
levels, the jet streak to the north will slowly sink south
allowing the right entrance region to become positioned over
southern GA and northern FL, promoting additional lift. Mean wind
around 20 knots from the southwest will become aligned with
corfidi vectors in terms of both speed and magnitude for a period
of time Wednesday afternoon/evening. This will likely result in
shower/thunderstorm activity training across portions of the
northeast Gulf Coast region along the FL coast between Tampa and
Tallahassee. Given this, there is the potential for isolated flash
flooding with high flash flood guidance taken into account.
Therefore, the Marginal Risk was maintained.

...Four Corners...
Mid/low-level monsoonal flow around the persistent Four Corners
high will provide moisture with instability and upslope flow to
promote convective initiation along higher terrain of the Four
Corners states Wednesday afternoon/evening. Continued light mean
flow will mean slow storm motions and the potential for the
activity to spread off the terrain into the plains. Therefore, the
Marginal Risk was maintained across portions of AZ and the Slot
Canyon region of southern UT. Southerly flow up the eastern sides
of the CO/NM Rockies will again provide moisture to storms firing
over the high terrain, so the Marginal Risk remains centered on
the San Juan mountains.


Day 3

The Day 3 outlook will be updated by 0830Z.

Day 1 threat area:
Day 2 threat area:
Day 3 threat area: