Quantitative Precipitation Forecast
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000
FOUS30 KWBC 270758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Tue Jul 27 2021

Day 1
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021

...THERE A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...

...Southeast...
As a surface low tracks along the surface boundary through the
region, expect to see periods of heavy rainfall. PW values sit
near 2.25 inches sitting near 2.5 std deviations above normal for
this area. Weak steering flow and possible training storms, could
lead to flash flooding. Latest model guidance places signals of 2
to 4 inches for portions of central to southern GA up along the
coast of of SC. The Slight Risk was left in play and adjusted
slightly to account for latest model guidance. Much of the
Southeast will see periods of heavy rain with this system, with
much of this area seeing 150-400% of normal precipitation over the
past two weeks, making flash flooding more probable for this
portion of the Country. The Marginal Risk area was also adjusted
slightly to account for latest Hi-Res guidance and forecast QPF.

...Great Basin through the Southern Rockies...
Monsoonal moisture will bring the next round of showers and
thunderstorms across the Intermountain West. Areas west of the
ridge axis will continue to see well above normal moisture,
particularly over the Great Basin and over far north CA into
central and eastern OR where precipitable water values of 1" to
1.25" are 2 to 3 standard deviations above normal. The Marginal
Risk area was maintained over terrain from central OR to southern
AZ by way of UT with only minor adjustments made to account for
latest guidance.

...Northeast...
A northern stream trough will pivot through the region Tuesday
night with an associated cold front becoming oriented northeast to
southwest opening the window for training convection. Ahead of the
trough axis, southerly flow will increase ushering in higher
precipitable water values which will exceed 1.75 inches aided by
35+ knot 850mb west-southwesterly winds. This low level flow will
become aligned with the frontal zone allowing for multiple rounds
and/or training convection over any given location. With
instability climbing above 1000 J/kg and enough mid/upper level
lift, convective coverage should be widespread. With the available
moisture, instability, and large scale lift, expect hourly rain
rates to exceed 1.5 inches in some locations. Due the progressive
nature of the activity, only areas of repeating convection should
have excessive rain, so the flash flood risk should be isolated.
The Marginal Risk area was maintained and expanded a bit to
include all of the eastern Great Lakes.

Chiari/Jackson




Day 2
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER GREAT LAKES INTO THE UPPER MIDWEST...

...Upper Midwest/Great Lakes...
The next shortwave trough makes its way across the area during the
period with sufficient instability and PW values reaching upwards
of 2.0 inches to provide heavy rounds of precipitation for the
area. Soils will be primed during the day 1 period as mesoscale
features make their way across the region. Soil moisture are
already sitting in upwards of the 98th percentile and with day 1
QPF, the day 2 periods will see much lower FFG than what is
currently seen, which is already low. Flash flooding concerns are
on the higher end confidence wise and thus, a Slight Risk area has
been hoisted for portions of WI into MI. The Marginal Risk area
already in play for this region was expanded slightly to account
for differences among the models and latest QPF forecasts.

...Great Basin/Intermountain West...
Monsoonal moisture continues to surge north across the region with
weak mid-level impulses helping to focus convection farther north
than has been observed in weeks. This becomes no more evident than
by the precipitable water values remaining above 1.25 inches in
some locations under broad southerly low level flow. With plenty
of diurnal heating, anticipate MUCAPE to climb above 1000 J/kg
allowing for differential heating across the terrain. Therefore,
afternoon convection will largely be tied to the mountains and
eventually shift off into the valleys/deserts through the late
afternoon/evening. Hourly rain rates could reach between 0.5-1
inches, though isolated higher amounts are possible, especially
with slow moving convection. Given the complex terrain, wet
antecedent conditions across the Southwest/Central-Southern
Rockies, and burn scars, localized flash flooding could occur and
the Marginal Risk area has been maintained.

...Southeast...
A boundary should stall near the FL/GA line extending west to the
central Gulf Coast. Convergence of tropical air along this
boundary should allow heavy enough rainfall to locally exceed the
currently high flash flood guidance and combined with day 1 QPF,
the Marginal Risk area was kept in play.

Chiari/Jackson

Day 3
Valid 12Z Wed Jul 28 2021 - 12Z Thu Jul 29 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN THE
SOUTHWEST THROUGH THE NORTHERN ROCKIES AND THE NORTHEAST DOWN INTO
THE OHIO VALLEY...

...Southwest into the Northern Rockies...
Monsoonal moisture will continue to cause flooding concerns for
portions of the Southwest northward into the Northern Rockies. Low
FFG and periods of rain leading up to this period all lead to
concerns for flooding. A Marginal Risk area was hoisted extending
from southern AZ northward into UT and western CO, extending into
portions of central NV and west-central CA, with a northward
extent into WY and southern MT. PW values continue to exceed 1.00
inches with anomalies reaching 2.5-3 std deviations above climo
for these areas.

...Northeast south through the Ohio Valley...
The upper trough will continue to bring periods of heavy rains to
this region with latest guidance showing signals of 1 to 3 inches.
Much of this area sits near 150-400% of normal for precipitation
in the past few weeks and with antecedent conditions priming soils
even more, flooding could be of concern for this period. PW values
sit near 1.5-1.75 inches for much of this area during this time.

Chiari


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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