Quantitative Precipitation Forecast
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FOUS30 KWBC 312011

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Sat Jul 31 2021

Day 1
Valid 16Z Sat Jul 31 2021 - 12Z Sun Aug 01 2021


16Z update... The convective complex moving southeast through
Missouri and Illinois have been producing hourly amounts of 1 to 3
inches this morning and will continue tracking toward
Kentucky/Tennessee this afternoon. Additional convection is
expected redevelop along and ahead of the warm front lifting into
the Mid-Mississippi Valley, capable of hourly rain rates of 1 to
2+ inches per hour over nearly the same path as the first round.
The Slight Risk area was expanded to the east, which places it
further into southern Illinois and western Kentucky; which is
supported by the latest hi-res guidance that has increased amounts
over northeast Missouri and western Illinois. The Marginal Risk
area was also adjust over central Illinois and central Kentucky to
account for the initial round of heavy rain.

The Slight Risk area that covers parts of Idaho and Oregon largely
covers the areas with current convection and the forecast QPF.
Showers and thunderstorms have started to pop up over eastern
Oregon this morning and the latest hi-res guidance is also showing
a northwest shift and increasing QPF trend there. The Slight Risk
was expanded over eastern Oregon to account for this trend.


...Western United States West of the Rockies...
Overall, the primary threat for excessive rainfall will once again
be focused out over the western United States where anomalous
moisture will continue to circulate in and around a broad upper
high.  With monsoonal moisture surging northward further into the
Great Basin and the Rockies, PW values of 1-1.25 inches over the
region will be on the order of 2 to 4 standard deviations above
normal. For some locations these values may be on the upper
extreme of the anomalies. Short wave energies lifting through the
region will trigger scattered to widespread convection capable of
producing rainfall rates greater than 1 inch per hour with
localized maximum amounts of a couple inches being possible.
Placement of the additional Slight Risk area made on Friday looks
good with both the ECMWF and GFS having the axis of maximum
precipitable water moving northward and approaching the entrance
region of an upper jet by 31/06Z over the Sierra Nevada
northeastward toward far western Wyoming,

...Rocky Mountains and Adjacent High Plains...
Numerical guidance has started to favor the areas in and near the
higher/complex terrain of central and southern Colorado as well as
a signal for heavy rainfall to extend out over the adjacent high
plains.  The axis of the broad upper ridge should be very near
this area with resulting weak to minimal steering flow to move any
convection that should fire in the afternoon or evening.  Moisture
anomalies are not going to be quite as big as areas to the west,
but sufficient to support heavy to potentially excessive
rainfall...especially in favored terrain and over any fresh
burn-scar areas. Spaghetti plots show 1 and 2 inch amounts shown
by ARW and NMM members of the SREF and by the GEFS ensemble.  Saw
little reason to change the Slight Risk that was out for the
Colorado and a small part of New Mexico that was previously

...Mid Mississippi Valley...

The degree of spread amongst the model guidance continued to
decrease through the 31/00Z model cycle...and they still depict an
axis of 1 to 3 inch rainfall amounts mainly focused over
Missouri...but extending into parts of Tennessee.  Except for a
few minor adjustments based on latest guidance, few changes were
made to the on-going ERO here.


Day 2
Valid 12Z Sun Aug 01 2021 - 12Z Mon Aug 02 2021


21Z update... Increasing southeasterly flow further into
Washington will increase the likelihood of moderate to heavy rain
especially near the Cascades. The Marginal Risk area was adjusted
to the west as well as the northern bound of the Slight Risk area
in northern Idaho and far eastern Washington. The signal for
increasing coverage and intensity over far southern Idaho,
northeast Nevada and northwest Utah warranted an adjustment of the
southern bound of the Slight Risk area. Model guidance is favoring
west-central Texas for higher QPF, mainly in the 1 to 3 inch range
but local higher values possible. The Slight Risk area expanded
toward into west-central Texas and the Marginal was adjusted south
as well.

No changes were made to the Marginal or Slight Risk areas in the
Northeast. Moderate to heavy rain is still expected to congregate
near the Mid-Atlantic/Carolina Coast. 1 to 3 inches are forecast
from Charleston, South Carolina to the Del-Mar-VA area; the higher
amounts will likely focus near the Outer Banks of North Carolina
and Virginia.


...Northwest/Intermountain Region to the Central and Southern High

The threat for heavy to excessive rainfall continues on Sunday
over many of the same areas where heavy rainfall has been a threat
in preceding days.  The significant PW surge into the Pacific
Northwest/Intermountain West will continue with anomalies ranging
from 2 to 4.5 standard deviation above normal for early August.
Over western Montana, that represents values in excess of the 95th
percentile.  The latest guidance is showing areas with 1 to 2
inches of rainfall during the 24 hour period, with some of the
highest values over Idaho into western Montana, eastern Oregon and
as far south and southeast into Nevada. Areal average amounts from
the models are not overly impressive, but the ability for any
convection to produce very heavy rainfall rates is present from
the afternoon into the late evening.

For the Central/Southern Rockies, the deterministic model guidance
is suggesting the higher amounts will be more eastern New Mexico
and northern Texas but the heavy rain expected just prior to this
period will keep parts of Colorado and northern New Mexico with an
elevate risk for flash flooding. Looking at spaghetti plots of 24
hour QPF suggests the need to expand the Slight Risk a bit farther
east into Texas than in previous outlooks while maintaining the
Colorado/New Mexico area with few changes. In fact, there is
decreasing separation between the High Plains and a second system
capable of producing a couple inches of rain over the South.  Will
combine the Marginal Risk areas into one for the time being.

...New York...

Maintained the Slight Risk over portions of New York, but confined
the Slight to areas that had heavy rain and flooding on Thursday
and still appear to be sensitive to additional rainfall in the
near-term.  Soil moisture was lower elsewhere and appears to be in
a better position to handle the type of rainfall associated with a
synoptic system. Kept part of the Marginal Risk area in place near
the heaviest rainfall amounts in regions of terrain.


Day 3
Valid 12Z Mon Aug 02 2021 - 12Z Tue Aug 03 2021


21Z update... Very minor adjustments were made to the Slight Risk
areas in the West, but overall periods of heavy rain are still
expected that may lead to rapid run off, localized flooding and/or
debris flow near recent burn scars. With QPF trends increasing
over southeast New Mexico and western Texas, the Marginal Risk
area was expanded south to reflect this. The moderate to heavy
rainfall that is expected across the eastern Gulf Coast and the
Southeast largely remains covered with the current Marginal and
Slight Risk areas. Minor adjustments were made on the Marginal
Risk, bringing it further north into central Georgia and South

Showers and thunderstorms will fire up across South Florida during
this period and will be capable of producing up to 2 inches per
hour rates. The Marginal Risk area that was already in effect
largely covered the locations where the heavy rain is expected. A
small tweak was made to extend the northern bound into Martin
County, Florida.


...Western US...
The focus for heavy to excessive rainfall persists over parts of
Montana, Idaho and Wyoming as the ridge persists across the region
but has started to show signs of shifting eastward.  The moisture
remains 2 to nearly 4 standardized anomalies above climatology
with sufficient instability for cells to produce some intense
downpours.  The locally intense rainfall rates and slow moving
cells could result in flash flooding...with the complex terrain
and burn-scar areas being especially susceptible to life
threatening flooding.

Farther south, deep moisture trapped by the ridge combined with an
easterly low level component to the wind will result in the threat
of mainly afternoon and evening excessive rainfall across
central/southern Colorado into New Mexico. The Colorado portion of
the area looks to be most prone to excessive rainfall on Monday
afternoon and evening before the focus shifts into New Mexico
later in the evening as southeast low level winds draw
precipitable water values in excess of 1.5 inches back towards the
mountains once again.

...Southern Georgia to the Carolina Coasts...
A surface cold front which is strengthening in response to an
amplifying upper level trough will provide the focus for afternoon
and evening thunderstorms over parts of the Southeast U.S. Monday
night.  Precipitable water values in excess of 2 inches will be
located south of the boundary.  With the entrance region of a 100
kt upper level jet to aid deep vertical motion, storms which form
in the area should be able to produce some 1 to 2 inch rainfall
rates.  The storms themselves should be progressive enough, but
the potential for repeat convection will be present.  Model
guidance does show some potential for 2 to 4 inch rainfall amounts
although the confidence in where the focus for heaviest rainfall
will be due to a wide variety of model solutions.


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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