Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed Aug 17 2022
Day 1
Valid 1833Z Wed Aug 17 2022 - 12Z Thu Aug 18 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHEASTERN NEW MEXICO INTO FAR SOUTH-CENTRAL COLORADO, SOUTHERN
ARIZONA AND NEW MEXICO INTO FAR WEST TEXAS, AND A SMALL PORTION OF
THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST...
...1830Z Update...
Amended the Day 1 ERO to expand the western boundary of the
Marginal Risk area in parts of southern Califonria. Also made an
adjustment to the eastern boundary in West Texas. Both areas were
modified in response to short term radar trends and an increasing
signal for heavy to potentially excessive rain. With respect to
the California adjustment...refer to MPD 775 for additional
details while an MPD will likely be needed soon in the far
southern portion of West Texas.
Bann
...16z Update...
The inherited Slight Risk areas were maintained for the D1 update
with only minor adjustments/tweaks needed for northeastern
NM/south-central CO and the Lower MS Valley/Central Gulf coast
based on the latest hi-res CAM data. Marginal Risk edits were also
minimal, but included 1) a minor expansion in the Southwest to
cover more of southwestern AZ and southeastern CA and 2) trimming
the northern extent over the Lower MS Valley where heavy rainfall
has come to an end.
The only significant change with this update was to introduce a
Marginal Risk area for portions of north-central MN into
northwestern WI, as the latest hi-res CAM guidance (12z HREF
suite) indicates higher neighborhood probabilities for 2-3"
exceedance (as high as 20% over 3-hr) this evening into tonight
(primarily from 00z-12z Thursday). Given the localized occurrence
of 2-3" totals yesterday near this area with little change in the
overall synoptic pattern (including PWs of 1.2-1.5", near the 90th
percentile per MPX sounding climatology), an isolated instance or
two of flash flooding appears possible (despite the rainfall being
largely beneficial for the region).
Churchill
...Previous Discussion...
...Southwest into southern Rockies and High Plains...
Upper high centered across the central Great Basin will expand a
bit today with a larger coverage of >591dm heights, thus promoting
a bit more stability to parts of the Great Basin and central
Rockies. Precipitable water values will remain above normal for
the rest of the Southwest and southern Rockies, anywhere from +1
to +3 sigma (actual values around an inch over the Great Basin to
two inches near the Mexican border). There, decaying, compact
mid-level inverted trough/convectively enhanced vort lobe will
still provide an influx of moisture to southeastern AZ/southern
NM/far West Texas with modest IVT values from the southeast at
least initially, before likely being disrupted by the higher
terrain of Mexico south of the Big Bend. Based on the latest
guidance trends, including the latest (00Z) HREF exceedance
probabilities, have expanded the Slight Risk a bit to include more
areas in south-central and southeast NM, along with West Texas.
These areas will see an additional 0.50"-1.25" of areal-average
rainfall, with locally higher amounts. Farther west across
southwest NM and southeast to south-central AZ, greater deep-layer
instability and higher PWs will lead to spotty 3-5" totals, though
the spotty coverage is such that the overall flash flood threat is
expected to remain below the Moderate Risk level.
Over northeastern NM into south-central CO, have re-shaped the
Slight Risk area to account for the antecedent heavy rain that
fell on Tuesday. Current 1 hour FFG over much of this area is 1"
or less as a result. Axis of higher that normal PW values will
sink south of this area after 00Z Thu, but this will leave room
for afternoon convection to capitalize on high moisture content in
the column to yield some heavier rainfall rates of 1-2"/hr per the
00Z CAM guidance (at least matching current FFG values).
...Lower MS Valley into the Central Gulf Coast and parts of the
Southeast...
A frontal boundary draped from north of the ArkLaTex ESE to the GA
coast early this morning will sink slowly toward the I-10 corridor
later today and Wed night as the southern periphery of the
mid-level trough drops toward the Gulf Coast as well. As a
~90-100kt jet streak traverses the trough base across the central
Appalachians/Lower Mid-Atlantic, a pocket of broader-scale lift
will track W-E along the front. Afternoon heating will promote
showers and thunderstorms near and south of the front, where
precipitable water values are forecast to be above normal by about
+1 to +1.5 sigma (values near and just over 2 inches). Soil
moisture varies through the region, and FFG values are about
2-3"/hr with pockets of lower values in northern AL. 00Z guidance
indicates potential for localized 2-5+ inch rainfall, however the
areal spread remains high. We added a narrow Slight Risk area from
a small portion of northeast LA southeast through parts of
southern MS and AL, based on the aforementioned forcing and
favorable thermodynamics over this region (higher MUCAPEs
~2000-2500 J/kg and PWs aoa 2.00"), along with the fact that many
locations within the Slight Risk area had 1-2+ inches of rain
yesterday (with pockets of 3-hourly FFG of 2-2.5").
Hurley/Fracasso
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Wed Aug 17 2022
Day 2
Valid 12Z Thu Aug 18 2022 - 12Z Fri Aug 19 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN ARIZONA AND SOUTHWESTERN NEW MEXICO ALONG WITH PARTS OF
THE SOUTHEAST COAST...
...Arizona and New Mexico into the Great Basin...
Remnants of the Day 1 mid-level low will continue traversing the
CONUS/Mexican border Day 3, adding some instability to the already
wetter monsoonal environment. Precipitable water values will
remain near 2 inches close to the Mexican border which is about +2
to +3 sigma above the mean. Coupled with afternoon heating, this
will provide the ingredients for another day of heavier rainfall
in association with the system, perhaps including additional
moisture from the western side of Mexico through the Sea of
Cortez. High-res CAM consensus shows isolated areas of 2-5+ inches
across south-central and southeast AZ into southwestern NM, and as
such only minor changes were made to the Slight Risk area from
yesterday's Day 3 ERO.
...Southeast...
The stationary front will linger west to east through the
Southeast Thursday on the southwest side of broad troughing in the
Great Lakes/Northeast. Precipitable water values will remain above
normal (+1 to +2 sigma, actual values around 2 inches) so the
environment will be capable of at least isolated to scattered
convection and heavier rainfall. Best upper-level support on the
synoptic scale will be across portions of the Southeast coast,
within the right-entrance region of the 90-100 kt upper level jet
streak draped over the southern Appalachians and Piedmont. PWs of
2.00-2.25" along a quasi-stationary front with a slow-moving, weak
surface low reflection per the guidance consensus would support a
targeted Slight Risk along the coast and parts of the coastal
plain from far northeast FL into eastern GA and far southern SC.
Most of the CAMs are rather prolific with the QPF in this area,
especially along the coast, with the 12hr HREF probability-matched
mean indicated a swath of 3-5+ inches of rain from 12Z Thu-00Z Fri
between JAX and south of CHS. The 00Z HREF in fact indicated
probabilities of >2"/hr rates between 30-50+ percent from 18Z Thu
through 00Z Fri, with probs of 3 hour QPF exceeding current 3 hour
FFG between 30-40%.
Hurley
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
458 AM EDT Wed Aug 17 2022
Day 3
Valid 12Z Fri Aug 19 2022 - 12Z Sat Aug 20 2022
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS
SOUTH-CENTRAL AND SOUTHEAST ARIZONAN INTO SOUTHWEST NEW MEXICO...
...Southwest and Four Corners-Eastern Great Basin Areas into
Central-Southern Rockies...
Upper pattern will be conducive for potentially a more impactful
heavy rainfall event over parts of the Southwest in what has been
an active monsoon season thus far. Shortwave energy is expected to
elongate across the SW and eastern GB Fri-Fri night, drifting
slowly while caught within the mid-level COL/broad deformation
zone. QC forcing progged from the models is quite robust,
particularly over southern AZ-NM, with focused areas of DPVA and
upper divergence per the guidance. At this point (day 3 forecast),
there's always a question as to how unstable the environment will
get during the peak heating hours (predicated on the degree and
depth of mid-upper level cloud debris), however considering the
favorable (and potentially prolonged) synoptic support, with
850-700 mb moisture flux anomalies between +3 and +4 per the
ensembles, along with the continued anomalous PW values, the
signal is certainly there for a more focused, elevated ERO risk as
the bulk of the guidance (outside of the CAM windows) show more
widespread 1-2+ inch areal-average QPF across much of central and
south-central AZ into southeast AZ and southwest NM. Per
collaboration with WFOs TWC, PSR, FGZ, ABQ, and EPZ, have hoisted
a Moderate Risk over these areas on Day 3.
...Upper Texas Coast into Parts of the Central Gulf Coast,
Southeast, and Lower Mid Atlantic...
Aforementioned quasi-stationary front lifts slightly northward
Fri-Fri night across the Lower MS Valley-Lower TN Valley,
Southeast, and Lower Mid Atlantic. Lingering right-entrance region
forcing will continue to foster areas of more organized convection
and heavy rainfall, migrating northward into the Carolinas as the
upper jet streak lifts north across the mid Atlantic and northeast
coast. At this point, while there is consensus among the guidance
in terms of the heavy rainfall signal, at this point (Day 3) there
is too much areal spread to support more than a Marginal Risk in
the ERO.
Hurley
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
