Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Jul 14 2026
Day 1
Valid 01Z Wed Jul 15 2026 - 12Z Wed Jul 15 2026
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN THE TEXAS HILL
COUNTRY...
01Z Excessive Rainfall Discussion Update...
Introduced a targeted Slight Risk area in Montana given trends in
satellite and radar imagery during the late afternoon combined with
the anomalously deep moisture in place. Mesoscale Precipitation
Discussion 722 had recently been issued. Convection had been
struggling to produce much in the way of intense rainfall rates or
rainfall accumulations as it crossed into parts of New England from
Canada. It was presumed that cloud cover and smoke aloft had
limited the instability. As a result...the Marginal Risk was
removed.
Texas Hill Country remained the prime candidate for high impact
and potentially life-threatening flash flooding throughout the
night and even beyond the end of the Day 1 period at 12Z on
Wednesday. Refer to the Day 2 ERO/ERD for details of the
continuation of the High Risk.
Beyond that...there were some minor adjustments to the previously
issued outlook areas but there was no fundamental shift in forecast
reasoning. The expectation was that the convection across much of
the Western US will fade later tonight but that it was too soon to
remove from the ERO.
Bann
Previous Excessive Rainfall Discussion...
...Texas Hill Country...
Portions of the TX Hill Country witnessed torrential rainfall last
night and this morning that will continue to unfold through
midday. Radar estimates show anywhere for 6-10" of rain have fallen
in Medina, Uvalde, and Bandera counties (locally around 12") and
have already led to reports of numerous flooded roadways and water
rescues. Looking ahead, the atmosphere is primed for additional
rounds of Excessive Rainfall with hourly rates between 2-4" likely.
The 12Z RAOB out of Del Rio, TX showed an abundance of moisture
aloft (2.28" PW, well above the 90th climatological percentile) and
sufficient instability for thunderstorms to work with (~1,600 J/kg
of MLCAPE). The MCV north of Del Rio is the catalyst for this
event, tapping into the rich moisture and unstable air-mass to
generate numerous showers and thunderstorms. Overnight CAMs
struggled with the placement and duration of the storms this
morning, although new 12Z guidance is catching on better and is in
largely good agreement that storms will continue through early
afternoon. Many new 12Z CAMs suggest a brief lull may occur this
afternoon, but the presence of the MCV is likely to keep some
storms in the heart of TX through early evening.
By tonight, the LLJ over South TX will strengthen and act to re-
supply additional low-level moisture towards the MCV, which is
forecast to hover near Del Rio, TX. The combination of upscale
ascent supported by the MCV, the LLJ intersecting a nearby stalled
frontal boundary, and natural topographic upslope into the Hill
Country west of I-35 will reinvigorate thunderstorm activity
tonight. It is worth noting that all CAMs do have an axis of
Excessive Rainfall with amounts in the 2-4" range overnight (locally
higher than 6"), but they disagree on placement. The 12Z HREF and
06Z REFS are keying in on areas closer to the MCV, essentially from
Medina/Bandera counties on WNW towards Del Rio with moderate
chances (40-60%) for additional rainfall totals >3" and low
chances (10-30%) for rainfall >5" between 00-12Z. Some members,
such as the 12Z HRRR and 12Z RRFS, show the axis of heavy rainfall
encroaching upon the western San Antonio suburbs. Given the
rainfall that the current atmospheric environment has already
produced, additional rainfall totals of 3-6" are forecast for the
remainder of today and through tonight in the TX Hill Country with
localized amounts over 10" possible. Reminder that this discussion
strictly focuses on 16Z today through 12Z Wednesday (Day 1), so
this does not take into account the rainfall still to come for Day
2 and the second half of the week.
In collaboration with the NWS San Antonio/Austin office, given the
likelihood for additional significant rainfall in areas slammed
with heavy rainfall and ongoing flooding this morning, a High Risk
was introduced for the 16Z update. Significant to locally
catastrophic flash flooding is forecast to continue today and, with
additional heavy rainfall tonight, likely to persist into tonight
and early morning Wednesday. Potential impacts include
flooded/impassable roads, flooded creeks/streams, and potential
inundation of homes. Please have a reliable means of receiving
weather warnings and information from local/state officials.
Elsewhere, some minor tweaks to the inherited Moderate and Slight
Risks were made to account for latest 12Z guidance, but otherwise
the setup featuring a pooling of anomalous moisture along a stalled
frontal boundary will continue to trigger additional strong storms
and scattered areas of flash flooding through this evening.
...Coastal South Carolina...
A Slight Risk was introduced along the South Carolina coast with
this update. The 12Z CHS sounding measured a 2.26" PW (above the
90th climatological percentile), MLCAPE over 1,000 J/kg, and a warm
cloud layer over 14,000ft deep. Storms have already flared up along
a frontal boundary both over land and at sea, and as daytime
heating ensues, additional strong thunderstorms are likely to
develop. Storms will be generally slow moving and can produce 3"/hr
rainfall rates with locally more intense, shorter bursts of
torrential rainfall possible. The 12Z HREF shows low-to-moderate
chances (20-50%) for localized areas of the SC coast receiving over
5" this afternoon. The area has dealt with locally heavy rainfall
in recent days and is a little more susceptible to rates
approaching 3"/hr, particularly in the more urbanized communities.
Mullinax
---Previous Discussion---
...Southeast into Southern Mid Atlantic...
General consensus on widespread convective development across the
southern Carolina's into the Southeast lends to at least a broad
MRGL risk for flash flooding in any area north of I-4 with a
relative max for heavy rain situated across portions of MS/AL
thanks to the presence of a meandering upper level disturbance over
the area. WV satellite this evening pin-points a weak upper level
circulation centered over northern MS with an extremely slow
propagation to the west-southwest over the past several hours. This
particular upper circulation will be a focal point for enhanced low
to mid-level convergence later this afternoon interacting with what
is a formidable moist/unstable environment across the Southeast
CONUS. Scattered to widespread showers and thunderstorms will
materialize across northern LA over into MS and AL during peak
diurnal destabilization leading to pulse/multi-cell convective
development over the region with heavy rain prospects likely very
high considering the environment in place. Wetbulb zero heights
>14k ft on forecast soundings across MS/AL indicate an effective
warm cloud layer that will likely present efficient rainfall rates
with even pulse variety convection yielding 2-3"/hr rainfall
potential during its mature phases. Slow cell motions under a weak
steering pattern situated across the Southeast will only act as an
aid to flash flood prospects over the region as cells will be more
likely to produce heavy bouts of rain for multiple hours before
drifting away from an area, or even just decaying overhead. 00z
HREF and REFS neighborhood probabilities for >3" are solidly
between 50-80% across central and south-central MS into western AL
with the main area of interest lying along and north of I-20 in
both areas. HREF blended mean outputs show a scattered 2-3" precip
signal over the aforementioned area with individual deterministic
even pushing close to 5" in the hardest hit locales. This is more
than sufficient for maintaining the previous SLGT risk forecast, as
well as wording for a high-end SLGT threshold likely over that area
encompassed within central MS to western AL.
...Southwest U.S. into the Great Basin...
Continued impacts expected from lying into the western flank of
the ridge centered over the Northern U.S. as prevailing southerly
flow will just maintain a prevalent moisture advection regime
poleward over the Desert Southwest into the Great Basin. Models
remain consistent in their interpretations of locally heavy
rainfall over both regions with the heaviest QPF centered over AZ,
but potentially greater impacts in those more flash flood prone
areas located in the interior west, including those slot canyons
over UT/NV. Rainfall rates will be the driving factor for flash
flooding in this scenario with instability generally favorable
(>750 J/kg MUCAPE) over a large portion of the Western CONUS. The
combination of PWAT anomalies >2 deviations above normal and
elevated instability favors at least widely scattered flash flood
potential, which this setup is leaning into the textbook signature
as forecast. 00z HREF neighborhood probabilities for >1" have a
smattering of 40-60% probs within areas of the Great Basin and
Four Corners with a more widespread 40-70% probability oriented in
the terrain from southeastern AZ up through the Mogollon Rim into
the area of the Grand Canyon. Any of these cells in the terrain
will be conducive for heavy rainfall between 0.5-1"/hr, locally
higher across the terrain with more of the general 0.25-0.75"/hr
over the Great Basin. Given the consistency in the model output and
anomalous moisture presence across the West, the previous MRGL
risk was maintained.
...Montana...
PWATs encroaching the 90-99th percentile over much of MT coupled
with favorable instability and approaching mid- level troughing to
the west will allow for scattered to perhaps widespread convection
to materialize over the Northern Rockies into Central MT later this
afternoon and evening. Ample shear and elevated moisture presence
will amplify the convective threat and proceeding heavy rainfall
posture across portions of Big Sky country as we move into the time
frame of peak diurnal destabilization. Organized cell clusters,
including a few supercells, are likely to initiate over the terrain
in the Central portion of MT with an eventual rapid storm motion
off to the northeast as the steering flow will allow for very quick
propagation(s) through the High Plains of MT by later in the
evening. The best threat for heavy rainfall suitable for flash
flooding will likely occur where the initiation point occurs and
just downstream of the eventual storm motions to the northeast.
HREF probabilities for >2" are generally modest in this area with
consistency in the CAMs for cell initiation and eventual impacts in
this region northwest of Billings up through Glasgow to the
Canadian border. The previous MRGL risk was maintained given the
threat.
...Northern New England...
A strong closed low analyzed over Northern Quebec this evening
will pinwheel southeast, well north of the Canadian border in New
England, but will aid in a broad brush of robust mid-level
vorticity over the northern tier of both VT/NH into western ME
during peak diurnal heating into the coming evening. MUCAPE
between 2500-3500 J/kg will be located across the northern tier of
all three states leading to a targeted area for locally heavy
rainfall with rates likely to approach close to 2"/hr as noted in
the latest HREF prob fields (10-25%) with 1"/hr rates a greater
certainty (30-50%). A quick 1-3" is forecast across this small
area of New England, however this area is notorious for local
terrain influences that could cause issues with any appreciable
rainfall in that short period of time. The previous MRGL risk was
expanded further north to include western ME given the trends in
guidance for heavy rain to impact this area as cell maturation over
neighboring Canada will likely motion into the area as we step into
the late-afternoon and early evening hours today.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Jul 14 2026
Day 2
Valid 12Z Wed Jul 15 2026 - 12Z Thu Jul 16 2026
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE EDWARDS PLATEAU, CENTRAL RIO GRANDE VALLEY, AND HILL COUNTRY...
...Texas...
Similar to Tuesday morning, just about all guidance is on board
with a cluster of robust thunderstorm activity producing excessive
rainfall from portions of the TX Hill County and central Rio
Grande Valley to the Edwards Plateau. Guidance agrees that the
lingering MCV responsible for Tuesday's flooding will continue to
organize rounds of thunderstorms that have the potential to train
over areas hit from Tuesday's flood-producing storms. Anomalous PWs
surpassing 2" and sufficient instability will keep the atmosphere
in ripe condition to support torrential rainfall. The main
disagreement between guidance is the placement of the heaviest
rainfall. 12Z HREF members suggest the placement of the MCV is
farther north and west with a more westerly-positioned LLJ over the
Rio Grande. This is allowing for several HREF members to focus
heavier rainfall rates over the Edwards Plateau. Meanwhile,
guidance such as the HRRR and RRFS (REFS probabilistic guidance as
well) suggest the southern extent of the thunderstorm activity may
bring the MCV farther south and closer to the San Antonio metro.
The LLJ is also notably stronger and oriented more at the TX Hill
Country.
Ultimately, both placements of storms can have merit. The southern
extent of Tuesday's thunderstorms is likely to drop a remnant
boundary where, as the LLJ strengthens Tuesday night, could be a
trigger for thunderstorms near and west of the I-35 corridor
Wednesday morning and potentially redevelop Wednesday night.
Meanwhile, the mesoscale-enhancement near the MCV would also
support a pivoting axis of strong storms near and north of Del Rio
that could even reach the Big Bend. While the exact location is
unclear, the dangerous combination of prolific rainfall rates over
highly saturated soils is a recipe for significant to even
catastrophic flooding. This set up for Day 2 (Wednesday) is not too
dissimilar to the setup that produced 6-12" of rainfall in parts
of the Hill Country Tuesday morning, given the same atmospheric
phenomena are on the playing field again on Wednesday. When
utilizing both HREF and REFS probabilistic suites, they both paint
a potentially dangerous situation for life-threatening flash
flooding Wednesday morning.
In collaboration with the San Angelo and San Antonio WFOs, a High
Risk was introduced on Day 2. A Moderate Risk (threat level 3/4)
also extends from the Big Bend to as far east as the San Antonio
metro area. While there remains some uncertainty in the exact
placement of heaviest rainfall, antecedent soil conditions coupled
with rainfall rates ranging between 2-4"/hr are likely to cause
numerous flash floods. Life threatening and catastrophic flash
flooding is likely in the hardest hit areas. Be aware that flooding
is likely to occur overnight and into the early morning hours both
Wednesday and Thursday.
Mullinax
---Previous Discussion---
...Mogollon Rim into Central Arizona...
Broad upper ridge centered over the Northern U.S. will continue to
provide significant moisture advection into the interior west
leading to scattered convective signals from the Canadian border
down into the Desert Southwest with flash flood concerns in those
more susceptible areas out west. Increased confidence in a locally
favorable convective maxima has allowed for not only a continuation
of the previous SLGT risk positioned across the Mogollon Rim in AZ,
but also an expansion further northwest and north, including the
addition of Flagstaff proper. 00z HREF at the end of its temporal
range signaled widespread thunderstorm genesis over the terrain
after 20z Wednesday with some formidable heavy rain cores likely to
materialize out of the initiation. Modest neighborhood
probabilities for >1" of rainfall exist in the small window between
18-00z Wed/Thu with the highest probs situated in that terrain area
between Phoenix and Flagstaff within the Mogollon Rim across
central AZ. In coordination with the local Flagstaff WFO, have
expanded the previous SLGT risk forecast to areas northwest of
Flagstaff within the western periphery of the Mogollon Rim, as well
as a further north expansion to include Flagstaff proper given the
recent trends in guidance, and the premise of urbanization factors
providing more suitable ground conditions for flash flood
prospects.
...Lower Mississippi Valley into Southeast...
Our meandering upper level disturbance will continue to churn over
the Southern U.S. with sights on the Lower Mississippi Valley by
Wednesday afternoon. Models are within close proximity of each
other on the positioning of the disturbance with the centroid of
the circulation likely to be focused somewhere over Arkansas and
the southwest corner of TN. Elevated moisture and general diurnal
destabilization across the Lower Mississippi and Southeastern U.S.
will lead to another round of slow-moving thunderstorms capable of
producing locally heavy rainfall with rates of 2-3"/hr likely in
those stronger cell cores. The setup is not as prolific within the
QPF means compared to the previous few periods, so the threat
remains within the MRGL risk threshold. However, will continue to
monitor trends closely as we move forward in time as the chance for
a targeted upgrade, likely underneath the upper level disturbance,
is plausible.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Jul 14 2026
Day 3
Valid 12Z Thu Jul 16 2026 - 12Z Fri Jul 17 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER TRANS PECOS, EDWARDS PLATEAU, AND RIO GRANDE
VALLEY...
20Z Update...
The biggest adjustments made from overnight was to expand the
Moderate Risk farther north through the Edwards Plateau based on
recent wetter trends in WPC QPF. Impacts will be heavily dependent
upon where the pesky MCV will be located, but most guidance shows
its gradual north and west movement by Thursday. Following the
active day on Wednesday as well, portions of the Edwards Plateau,
the Lower Trans Pecos, and Rio Grande Valley will likely also see
antecedent soil conditions grow more susceptible to excessive
rainfall rates.
...Ozarks & Middle Mississippi Valley...
A 500mb trough pivoting overhead will also have anomalous PWs in
excess of 2" throughout the generally flashy areas of the Ozarks on
Thursday. Model soundings across the board show highly saturated
profiles combined with skinny MLCAPE values in excess of 1,000
J/kg. Storm motions appear to be fairly slow and vertical wind
shear is weak, so storms will behave more pulse-like in nature.
That said, the atmospheric environment could support hourly
rainfall rates up to 3"/hr in some of the more intense cells, and
1-hr FFGs are likely to range between 2-3". Given these factors,
localized flash flooding is possible on Thursday with poor
drainage areas and complex terrain most prone to potential
flooding.
Mullinax
---Previous Discussion---
...Texas...
Convection will continue across the Edwards Plateau and surrounds
for at least one more period as the regional pattern maintains a
persistence in its evolution with yet another round of convection
lingering through the morning from the previous period, and one
last round of nocturnal convection anticipated the evening of
Thursday into early Friday morning. There is some uncertainty on
the exact location of the final convective wave that will transpire
with some guidance positioning the greatest threat a little further
north of the area likely to see the greatest impacts from the prior
periods. In any case, the threat is still very much robust for
wherever this transpires as the axis of heavy rainfall will likely
produce more totals bordering between 2-4", locally higher in the
hardest hit locations. Considering the nature of anticipated
compromised soils and ongoing flooding across portions of the RGV
and Edwards Plateau, very little additional rainfall would cause a
myriad of problems, including an exacerbation of any remnant flood
waters, especially if impacted by additional heavy rainfall >1"/hr.
There's enough of a signal to warrant an addition of a MDT risk
across the area between the Big Bend over into the Edwards Plateau
with emphasis on Val Verde county extending up to the neighboring
counties to the north, including the I-10 stretch in central
Crockett county. Considering the nature of the situation, this
period will be monitored very closely and its susceptible to larger
scale changes as the setup evolves. Heavy rain will likely enhance
life-threatening flash flooding to portions of the region for yet
another period before the setup finally looks to break after the D3
time frame.
...Southwest U.S into the Great Basin...
Another round of afternoon and evening convection will transpire
across the Southwest CONUS into the Great Basin on Thursday with
the threat carrying into the early morning periods of Friday in
some locations. Models are keen on a more robust moisture advection
pulse and general instability axis across the Desert Southwest on
Thursday with a broad axis of heavy rain prospects from the
southern border all the way north into southern UT. Ensemble bias
corrected output is very bullish on the threat in and around the
slot canyons of southern UT, along the Mogollon Rim, and down
towards the terrain embedded within Cochise, Santa Cruz, and Pima
counties. Areal average of 1-2" is forecast for the region along
the southern border with 0.75-1.25" forecast across the Mogollon
Rim and southern UT. Considering the environment maturing closer to
1000-2000 J/kg MUCAPE over much of AZ into southern UT/NV,
instability and anomalous moisture within the 90-99th percentile of
climatology across the region will produce more widespread heavy
rain prospects leading to scattered flash flood occurrences in
those more susceptible locations in the terrain. There is even some
opportunity for thunderstorm genesis off several outflows that
could impact some of the larger urban corridors within AZ,
including the Phoenix metro as the environment favors this type of
potential. A broad SLGT risk was introduced for much of AZ into
southern UT with the eastern extent out towards the NM/AZ border
into the immediate Four Corners, outlining the mean QPF closer to
0.5" within the ensembles.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Jul 14 2026
Day 4 and Day 5
Valid 12Z Fri Jul 17 2026 - 12Z Sun Jul 19 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST ON FRIDAY...
2030Z Update...
No major changes were made to either day or to either area of
concern. Having said that...felt the on-going excessive rainfall
over West Texas in the short term would warrant a Slight Risk
going into the Day 4 period. This resulted in an expansion of the
Marginal Risk area northward and a bit eastward. The 14/12Z GEFS
QPF did not show blockbuster values but any MCVs in such a moisture
rich airmass could result in continued or renewed flooding.
Only subtle shifts were made to the periphery of the Marginal Risk
area across parts of the Intermountain West/Southwest U.S. and the
Great Lakes/Mid-Atlantic.
Bann
Previous Discussion...
...Western U.S...
A broad active monsoonal pattern will continue across much of the
western U.S. Friday into Saturday. An anomalous PW axis stretching
from AZ northward into MT is forecast to remain above the
climatological 90th percentile, and locally higher in some spots.
This airmass will sustain a localized flash flood threat across an
expansive corridor spanning from portions of eastern CA, AZ, and NM
northward to MT.
We did go ahead and upgrade much of AZ into southwest UT and
southwest NM into a Slight risk for D4 (Friday). While the
prolonged monsoonal pattern will bring at least a localized flash
flood threat to the region each of the next 5 days, current model
guidance indicates Thursday and Friday will see a peak in both PWs
and CAPE. Model consensus and NBM QPF are also higher on these
days. Of course, mesoscale details (which we can't pin down at this
lead time) may end up determining the eventual flash flood
coverage. But there seems to be enough of a signal to suggest the
risk may be relatively higher those two days. The isolated threat
continues into D5 (Saturday), but will hold that at a Marginal for
now with a modest decrease in PWs/CAPE/QPF noted in the models
compared to Friday.
...Texas...
The multi-day heavy rainfall event over portions of central and
west TX should be on a declining trend by Friday. PWs gradually
decrease and lower level convergence seems to wane, with both the
AI models and GFS/ECMWF showing a decrease in convective coverage.
With that said, we will still have strong 850mb southerly moisture
transport across the state, and some leftover shortwave energy. So
the event is likely not over on Friday, just probably less
widespread and hopefully not as high end. The setup into Friday is
still favorable enough that can not rule out Slight risk upgrades
on future updates, especially if the convective footprint overlaps
hydrologically sensitive areas from previous rainfall. The risk
should lower further by Saturday, but again some risk of a locally
heavy rainfall threat continues from the Edwards Plateau and points
west.
...Great Lakes into the Mid-Atlantic...
On Friday a Marginal risk was introduced from portions of the TN
Valley northward into the Great Lakes. The strong mid to upper
level ridge currently anchored over the Plains/Midwest is forecast
to break down and translate southeastward. This synoptic shift will
allow the high PW airmass currently situated over the South to
advect north-northeastward. By Friday afternoon much of the OH and
TN valleys will have PWs around 2" and CAPE potentially exceeding
3000 J/kg in spots. This unstable and saturated environment will be
conducive for high rainfall rates. While weaker large scale
forcing across the southern portion of the risk area may limit
overall convective organization isolated instance of flash flooding
will be possible. Further north, stronger synoptic forcing should
support organized convection across portions of WI into northern
MI. These convective cluster may lean toward a progressive mode,
but the degree of instability and PWs should support some flash
flood risk.
On Saturday the core of the high PW airmass shifts eastward into
the Mid-Atlantic. Amplifying troughing building into the
Northeast, accompanied by embedded shortwave energy, should
support a organized convective threat from OH into the northern
Mid-Atlantic and possibly southern NY. Spatial uncertainties exist
regarding how far north the surface front and axis of better
instability will get, which will ultimately dictate the northern
bound of any flash flood risk. Further south and west across the OH
Valley into the southern Mid-atlantic large scale forcing may be
weaker, but plentiful moisture and instability may still drive
localized areas of heavy rainfall.
Chenard
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Jul 14 2026
Day 4 and Day 5
Valid 12Z Fri Jul 17 2026 - 12Z Sun Jul 19 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST ON FRIDAY...
2030Z Update...
No major changes were made to either day or to either area of
concern. Having said that...felt the on-going excessive rainfall
over West Texas in the short term would warrant a Slight Risk
going into the Day 4 period. This resulted in an expansion of the
Marginal Risk area northward and a bit eastward. The 14/12Z GEFS
QPF did not show blockbuster values but any MCVs in such a moisture
rich airmass could result in continued or renewed flooding.
Only subtle shifts were made to the periphery of the Marginal Risk
area across parts of the Intermountain West/Southwest U.S. and the
Great Lakes/Mid-Atlantic.
Bann
Previous Discussion...
...Western U.S...
A broad active monsoonal pattern will continue across much of the
western U.S. Friday into Saturday. An anomalous PW axis stretching
from AZ northward into MT is forecast to remain above the
climatological 90th percentile, and locally higher in some spots.
This airmass will sustain a localized flash flood threat across an
expansive corridor spanning from portions of eastern CA, AZ, and NM
northward to MT.
We did go ahead and upgrade much of AZ into southwest UT and
southwest NM into a Slight risk for D4 (Friday). While the
prolonged monsoonal pattern will bring at least a localized flash
flood threat to the region each of the next 5 days, current model
guidance indicates Thursday and Friday will see a peak in both PWs
and CAPE. Model consensus and NBM QPF are also higher on these
days. Of course, mesoscale details (which we can't pin down at this
lead time) may end up determining the eventual flash flood
coverage. But there seems to be enough of a signal to suggest the
risk may be relatively higher those two days. The isolated threat
continues into D5 (Saturday), but will hold that at a Marginal for
now with a modest decrease in PWs/CAPE/QPF noted in the models
compared to Friday.
...Texas...
The multi-day heavy rainfall event over portions of central and
west TX should be on a declining trend by Friday. PWs gradually
decrease and lower level convergence seems to wane, with both the
AI models and GFS/ECMWF showing a decrease in convective coverage.
With that said, we will still have strong 850mb southerly moisture
transport across the state, and some leftover shortwave energy. So
the event is likely not over on Friday, just probably less
widespread and hopefully not as high end. The setup into Friday is
still favorable enough that can not rule out Slight risk upgrades
on future updates, especially if the convective footprint overlaps
hydrologically sensitive areas from previous rainfall. The risk
should lower further by Saturday, but again some risk of a locally
heavy rainfall threat continues from the Edwards Plateau and points
west.
...Great Lakes into the Mid-Atlantic...
On Friday a Marginal risk was introduced from portions of the TN
Valley northward into the Great Lakes. The strong mid to upper
level ridge currently anchored over the Plains/Midwest is forecast
to break down and translate southeastward. This synoptic shift will
allow the high PW airmass currently situated over the South to
advect north-northeastward. By Friday afternoon much of the OH and
TN valleys will have PWs around 2" and CAPE potentially exceeding
3000 J/kg in spots. This unstable and saturated environment will be
conducive for high rainfall rates. While weaker large scale
forcing across the southern portion of the risk area may limit
overall convective organization isolated instance of flash flooding
will be possible. Further north, stronger synoptic forcing should
support organized convection across portions of WI into northern
MI. These convective cluster may lean toward a progressive mode,
but the degree of instability and PWs should support some flash
flood risk.
On Saturday the core of the high PW airmass shifts eastward into
the Mid-Atlantic. Amplifying troughing building into the
Northeast, accompanied by embedded shortwave energy, should
support a organized convective threat from OH into the northern
Mid-Atlantic and possibly southern NY. Spatial uncertainties exist
regarding how far north the surface front and axis of better
instability will get, which will ultimately dictate the northern
bound of any flash flood risk. Further south and west across the OH
Valley into the southern Mid-atlantic large scale forcing may be
weaker, but plentiful moisture and instability may still drive
localized areas of heavy rainfall.
Chenard
