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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2009Z Aug 07, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Sun Aug 07 2022

Day 1
Valid 1738Z Sun Aug 07 2022 - 12Z Mon Aug 08 2022

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHEAST IOWA, SOUTHERN WISCONSIN AND NORTHWEST ILLINOIS...

...Great Lakes...
1600 UTC Update -- Per collaboration with WFO LOT, have expanded
the Moderate Risk a little farther E-SE to include ORD and much of
the Chicago metro area (northern 2/3rds...north of I-80). This
based of of current convective trends with the weakening MCS and
with the potential for more heavy rainfall overnight in advance of
the next shortwave/main surface front. Have also expanded the
Slight Risk a bit into northern IN and southwest Lower MI based on
the latest trends with the guidance (especially CAMs), along with
the increased exceedance probabilities per the 12Z HREF.

Previous dicsussion...

An ongoing flash flood risk exists to start the new day 1 period
across portions of eastern IA, southwest WI and northwest IL.
Northwest to southeast Training convection will likely continue
into the morning hours, although the organization and intensity of
activity should generally be on a downward trend after ~15z or so.
Leftover outflows and/or MCV will track downstream across the
Great Lakes through the day and should trigger some additional
convective development across portions of MI/IN and OH this
afternoon and evening. This convection should generally be
progressive, but quick bursts of heavy rainfall and some brief
training could still result in isolated flash flood concerns.

Then attention turns to what happens tonight over portions of
IA/MN/WI and northern IL. Much stronger synoptic forcing pushes
across the area, with a well defined mid level shortwave and upper
jet favorably positioned to the north. Low level moisture
transport ramps up as well, increasing lower level convergence and
helping lift the stalled out boundary and instability pool back
north. The system is progressive as a whole, which will put a
limit on rainfall duration. However thickness diffluence and weak
Corfidi Vectors support some backbuilding/training into the
increasing low level jet...so some increased rainfall duration
appears probable. Meanwhile PWs should remain over 2" and near
record values, with a continued connection in the mid/upper levels
to the previous Monsoon moisture that made its way around the
mid/upper ridge. Thus the ingredients seem to be there for another
round of potentially significant rainfall tonight.

There remains above average uncertainty with the details of this
overnight convection. The better forcing should end up over
eastern MN into central and northern WI, but instability recovery
here is questionable. So while we will see locally heavy
convection here, the lower instability and progressive cell
motions could limit event total magnitudes. If there is some
better west to east training it will probably be on the southern
edge of the forcing, along the instability and low level moisture
gradient. Convection this morning may play a role on where this
ends up and exactly how strong of a gradient we end up with.
Possible that the convection this morning limits recovery to some
degree, keeping the upstream instability pool a bit muted and
reducing the significant training threat tonight. However
certainly a chance we are able to regenerate significant
instability and a stronger convergence/instability axis results.
In this case a more pronounced training scenario could evolve
somewhere across northeast IA, southern MN and central/southern
WI. Given antecedent rainfall, this scenario would result in the
likelihood of more widespread hydrologic concerns.

We opted to expand the MDT risk with this update into portions of
northeast IA, southwest WI and far northwest IL. This is based on
the rain that has already fallen, the additional rain that is
expected through mid morning, and then the additional convection
forecast tonight. With hydrologic conditions becoming more
sensitive with time today into tonight, at least scattered
instances of flash flooding appear likely. The threat of a higher
end event is probably dependent on what happens tonight, and as
mentioned above that remains uncertain. Either way locally
significant flash flooding could still evolve.

...West...
An isolated to scattered flash flood risk exists from portions of
AZ/NM into CO. One Slight risk this period will be focused over
portions of northeast NM into eastern & central CO.  This is
likely being driven by an approaching front enhancing low level
convergence and some easterly upslope flow in the wake of the
front within an exceptionally moist airmass for the area of
1.25-1.75". Combine that with upper level divergence in the right
entrance region of the upper jet and the synoptic setup looks
pretty good for heavy rainfall. Sufficient instability and PWs
increasing over the 90th percentile should support heavy rainfall,
which combined with the expected convective coverage, continues to
support the Slight risk over this area. HREF 3" neighborhood
probabilities are over 40% across this area, indicating the
likelihood of scattered 3"+ rainfall totals with this event. FFG
exceedance probabilities from the HREF are also over 40%, and
portions of this area received heavy rainfall yesterday. Thus
seems likely that scattered flash flooding will occur today over
this region, with a minor eastward adjustment to the Slight risk
to account for HREF trends.

A Slight risk remains over portions of central and southern AZ. 
PWs are forecast to increase quite a bit over southern AZ compared
to Saturday, with ~1000-2000 J/KG of CAPE forecast as well. The
overall synoptic pattern will remain favorable for terrain based
convection, potentially aided by any shortwaves rounding the base
of the upper ridge, with some eventual propagation into the valley
locations given mean easterly flow. The overall signal today
appears better than Saturday, so the Slight risk seems warranted.
The risk was expanded a bit based on trends seen in the 00z HREF.

...East...
A Marginal risk was maintained across portions of the OH and TN
valley into the central and southern Appalachians. Continued
isolated to scattered convection is expected today, and while the
coverage and magnitude of rainfall will probably be less than
Saturday, indications are that localized 1-2" rainfall will again
be possible (locally 3" in the southern/central
Appalachians)...which may approach or exceed some of the
compromised FFG values over sensitive hydrologic areas.

Hurley/Chenard


Day 2
Valid 12Z Mon Aug 08 2022 - 12Z Tue Aug 09 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST U.S. AND THE NORTHEAST...

...Southwest into the Western Great Basin...Sierras...and Oregon
Cascades...
2000 UTC Update -- Minimal changes made to the Day 2 ERO out west,
other than expanding the Marginal Risk area well north-northwest
to include western NV, northeast CA, and parts of western-central
OR from the eastern Cascades to the High Deserts. The guidance
depicts a northward expansion of the monsoonal convective QPF
footprint east of the offshore closed low and west of the upper
ridge axis -- where the synoptic scale along-stream forcing, while
subtle/weak overall, would support such expansion of
isolated-scattered convection during Monday afternoon-evening.
MUCAPEs peaking between 500-1000 J/kg on average along with the
increase in PW standardized anomalies (+ 2 to +2.5 per the GEFS)
will support peak sub-hourly rainfall rates of 0.50+ inch
underneath the strongest cores, supported by the uptick in HREF
exceedance probabilities.

Previous discussion...

Isolated to scattered convection will be likely Monday/Monday
night across portions of southern CA into AZ, NM and southern CO.
Overall seems more in line with a typical Monsoon day, so likely
isolated flash flood issues, but currently not looking like
anything too widespread or organized. Model and ensemble QPFs seem
a bit lower overall compared to Sunday, and overall think a
Marginal risk should generally suffice. Although with PWs and
instability remaining elevated, will need to continue to monitor
the potential for a Slight risk over portions of CA and AZ. 

A Slight risk remains over portions of northern NM into far
southern CO. This is probably a lower end Slight at this
point...but there is some clustering of higher QPF amongst the
deterministic and ensemble solutions, and soil conditions are more
saturated over this area, so opted to maintain the Slight risk
here.

...Ohio Valley into the Northeast...
2000 UTC Update -- Few changes to the Day 2 ERO in these regions
based on the guidance trends. First, given the faster eastward
progression per the multi-model consensus, have pulled the western
periphery of the Marginal Risk area eastward across the Upper
Midwest (MI specifically). Also, have added the remainder of
northern ME to the Slight Risk area, again based on the (wetter)
guidance trends. 12Z HREF neighborhood probabilities of 24hr QPF
exceeding 3" reach 50+ percent across portions of northwest ME,
with a broad area of 80%-plus probs of 24hr QPF > 2" over northern
VT into northern ME.

Previous discussion...

A cold front moving across the mid MS Valley, OH Valley and
Northeast will likely pose at least an isolated flash flood risk
on Monday. The southern extent of this front from MO into the OH
valley should have 2" PWs to work with along with a gradual
slowing of the frontal progression as the stronger forcing shears
off to the north. Upper level flow over this corridor will be
quite diffluent as well, which should help with the maintenance
and organization of convection to some degree. Thus could end up
with some locally heavy rainfall this period and at least some
flash flood risk. Could envision a Slight risk over parts of this
area...but at this point model QPF forecasts don't quite seem to
that level...and while the front slows, it is currently forecast
to still progress southward enough to generally limit the duration
of favorable ingredients. Thus at this point think a Marginal risk
should suffice.

Over the Northeast we will see even greater forcing, with a closer
proximity to the mid level wave and upper jet. PW values should be
pretty impressive...with the same moisture source that will be
over the Plains today and Great Lakes tomorrow reaching the
Northeast by Monday. While pretty well removed from the source by
this point, at least some of this moisture could be traced back to
the Monsoon surge over the Southwest being pulled up and over the
ridge and then eastward across the northern tier of the country.
Thus PW values could end up approaching record values over
portions of the Northeast this period.

The system is progressive, which will certainly be a limiting
factor for excessive rainfall. However some indications that a
corridor of enhanced 850mb moisture transport will be strung out
enough to allow for an elongated corridor of increased
convergence. This could allow for some west to east training of
cells as the system as a whole progresses eastward. So even with
the quick movement, rainfall duration could be long enough in
spots to result in some 2-3" rainfall totals across portions of
northern New England. There remains some uncertainty with the
exact location and magnitude of highest rainfall...but with not
much change in the guidance from yesterday, and continued support
of a Slight by our CSU machine learning ERO products, we will
maintain the Slight risk. Will continue to monitor and some chance
we will eventually need a southward expansion or shift of this
outlook into portions of central New England. Overall though this
currently appears to be on the lower end of the Slight risk
range...as it remains unclear whether the better
forcing/convergence will overlap with the better instability. Some
chance the higher QPF ends up in an area of limited instability,
and thus we see lower rates. While in the area of higher
instability further south cells stay more disorganized and quick
moving, keeping the flash flood risk only isolated. So certainly
will need to monitor trends.

Hurley/Chenard

Day 3

The Day 3 outlook will be updated by 2030Z.


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