Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
140 PM EDT Fri Jul 19 2019
Day 1
Valid 16Z Fri Jul 19 2019 - 12Z Sat Jul 20 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MIDWEST/GREAT LAKES & NORTHERN MID-ATLANTIC STATES...
...Northern Plains...Upper Mississippi Valley into the Great
Lakes...
A northward shift was made to the previous elongated marginal and
slight risk areas stretching from portions of the Northern
Plains...Upper Mississippi Valley and into the Great Lakes per 700
hPa temperature forecasts and corresponding, logical shifts in the
12z mesoscale guidance. The surface frontal boundary is expected
to become oriented in a west southwest to east northeasterly
direction across these regions this period and remain a focus for
additional convective development Friday afternoon into Friday
night and the early hours of Saturday. Hi res model consensus is
for activity in northeastern-most SD to broaden east-west with
time into a broader organized area of convection to form along and
just north of the west to east oriented front in the vicinity of
central MN late Friday afternoon...ahead of a surface wave moving
along the front...and push downstream across northern WI...the
U.P. and northern L.P. of MI.
...Northern Mid-Atlantic States...
Thunderstorms that moved out of MI into southeast Ontario are
expected to move into western NY in the next 2-3 hours, with a
tail of convection possible across southeast MI in an area where
the guidance suggests a weakness in the mid-level cap. The
activity should move roughly east to east-southeast with time.
The mesoscale guidance has a scattered but agreed upon signal for
local 3" amounts from Detroit MI into northeastern-most OH,
northwest PA, and western NY. Added a slight risk of excessive
rainfall to this area.
...Southeast...
Pulse convection within an uncapped atmosphere with negligible
effective bulk shear is flaring within a moist and unstable
airmass: precipitable water values are 2-2.25" and ML CAPE of
2000-3000 J/kg. Thunderstorms should pinball across the region
with local amounts of 3-4" possible where storms collide/merge.
Drew in a marginal risk area as a number of flood advisories and a
flash flood warning have come out within this area as of late.
Roth/Oravec
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Fri Jul 19 2019
Day 2
Valid 12Z Sat Jul 20 2019 - 12Z Sun Jul 21 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST CENTERED ON IOWA AND SOUTHERN WISCONSIN...
...Midwest into Michigan...
2000 UTC Update -- Based on the model trends, including the
high-res CAM windows which now extend through the entire day 2 ERO
period (12Z Sat-12Z Sun), have adjusted the Slight and Marginal
Risk areas a bit farther south compared to the previous outlook.
This aligns well with the favorable pre-frontal thermodynamic
profile per the growing model consensus (robust deep-layer CAPE
along an axis of PWs aoa 2.00"). Strong deep-layer shear should
limit the potential for cell training; however, the strength of
the thermodynamic profile will likely lead to robust short term
rainfall rates -- likely aoa 2.0"/hr underneath the strongest
cells.
Previous discussion...
Early Saturday morning, ongoing isentropic ascent across the
stationary boundary will be growing upscale to a solid MCS with
and MCV crossing the North Central Great Lakes with ample
mid/upper level support. Unseasonably strong 250mb jet will shift
northeast centered over Ontario which will provide solid right
entrance region dynamics across the Upper Midwest and support
height-falls and tail end of shortwave energy to amplify and press
eastward across the Upper Midwest. This will help the front to
advance south through the day. The challenge will be the
convective initiation during the afternoon which will be largely
focused along the boundary, but also any residual outflow
boundaries left during the aforementioned convective activity.
There is no shortage of moisture and instability pooling along and
just south of the front. Precipitable water values surge as high
as 2+ inches in the early morning associated with the MCS into the
Great Lakes region. During the afternoon/evening with strong
moisture transport out of the Gulf and strengthening low level jet
there is strong evidence of 2.25" precipitable water centered in
Iowa into Wisconsin which is 2 standard deviations above the mean.
Instability is also a factor as MUCAPE values climb to over 4500
J/kg across this region.
Areal average QPF based on a blend of the GFS, ECMWF and our
in-house bias corrected model results in values of 1-1.5 inches
with locally higher amounts expected. While training is possible
as determined by the mean wind aligning with the corfidi vectors
for a time, cold pool/outflow dominated convection in the late
evening into the overnight hours may limit this potential. Also,
mean wind between 850mb-300mb is generally 20-25 knots which is
another limiting factor, though it is hard to discern the
propagation pattern of convection this far out. Regardless, with
expected convection and hourly rain rates at times well above 1.5
inches/hour, anticipate localized flash flooding across portions
of Iowa into Wisconsin during the late afternoon through the
overnight hours. This is further evident by flash flood guidance
as low as 1 inch/hour. Therefore, kept the Slight but modified
for per the latest QPF and model trends. If models come into
better agreement in terms of placement of the heaviest rain and
the overall rain amounts, especially if they go up, a Moderate
Risk may need to be considered.
...Front Range of Colorado/High Plains of NW Kansas...
As the front drops south across the High Plains, remaining
moisture banked up along the Range will be available for diurnal
convection. Therefore any thunderstorms that do form assisted by
upslope flow has the potential for producing heavy rainfall.
Initially slow development moving off terrain with chaotic cell
motions and intersections of outflow boundaries may pose
short-term stalled downdrafts capable of isolated rainfall totals
of 2"+. While confidence is not particularly high in one given
location, the environment is supportive of this flooding threat
and so a Marginal Risk was maintained to incorporate this threat,
particularly along the Front Range of CO and SE WY.
...Portions of the Lower Tennessee Valley to the Gulf Coast...
Included a Marginal Risk across eastern MS, much of AL, and far
western portions of GA and the western FL Panhandle. A weak
mid-level shear axis/vort lobe is indicated on the global runs,
and is reflected in the sensible data with the uptick in QPF. The
activity will be heavily weighted toward the peak heating hours
later Sat morning through the afternoon given the weak lapse rates
aloft (<6.0 C/KM between 700-500 mb). Moreover, the 0-6km bulk
shear values aob 20 kts would suggest more pulse-type activity --
which would limit the duration of any intense rainfall rates.
Hurley/Pagano
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 PM EDT Fri Jul 19 2019
Day 3
Valid 12Z Sun Jul 21 2019 - 12Z Mon Jul 22 2019
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
CENTRAL AND SOUTHERN ROCKIES AS WELL AS THE CENTRAL PLAINS INTO
THE OHIO AND TENNESSEE VALLEYS...
...Southern Rockies...
As a front continues to dive south through the Central Plains and
Mid-MS Valley, a trailing boundary along the Front Range will act
as a catalyst for convection during the afternoon into the
overnight hours Sunday into Monday. Mid-level high pressure will
build overhead with impulses rounding this feature which too will
promote vertical motion. At the surface, as the Canadian high
builds south into the Northern Plains, easterly flow will enhance
upslope during the afternoon. Moisture that pooled south of the
aforementioned front will bank against the high terrain with
instability of over 2000 J/kg available during the
afternoon/evening hours. These factors should promote convective
initiation along the windward slopes of the Rockies in CO and NM.
Given fairly weak flow, convective cells should have fairly slow
storm motion along the slopes with more erratic propagation as
they grow upscale. Regardless, expect heavy rainfall is given
decent moisture transport across this region. Hourly rain rates
may approach 1 inch/hour which is just shy of the one hour FFG
(1.5 inches) across this region. Again, given the uncertainty
with respect to the evolution and storm motion, feel there could
be some locations that see as much as 2-3+ inches when all is said
and done. Thus a Marginal Risk was introduced across this region.
If QPF amounts come up, a Slight Risk may be justified.
...Central Plains, OH/TN Valley...
2000 UTC Update -- Based on the latest model trends, removed the
Marginal Risk area north of ~42N, as the expectation is that the
surface front and best pre-frontal instability and moisture
transport will have pushed to the south.
Hurley
Previous Discussion...
A front will continue to slowly move south and east Sunday into
early Monday. While all model guidance illustrates this feature,
the overall orientation and timing of the front varies. The GFS
and the ECMWF phase the mid-level energy round the closed low in
Ontario differently which plays a large role in the surface front
and expected organized convection. Regardless, the focus for
shower/thunderstorm activity will be along this feature. Moisture
and instability will pool right along and south of the boundary,
with precipitable water as high as 2 inches and MUCAPE values over
3500 J/kg. Since there is bit of uncertainty with respect to the
mid/upper level pattern, it is hard to discern if organized
convection will actually occur. The 00Z GFS has a stronger signal
for mid-level vorticity and upper level divergence across the
Central/Southern Plains into the Mid-MS Valley as compared to 00Z
ECMWF or even the 00Z NAM which is reflected in the QPF output.
The GFS is by far higher in its precipitation amounts given its
overall synoptic forcing resultant MCS. While there is little
model consensus for this and thus no strong signal to suggest such
organized convection along the frontal boundary, it is still taken
into consideration given many of the ingredients are available.
We will have to monitor the evolution of the upper trough axis and
mid-level energy which would most certainly help to initiate
organized convection. At this time, forecasting weak mid-level
impulse atop a surface boundary which should promote convection
along the front from the Central Plains across the OH/TN Valley
and into the northern tier of New England.
Areal average QPF generally ranges between 0.5-1.5 inches with
locally higher amounts associated with convection. The most
sensitivity areas will likely be across portions of the OH/TN
Valley into the central Appalachians where FFG values are as low
as 1 inch in 3 hours. A Slight Risk was considered across this
region, though confidence in amounts and coverage are too low at
this point. Also, with respect to the potential MCS across the
Central/Southern Plains, the soils are not as sensitive per FFG
and the NWM Soil Moisture. Therefore, felt a Marginal Risk would
suffice for this forecast package with the understanding that a
Slight Risk will likely be hoisted if models come into better
agreement about organized convection and thus higher QPF amounts.
Pagano
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
