FOUS30 KWBC 230058

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Thu Oct 22 2020

Day 1
Valid 01Z Fri Oct 23 2020 - 12Z Fri Oct 23 2020


...Northeast Iowa to Wisconsin and Michigan...
Only modest changes were made to the excessive rainfall outlook
with some local adjustments to the Slight Risk area to account for
the latest radar trends and HRRR guidance. The threat of heavy
showers and thunderstorms continues across areas of northeast IA
and extending northeast across central WI and into northern lower
MI for the overnight period.

The combination of approaching trough and increasingly-curved
upper jet (both along-stream and cross-stream components) will
continue to result in a dynamic as well as thermodynamic
environment conducive for heavy rainfall across portions of the
Upper Midwest and Upper Great Lakes, with a focus from northeast
IA across south-central to eastern WI, and into northern lower MI,
where areal average rainfall totals of 2-3 inches are forecast
going through early Friday morning. MUCAPEs from the CAMs get as
high as 500-1000 j/kg, while PWs peaking between 1.25-1.50"
average around 2 standard deviations above normal for late
October. Training will be possible, particularly by late this
evening and overnight, as the strong southwesterly low-level
inflow (50-65 kt at 850 mb) aligns nearly parallel while nearly of
the same magnitude of the mean 850-300 mb layer wind. This will
result in a weakening of Corfidi vectors through the evening,
which the CAMs show turning northeasterly (under 10 kts) after
02-06Z from south-central to eastern WI, then into northwest Lower
MI. The increased upwind propagation will allow for an enhanced
threat for cell-training for at least a few hours late this
evening into the overnight, and while the aforementioned
thermodynamic profiles (deep-layer instability and PW) are modest
at best, there remains a threat of some FFG exceedance given the
repeating nature of the rainfall already seen earlier this
afternoon and this evening.


Day 2
Valid 12Z Fri Oct 23 2020 - 12Z Sat Oct 24 2020


...Southeast Florida...
...20Z Update...
Based on the 12Z model guidance, it appears most of the solutions
are signally a suppression of the axis of heaviest rain/moisture
convergence which will now reside across the Florida Keys. This
will reduce the potential for heavy rain and isolated flash
flooding across the mainland of southern FL. Therefore, the
Marginal Risk was adjusted south.  

...Previous Discussion...
Deep moisture and instability focused on a low level east
southeast flow is expected to support convection capable of heavy
to locally excessive rainfall across a portion of southeast FL
during Day 2.

Tropical waves in the low to mid level easterly flow across south
FL interact with deep moisture (with precipitable water values
between 2.00/2.25 inches, which is between two and three standard
deviations above the mean) and 1000/2000 J/KG of MUCAPE to spark
convection in the low level convergent flow that crosses southeast
FL. mainly from Miami south. While there is not a strong high
resolution model consensus, given the depth of the moisture and
instability, local rainfall amounts of 4.00+ inches are possible
in this area (which is shown by the 00z NAM CONUS Nest). Should
the tropical waves focus the convection, this amounts are
certainly possible. However, if the focus is not enhanced by the
tropical wave, the convective coverage could be considerably less.

For the most part, the best convective threat in this flow regime
is expected between 23/18z and 24/00z (during the best synoptic
scale lift with short wave energy in the mid levels crossing south
FL), and the timing is typical across FL. There is some indication
from the WRF ARW runs that the threat could extend until 24/06z,
and in this flow, that cannot be discounted. In any event, the
greatest threat appears to be from Miami south, and this is shown
in much of the high resolution/regional guidance.

Portions of southeast FL (generally south of Miami) received
3.00/5.00 inches of rainfall Wed, and has generally been wet over
the past 7/14 days (with rainfall amounts in that period over 300
percent of normal). Wet antecedent conditions could play a role in
the flash flood process, and despite the high resolution guidance
spread, a Marginal Risk was placed here for Day 2 (which was
collaborated with WFO MFL).

...Lower Mississippi Valley into the Ohio Valley...
...20Z Update...
After a detailed assessment of the 12Z guidance, also decided to
not introduce a Marginal Risk area at this update given the
progressive nature of convection moving along/ahead of a surface
cold front. And while heavy rain may move over saturated soils,
the flash flooding impacts will likely be very isolated. However,
if the front slows and/or training occurs, then we will need to
reevaluate the risk potential. 

...Previous Discussion. 
Moisture and instability ahead of a cold front crossing the Ohio
Valley into the Lower MS Valley will fuel storms that could
produce heavy rainfall during Day 2. For the most part, given the
mid level flow, the front is expected to remain progressive, and
the convection should follow suite. In fact, simulated radar from
virtually all of the high resolution model solutions showed cell
motions greater than 10 knots. It is possible that 1.00 inch
rainfall rainfall could occur due to the 1.25 inch precipitable
water air in the column, but the storm motions should preclude
much higher values.

Some of the high resolution guidance shows local 1.50 inch
rainfall amounts extending from northeast AR into northern KY/far
southern IN, mainly in the 23/18z to 24/00z time frame. Normally
this would not be a concern, but portions of far southern IN/far
northern KY did receive between 3.00/4.00 inches of rainfall
during the last event, and the wet antecedent conditions could
make this area more vulnerable to even a short period heavy
rainfall. Based on the progressive nature of the convection, and
after collaborating with WFO LMK, it was decided to forego the
Marginal Risk option this forecast cycle. However, if there is a
greater model signal for heavy rainfall here, a Marginal Risk
could be needed in subsequent forecasts.


Day 3
Valid 12Z Sat Oct 24 2020 - 12Z Sun Oct 25 2020


...20Z Update...
Multi-model agreement on the potential for another resurgence of
heavy rain into portions of southern FL, especially along the
southeast coast with the better surface/frictional convergence and
onshore advection of moisture/instability into the region. Some
model guidance is quite robust with QPF totals, but given some
uncertainty with respect to the QPF coverage/amounts, decided to
forgo an upgrade to a Slight Risk.  However, if trends continue,
given the wet antecedent conditions, this may need to be
considered at subsequent forecast updates.

...Previous Discussion...
Much like Day 2, deep moisture and instability in a low to mid
level southeast flow could supply convection capable of producing
heavy to locally excessive rainfall across portions of southeast
FL during Day 3. Within the more  typical convection during the
best instability (between 24/18z and 25/00z), scattered storms
could produce hourly rainfall rates in excess of 2.00 inches,
especially from Miami south. These areas saw heavy rainfall Wed,
and have been much wetter than normal during the past 7/14 days.

Though the convection may not be focused enough during the
abovementioned time frame to pose a more bonafide flash flood
threat, the 00z non-NCEP guidance (particularly the UKMET/ECMWF)
have become more robust with the next wave in the easterly mid
level flow. Both models suggest that the next influx of deep
moisture with the wave could occur at the tail end of Day 3 (or
possibly early Day 4), but timing of the wave is still in flux.
Should it arrive just a bit earlier than these solutions indicate,
a larger area of convection capable of producing hourly rainfall
rates of 2.00 inches or greater could arrive on the southeast FL
coast between 25/06z and 25/12z.

With these areas having been heavy rainfall recently, a Marginal
Risk was placed across southeast FL for Day 3 (mainly for areas
south of the Mia metro area), which was collaborated with WFO MFL.


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