FOUS30 KWBC 020025

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM EDT Mon Jun 01 2020

Day 1
Valid 01Z Tue Jun 02 2020 - 12Z Tue Jun 02 2020

The probability of rainfall exceeding Flash Flood Guidance is less
than 5 percent.


Day 2
Valid 12Z Tue Jun 02 2020 - 12Z Wed Jun 03 2020


...Upper Mississippi Valley across the Great Lakes...
Quasi-zonal flow across the northern U.S. will be in place late
Tuesday into Wednesday with a low amplitude shortwave tracking
from the Northern High Plains into the Upper Great Lakes. At the
surface, a cold front will drop slowly south across MN/WI, prior
to a surface wave advancing along the boundary toward the Mid-MO
River Valley by 00Z. Thunderstorms are expected to form near the
surface low ~21Z, increasing in coverage through 00Z. The latest
model consensus of CAPE values showed a narrow west-east region of
MLCAPE in excess of 3000 J/kg extending from SD/NE into WI.

Initial cells in the vicinity of the tri-state region of SD/NE/IA
could show brief training and mergers as 850 mb winds begin to
increase ahead of the low. However, the passage of the cold front
should keep this threat limited to a few hours near peak heating.
Downstream, convection near the slow moving front extending across
southern MN/WI into MI will be accompanied by 30-40 kt of 850 mb
flow, favoring repeating cells within the unidirectional LFC-EL
layer, beneath the right entrance region of a 80-100 kt upper
level jet max. 3 hour flash flood guidance (FFG) values of 1.5 to
2.5 inches are common across much of the Marginal Risk area,
extending from the Missouri Valley into Lower Michigan. While the
details remain uncertain, there remains good multi-model support
for QPF to exceed FFG from the NE/IA/SD border into portions of
Lower Michigan, but with some north-south spread along with minor
timing differences.

...Central and Southern Rockies...
Diurnally driven convection forming on the east side of a
mid-level ridge along the higher terrain and near a weak mid-level
perturbation will focus for thunderstorms from the Central and
Southern Rockies into the adjacent High Plains Tuesday
afternoon/evening. LFC-EL layer mean winds are only forecast to be
5-10 kt or less which will contribute to slow storm motions.
However, moisture values are only expected to be weakly anonymous
and CAPE values generally under 1000 J/kg. There is little to no
signal for excessive rainfall in the latest guidance with the 12Z
NMMB, ARW, NSSL and NAM_nest showing potential for localized 24
hour totals in the 1 to 1.5 inch range, with most of this likely
falling between 18Z and 03Z. While this area will continue to be
monitored as newer model guidance comes in, no excessive area
continues for this region of the country.  


Day 3
Valid 12Z Wed Jun 03 2020 - 12Z Thu Jun 04 2020


...Mid Mississippi Valley to the Ohio Valley...
Zonal flow will be in place across the northern tier of the U.S.
for Wednesday into Thursday, which typically makes for a
challenging forecast for Day 3 during the warm season. A surface
cold front is expected to sink southward into the OH Valley during
the afternoon and stall into morning. Precipitable water values
are forecast to range between 1.5 and 2 inches along the boundary
translating into standardized anomalies of +2 to +3 across the
Mississippi into the Ohio Valleys. While some degree of ongoing
activity appears likely ahead of the cold front early Wednesday,
this activity is expected to be on the decline. The main flash
flood threat will come later in the afternoon as instability
increases, with a 12Z NAM/GFS/ECMWF consensus supporting anywhere
from 1000-3000 J/kg CAPE from the MS Valley into IL/IN/OH into
portions of WV.

Mean storm motions are forecast to parallel the orientation of the
frontal boundary as 850 mb flow rises into the 20-40 kt range.
However, the extent of these relatively stronger 850 mb winds
appears to be low compared to the previous day, likely resulting
in more of scattered nature to the flash flood threat, especially
from the MO Valley into IN where forecast speeds stay near or
below 20 kt. However, it is across western sections of the
Marginal Risk area that should have greater instability available.
Farther east, where lower level winds are expected to be higher,
CAPE values will be somewhat reduced. Upper level support will be
in the form of an upper level speed max approaching 100 jt,
forecast to track through Lower Michigan into PA. Three hour flash
flood guidance (FFG) values of 1.5 to 2.5 inches cover a broad
section of the Midwest and OH Valley. Though FFG values are not
particularly low near the Kansas City Metro, portions of this area
has received over 300 percent of normal rainfall over the past
week which may leave portions more susceptible to flash flooding.

...Lower Mississippi Valley into Florida...
21Z update...No changes to previous thinking below based on the
latest model guidance...

A developing long wave trough extending from the Lower MS Valley
into the central Gulf of Mexico move slowly eastward during Day 3,
possibly closing off over LA/MS by the end of the period. Ahead of
the mid level system, deep moisture and instability will combine
to support convection developing along the Gulf Coast from MS/AL
to the FL west coast. From this distance, it appears as though the
strongest convection (and the heaviest rainfall) remains just
offshore, possibly affecting the FL west coast most directly.

Though there is a signal for heavy rain with this feature, the
threat remains nebulous, and for that reason, no excessive area
was assigned. Should the model signal for heavy rainfall increase
for the FL Panhandle down along the FL west coast, a Marginal Risk
could be needed in later forecasts.


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