Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Mon Aug 20 2018

Day 1
Valid 15Z Mon Aug 20 2018 - 12Z Tue Aug 21 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE MID TO
LOWER MISSISSIPPI VALLEY, LOWER OHIO VALLEY AND LOWER TENNESSEE
VALLEY...

1500 UTC Update...
Adjustments based on recent observation trends as well as the
hi-res model runs include the western extension of the Marginal
and Slight Risk areas back across southern Missouri into the
southwestern portion of the state.  Observations show developing
convection ahead of wavy cold front pushing into the region, with
the latest hi-res guidance showing an increasing signal for heavy
rainfall rates developing across the region by the late morning
and continuing into the afternoon hours as the front pushes
through.

Elsewhere, made only minor adjustments based on the recent trends.

Previous discussion...

...Central to northern Iowa..southeast Minnesota...southern
Wisconsin into northern Illinois...
The latest model guidance is very similar with the evolution of
the strong closed low initially over the lower Missouri Valley.
All models show a steady east northeast push into the mid
Mississippi Valley region Monday and toward the southwest Lower
Lakes region Monday night/early Tuesday.  With good mass field
agreement among the models...there is also good agreement on an
axis of heavy precipitation associated with what will likely
remain a well defined comma head/deformation precipitation band to
the north of the track of this mid to upper level closed low. 
Precipitation areas in this comma head/deformation band will be
slow moving...pivoting this morning over central to northern
Iowa...and this afternoon/evening from southeast
Minnesota/northeast Iowa into southern Wisconsin and far northern
Illinois.  In this pivoting regions..there will be widespread
areal average totals in the 1-3" range...with isolated heavier
totals reaching 4-6".  No significant changes were made to the
previous outlook for this forecast period.

...Middle to Lower Mississippi Valley...Lower Ohio Valley...Lower
Tennessee Valley
To the south of the expected well defined comma head/deformation
precipitation band with the strong moving into the mid Mississippi
Valley today...additional pre-frontal/frontal convection will
likely reform this afternoon across the Mid to Lower Mississippi
Valley in a region of defined upper difluence and boundary layer
convergence.  This activity is expected to then push eastward into
the Lower Ohio and lower Tennessee Valley region by Monday
evening.  This activity will likely be fairly progressive---but
with unstable conditions ahead of this front...mu-cape values
1000-2000+ j/kg and pw values above average...locally heavy
rainfall amounts possible. Changes to the previous outlook for
this period were to expand the slight risk area southward into
more of the lower Mississippi Valley. 

...Southern Mid-Atlantic...Southern Appalachians into the Central
Gulf Coastal region...
A broad marginal risk area depicted day 1 in the moist unstable
airmass from the Central Gulf Coastal region into the Southern
Appalachians and Southern Mid Atlantic.  Over the Central Gulf
coastal region...the previous marginal risk area was expanded
farther to the south into the Central Gulf Coastal Region to cover
areas that have received heavy rains over the past few days from
far eastern Louisiana into southern Mississippi and far southern
Alabama..resulting in the lowering of ffg values and increasingly
anomalous stream flow.  Additional localized heavy rains possible
across these regions this period. Farther to the east...the
marginal risk area was extended east from the Southern
Appalachians and across the remainder of North Carolina. 
Additional heavy rains possible in the axis of instability to the
south of the west to east front across the Mid-Atlantic. 

...Northern Rockies...
The mid to upper level trof moving east from the Pacific Northwest
into the Northern Rockies region will enhance upper difluence
across the Northern Rockies this afternoon.  This and post frontal
east northeasterly flow will support increasing precipitation this
afternoon from north central Idaho into western to southwestern
Montana and northwest Wyoming.  With pw values rising to 1 to 1.5+
standard deviations above the mean and the well defined upper
difluence...locally heavy rainfall amounts and isolated runoff
issues are possible.


Pereira/Oravec


Day 2
Valid 12Z Tue Aug 21 2018 - 12Z Wed Aug 22 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL NORTH FROM THE
CENTRAL APPALACHIANS AND IN ARIZONA...

...Eastern Great Lakes to Mid-Atlantic Region..
A shortwave trough will be drawn northeast across the Great Lakes
to the base of a larger-scale trough extending from Hudson Bay
Tuesday. Southerly flow, deep moisture and the formation of a
mid/upper level jet streak will set the stage for afternoon and
night time storms capable of producing heavy rainfall south from
the eastern Great Lakes. A Slight Risk was maintained over
portions of the Eastern Great Lakes to the central Appalachians
given the Precipitable Water Values of 1.75 to 2.00 inches (2 to
2.5 standardized anomalies above climatology) expected. Eastern
portions of this area have had above normal rainfall in the past
month.

Farther south over the southern Appalachians and VA/NC Tidewater
forcing and instability remain limited, so a Marginal Risk was
maintained despite well above normal moisture.

...Desert Southwest....
A plume of deep moisture continues to advance north on return flow
west of the high centered over NM/TX Tuesday. Precipitable water
values of 1.75 inches drawn from southwest Arizona will cross the
higher terrain of central AZ. Precipitable Water values of an inch
or more (generally two standard deviations above normal in this
high terrain area) will reach the southern Wasatch by late Tuesday
afternoon. This will support scattered late day and evening
convection that will be capable of producing locally heavy
rainfall across the four corners region. Maintained a Slight Risk
over portions of Arizona where the moisture will be deepest and
where shortwave energy is expected to rotate around the periphery
of a mid/upper level high to the east. Will have to continue to
monitor smaller scale features and timing for the most at risk
areas.

...South-Central High Plains...
A shortwave riding zonal flow south of the northwestern CONUS
trough will encounter the nocturnal low level jet over the Plains
Tuesday night. 1.5 inch PW (1 to 1.5 standard deviations above
normal) will allow thunderstorms to organize (most 12Z guidance
suggest two tracks across KS and the Panhandles). A broad Marginal
Risk continues to cover both of these areas.

Jackson


Day 3
Valid 12Z Wed Aug 22 2018 - 12Z Thu Aug 23 2018

...MARGINAL RISK FOR EXCESSIVE OVER NEW ENGLAND...CENTRAL
PLAINS...AND THE SOUTHWESTERN U.S....

...Northern New England...
Maintained a Marginal Risk over northern New England as moist flow
from the Atlantic Ocean gets drawn inland by a developing low
pressure moving into Canada from the eastern Great Lakes
Wednesday. Plenty of confluent flow at the low levels being drawn
into the hilly terrain of northern New England is expected.
Instability looks quite limited per the 12Z GFS/ECMWF, so the
Marginal Risk was maintained for now.

...Southwest U.S....
Moisture being drawn northward across AZ shifts east a bit
centering on the four corners. Ample moisture (generally 1.5
standard deviations above normal) and instability will allow heavy
thunderstorms. However, weak forcing suggests widely scattered
heavy rain with the Marginal Risk maintained for now.

...Central Plains...
Convection forming over the Central/Southern Plains early
Wednesday morning will taper off at the time of convective minimum
over parts of Kansas or Oklahoma, followed by an additional round
of convection forming during the latter portion of Wednesday and
the early-hours of Thursday morning as 850 mb winds ramp up in a
low-level jet of 25 to 35 knots. 12Z guidance converged a bit on a
QPF max just north of the KS/OK border which has been fairly wet
over the past week. Maintained a Marginal Risk.

Jackson


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