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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1458Z Aug 20, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1058 AM 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 FROM THE EASTERN
GREAT LAKES REGION TO THE CENTRAL APPALACHIANS...

...Eastern Great Lakes to Mid Atlantic Region..
A shortwave trough rotating rough the base of a larger-scale
trough in the Great Lakes should be moving east/northeast on
Tuesday before it gets drawn northward by the larger scale
dynamics. Southerly flow, deep moisture and the formation of a
mid/upper level jet streak will set the stage for another round of
afternoon and night time storms capable of producing heavy
rainfall. 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.  Maintained
the Slight Risk across parts of the Eastern Great Lakes into parts
of Pennsylvania where the combination of heaviest rainfall
forecast and lowest 1- and 3-hour Flash Flood Guidance.  This area
had recently been soaked in the past week making the potential for
additional flooding greater.  To the south, the model guidance had
backed off on rainfall amounts and the degree of instability
suggests that a Marginal Risk would be more appropriate than a
Slight Risk.

...Desert Southwest....
A plume of deep moisture begins to move back into the Southwest
U.S. on the western periphery of deep high that drifts east. 12Z
consensus is for precipitable water values of 1.75 inches drawn
north through terrain from southwest Arizona Tuesday. 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. Hoisted a Slight Risk over portions of
Arizona where the moisture will be deepest and where shortwave
energy rotating around the periphery of a mid/upper level high to
the east.  There remains some uncertainty concerning about timing
but felt enough ingredients were in place in and near the complex
terrain of southern Arizona.

...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 locally heavy rain over the CO/KS High Plains
and OK/TX Panhandle. The models got into a little bit better
agreement on placement of the nocturnal convection, but some
spread still remained.  Continued to indicate a Marginal Risk area
in this area.

Bann


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....

...New England...
Placed a Marginal Risk over parts of New England as moist flow
from the Atlantic Ocean gets drawn inland by a deep-layered area
of low pressure moving into Canada.  There should be plenty of
confluent flow at the low levels being drawn into the hilly
terrain (especially in northern New England).  The big question
will be how much instability will be in place over the region.  At
this point, think the instability will be not be that impressive
and will be waning during the period.  Thinking is that this
precludes a Slight Risk at this point but will re-evaluate this
area in subsequent outlooks.

...Southwest U.S....
Moisture being drawn northward into the region on Day 2 will
continue to move into the area from the south, with the models now
showing precipitable water values running at least 2 standardized
anomalies moving northward across Arizona and Utah before being
turned anti-cyclonically towards parts of Colorado/New Mexico. 
Opted for a broad Marginal Risk given the weak forcing.

...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.  The biggest disagreement was on
whether the heaviest rainfall will be over southwestern portions
of Kansas/western Oklahoma or farther to the east.  Given the
placement of the strongest winds, focused the area closer towards
the western side of the guidance.  This was also the area of best
equivalent potential temperature advection,

Bann



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