Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Sat Aug 18 2018
 
Day 1
Valid 12Z Sat Aug 18 2018 - 12Z Sun Aug 19 2018 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE NORTHERN PLAINS FROM NORTHEAST WYOMING/SOUTHEAST MONTANA INTO 
CENTRAL SOUTH DAKOTA.  THERE IS A SLIGHT RISK OF EXCESSIVE 
RAINFALL ACROSS PORTIONS OF THE CENTRAL TO SOUTHERN PLAINS FROM 
EASTERN COLORADO..SOUTHWEST KANSAS..THE TEXAS-OKLAHOMA PANHANDLE 
INTO CENTRAL OKLAHOMA...

...Eastern Colorado...western Kansas...TX/OK panhandle into 
central Oklahoma...
An increasingly difluent upper level pattern will be spreading 
eastward across much of the High Plains this period as height 
falls amplify from the Northern to Central Rockies into the High 
Plains.  This along with strengthening low level inflow  after 
0000 UTC Sunday into the west to east oriented frontal boundary 
forecast to lie across the Southern Plains will support increasing 
overrunning convection to the north of this boundary late Saturday 
afternoon--continuing into Saturday night/early Sunday morning.  
While there is some spread among the latest guidance with the axis 
of the heaviest totals...there is a consistent signal for the 
potential for organized convection...heavy rains and localized 
runoff issues. While there has not been significant precip across 
most of this region recently...the national water model does show 
stream flow anomalies rising to much above to  high flow Saturday 
evening/night with this heavy rainfall potential.  Hi res guidance 
is showing max localized hourly rainfall amounts of 1.5-2" at this 
time.  There were only some minor changes to the slight risk area 
from the previous outlook with a southward expansion to cover some 
of the model qpf spread.

...Northeast Wyoming...southeast Montana into central South 
Dakota...
Model consensus is for a northern precipitation max closer to the 
primary mid to upper level center associated with the amplifying 
area of height falls moving out of the Northern to Central Rockies 
and into the High Plains.  Increasingly unstable conditions 
Saturday afternoon over the Northern Plains...mu-cape values 
rising to 2000-2500 j/kg will support convection and potentially 
heavy rains pushing eastward from northeast WY into western to 
central South Dakota.  This will then be followed by a slower 
moving area of comma head/deformation precipitation forming across 
southeast MT/northeast WY into western SD...which may pivot across 
these areas into the early morning hours of Sunday. Much of this 
area has seen above average precip over the past week...lowering 
ffg values and increasing runoff risk from these two precipitation 
events.  The slight risk area from the previous outlook was 
extended westward into northeast WY and southeast MT to cover 
areas where precip amounts have been above normal over the past 
week.

...Lower Mississippi Valley...Upper Tennessee Valley...Southern to 
Central Appalachians into the Mid-Atlantic...
A surface frontal boundary expected to move slowly southward 
through the northern Mid-Atlantic---Central Appalachians into the 
Tennessee Valley and Lower Mississippi Valley.  Mid to upper level 
troffing pushing eastward across these areas will enhance uvvs in 
the axis of above average PW values that will persist along and to 
the south of this front.  This should support a fairly broad 
region of scattered convection and localized heavy precip amounts. 
 A broad marginal risk area was maintained...decreased in size 
over portions of the northern Mid-Atlantic to reflect model 
consensus for a minima in precipitation there.

Oravec
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt