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Excessive Rainfall Discussion
(Latest Discussion - Issued 0122Z Aug 18, 2018)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
921 PM EDT Fri Aug 17 2018

Day 1
Valid 01Z Sat Aug 18 2018 - 12Z Sat Aug 18 2018


...Great Lakes into the Northern Mid Atlantic---Southern Tier of
NY state...northern NY state into Northern New England...
Organized frontal/pre-frontal convection occurring this evening
ahead of the trailing cold front from the low moving into northern
New England. Much above average precipitable water
values...standard deviations of 2-3+ above the mean for
mid-August...will support potential for widespread heavy precip
totals and the threat of runoff issues in an near terrain. With
increasing cell coverage continuing...pockets of 2-4" of rainfall
are probable. 

...Oklahoma and the Ark-La-Tex...
A more focused flash flood risk may be starting to evolve across
portions of eastern OK into western AR at this time. 
Strengthening low level confluence near the MU CAPE gradient in
southwest AR appears to be focusing this renewed activity.  The
850mb moisture transport increases into this axis overnight...and
there is a pretty good model signal for nocturnal development
here. This activity would tend to move southeastward into northern
AR, with some training possible over areas that received rain over
the past several hours.  Soils saturating across portions of
central AR into MS would be a growing concern with time.

...Lee of the Central Rockies into the Southern High Plains...
Organized convection is enhancing as it pushes southeastward this
evening across eastern Colorado...northeast NM into the
Texas/Oklahoma panhandle region within a region of upslope flow to
the southeast of an upper level trough.  A slight risk was
maintained across these regions...with the 12z HREF guidance still
showing localized 3" amounts across this corridor where cell
mergers occur.

...Southwest into the Southern Great Basin...
Widespread scattered monsoonal activity is slowly fading within a
persistent axis of above average PW values across these regions. 
A trough at 700 hPa has been keeping storms east of southern NV. 
With an upper trough moving north-northwest of the region, there
is a non-zero threat of a brief uptick of convection in the
06z-09z period, so have maintained a marginal risk.

...Portions of the Northern Rockies...
A marginal risk area over the Northern Rockies from central Idaho
into southwest Montana and far northwest Wyoming has been placed
where scattered convection has fired this afternoon and is
continuing into Friday night/early Saturday ahead of a well
defined shortwave moving east from central Oregon Friday
afternoon...into central ID and southwest MT Friday night/early
Saturday. Precipitable water values are around 0.75", which could
cause some localized heavy rain issues with hourly rain totals of
1.5" where cells merge, train, or backbuild.


Day 2
Valid 12Z Sat Aug 18 2018 - 12Z Sun Aug 19 2018


...Great Plains into the Rockies...
A shortwave trough moves east from Yellowstone Saturday, dropping
southeast over the north-central Great Plains starting Saturday
night. The low-level jet strengthens ahead of this trough with
convection breaking out across eastern Colorado and translating
downstream into the adjacent High Plains. Elevated instability is
expected across the central high Plains down into the
Oklahoma/Texas panhandles within an area of strong moisture
transport and anomalous precipitable water (1-2 standard
deviations above normal). Above normal confidence in the 12Z suite
warrants expansion of the Slight Risks over SD and farther into
the southern high plains. The Marginal Risk was also focused on
the southern Rockies.

Decreasing moisture in westerly flow over southern AZ warrants a
removal of the Slight Risk of excessive rainfall from the state as
well as southwest NM.

...Lower Mississippi Valley across the Mid-Atlantic into southern
New England...
Continued moisture pooling along a west-east boundary will provide
a focus for additional bouts of locally heavy rainfall from the
lower Mississippi Valley to the mid-Atlantic. Flash flood guidance
values in some locations are fairly low (especially across the
Central Appalachians and likely over MS/AL where activity is
currently occurring). The broad Marginal covers the expected
widely scattered heavy rain/flash flood threat.

Surface low development along the front is still uncertain over
the northern Mid-Atlantic into New England and is looking much
weaker in the 12Z consensus. Nevertheless low flash flood guidance
over portions of the northern Mid-Atlantic warrant a Marginal Risk
for now.


Day 3
Valid 12Z Sun Aug 19 2018 - 12Z Mon Aug 20 2018


...Great Plains region into the Southern Rockies...
A shortwave trough shifts southeast from the Western High Plains
late Saturday night through Sunday night. An associated focus for
heavy to potentially excessive rainfall is ahead of the track
where deep moisture will still be in place. A swath precipitable
water values of 2 inches expected to drift east over adjacent
parts of KS/OK/AR/MO Sunday in southerly 850mb flow of 25 to 30
kt. The general concept of organized convective clusters moving
southeast across the central plains to the Ozarks is evident in
all 12Z guidance with a preference to non-NAM blend. This area was
impacted last night through the time of writing providing wet
antecedent conditions. Farther north, a deformation zone is
expected to bring locally heavy rain to SD. The Slight Risk was
expanded over SD once again (where FFG is low) and maintained for
the south-central plains/Ozarks.

...Mid-Atlantic/Carolinas to the Gulf Coast...
Maintained the Marginal Risk area along and ahead of a ill-defined
boundary extending from the Carolinas southward towards the Gulf
Coast. Forcing is weak, but moisture is 1 to 1.5 standard
deviations above normal. Scattered slow moving heavy thunderstorms
are likely over a broad area from the Carolinas to the
Mid-Atlantic. The NYC area was added though should the trend of a
weaker trough over the northeast continue, the heaviest QPF would
be shunted farther south.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: