Mesoscale Precipitation Discussion 0966
NWS Weather Prediction Center College Park MD
1109 AM EDT Thu Oct 11 2018

Areas affected...North Carolina, Southwest Virginia, Far SE West
Virginia

Concerning...Heavy rainfall...Flash flooding likely

Valid 111508Z - 111900Z

Summary...Flash flooding is likely today from North Carolina into
Virginia, associated with heavy rainfall from Tropical Storm
Michael. The greatest risk from the late morning into the early
afternoon will be from western North Carolina into southwest
Virginia. However, flash flooding will become possible over much
of North Carolina and the southern half of Virginia during that
time. Some of the flash flooding could be significant and
life-threatening, particularly in areas of increasing terrain near
the NC-VA border.

Discussion...Flooding was ongoing in portions of the Carolinas as
a result of heavy rain that has already fallen from Tropical Storm
Michael. Therefore, the flash flooding threat would best be
described as expanding over the next several hours into additional
parts of NC and southern VA as Tropical Storm Michael continues to
track northeast into North Carolina. As one would expect near the
lingering core of a landfalling tropical cyclone, there is a
reservoir of deep tropical moisture feeding the heavy rainfall
threat. The 12Z sounding from Greensboro (GSO) had a precipitable
water value of 2.45 inches, which is very nearly the highest on
record at that station (dating back to 1945). Ahead of the center
of Michael, the cyclonic east to southeast flow was advecting this
tropical moisture plume to the northwest and impinging on the
terrain of the Appalachian Mountains. This had already generated
some significant rainfall amounts overnight in southwestern North
Carolina, with fairly widespread 3-5 inch amounts in 12 hours.

Downsloping and decreasing moisture on the back side of Michael's
circulation should allow precipitation to begin to diminish in SW
NC, but the heavy rain is likely to continue for another several
hours in NW NC, and should become increasingly focused in SW VA
into the early afternoon hours. Hi-res models are showing a very
high probability of over 3 inches of rain in 6 hours from NW NC
into SW VA (12Z HREF), and this would generally approach or exceed
flash flood guidance in most of those areas. A marginal amount of
instability (MUCAPE around 300-600 j/kg) may be sufficient for
some convective banding in addition to orographic enhancement, and
the combination of these factors with near record PWs means that
significant, life-threatening flash flooding could occur in some
of these areas with deeper inundation.

Further east, closer to the track of Michael and out into the
Piedmont and flatter terrain, there will be more convective
instability (MUCAPE closer to 1000-1500 j/kg). Therefore, tropical
rain bands may be more organized in these areas and produce higher
rain rates, approaching 3 in/hr at times. However, unlike in the
mountainous areas where terrain features lock the heaviest rain
bands in place, the rain bands further east may be more
progressive and not necessarily remain situated in the same areas
for hours on end. Flash flooding is still likely in these areas
given the heavy tropical rain rates, but the most notable impacts
may be more scattered. Flash flooding would be most likely where
tropical rain bands can intersect urban areas for at least 40-60
minutes.

Lamers

ATTN...WFO...AKQ...CAE...GSP...ILM...LWX...MHX...MRX...RAH...
RLX...RNK...

ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...

LAT...LON   38147971 37757854 37167798 36037779 35007818
            34507893 34477971 34798020 35068063 35078155
            34938218 35578243 36288248 37218201 37918102