Mesoscale Precipitation Discussion 0969
NWS Weather Prediction Center College Park MD
528 PM EDT Thu Oct 11 2018

Areas affected...Eastern Virginia and the Coastal Mid-Atlantic

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

Valid 112125Z - 120315Z

Summary...Tropical Storm Michael will move northeast from the
Triangle Region of North Carolina to just off the Delmarva
Peninsula this evening, spreading a risk of flash flooding up the
Mid-Atlantic Coast. Bands of heavy rainfall are likely to lead to
flash flooding, particularly where they intersect urban areas.
Additionally, severe, life-threatening flash flooding is
increasingly likely over south-central and east-central Virginia,
from the Farmville, to Richmond, to Northern Neck area where rain
rates may reach 3 in/hr in a strong tropical rain band.

Discussion...As of 21Z, regional radars continued to show a very
strong rain band situated to the north of the center of Tropical
Storm Michael, generally over south-central Virginia. MRMS and
KRAX/KAKQ dual pol estimates have occasionally reached as high as
3 in/hr rain rates with this band, that was being focused by
increasing low-level convergence. Although this was most evident
in the position of the rain band on the nose of a 50+ knot 850mb
jet (per KAKQ VWP), it was also beginning to manifest in the
surface observations. Locations to the northwest had cooled
several degrees in the past couple hours and were showing a
northerly component to their winds. Meanwhile, the inflow region
over SE VA and NE NC still had dewpoints over 70 degrees with
southeasterly winds. The increasing convergence and frontogenesis
should continue to focus a significant rain band into the early
evening hours just north of the projected path of Michael's
center. This left-of-track rainfall pattern is consistent with
conceptual models of a tropical cyclone interacting with an
approaching front in this way.

Multiple GPS-PW observations in the southeast quadrant of Virginia
were around 2.5 inches, and thus the supply of deep tropical
moisture should also be able to sustain significant rain rates,
exceeding 2 in/hr in many spots and reaching 3-4 in/hr in some
localized areas. This will likely lead to fairly widespread flash
flooding wherever the focused rain band traverses, but is also
high enough to potentially lead to more severe, life-threatening
flash flooding. Numerous roads in the impacted area are likely to
be flooded or impassible. Rain rates approaching 3 in/hr would be
almost certain to generate flash flooding in urban areas.
Additionally, the 18Z HREF blended mean shows 6-hr rainfall totals
of 5-7 inches associated with this rain band, and most individual
hi-res models show these type of amounts. Those totals would have
less than a 1 percent annual exceedance probability, a good proxy
for the potential for a significant event, and given the agreement
in the hi-res models it seems that more significant flash flooding
is increasingly likely in the aforementioned corridor.

Elsewhere, along and south of the track of Michael, the tail end
of some inflow rain bands should sweep through the eastern portion
of the NC-VA border region, affecting the Norfolk and Hampton
Roads regions. These bands should be more progressive and are less
likely to linger in any one spot, but the instantaneous rain rates
are likely to be high given the unstable air mass with deep
tropical moisture. Therefore, flash flooding cannot be ruled out.
Further north, a front was just beginning to take shape near or
just southeast of the I-95 corridor, and convection was developing
up the front toward southern Maryland and the D.C. Metro Area.
Additional convective rain bands are likely to develop into the
Delmarva Peninsula and perhaps up into southern New Jersey into
the evening, with an increasing threat of flash flooding as the
stronger forcing associated with Michael's core circulation draws
nearer. Therefore, the threat of flash flooding should spread up
the coast, particularly in the 00-03Z time frame when the primary
left-of-track rain band may begin to reach the Delmarva Peninsula.

Lamers

ATTN...WFO...AKQ...LWX...PHI...RAH...RNK...

ATTN...RFC...MARFC...SERFC...

LAT...LON   40107491 39777419 38837484 37997518 37217564
            36367638 36257755 36577854 37117887 37697866
            38537781 39367664