Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 290758

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
358 AM EDT Thu Sep 29 2016

An upper level low will settle over Kentucky today into Friday
before lifting back northward across the Great Lakes over the
weekend. Persistent onshore flow and periods of heavy rain will
occur across the Mid-Atlantic through Friday. The low is forecast to
pass to our north as it tracks east across the northern Mid-Atlantic
late in the weekend and into early next week, with high pressure
building into the area.

Satellite imagery indicates southward-moving upper-level low is
currently nearing its forecasted southern most latitude (which is
somewhere over Kentucky) it nestles itself between high
pressure over the Atlantic and ridging over the central CONUS.
Becoming quasi-stationary through tonight as it stalls out in a
blocking pattern. This will set up a continued fetch of warm/moist
air (from several tropical source regions) as it converges over the
area through the period.

Current radar trends show bulk of moderate/heavy rainfall is
currently located across the northern half of our area...with more
scattered showers across the south. HRRR-TLE shows high probability
of at least 0.5-1 inch/hr rain rates in the strongest cores as this
activity continues to develop and move across the northern half of
the CWA through the morning.

By afternoon...backing flow at 850 hPa strengthens to between 30-50
kts through the day leading to a significant upslope component. This
should allow for showers and scattered thunderstorms...heavy at develop over the higher terrain. Elsewhere...guidance is
a little less bullish after the initial activity during the
morning...keeping activity slightly more scattered in nature.
Despite this...convective elements coupled with efficient rain
processes and anomalously high PWATs suggest flooding concerns could
arise in locations that experience training activity.

SPC currently has SW portion of CWA in a marginal risk. While ML
CAPE remains relatively poor (At best 1000 J/KG), effective bulk
shear values AOA 40 kts suggest transient supercell structures are
possible across the marginal risk area...with isolated hail/wind
threats. Furthermore, nearby frontal boundary and strongly backed
low-level flow suggest an isolated tornado is also possible across
the far southwest.

By Friday...ridging across the central CONUS begins to weaken. This
will allow the upper-level low to gradually retrograde and lift back
to the north as it begins to become vertically stacked and
fills/weakens. Rain rates are expected to gradually become less
intense as activity becomes more scattered through the day. For
now...did not adjust the expiration time of the Flash Flood
Watch...though that will need to be revisited later today.
Current thinking is that flooding issues would be more localized

Gradual drying trend continues into vertically-stacked
upper-level low continues to weaken and drift northward. While not a
washout by any means...scattered shower activity will remain

Influence of upper-level low finally begins to diminish by late in
the weekend into early next week as it passes by to our north and
moves out to sea...while high pressure builds into the area in its
wake. Most of Sunday appears dry...aside from some possible
lingering early morning showers.

Remaining dry with temperatures near climatology as high pressure
remains over the area through early next week.

Upper-level low will continue to impact the area today through
Friday...bringing periods of IFR/LIFR to all the terminals. Heaviest
rain expected today into Friday. Isolated thunderstorms are also
possible today. Low ceilings are possible through Friday night with
persists onshore flow.

Gradual improvement Saturday...with a return to VFR by Sunday.

Gale Warning continues until 14z today for the main channel of the
Chesapeake Bay and eastern inlets (with SCA elsewhere). SCA
continues everywhere after 14z BUFKIT soundings show
winds remaining just below Gale. However...observations will
continue to be monitored as guidance might be underestimating
strength of low-level flow. SCA currently expires at 12z
Friday...though that will likely need to be extended into Friday
afternoon. Gradient should begin to relax by later Friday...with SCA
winds becoming less likely Saturday into the Weekend.


Tidal departures increased with the incoming high tide overnight,
assisted by gale force east winds. Anomalies running around 1.5
feet, although its a little less near Solomons and the mouth of
the Potomac River. Feel that there is enough coming up the Bay to
add southern Baltimore to the Coastal Flood Advisory.

Since an easterly component to the wind will be in place through
at least early Friday, water will only be reinforced. Pondered
going with a Watch for the lower reaches (St Mary`s Co primarily...
due to the susceptibility of St George Island) but ultimately
opted not to since flow should be pushing water away from that
part of the Potomac. However confidence is sufficient to extend
Advisories through the Friday morning tide cycle. That`s not
saying that moderate flooding is out of the question at some point
between now and the weekend; rather certainty is too low to issue
a Watch at this time.

We will continue to monitor. Water levels will continue to be an
issue through at least Saturday.


DC...Flash Flood Watch through Friday morning for DCZ001.
     Coastal Flood Advisory until noon EDT Friday for DCZ001.
MD...Flash Flood Watch through Friday morning for MDZ003>006-011-
     Coastal Flood Advisory until 10 AM EDT Friday for MDZ011-016.
     Coastal Flood Advisory until 8 AM EDT Friday for MDZ014-017-
VA...Flash Flood Watch through Friday morning for VAZ025>031-
     Coastal Flood Advisory until 10 AM EDT Friday for VAZ057.
     Coastal Flood Advisory until noon EDT Friday for VAZ054.
WV...Flash Flood Watch through Friday morning for WVZ050>053-055-
MARINE...Small Craft Advisory from 10 AM this morning to 8 AM EDT
     Friday for ANZ530>534-539>541-543.
     Gale Warning until 10 AM EDT this morning for ANZ530>534-
     Small Craft Advisory until 8 AM EDT Friday for ANZ535>538-542.


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