Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 260747
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
347 AM EDT Mon Sep 26 2016
High pressure to the north will move off the coast today. A cold
front will pass through the area tonight into Tuesday. An upper-
level low will impact the area during the middle to perhaps latter
portion of the week.
.NEAR TERM /THROUGH TONIGHT/...
High pressure over New England extends southwest toward the Mid-
Atlantic Coast early this morning. A southeast flow has developed
with the ridge axis to our east. Warm and moist air is overrunning
the cooler air in place this morning...resulting in plenty of
clouds. Patchy drizzle is expected as well due to the isentropic
A cold front will approach from the west later today while high
pressure continues to move off to the east. A south to southeast
flow will continue over our area...and weak isentropic lift will
continue during this time. This will likely keep clouds in place
throughout the day. Cannot rule out a few breaks of sun...but much
of the day will turn out cloudy. A few popup showers and isolated
thunderstorms are expected west of the Blue Ridge Mountains this
afternoon where there may be a little more instability closer to
the cold front.
The cold front will slowly cross through the area tonight. More
widespread showers and possible thunderstorms are expected across
all areas. The best chance for widespread precipitation will be
early this evening near the Allegheny Highlands...late this
evening across the Shenandoah Valley toward the Blue Ridge
Mountains...and overnight across the Washington and Baltimore
Metropolitan areas and points south and east. There will be enough
moisture for moderate to locally heavy rainfall at times...but
this system will be progressive. Average rainfall amounts around a
quarter to one-half inch are expected with locally higher amounts.
.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
The cold front will get hung up just to our south and east Tuesday
while high pressure builds overhead northern and western areas.
However...a cutoff upper-level low will be tracking through the
Great Lakes and our area will likely remain in the right entrance
of the upper-level jet over New England. This may cause clouds and
precipitation to linger into the morning hours near the I-95
corridor and throughout the day across southern Maryland.
Weak high pressure will build overhead for most areas Tuesday
night into Wednesday...allowing for dry and seasonable conditions.
Showers may remain hung up across extreme southern
Maryland...closer to the stalled boundary.
Cutoff upper-level low drop south into the Ohio Valley later
Wednesday through Wednesday night. Guidance still differs in some
of the details...but confidence of this system impacting our area
has increased since guidance has come into better agreement.
This system will be strong enough to have surface low pressure
associated with it over the Ohio Valley. This will allow for the
boundary just to our south and east to return north and west as a
warm front. A southwest flow aloft will also usher in more
moisture across our area...bringing more clouds along with the
chance for showers later Wednesday and Wednesday night.
.LONG TERM /THURSDAY THROUGH SUNDAY/...
Uncertainty remains rather high with the evolution of upper low as
it drifts southward and becomes cutoff from the main belt of
westerlies Thursday and possibly into the weekend. While ensemble
guidance appears to be in the early stages of converging on a
solution that keeps the upper low to our southwest/west through at
least Friday...there are still several members (especially of the
GEFS) that quickly move it off to our north. Ultimately,
spatiotemporal evolution of upper low will have a significant impact
on sensible weather elements through much of the long term.
This forecast leans heavily on a forecast blend, with greatest
weight placed on the ECMWF (and EPS)...as it has exhibited the
greatest run-to-run consistency. Thus, showers and possibly a few
thunderstorms appear likely Thursday...with shower chances
continuing through at least Saturday. Temperature forecast prone to
large errors...as amount of insolation remains in question.
.AVIATION /07Z MONDAY THROUGH FRIDAY/...
High pressure will continue to move off to the east and a
southeast flow will increase today with gusts around 15 to 20
knots this afternoon. Low clouds have developed across most areas
early this morning due to the southeast flow. The low clouds will
continue to expand north and east this morning and lower as well.
Most terminals will have MVFR conditions along with patchy
drizzle...but will continue with IFR conditions this morning for
Low clouds will likely hang around throughout the afternoon and
possibly even through most of tonight. MVFR conditions are most
likely...but cigs may be close to 3kft this afternoon into this
evening. A cold front will pass through the terminals late this
evening through the overnight. Showers are likely during this time
with isolated thunderstorms possible. A northwest flow behind the
boundary should clear things out overnight across the northwest
terminals and early Tuesday morning farther south and east.
Weak high pressure will build overhead for later Tuesday into
early Wednesday. An upper-level low will impact the terminals
later Wednesday through Wednesday night...bringing the chance for
showers. Subvfr conditions are possible...especially Wednesday
Sub-VFR conditions probable Thursday with showers and perhaps a few
thunderstorms. Periods of Sub-VFR conditions could linger into
Saturday as upper low remains near the area...though, uncertainty
beyond Thursday remains high.
A southeast flow has developed over the waters early this morning
as high pressure moves off to the east. A cold front will approach
the waters later today through this evening before passing through
overnight into early Tuesday. The southeast flow will strengthen
ahead of the boundary...and a small craft advisory is in effect
for the waters this afternoon through tonight. Will have to
monitor winds over the waters this morning. Gusts around 15 knots
are expected and winds may even approach SCA levels across the Bay
and lower Tidal Potomac River later this morning.
Showers are expected late tonight into early Tuesday along with a
possible thunderstorm. Weak high pressure will build over most of
the waters later Tuesday into Wednesday...but clouds and showers
may get hung up across the middle portion of the Bay Tuesday into
Tuesday night since the boundary will stall just to our south and
An upper-level low will drop through the Ohio Valley later
Wednesday into Wednesday night. Surface low pressure will develop
over the Ohio Valley as well...and the gradient between high
pressure over New England and the lower pressures over the Ohio
Valley will strengthen. A Small Craft Advisory may be needed for
portions of the waters Wednesday night.
Small craft advisory possible Thursday into Friday as coastal low
possibly develops off the Mid-Atlantic coast.
Tidal anomalies have dropped closer to one-half foot above normal
early this morning due to the light flow. However...southeast
winds will increase today into tonight and tidal anomalies will
increase as well. The next high tide cycle this afternoon will be
the lower of the two...so confidence is too low at this time for
an advisory. There is a better chance for minor flooding during
the high tide cycle tonight into early Tuesday.
Tidal Anomalies should drop later Tuesday behind the cold front.
MARINE...Small Craft Advisory from noon today to 6 AM EDT Tuesday for