Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
840 PM EDT Friday Sep 4 2015
a backdoor cold front will sink south across the area tonight.
High pressure will build overhead during the Labor Day weekend
before moving off the coast during the middle portion of next
Near term /through Saturday/...
back door cold front is sinking south of the Mason-Dixon line as
of 00z observation...extending from Eastern Lake Erie south and across
Chesapeake Bay. Isolated showers and thunderstorms are popping right
along the surface boundary...mainly over areas that have not been
worked over from earlier convection. Storm Prediction Center mesoanalysis and 00z kiad
sounding shows elevated layer of cape between 1500-2000 j/kg over
northern Virginia/central Maryland. With instability being the main factor in
the storm development...as shear remains on the weak
side...anticipating the storms to maintain composure pretty much
in the current area of instability maxima...generally east of the
main Blue Ridge line...with a weakening trend as any cells push
further west. Stabilizing airmass in the wake of the front off an
Ely flow will aid in diminishing any precipitation threat as the front
The backdoor boundary will stall out near the Potomac Highlands
into central Virginia and southern Maryland late tonight. A few
showers are possible near these areas...but precipitation amounts
should be light due to limited instability. Elsewhere...dry
conditions will prevail overnight but low clouds are expected as
moisture gets trapped underneath the subsidence inversion.
High pressure will continue to wedge farther south and west into
our area for Saturday. An easterly flow will allow for cooler
conditions compared to recent days and it will turn out noticeably
less humid across northern Maryland into the Washington
metropolitan area where some drier air will advect into the
region. Farther south and west...close to the boundary a shower or
two cannot be ruled out...but most of the time should be dry.
Low clouds will be around early Saturday morning as moisture
remains trapped underneath the subsidence inversion...but clouds
should burn off later in the morning into the early afternoon
Short term /Saturday night through Monday/...
high pressure will continue to slowly build south overhead
Saturday night through Monday. Most areas will remain dry during
this time...but there may be some low clouds and fog during the
overnight and early morning hours each day. There may be enough
instability for a shower or two over the Potomac Highlands and
extreme southern Maryland during the afternoon and evening hours
each day...but even across these areas most of the time will turn
Maximum temperatures will be near climatology for Sunday...but above normal
conditions will return for Monday due to more subsidence with the
high directly overhead.
Long term /Monday night through Friday/...
weak upper level troughing affecting middle Atlantic through early
half of period and again late Friday. Moisture associated with
surface ridging will work its way into Shenandoah Valley/Potomac
Highlands by middle week. Weak positive vorticity advection...afternoon heating and
orthographic lift may support for marginal convection during early
half of period. Additional dynamics when cold front crosses region
will increase chances of convection during latter half of period.
Used gfs20 to tweak timing.
Aviation /01z Saturday through Wednesday/...
VFR conditions initially this evening...with moderate-high confidence
of MVFR ceilings moving in aftr 03-05z at all taf sites. Confidence
slightly lower on the IFR ceilings...especially kbwi/kmtn...so have held with
MVFR ceilings for now there...and IFR ceilings elsewhere 07-13z. Low
confidence of fog development as stratus moves in...so have with
held from dropping visible less than MVFR at this time.
Low ceilings will linger for the morning...though slightly imprvg.
Expecting ceilings to improve to VFR by the afternoon...though the timing
could be off 1-3 hours than in tafs. Another round of MVFR or lower
ceilings possible Sat night with increased moisture on Ely flow.
Low clouds are possible again overnight Sunday into Monday
morning. There is a better chance for fog late Sunday night into
Monday morning as well.
The high should move off the coast during the middle portion of
next week...but it should be close enough for VFR conditions most
of the time. Patchy fog is possible during the early morning
Small Craft Advisory remains in effect for all the waters to account for a brief
period of gusts to 20 kts with a pressure surge along a back door
cold front. Persistent Ely flow should keep winds at Small Craft Advisory levels
everywhere except the northern tidal Potomac through Sat.
High pressure will build over the waters Saturday night through
Monday. No marine hazards are expected during this time. The high
will move off the coast during the middle portion of next week.
tidal anomalies are around one-half to three-quarters of a foot
above normal this evening. An easterly flow will strengthen
overnight which should cause tidal anomalies to increase. Have
held off from issuing a coastal Flood Advisory as the most
liking sites...Solomons and Saint George Creek look to be reaching
just below minor levels. Annapolis will approach but should remain
below the minor threshold as well.
The next chance for minor flooding will be with the higher of the
two high tide cycles Saturday night as tidal anomalies remain
elevated...generally around half of a foot.
District of Columbia...none.
Marine...Small Craft Advisory until 2 PM EDT Saturday for anz530>534-
Small Craft Advisory until 2 am EDT Saturday for anz535.