Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 200002
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
802 PM EDT Wed Apr 19 2017
High pressure off of Atlantic Canada will move further away
from the coast tonight and Thursday. A lee-side trof will setup
across the area tomorrow. A cold front will cross the area
Friday afternoon. Weak high pressure builds over the area Friday
night before another low pressure impacts the area Saturday
night and Sunday.
.NEAR TERM /THROUGH THURSDAY/...
Light rain over ncntrl MD will taper off later today with
patchy drizzle and areas of fog developing late tonight as mid-
levels dry out and moist air remains in place near the sfc. Low
clouds/fog dissipate by mid- morning Thu with risk of showers
and t-storms as lee-side trof sets up and provides lift and
instability builds as temps rise through the 70s to perhaps 80F.
.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
Shra/tsra activity will wane with loss of heating Thu evening,
but approaching cdfnt from the west could bring another round of
late night and early morning showers Thu night and Fri. Still
warm on Fri with sufficient moisture and instability Fri
afternoon for another round of showers/t-storms mainly across
the southern half of the fcst area. The 12Z GFS wants to push
the front all the way into North Carolina, but the latest Euro
keeps the front across srn VA keeping the risk of showers across
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
An area of low pressure will take shape over the south-central CONUS
Saturday morning, then slowly push east just south of the area
through the second half of the weekend. Models are in general
agreement with this setup, which should result in a beneficial
soaking rain across much of the area.
A cool area of high pressure will persist to the north following the
weekend system which should keep temperatures near to below normal
for this time of year during the first half of next week. Periods of
clouds or sprinkles will be possible in onshore flow as well.
.AVIATION /00Z THURSDAY THROUGH MONDAY/...
A band of MVFR ceilings (OVC025) will be crossing the metros
this evening. Behind that, clearing was evident on recent
visibile satellite pictures. It will be interesting to see if
those trends continue into the nighttime hours. RAP thinks so.
However, the first nighttime images suggest that this area in
the Potomac Highlands is starting to fill in again. LAMP has
been backing off on poor flight conditions overnight. Have done
likewise in the TAFs. It would take a solid MVFR cloud deck to
prevent LIFR fog/low clouds from developing. Do not have a
strong level of confidence, but am cautiously optimistic that a
brief period of predawn MVFR will be the biggest concern at
IAD/BWI, and maybe not even that for DCA. CHO, on the other
hand, more likely to see some IFR.
More recent guidance suggesting that scattered to perhaps even
numerous showers and thunderstorms will be developing Thursday
afternoon in daytime heating combined with lift from a warm
front. This timeframe now entering the TAF window. Am taking no
restrictions at this time due to spatial uncertainties. The
associated cold front will cross the terminals on Friday. Again,
showers and thunderstorms will be a possibility. Its unclear
whether coverage will be as much as tomorrow.
Sub-VFR likely over the weekend with rain. Winds will
generally be out of the northeast around 10 knots or so.
Canadian HRDPS suggests winds will increase across
the waters tonight, but other reliable guidance such as the GEFS
and SREF indicate winds will remain steady and perhaps even
diminish tonight. Solid SCA not likely until Sun afternoon when
low pressure moves offshore.
Northeasterly winds of 10 to 15 knots are expected across the
waters over the weekend as low pressure passes to the south.
Gusts would exceed 20 knots for a time especially Saturday as
the pressure gradient tightens between low pressure developing
to the southwest and high pressure developing to the northeast.
Observations this afternoon/early have been coming in lower
than forecast...on the order of three-quarters of a foot lower
at Strait`s Point. Therefore, the Coastal Flood Advisory for St.
Mary`s County has been cancelled. Forecasts elsewhere seem to be
doing ok, but have infused some of the latest guidance to fine-
tune these projections.
Water levels will continue to be elevated through Thursday.
Departures will be running about a foot above normal. Some
Action Stages will be threatened. Since levels rose within the
past day, and the morning tide cycle will be astronomically
higher, would not be surprised if an upward adjustment
ultimately will be needed to the Thursday AM tide cycle.
Corrections likley won`t come before a cold front on Friday.