Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200106

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
906 PM EDT Wed Apr 19 2017

High pressure off of Atlantic Canada will move further away
from the coast tonight and Thursday. A warm front will cross the
area Thursday. A cold front will follow Friday afternoon. Weak
high pressure builds over the area Friday night before another
low pressure impacts the area Saturday night and Sunday.


Latest surface analysis indicates, in spite of southerly flow,
forecast area still influenced by the southwestern periphery of
cool high pressure. The warm front is back in the Ohio Valley.
Thunderstorms have erupted along the cold front in the lee of
the Great Lakes. Guidance in agreement with respect to weakening
that line prior to its arrival in the Potomac Highlands toward
dawn. Have therefore extended the period of dry weather past
midnight before allowing PoPs (in the form of showers) to creep
subtly upward again.

It appears as though any late afternoon breaks in the overcast
have filled back in again, or have been covered by higher cloud
decks. We need a solid overcast to prevent radiational fog/low
clouds to develop. Those breaks were in the Potomac Highlands,
and since the mountains and its valleys will be nearer to the
warm front, and any advection fog, believe those areas have the
better chance to see any fog development. Am confining the
threat to patchy coverage, due to somewhat low certainty.

Any residual clouds and fog will break apart during the morning
as the warm front crosses the area. Isentropic lift from the
warm front, along with favorable moisture advection, increased
CAPE, decent mid level lapse rates, and modest shear suggest
that scattered to numerous showers and thunderstorms will
develop coincident with daytime heating Thursday. Have kept
likely PoPs west of the Blue Ridge; mountains will provide lift
as well.

Temperature forecasts on track this evening. Will have a
relatively uniform mid-upper 50s for low temps tonight. Will
touch up using latest LAMP shortly. On the warm side of the
front tomorrow, a bubble of 12-13C suggest that highs should
reach within a few degrees of 80F.


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
srn zones.


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.


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.


South winds maintaining a fairly steady 10-15 kt this evening.
Latest guidance suggests this will remain the case for much of
the night, with a slow drop off toward dawn.

Gradient flow should remain below Small Craft Advisory
thresholds Thursday (from the south) and Friday (veering west
behind a cold front). However, scattered showers and
thunderstorms are anticipated both days, primarily in the
afternoon and evening on Thursday, and during the daytime hours
on Friday. A few of these storms could be on the stronger side,
containing gusty winds. Special Marine Warnings may be required.

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.




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