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Area Forecast Discussion (AFD)

Issued by Sterling, VA (LWX)

FXUS61 KLWX 170749

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
349 AM EDT Thu Aug 17 2017

A warm front will pass through the area today and a cold front
will approach from the west Friday before passing through Friday
night. Weak high pressure will build into the area for the
weekend before moving offshore early next week. A stronger cold
front will pass through the area during the middle portion of
next week.


Partly to mostly clear skies and light winds combined with abundant 
low level moisture has led to areas of fog again early this morning. 
Due to the presence of some cloud cover and being one day removed 
from widespread rainfall, density is significantly less than 
yesterday, however there will still likely be localized areas where 
dense fog does develop.

Warm frontal boundary is also positioned near the region early this 
morning, and this is continuing to help sustain a few rain showers 
in central VA and possibly into southern Maryland. There could be 
additional showers that develop along or south of the boundary 
into the morning hours.

During the day today, warm frontal boundary will gradually lift 
northward, providing for a warm and humid day. Once any fog lifts, 
partly sunny skies are expected, but will likely give way to more 
clouds than sun through the day. Highs today should reach the upper 
80s to around 90F. This combined with surface dew points in the low 
70s will allow for the development of 1000-1500 J/KG of MLCAPE, 
especially west of I-95 where warm front will clear first. While 
CAPE profiles are not that conducive for widespread strong to severe 
convection with tall/skinny CAPE depicted, 0-6KM shear of near 30 
knots and the presence of a warm frontal boundary and its 
directional shear along it, may be enough for a few isolated strong 
to locally severe thunderstorms. Convection-allowing models differ 
on evolution and coverage, but highest threat area appears to be 
west of the metros. In addition, precipitable water values 
increase markedly as warm front lifts northward, with values 
near or exceeding 2" by late in the day. This combined with 
11-12K foot warm cloud depths, wet antecedent conditions, and 
the potential for slow movement/repeated convection sets up the 
possibility for an isolated flood risk as well.


For tonight, warm and muggy conditions will persist. Convection
from this afternoon may continue to propagate eastward, with 
additional scattered development also possible as low level jet 
moves towards the region overnight. Potential for heavy rain 
will exist with any showers/storms overnight as well. Lows 
tonight low to mid 70s.

Area of low pressure will push into southern Canada on Friday,
with its trailing cold front entering the Mid-Atlantic states 
during the day. This will serve as another focus for scattered 
showers/thunderstorms during the day Friday. A severe threat 
will exist again on Friday, with potentially greater 
coverage/more intense storms than today. Temperatures and low 
level moisture will surge out ahead of the front, leading to the
development of 1500- 3000 J/KG of MLCAPE, with 30-35 knots of 
shear moving overhead. In addition to the severe threat, flash 
flooding is also a concern as precipitable water values surge 
over 2", possibly exceeding 2.25", and potential for training 
convection exists. Highs reach the upper 80s to around 90F. 
Convection will push eastward and gradually wane Friday night.

Model guidance has come into better agreement regarding
progression of the front for Saturday. It now appears more 
likely that the front will clear most of the region before 
stalling out along the Atlantic coastline and into southeastern 
Virginia. This will provide for a drier day Saturday, although a 
few isolated- scattered showers/storms are still possible, 
mainly near the front, as well as in the higher terrain as an 
upper trough moves across later in the day. Highs will still be 
in the upper 80s, although dew points will drop back into the 
60s behind the front.


High pressure will remain overhead Sunday...bringing dry
conditions for most areas along with warm conditions. A
downsloping low-level flow along with some sunshine will cause
max temps to range from the mid to upper 80s in northern
Maryland to the lower 90s in central Virginia. 

High pressure will slowly move offshore Sunday night through
Monday. A return flow will develop and this will allow for
humidity to increase along with very warm conditions. The
increased humidity may trigger some showers and
thunderstorms. Coverage should be isolated to scattered since 
there will not be a strong lifting mechanism. A southerly flow
will continue Tuesday with more warm and humid condtions along
with isolated to scattered showers/thunderstorms. 

A cold front will approach the area from the north Wednesday
before passing through Wednesday night. Showers and
thunderstorms are possible ahead of the boundary...especially
during the afternoon and evening hours. More warm and humid
conditions will persist ahead of the boundary...but noticeably 
cooler and less humid conditions are expected behind the


Reductions possible again this morning in areas of fog, with 
greatest risk for IFR at MRB/BWI. DCA has lowest probabilities
of seeing fog/br. Once any fog lifts, scattered to broken 
clouds expected, although mainly VFR. Isolated to scattered 
showers/thunderstorms then develop this afternoon, possibly 
lingering into tonight, and some may bring heavy rain/gusty 
winds. Some patchy fog and low clouds are possible again 

Additional showers/thunderstorms are then likely Friday 
afternoon/evening with potential additional reductions and gusty  
winds. Conditions improve for Saturday.

High pressure will remain over the terminals Sunday. VFR
conditions are likely. The high will move offshore Sunday night 
into Monday. Warm advection may bring an MVFR deck of lower 
clouds later Sunday night into Monday. A few showers and 
thunderstorms are possible...but coverage should remain isolated
to scattered. 


Sub-SCA conditions exist on the waters this morning, but south
winds will be on the increase later today, tonight, and 
through Friday ahead of a frontal system. A Small Craft Advisory
is in effect for portions of the central Chesapeake and lower 
Tidal Potomac from late this afternoon and through Friday. 
Additionally, isolated to scattered showers and thunderstorms 
are possible both today and Friday, mainly in the 
afternoon/evening, and Special Marine Warnings may be necessary.

High pressure will remain just to the north and west Sunday
while a weak surface trough remains over the waters. Light winds
are expected. The high will move offshore Sunday night into
Monday and a return southerly flow will develop. The southerly
flow will continue through Tuesday. Increased moisture from the
southerly flow may trigger a few showers and thunderstorms...but
coverage should remain isolated to scattered. 



Elevated water levels continue early this morning. However...the
next high tide will be the lower of the two so no minor flooding
is expected today.

The high tide cycle tonight into early Friday will be the higher
of the two. Also...southerly winds may increase a bit ahead of a
cold front over the Ohio Valley. This may cause minor flooding 
for sensitive areas such as Annapolis...Straits Point and 
possibly Washington DC. Coastal Flood Advisories may be needed.
The southerly flow will continue through Friday evening before
turning offshore behind the boundary. More minor flooding is
possible with the high tide cycle Friday night into early
Saturday for sensitive areas. Anomalies should decrease later
Saturday due to the offshore flow. 


MARINE...Small Craft Advisory from 4 PM this afternoon to 6 AM EDT 
     Friday for ANZ533-534-537-541-543.