Area Forecast Discussion

FXUS61 KLWX 181510
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1110 AM EDT Fri Aug 18 2017
A cold front will approach from the west today before crossing 
the region tonight. Surface high pressure will build in 
Saturday, but an upper level disturbance will cross the region 
Saturday night. A warm front will advance back north across the 
region early next week, followed by a strong cold front in the 
middle of the week.
Low clouds have struggled to break this morning, but are slowly
doing so. Otherwise, forecast is generally on track. Added in
enhanced wording for this afternoon/evening t-storms, and 
pondering a flash flood watch along and east of I-95. Will 
decide on this potential over next couple hours.
...Previous discussion...
The cold front will approach our area from the west later today
while a surface trough develops overhead...most likely between
the Appalachian Mountains and Blue Ridge Mountains. A southerly
flow ahead of the cold front will continue to usher in hot and
very humid conditions. Dewpoints are likely to be in the upper
60s and lower 70s this afternoon west of the Blue Ridge
Mountains with mid to upper 70s most likely east of the Blue
Ridge Mountains. Low-level temps are progged to be about a
degree celsius higher compared to yesterday. Therefore...max
temps are forecast to be a couple degrees hotter for most
locations. This will cause max temps to top off in the mid to
upper 80s for valley locations west of the Blue Ridge MOuntains
with lower 90s near the Washington and Baltimore Metropolitan
areas into central Virginia and southern/central Maryland. The
combination of heat and very high humidity will cause heat
indices to top off near 105 degrees near the urban Corridor from
Baltimore through Washington DC toward Fredericksburg. A Heat
Advisory has been issued for these areas. 
The high heat and humidity will cause moderate to high amounts
of instability to develop this afternoon. Showers and
thunderstorms are likely to develop along and ahead of the cold
front...especially this afternoon and evening. Shear profiles
will strengthen a bit as an upper-level trough digs over the
Great Lakes. The strengthening shear profiles along with the
moderate to high instability suggests that some storms may
become severe...with damaging winds and large hail being the 
primary threats. An isolate tornado cannot be ruled
out...especially toward the waters where surface winds may be 
backed a bit. Confidence is low since the magnitude of low- 
level shear profiles should be weak.
PWATS are progged to be near or even above 2 inches...especially
east of the Blue Ridge Mountains where the higher dewpoints are.
Therefore...thunderstorms will contain torrential downpours and
isolated flash flooding is a concern. Did consider a Flash Flood
Watch with the update this morning. There is a chance for the 
outflow boundary to become parallel to the storm motion and this
could enhance the threat for flash flooding. The farther 
east...the better the chance. Latest hires guidance (HRRR and 
NCAR ensembles) suggest that the highest rainfall amounts will 
be near or even just east of the Bay. This combined with the
fact that storms should be moving due to the increasing shear
are reasons why we held off on a watch with this cycle. will have to be monitored closely throughout the  
The best chance for storms will be during the middle afternoon
hours west of the Blue Ridge Mountains...late this afternoon
into early this evening for the Washington and Baltimore
Metropolitan areas...and this evening across southern Maryland. 
Drier air should move in behind the cold front tonight...but
winds will be light. Therefore...patchy fog is possible
Drier and less humid conditions are expected behind the front
Saturday...but an upper-level disturbance will pass through late
in the afternoon into the evening. An isolated
shower/thunderstorm cannot be ruled out. However...much of the
day Saturday will turn out dry with sunshine. 
High pressure will build overhead overnight Saturday through
Sunday...bringing dry conditions along with lower humidity. The
high will begin to shift offshore Sunday night and a return flow
will develop.
Ridge of surface high pressure will slide offshore on Monday 
allowing for the development of return southerly flow and increasing 
low level moisture. Thus, we'll some increase in chances for 
isolated-scattered afternoon/evening showers/thunderstorms. In 
addition, warmth/humidity will also trend back upward, with highs 
back to near or above 90F and dew points in the 70s for both Monday 
and Tuesday.
A potent upper trough and cold front will then move through the 
Great Lakes on Tuesday and across the Mid-Atlantic and Northeast on 
Wednesday, with additional chances for showers/thunderstorms. 
Cooler and drier high pressure will follow the front for later next 
Cigs/Vsbys have been slow to improve, but are slowly doing so 
from west to east. However...showers and thunderstorms are 
likely later this afternoon into this evening. Some 
thunderstorms will produce gusty winds and torrential rainfall 
that can briefly reduce cigs/vsbys to IFR or below. Drier air 
will move in behind a cold front tonight...but winds may become 
light and this will allow for patchy fog to develop toward 
morning. VFR conditions are expected later Saturday through 
Sunday night for most of the time. Cannot rule out an isolated 
shower/thunderstorm late Saturday afternoon and evening...but 
most areas should be dry. 
Mainly VFR is expected for Monday and Tuesday, although some
brief reductions are possible in isolated to scattered 
showers/thunderstorms each afternoon/evening. There is also the 
possibility for patchy late night/early morning low clouds/fog.
Have added SCA to all waters this afternoon for gusty winds
ahead of and along front/storms. Special Marine Warnings will
likely be needed as well. 
Southerly winds continue to channel up the middle portion of the
Bay and lower Tidal Potomac River early this morning. A Small
Craft Advisory remains in effect. Will continue with the SCA 
for later this morning into this afternoon across the middle 
portion of the Bay and lower Tidal Potomac River...but there may
be a lull in the gusts during this time. Winds should pick up 
ahead of a cold front late this afternoon. The cold front will 
pass through tonight and an SCA is in effect for most of the Bay
and lower Tidal Potomac overnight into early Saturday for 
northwest winds. will be marginal. 
Showers and thunderstorms are likely ahead of the cold
front...especially late this afternoon and evening.
Thunderstorms will be capable of producing gusty winds. An
isolated waterspout or thunderstorm with large hail cannot be
ruled out as well. Special Marine Warnings may be needed. 
High pressure will build toward the waters later Saturday
through Sunday night. upper-level disturbance will
pass through the waters Saturday evening. An isolated
shower/thunderstorm cannot be ruled out...but most areas will 
be dry.
Sub-SCA conditions likely to continue Monday, although southerly 
return flow will be on the increase, and SCA conditions then
become possible Monday night and Tuesday. Additionally, a few 
afternoon/evening showers/thunderstorms are possible both Monday
and Tuesday.
Southerly flow will continue to cause elevated water levels
through today. Anomalies are high enough to cause Straits to
touch with the midday high tide, so issued CFA for St. Mary's
through tonight. A second round of minor flooding is possible 
with tonight's high tide cycle at our other more sensitive 
sites of Annapolis and DC Southwest Waterfront. Will need to
watch the less sensitve sites as well as Baltimore also touched
in the last high tide cycle.
DC...Heat Advisory until 8 PM EDT this evening for DCZ001.
MD...Heat Advisory until 8 PM EDT this evening for MDZ011-013-014-
     Coastal Flood Advisory until 3 AM EDT Saturday for MDZ017.
VA...Heat Advisory until 8 PM EDT this evening for VAZ052>057.
MARINE...Small Craft Advisory until 9 AM EDT Saturday for ANZ531-532-
     Small Craft Advisory until 9 AM EDT Saturday for ANZ533-534-
     Small Craft Advisory until 1 AM EDT Saturday for ANZ530-535-