Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210727

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
327 AM EDT Tue Aug 21 2018

A warm front will lift north across the Mid-Atlantic today,
followed by a cold front moving west to east tonight. A
secondary cold front will follow Wednesday. Strong high pressure
will build in from the Ohio Valley Thursday, then cross the
region Friday into the weekend.


A warm front remained stalled near Raleigh NC as of early this
morning. With widespread cloud cover, a weak low-level jet and a
~1023 mb surface high situated over northern New England, find it
hard pressed that this front will budge much until later this
morning. After that time, the surface high pushes offshore and low-
level flow increases some. This should allow the front to inch
northward, eventually nearing the Mason-Dixon line by this evening.

Meanwhile, low pressure near Chicago will occlude as it traverses
the Great Lakes beneath a neutrally-tilted mid/upper trough through
the afternoon, then depart up the St. Lawrence River Valley tonight.
Its surface cold front will approach the region this evening. With
extensive cloud cover, appreciable heating will be hard to come by,
especially east of the Blue Ridge where clouds are often a bit more
persistent. Mid-level lapse rates are modest. Even so, abundant
moisture (PWATs 2+ inches, surface dew points 70+ F) will result in
modest CAPE even without much heating. This coupled with 30-40+ kts
of effective shear ahead of the approaching trough will result in an
environment capable of producing at least isolated strong to severe
convection, with damaging wind gusts the primary threat.

Precipitation will likely come in two different waves: 1) along and
ahead of the northward advancing warm front late this morning into
this afternoon, and 2) ahead of the approaching cold front this
evening. Showers are likely (~60-70% coverage) associated with the
warm front. Though thunderstorms may be more isolated, increasing
low-level shear could lead to a few organized cells with brief
rotation. Precipitation may ultimately be more likely with the warm
front east of the Blue Ridge given questions of instability later
this afternoon and evening. An organized line of convection seems
probable ahead of the cold front late this afternoon, developing
over central WV before progressing eastward/northeastward. The best
mid and upper forcing will be across PA and adjacent eastern
WV/western MD nearer to the synoptic system, but if higher
instability can be realized, the line may extend further south and
eastward into northern/northwestern VA and central MD. Loss of
heating and the departure of the synoptic lift should result in a
steady decrease in showers and thunderstorms by late this evening as
it approaches the I-95 corridor. The 00Z WRF-ARW is most aggressive
with the evening convection, but most other CAM/HREF guidance does
not bring organized convection as far south and east. Therefore,
have kept the highest PoPs/enhanced wording for gusty winds across
eastern WV/western MD for now where confidence is highest, but most
areas west of the I-95 corridor are under a Slight Risk for severe
convection from SPC.

Additionally, if repeated rounds of thunderstorms are observed over
the highlands (where FFG is relatively low), flooding may become a
concern given the high PWAT environment and warm-cloud layers of 13-
14 kft, despite relatively swift storm motions. Eastern WV/western
MD are under a Slight Risk of excessive rain from WPC.


A secondary shortwave and associated surface cold front will cut
across the Great Lakes and then traverse the Mid-Atlantic on
Wednesday. Low-level flow should be northwesterly by this time,
reducing the risk of deeper convection, but gusty showers seem
plausible given the potency of the upper wave.

After the passage of this secondary front, a strong late summer high
pressure system will build in from the Ohio Valley on Thursday. High
temperatures may stay below 80 for much of the area, and with dew
points dropping into the 50s it`ll feel like an early taste of


Surface high pressure will be overhead Friday morning, moving
offshore by Saturday. Temperatures will remain about 5 degrees below
normal Friday, rebounding a few degrees on Saturday as southerly
flow develops. A weak low will be developing offshore from North
Carolina late Saturday and Saturday night, while shortwave energy
moves through the Great Lakes. There`s low probability that any
precipitation from either system affects the forecast area.

While no well-defined frontal systems will be affecting the area
Sunday and Monday, there will be some potential for shortwave
troughs to impact the area, with upper level ridges centered
offshore and near the lower Mississippi Valley. Temperatures will
also be rising back above normal, with dew points returning to
muggier levels in the mid to upper 60s. Thus there is some chance
for showers and thunderstorms, but no organized threat at this time.


IFR/MVFR CIGs are expected north of a slowly advancing warm front
through mid-morning, with MVFR possibly lingering through midday.
There is a moderate level of uncertainty in the convective evolution
and coverage today. Most terminals likely see showers late this
morning through mid afternoon as the warm front crosses. Isolated to
scattered thunderstorms are expected during this time as well as
instability increases. Carrying VCTS ~17-21Z due to scattered
nature, though likely wouldn`t last more than 30 mins at any given
TAF site. More uncertainty in convective evolution this evening. An
organized line will likely develop over central WV this afternoon,
but instability over the area may not allow it to maintain an
organized form as it enters our region this evening. Rather than
carry 8-12 hrs of VCTS, instead mention VCSH this evening due to
lower confidence. Most likely TS would be near MRB 22-03Z with
the second round/cold front.

Flow will generally be SE around 10 kts much of the day, veering to
SW briefly ahead of a cold front this evening then becoming NW late.

Patchy fog is possible later tonight where rain is observed, clouds
clear and winds go light, but the uncertainty in the juxtaposition of
these factors precludes any fog mention from the TAFs attm.

Mainly VFR Wed-Thu, though scattered showers are possible Wed. NW
flow expected.

Hazardous aviation conditions are unlikely Friday and Saturday as
high pressure continues to move across the area.


Veering low-level flow and cloud cover should result in poor mixing
outside of any showers or thunderstorms much of the day today. Low-
level flow increases to 20+ kts under 1000 ft by late afternoon as a
slowly northward advancing warm front crosses. Though lapse rates
remain poor in the boundary layer, given the magnitude of the low-
level winds, have an SCA running for the more open waters tonight.
Gustier winds are possible in any showers and scattered storms.

Gusts may linger on NW flow Wednesday which could require an
extension or expansion of the current SCA. Gusty showers are
possible Wednesday as well as a secondary cold front crosses.

Lighter winds are expected later Wednesday night into Thursday.

Light and variable winds on Friday will become southerly on Saturday
as high pressure moves offshore.


Onshore flow has promoted tidal anomalies between 1 and 1.5 feet,
which will likely be maintained as southerly flow develops behind a
warm front later today. A few Coastal Flood Advisories remain out
for this morning`s high tide. Strait`s Point has the greatest chance
of reaching minor flood stage for the next cycle, though confidence
is low since it`s lower astronomically. It`s likely a few locations
reach minor flood again tonight before a cold front passes and
northwest flow lowers anomalies on Wednesday.


DC...Coastal Flood Advisory until 7 AM EDT this morning for DCZ001.
MD...Coastal Flood Advisory until 7 AM EDT this morning for MDZ011-
MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT
     Wednesday for ANZ532-533-540>542.
     Small Craft Advisory until 6 AM EDT early this morning for
     Small Craft Advisory from 6 PM this evening to 10 AM EDT
     Wednesday for ANZ534-537-543.


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