Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 110123

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
923 PM EDT Thu Aug 10 2017

High pressure will shift offshore this evening and tonight. A
warm front will lift north towards the region on Friday while a
cold front will approach the region from the northwest Saturday,
then stall south of the region Sunday. A wave of low pressure
could move from southwest to northeast along the stalled front
early next week.


An upper level disturbance is exiting the Mid-Atlantic region
this evening. Clouds have cleared out behind it with the
exception of low-level clouds across the Appalachains tonight.
Moisture has returned due to the departing sfc high and low-
level clouds will likely increase tonight. Can not rule out an
isolated shower mainly west of the Blue Ridge tonight.

Previous discussion...

Lows tonight in the 60s to locally near 70F in the urban
centers and along the immediate shores of the tidal Potomac/Chesapeake.


Upper level forcing increases for Friday as the upper trough
enters the Great Lakes region and southwest flow increases
aloft. At the same time, broad moisture advection will continue
and the weak warm front will make slow progress northward
towards the area. Southwest low level flow will also be on the
increase along and west of the Appalachians ahead of an
a surface cold front dropping into the Ohio Valley, while at the
same time, southeasterly flow will still be ongoing across the
coastal plain due to the ridge offshore. This is all expected
to help create an area of broad low level convergence, leading
to the development of showers and scattered thunderstorms on
Friday. Initially the most widespread activity is likely to be
focused west of I-95, but should propagate eastward later in the
day and into the evening and nighttime hours. Poor mid level
lapse rates and CAPE profiles are not conducive for widespread
strong to severe thunderstorms, but given 500-1000 J/KG of
MLCAPE and 0-6KM shear of around 35 knots, a stronger storm or
two cannot be ruled out. Of more concern may be the potential
for localized flooding in any areas that see repeated convection
given recent rainfall and as precipitable water values approach
2 inches and warm cloud depths exceed 10K feet. Highs should
reach the upper 70s to low 80s, with lows Friday night in the
60s to around 70F.

The cold front will drop southeastward into the area on
Saturday, with additional scattered shower and thunderstorm
development expected. Some drying may occur aloft, which could
limit overall coverage of convection. Poor mid level lapse
rates will still exist, but with MLCAPE reaching 1000-2000 J/KG,
0-6KM shear values from 30-40 knots, and the presence of a cold
front, there`s the possibility for some stronger storms. Highs
Saturday reach the low to mid 80s.

The cold front will slowly push southeast through the area and
now appears that it will clear the region by Sunday morning.
This should allow for drying and some clearing by morning. Lows
60s to around 70F.


Based on the latest guidance, Sunday may turn out to be a drier day
in the stretch of unsettled weather. High pressure moving across the
Great Lakes will keep the slow moving front to our south. However,
there may be non-zero rain chances, especially across southern
portions of the area. Upper level trough will dig southeast from
Canada Monday into Tuesday. The surface front will likely lift back
to the north some, and low pressure may develop and track across the
area. While timing is uncertain, rainfall looks like a good bet at
some point during this two day period. The trough will push the
front to the southeast, allowing high pressure to build across the
area Wednesday into Thursday.

Temperatures will be warmer/near normal on Sunday due to increased
sunshine and downslope flow, but with slightly lower humidity.
Unsettled weather will bring high temperatures below normal Monday
and Tuesday, with a relatively cool post-frontal airmass remaining
in place Wednesday and Thursday.


Dry conditions and VFR expected through much of tonight with
increasing clouds. There may also be a period of MVFR ceilings
tonight into early Friday, with the greatest risk at CHO.
Showers and thunderstorms will increase in coverage on
Friday/Friday night, with potential for reductions in both
ceiling and visibility. Additional convective activity possible
Saturday as well. Conditions improve Saturday night.

VFR conditions are more likely on Sunday as the slow moving front
slips to the southeast. The front will return north Monday into
Tuesday. A period of sub-VFR conditions is possible with low clouds
and rain.


Southeast flow will gradually increase through the rest of
tonight, and into Friday. Wind speeds may approach or exceed
Small Craft Advisory criteria for a time later Friday, but
confidence low at this time. In addition, showers and
thunderstorms become increasingly likely Friday and Friday
night. Ahead of an approaching front on Saturday, an additional
window will also exist for some SCA winds. There is potential
for more showers and thunderstorms Saturday as well.

Light NW winds on Sunday with front just southeast of the area.
Front returns north Monday into Tuesday with some uncertainty about
strength and location of any low pressure center, although overall
conditions should remain below SCA levels. Some thunderstorms may
also occur during this period.




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