Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210801

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
401 AM EDT SAT MAY 21 2016

Low pressure will track up the Mid-Atlantic coast today. Upper
level low pressure will persist along the Mid-Atlantic coast
tonight through Monday night. High pressure returns Tuesday and
persists through mid-week.


High pressure is retreating off the New England coast this morning
as low pressure moves toward the Smokey Mountains. An area of
moderate to locally heavy rain is overspreading the area in
association with convergence/moisture advection from a low level
jet. The rain is having to overcome some dry air, so the better
rates are delayed from the onset despite radar appearance. A few
observations suggestion locations south of Charlottesville have
already received an inch or more.

Expect this shield of rain to progress northeastward during the
morning hours. The heavier rain looks progressive enough to preclude
much in the way of flooding. The main question of the day revolves
around a mid-level dry slot. This is already working into SW VA, and
indications are that the rain will shut off fairly quickly in the SW
CWA. However, energy will be transferring to a coastal low by this
afternoon, and may keep areas near and east of I-95 fairly wet in
wrap-around (but lighter) moisture. Even where higher QPF isn`t
depicted, wouldn`t rule out some light rain or drizzle at times.
Meanwhile the upper level trough is expected to spark convection
this afternoon in WV, and some of this activity may work toward
Highland Co. The thick low clouds will mean little movement to
temperatures...with highs struggling to reach 60F in many areas.


The best way to describe the short term period is
"unsettled"...although gradually less so with time through Monday
night. As energy rotates into the upper trough Saturday night, a
closed low will eventually develop, reaching the Outer Banks by
Monday, then perhaps drifting offshore by Monday night. With the
subtlety in disturbances within this pattern, timing and placing the
highest rain chances will be difficult. Have favored north/west
Saturday night where greatest height falls and moisture will be. On
Sunday, have brought a period of likely POPs across the area as
trough axis/upper low moves overhead. Can`t rule out some thunder,
although looks like a very minimal in the far west.
Have calculated a "storm total" through Sunday, which ranges from
less than one inch NE of Baltimore, with 1.5 to 2 inches from DC
south/west. Of course, locally higher is possible.

Expect shower chances to diminish Sunday night, only to renew
Monday, with highest chances in the east closer to the upper low.
There`s a little better chance of a thunderstorm or two on Monday as
it looks like there will be a better chance of seeing some breaks of
sun. The drying trend should begin in earnest Monday night.

Sunday`s temperatures will largely depend on where it rains the
longest, and models differ strongly regarding this. Splitting the
difference maintains continuity of lower to mid 60s. Right now
Monday is looking fact guidance is has widespread 70s.
Given unsettled pattern, played conservatively closer to 70.
Overnight lows remain fairly constant in the 50s.


Upper level low pressure will be departing eastward on
Tuesday...with convergence and warming temperatures aloft. Temps
will rise and clouds will break, but still a slight risk of a
shower is showing up in guidance. A ridge axis will approach from
the west Wednesday, continuing the confluence and the warming.
This ridge axis will weaken Thursday as southwesterly flow
develops aloft, but heights will continue to build resulting in
more warming. Instabiilty may result in convective showers or
thunderstorms developing over the mountains and possibly moving
east into the metro. Another weak ridge axis will cross the area
Friday with heights remaining high, though a trough will be
approaching from the west. More showers and storms may form over
the mountains and advect into the metro.


Shield of rain advancing NE across the area. Restrictions are taking
some time as atmosphere needs to saturate. MVFR is eventually being
reached, although IFR has been harder to come by so far. However,
lower cigs are poised to our SW, and actually expect a decreasing
trend through the day. Moderate rain will taper off by midday or so,
but expect some light rain to continue at times through the
afternoon, especially the metros.

The forecast for tonight through Sunday night is rather uncertain
with upper level low pressure moving through the region. It will be
unsettled to say the least with MVFR looking likely for much of the
time with occasional showers. Don`t have confidence to time in any
IFR. Monday should see improved conditions but a lingering chance of
showers, especially afternoon.

VFR Tuesday and Wednesday with high pressure regaining control.


Expect SE to E winds to increase this morning as low pressure
develops off the coast. The strongest winds may actually occur
through midday until clouds and rain can sufficiently limit mixing.
However, no changes to the SCA at this time. Wind should diminish
tonight, but will have to monitor for northerly channeling. N to NW
flow will continue through Monday around 5-10 kt. Light and variable
by Monday night.


MARINE...Small Craft Advisory from 8 AM this morning to midnight EDT
     tonight for ANZ530>533-538>541.
     Small Craft Advisory until midnight EDT tonight for ANZ534-537-
     Small Craft Advisory from 8 AM this morning to 7 PM EDT this
     evening for ANZ535-536.


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