Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 310108

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
908 PM EDT MON MAY 30 2016

A weak cold front will enter the region from the west tonight and
stall near or just south of the area on Tuesday. Another cold front
will move in from the north Tuesday night and Wednesday, then also
stall out. A third cold front will push in from the west Thursday
night and Friday.


CWA is under the influence of two systems this evening. First,
subtropical moisture feed around offshore ridge/remnants of Bonnie
has been slowly edging east, allowing persistent showers to exit
southern MD. Second, a weak cold front is crossing the
Appalachians, with scattered convection (albeit weakening
lightning trends) ahead of it. Despite analyzed instability ahead
of it (500 J/kg MLCAPE on 00Z IAD sounding), most of the
convection has had a very difficult time pushing east of
I-81...although some shower activity is now reaching the Blue
Ridge. There is another cluster of heavy showers in SE PA which
may clip NE MD before dissipating. RAP/HRRR have had very poor
handle on this convection, so have largely based forecast on radar
trends and the idea that any lingering showers will gradually
diminish with nocturnal cooling (which the models do show). Will
need to keep an eye on radar though with the weak boundary and
lingering instability.

The other issue of the night will be fog development, as dewpoints
will be slow to fall, if at all, over most of the area...and winds
will be nearly calm. Guidance paints a muddled picture, and
persistent clouds may hinder denser fog from developing. Will
maintain patchy wording for now. Don`t see a need to adjust lows,
which are forecast in the 60s in much of the area.

On Tuesday, the front will be even more diffuse as it settles
south across the region. Drier and sunnier conditions will be more
likely the farther north and west you go. However, a few showers
and thunderstorms may still develop over southern parts of the
area. Highs in the lower to mid 80s expected.


Moisture from Bonnie will linger not far to the south for the next
few days...but forcing will be weak. A backdoor front will try to
come southward Tuesday night and Wednesday which will try to
suppress the moisture further south and cool our area a bit.
Temperatures will be warmest Tuesday ahead of this front with some
sun...then drop a few degrees each Wednesday and Thursday. Lingering
moisture and some sun-fueled instability...not to mention
potential forcing of the stalling boundary itself...will still try
to get some spotty showers and thunderstorms going each of the
next few days...but coverage does not look high. More pop-up than
anything organized. Things get a little more interesting late
Thursday as the next front approaches but instability still looks
relatively meager. Highs will stay in the 80s with lows mostly in
the 60s.


A weak cold front, associated with surface low well to our north
over Canada, will move through the area during the day Friday
bringing a chance for showers and thunderstorms to much of the

Model divergence increases by next weekend, as long term guidance
differs with position/strength of mid and upper level trough digging
into the NE. Will maintain low-end pops for the possibility of
unsettled conditions next weekend into early next week, with best
shower and thunderstorm chances Sunday into Monday as cold front
moves through.


Convection to the west is dissipating, with perhaps a sprinkle
reaching MRB. Shower developing along Blue Ridge west of IAD will
be monitored. Meanwhile, MVFR clouds have pushing into the I-95
corridor. Guidance hints these should break apart within a few
hours of midnight. Patchy fog is a concern tonight. MRB and CHO most
likely to go down to IFR but its possible at the other terminals
as well. Have handled vsbys conservatively due to muddled guidance
(although notably MOS hits vsbys harder) and potential lingering
cloud cover. Spotty shower/storms again Tuesday and Wednesday
afternoons but coverage looks low...with slightly higher chances
near CHO. Winds generally less than 10 kt through the period.

Cold front moves through the area late Thursday into the
day Friday, with a scattered showers and thunderstorms expected
across much of the area.

Confidence in the forecast going into the weekend remains low as
upper level trough digs into the Northeast.


Southerly channeling has resulted in a few 15-20 kt gusts over
the central part of the MD Bay, but these have been isolated and
brief, with 10-15 kt on average. Weak front will turn winds to the
north by morning. Will need to watch possible surge behind
backdoor front Tuesday night into Wednesday. Storms may impact
waters (more-so with southern extent) over each of the next
several days but odds are not great.

Thunderstorm chances will be the main concern over the waters
Friday (and perhaps into next weekend).




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