Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 281252

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
852 AM EDT Fri Apr 28 2017

High pressure builds over the area today, then slides off the
southeast coast Saturday. A cold front will approach the region
from the north Saturday night into Sunday before lifting back
northward late Sunday. A cold front will move through the region
from the west Monday night. Canadian high pressure will then
build over the area through the middle of next week.



High pressure builds into the area today. Dewpoints fall into
the U50s. Despite frontal passage...highs only a few degrees
cooler than yesterday (U70s/L80s across the area).

Front lifts back north during the late afternoon/early evening. This
will allow moisture to surge back into the area...with
dewpoints rising into the U60s by 12z Saturday. Increasing
moisture will lead to very warm nighttime temperatures...with
high minimum records in jeopardy (see climate section below for
more details).


Mid-Atlantic remains sandwiched between Bermuda high and low
pressure over the plains through the short term. This will create a
constant fetch of moisture rich low-level air with source regions
across the Gulf of Mexico and Tropical Atlantic. At the same time, a
frontal boundary will be located nearby...with embedded shortwaves
riding the periphery of ridge aloft. These features will provide the
necessary ingredients for deep moist convection across the Mid-
Atlantic. Where exactly storms form will be largely dependent on
frontal location. Guidance continues to struggle with handling the
spatiotemporal evolution of front...which makes front
will likely be reinforced by convection. Thus, Sunday`s forecast
remains very uncertain. Additionally, it remains unclear if
convection can develop in the warm capping inversion due
to nearby ridging may stunt convective initiation. Though...terrain
circulations may provide enough additional lift to allow parcels to
reach their LFC.

If storms can develop over the area the favorable juxtaposition of
CAPE/SHEAR parameters imply locally strong storms would be possible.
Best chances for storms generally from DC northward and across the
higher terrain. This fits well with current Day 2 SPC Marginal
Outlook. If it appears convection is more likely across our area
would expect threat level to increase somewhat.

Additionally, unseasonably warm weather will overspread the region
south of frontal boundary. Record highs are possible Saturday (highs
near 90F) and record high minimums Saturday night and Sunday night
(see climate section below for more details). To the north of the
front temperatures/dewpoints will be ~10 F cooler than to the


By Monday morning, the forecast area will be in the warm
sector, with the warm front approaching the Canadian border and
the attendant cold front crossing the Midwest. Fairly well
stacked low pressure will be emerging from the Plains. GFS/ECMWF
in decent agreement on synoptic features, with a Monday evening
cold frontal passage. The air mass prior to fropa will contain
plenty of shear, but instability isn`t jumping off the charts.
GFS holding onto a line of QPF across area, while ECMWF
weakening system as energy crosses Great Lakes. Am fairly
comfortable forecasting thunderstorms, especially during the
late afternoon and evening hours, but the strength and areal
coverage both up for debate.

Zonal flow aloft will dominate Tuesday and Wednesday, as the upper
low migrates across the Great Lakes to the St Lawrence Valley.
Several pieces of vorticity potentially could eject east in this
fast flow. Thus, it won`t be purely sunny. Deep westerly flow
suggests that clouds/precip may get caught up in the mountains.

By Wednesday night and Thursday, guidance is hinting at a pattern
change, as a trough axis cuts across the central CONUS. Low level
moisture in advance of the trough advects off of the Gulf of Mexico,
with a plume of precipitation approaching the Mid Atlantic.
Interactions potentially could even induce cyclogenesis by the end
of the forecast period.

Monday`s temperatures will be relatively similar to Sunday. Weak
cold advection will have in impact for midweek, as highs slowly ease
closer to normal. That subtlely will end by Wednesday night and
Thursday in what possibly could be a wet and cool period.


VFR will continue through at least Saturday high
pressure influences the weather. Cold front will approach the area
from the north Saturday. If it makes it into the area thunderstorms
would be possible (mainly at MRB/BWI/MTN). Front may sag further
southward into the area late Saturday into Sunday with another
possibility of chance along/north DCA/IAD. If a
storm impacts a terminal it could be locally strong with brief sub-
VFR conditions.

A cold frontal passage Monday PM will likely result in
thunderstorm development. The areal coverage and strength both
up for debate. However, local/brief IFR possible within
thunderstorms. VFR expected behind the front on Tuesday.


Relatively light winds expected over the waters Friday into Sunday.
If a cold front can slide into our area from the north...shower and
thunderstorms would be possible Saturday afternoon/evening (mainly
for northern Chesapeake Bay). Another round of storms possible
Sunday as front remains nearby...again best chance on the northern
waters. Any storms that develop during this period could be locally

Gradient winds (from the south) will increase ahead of a cold front
Monday. Its unclear how well vertical transport will overcome the
cooler waters, but Small Craft conditions at least possible. Then,
with the cold frontal passage, some thunderstorms with strong gusts
possible. West flow behind the front Tuesday will provide a high
confidence Small Craft Advisory day.


Tidal anomalies remain elevated across the area as southerly flow
has kept water from evacuating the estuary. Coastal Flood Advisory
continues for Straits it is currently exceeding minor
flood stage. Issued a Coastal Flood Advisory for this tide cycle at
Annapolis since forecast of minor tidal flooding is within the
margin of error (a tenth of a foot or so). Minor flooding is also
possible this morning at DC. Though, confidence is too low to act on
it now...will continue to monitor.

Tidal anamolies are expected to decrease slightly over the next
several days as wind flow decreases. However, return of southerly
flow later today into the weekend will keep anamolies elevated.


Record highs/warm lows through the weekend:

     Washington DC...
Friday 28 April...92 (in 1957)/66 (in 1990)
Saturday 29 April...91 (in 1974)/68 (in 1956)
Sunday 30 April...92 (in 1942)/67 (in 1983)

     BWI Airport...
Friday 28 April...90 (in 1957)/67 (in 2009)
Saturday 29 April...91 (in 1974)/67 (in 1956)
Sunday 30 April...92 (in 1910)/63 (in 1983)

     Dulles Airport..
Friday 28 April...88 (in 1990)/62 (in 2009)
Saturday 29 April...87 (in 1996)/62 (in 1996)
Sunday 30 April...86 (in 2007)/64 (in 1983)

The only day when record high temperatures will be challenged
appears to be Saturday. Record warm low temperatures, on the other
hand, may be broken (or come close) each day.


MD...Coastal Flood Advisory until 9 AM EDT this morning for MDZ014.


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