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Area Forecast Discussion (AFD)

Issued by Sterling, VA (LWX)

FXUS61 KLWX 280752

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

A weak cold front crosses the area through morning. High pressure 
builds over the area for the remainder of 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.


Weak cold front currently crossing the area...surface analysis place 
north/south oriented front just west of the Blue Ridge...with wind 
shift/dewpoint gradient marking the boundary. The front will push 
through the area and stall just to our south today across central 
VA. It will then lift back north of our area late today. 

Main impact of frontal passage will be drier air infiltrating the 
area...with dewpoints falling from the L/M60s back 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 quality 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


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 sense...as 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 sector...as 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 within 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 morning...as 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 thunderstorms...best chance along/north DCA/IAD. If a 
storm impacts a terminal it could be locally strong with brief sub-
VFR conditions.

Could be some fog around again tonight as low levels moisten. 
Though, confidence is very low at this point (not in TAF) especially 
considering light southerly flow remains overnight.

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 Point...as 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.
     Coastal Flood Advisory until 6 AM EDT early this morning for