Area Forecast Discussion
Issued by NWS Sterling, VA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS61 KLWX 301358

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
958 AM EDT Sun Apr 30 2017

A front which is draped across the area will lift north as a
warm front later today. A stronger cold front move east of the
Mississippi River tonight and approach the Appalachians by
Monday morning. This front will cross the region Monday evening.
High pressure will bring cooler and drier weather during the
middle of the week.


At 9 AM surface front in form of wind shift is located from near
Franklin WV to Warrenton to Chesapeake Beach. Low clouds are
farther north than that line. While it appears the southern edge
is trying to scatter into cumulus, am a little concerned erosion
may be on the slow side since winds north of the boundary are
verifying with a more northeasterly component than originally
progged. While this could have some effect on the temperature
gradient, the already-warm start means portions of the Baltimore
area are already nearing their highs despite the clouds.
Temperatures are on their way to the 80s south of the boundary.

There is still a bit of uncertainty in the details of how
convection will transpire today. In the bigger picture, though,
it will be safe to say that north of the boundary surface-based
instability will be nil. To the south, SBCAPE will be in the
neighborhood of 1500-2500 J/kg with more than adequate deep
shear (30-35 kt bulk 0-6km layer). The struggle once again is
finding the trigger. Believe that terrain and frontal
convergence will be that mechanism, suggesting that the Potomac
Highlands will be the favored location. 12Z RNK sounding has
very dry mid-level air which is expected to advect northward,
and all regional soundings show multiple stable layers. Thus am
continuing the previous thinking that coverage will be scattered
at best. If a stronger updraft can develop, gusty winds will be
a threat.


Instability quickly dissipates with the loss of diurnal heating,
and have removed PoPs for the nighttime hours. Area will be
north of the warm front, in an air mass with 60s dewpoints, so
patchy fog development will once again be a concern. Otherwise,
it should be a quiet night. Lows won`t be far removed from

On Monday, a well defined cold front will be approaching the
area. There are still some timing differences amongst guidance.
This bigger disparity, though, is whether thunderstorms will
develop along the front or along a pre-frontal trough. The GFS
is the quickest solution, keying on pre-frontal convection in a
diurnally favored timeslot. If this happens, then strong to
severe thunderstorms will be possible. Other solutions suggest
thunderstorms will be nearer the front, and timed more in the
evening. The challenge will be how much instability will
generated; more than ample shear is present. PoPs will be likely
to categorical nonetheless, with emphasis on the later afternoon
and evening hours.

Cooler and drier air will overspread the area Monday night
(overnight) into Tuesday morning. We will have a glancing blow
from the upper trough axis during the day Tuesday, which will
sustain gusty winds much of the day. But since the air will be
drier and flow will be downsloping, don`t believe that clouds
will be numerous. Temperatures will be closer to normal during
the day, with nighttime low in the 40s and 50s again.


High pressure will build into the region Wednesday. Cooler
conditions expected mid-week as northerly winds usher in cooler
and dryer conditions. High temps in the U60s/L70s expected Wed.
A storm system will be coming together across the Mississippi
Valley Wed night. Warm air advection will begin across the
region Wed night-Thursday and clouds and showers are expected.
More showers are expected to move into the region Thu night-
Friday as the system becomes more dynamic. The upper level low
becomes a closed low Thu night and sfc low pressure will
continue to deepen across the Ohio Valley. This system may bring
thunderstorms capable of heavy rain as the wind field
strengthens and dewpts rise rapidly Friday. Timing is uncertain
and it is unknown at this time if conditions will be hazardous.
Low pressure will likely move overhead Fri-night- Saturday.
Clouds and showers will likely keep temperatures below normal
mid-late week.


A backdoor cold front has slipped south of the metro terminals
but not to CHO, providing a complicated flight category regime.
MVFR seems to be the rule on the north side of the boundary.
Have rather low confidence at how quickly the restrictions will
erode, especially since winds are verifying from a more
northeasterly direction. However, satellite shows the southern
edge of the clouds trying to erode. Expect BWI/MTN to be
affected the longest. Scattered thunderstorms should develop
south of the front, but direct impact at an airfield too limited
to forecast at this thinking the highest chance
will be near MRB.

Anything that does develop will dissipate with sunset tonight.
The air will be humid enough for more patchy fog development
overnight into early Monday morning. Will be keeping forecasts
at MVFR. Lower results certainly possible.

A well defined cold front will approach the terminals Monday,
and cross the region late Monday into Monday night. Numerous
thunderstorms anticipated along/ahead of the front, with brief
aob IFR along with locally gusty winds.

The front will clear the area late Monday night. VFR expected
through Wednesday. Rain will spread east Wednesday night into


A backdoor front has slipped to around Chesapeake Beach, and
further southward progression may be limited before gradually
returning northward. Flow will be northeast/east to its north,
and southeast/south to its south and shouldn`t be much more
than 10 kt sustained, so Small Craft Advisories are not
anticipated. This front will return to the north this evening.

Gradient flow (southerly) will increase overnight into Monday
ahead of a stronger cold front approaching from the west. A
Small Craft Advisory goes in effect after midnight tonight for
southern waters, and then for all waters on Monday. The front
itself will impact the marine area Monday night, although some
thunderstorms may develop ahead of the front Monday afternoon.
These thunderstorms may contain gusty winds.

Behind the front flow turns westerly. Mixed profiles appear
deep, and expect Small Craft conditions to extend into Tuesday.

A storm system may impact the waters late in the work week.
Showers and thunderstorms are possible as well as gusty winds
that may produce SCA conditions.


Water levels running very close to astronomical normals this
morning. Anticipate that will remain the case through the next
two tide cycles. ESTOFS does suggest that water levels will rise
in the northern Bay, but believe that is a result of a model
bias and am neglecting that solution.

After that, uncertainty grows as model solutions become more
divergent. Am providing a forecast of water levels below caution
stage Monday. South flow could sustain an increase. Some
solutions suggest these elevated levels could linger into
Tuesday, as winds will turn gusty but will be from the
west...and thus won`t prompt a blowout.

Low pressure will come up the coast Thursday, which likely will
increase water levels from wherever they stand up to that


Record highs were set at DCA and IAD yesterday, and record warm
lows were set at all three airports (DCA, IAD, and BWI). The
record warm low at DCA was also a record for the month of April.

Records highs are less likely today as a backdoor front drops
down. However, the record warm lows are a possibility. Here are
the record highs/warm lows for Sunday:

Washington DC... High 92 (in 1942); Warm low 67 (in 1983).
BWI Airport... High 92 (in 1910); Warm low 63 (in 1983).
Dulles Airport.. High 86 (in 2007); Warm low 64 (in 1983).


MARINE...Small Craft Advisory from 8 AM to 6 PM EDT Monday for
     Small Craft Advisory from midnight tonight to 6 PM EDT Monday
     for ANZ534-537-543.
     Small Craft Advisory from 4 AM to 6 PM EDT Monday for ANZ533-


CLIMATE...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.