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 211840

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
240 PM EDT Sat Apr 21 2018

High pressure over the Great Lakes will slowly move east into
the Northeast U.S. by Sunday night. This high will move offshore
Monday. A low pressure system will bring unsettled weather to
the region Monday night through the middle of the week.


Latest surface analysis depicts high pressure centered over the
Great Lakes, with a second center, slightly weaker, located near
Norfolk. This high will dominate our weather into Monday,
providing mostly dry and slightly cooler than average
conditions, though temperatures will be slowly moderating
towards normal.

Tonight, while the high will be overhead, a shortwave will be
advancing toward the region from the northwest. This shortwave
should bring a band of clouds with it as it passes, and while
they will at midlevels, it will likely be enough to prevent
temps from getting too close from freezing in the growing season
region. Thus, no frost or freeze headlines. Temps will get
chilly, with colder spots in the 30s, but along and east of I-81
it should generally be no lower than the upper 30s.

Clouds will remain through much of Sunday, and in fact lower a
bit. As the axis of the shortwave itself passes the region late
in the day, there could even be a stray sprinkle or light
shower. Guidance depicting this potential concentrate it near
the Appalachians of eastern West Virginia and western Virginia,
though one or two models do show a little light precip near
southern MD. For the time being have the forecast dry since odds
remain low, but if this trend continues, may have to introduce
an isolated shower/sprinkle into the forecast. While clouds will
dominate on Sunday, there should be enough insolation, coupled
with the higher starting point, to bring temps back to the 60s
for mores spots once again.


Odds of precip diminish Sunday night as a weak ridge moves
overhead. Clouds should also diminish, but there will likely
still be enough, combined with the continued moderation of the
air mass, to keep temps above frost/freeze thresholds, and in
fact, it should mostly stay in the 40s.

The ridge aloft pushes northeast past us on Monday as the closed
low moves across the lower Mississippi Valley. At the surface,
the high will move off the coast. This will allow clouds to
return, and in fact guidance is now edging precip into west-
central VA by day`s end. However, there will still be enough
insolation to boost temps into the 60s for most of the region.

As the low slowly pushes northeastward toward us Monday night,
the chance of precip will continue to increase from southwest
to northeast as the night progresses. Temps should be warm
enough for plain rain even at the higher elevations, though some
spots could edge into the upper 30s. Otherwise, temps will
mostly stay in the 40s with the clouds preventing any
radiational cooling.


Early on Tuesday, we will be dealing with a couple different
features that will act together to bring precipitation to the
region. First, the surface low moving out of the southeast and
developing off the North Carolina coast. The second is a cut-off
upper low that will move out of the southeast into the southern
Ohio Valley. Recent model runs are relatively consistent with
one another, and begin to merge this cut-off upper low with the
main axis of the jet stream. As this occurs, the resulting
shortwave moves into our area from the west Tuesday evening. The
combination of lift provided by the shortwave and the surface
low moving up the eastern seaboard should bring a period of
moderate rainfall Tuesday night, and continuing into Wednesday
as light to moderate rainfall. Temperatures on Tuesday and
Wednesday will remain slightly below average thanks to this
unsettled pattern.

Things should taper off to lingering showers on Wednesday
evening, as the low pressure system lifts to the north. However,
unsettled weather will continue in the form of light showers on
Thursday, as a relatively strong shortwave moves south of the
area. A weak surface low begins to form off the NC/VA coastline,
in association with this shortwave, late Thursday. This low
should quickly be taken to sea and have little effect on the
area. Temperatures on Thursday will be very similar to Tuesday
and Wednesday. With so many disturbances moving through the
area, it is going to be hard to get really warm.

A few lingering showers very early on Friday should taper off,
and skies should clear early in the day. A more potent low
pressure system will be moving along the US/Canada border
through the day Friday, but the cold front associated with this
system will affect us sometime during the day on Friday. Model
spread is great with exactly when the front will pass through at
this time, but a period of precipitation is possible as this
moves through. The timing of this front will be a big
determining factor when it comes to temperatures on Friday
afternoon, but for now the latest thinking is that the front
will move through early to mid-afternoon, allowing for
temperatures to at least get closer to average for this time of

The development of this low pressure system beyond Friday
introduces great uncertainty to the forecast. The GFS brings
precipitation to our northern areas as the low pressure system
strengthens in the northeast, while the Euro brings practically
nothing to the area on Saturday. Some ensembles do support the
possibility for some rain on Saturday, but the key is that it
does look to be light at this point if we do see any.
Temperatures look to return to below average.


VFR thru Monday all terminals as high pressure dominates. Winds
mostly light and variable, though becoming easterly Monday. Cigs
and vis may diminish in rain later Monday night, especially at
CHO. Least likely terminal to be impacted through Monday night
is BWI.

A very unsettled pattern will develop on Tuesday, and last
through Thursday, as several systems move across the region. Low
clouds will certainly be on the table throughout the long-term
period, so expect sub-VFR conditions throughout much of this


Winds light through Monday morning, generally 5-10 knots on the
waters. Southeasterly flow increases Monday as high pressure
slides off the coast and low pressure approaches from the
southwest. SCA level gusts may begin late Monday or Monday night
as the system approaches.

The primary concern in the long term will be out ahead of a
developing surface low on Tuesday along the NC coast, which will
move north into the area. Windy conditions can be expected on
the waters, bringing about the possibility for SCA. Wind should
be calmer by Wednesday.


Persistent onshore flow will develop Monday and continue Tuesday
with an approaching storm system. This will likely result in
increased tidal anomalies, and possibly some coastal flooding,
by the middle part of the upcoming work week.




TIDES/COASTAL FLOODING...RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.