Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 230128

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
928 PM EDT Sun Apr 22 2018

High pressure over the Great Lakes will slowly move east into
the Northeast U.S. tonight. This high will move offshore
Monday. A low pressure system currently over the lower
Mississippi Valley will bring unsettled weather to the region
Monday night through Wednesday. Another storm system may impact
the region at the end of the week.


Latest surface analysis depicts high pressure centered over the
eastern Great Lakes, with an extension southeast to a second
center east of Norfolk in the western Atlantic. The axis of a
shortwave trough aloft is passing overhead, and with it, clouds
have been clearing out. The forecast has been indicating clouds
lingering into the night, mainly due to a persistence of the
stratocumulus which has failed to occur. Clouds have been
redeveloping over the Eastern Shore in the past hour, so will
observe these trends before completely clearing out the night.
Otherwise, dry weather will be the rule for the remainder of
tonight. With dew points now in the mid 30s to lower 40s, we
should also stay comfortably above freezing. Lows will likely be
in the 40s for most, but could drop into the upper 30s in
clear, calm rural valleys.

On Monday, the high pressure system will shift eastward off the
coast of New England, while low pressure over the lower
Mississippi Valley at present will migrate northeast towards
central Tennessee. As this happens, isentropic lift and moisture
advection will increase with continued southeast flow from the
Atlantic, and rain is likely to start breaking out from
southwest to northeast over west central Virginia. However,
most of the region should remain dry through sunset, but with
increasing clouds. Despite the southeasterly flow, temperatures
should generally remain as mild as today, or perhaps moderate a
tad further, with 60s common. Coolest readings will be in west-
central VA where greater cloud cover and the threat of late-day
rain will hold temps down.


Closed low over western Tennessee early Monday night will
migrate east and north into Kentucky through Tuesday, finally
opening back up into a shortwave trough Tuesday night as it gets
punted eastward more rapidly thanks to another shortwave
droppingsoutheast across the northern Plains. In response to
this, surface low pressure over the Tennessee Valley will start
to transfer to a new low in the Carolinas on Tuesday. This
second low will move northeastward up the coast Tuesday night
and Wednesday. Isentropic lift, upper divergence, fgen and
moisture advection, combined with upslope low level flow from
the southeast, should provide adequate forcing for a decent dose
of rainfall. The question which we are stuck with is: Just how
heavy? Many areas had pretty significant rain last Monday, but
it has now been about a week, and will be 8 days past on
Tuesday, so while soils are still more moist than they were
before Monday`s rain, they have dried out a bit, and rivers have
also gone down. Right now, guidance is fluctuating a bit with
regards to how much rain will fall, but best estimate places
most of the region in the 1-2 inch range. Locally higher amounts
may result in isolated flooding, but confidence is not high
since the surface low should stay southeast of the region,
potentially limiting the convective elements. Thus, will hold
off on posting any mention of flooding in the HWO for the time
being, though it is noted that WPC has placed the region in a
marginal risk of excessive rainfall.

With the passage of the low pressure and accompanying clouds and
rain, temperatures Monday night through Tuesday night should not
fluctuate much, with 40s for lows Monday night, and temperatures
mostly in the 50s Tuesday and Tuesday night.


Low pressure will be moving northeast away from the area
Wednesday morning, leaving a chance for some residual showers in
its wake. Another quick moving shortwave will move through
Wednesday evening into Thursday morning. Some showers will be
possible with this system as well, especially Wednesday night.
Temperatures Wednesday will be below average, thanks to the
cloud cover and precipitation expected over the area.

Weak high pressure will move into the region in the wake of
this system on Thursday into Thursday night, bringing drier
weather into the forecast again, but temperatures remain below

A lot of uncertainty for Friday`s forecast at this time. One
thing that seems consistent among all guidance, is that a
shortwave will move across the southeast late Thursday into
early Friday. This shortwave spawns a surface low somewhere in
the southeast in that same time period. The uncertainty comes
with determining where exactly this system moves, and how strong
it will be. Model guidance has been very jumpy with this
system, thus not giving a clear pattern to run with. On one
hand, you have the European, which slides this system off the
coast and keeps precipitation chances here very low. However,
both the Canadian and GFS want to bring the low up the coast,
bringing precipitation chances to the area on Friday. These
models differ from one another in terms of strength as well,
with the GFS phasing the shortwave with the main trough axis,
thus bringing a stronger system to the area, while the Canadian
does not phase the systems, resulting in a weaker coastal low,
and less precipitation. The GFS had been more aligned with the
European the past few runs, so going to go with a more
intermediate approach and say that we will see some impact from
a coastal low, but not thinking it will be as strong as the
latest GFS suggests. The bottom line is, precipitation is
possible Friday and temperatures will greatly depend on the
evolution of the aforementioned surface low, but thinking we
will be below average once again.

Another low pressure system looks to swing through areas along
the Canadian/Northeastern U.S. border on Friday night into
Saturday. A cold front associated with this system looks to move
through the area on Saturday, bringing another chance for
precipitation. Model guidance has backed off a bit on the
strength of this front, but some light precipitation is
certainly possible. The time of frontal passage will determine
how warm we can manage to get on Saturday, but with lingering
clouds from Fridays system, and clouds associated with the
aforementioned front, temperatures will probably remain at, or
slightly below, average.


VFR all terminals through late Monday, though cigs may start
lowering towards MVFR across the region by then, especially at
CHO. At the moment, this looks to occur after 00Z Tuesday. Winds
mostly from the southeast, but light until Monday afternoon,
when the gradient between the high departing and the low
approaching will increase flow to 10-20 knots.

Sub-VFR likely by late Monday night through Tuesday night all
terminals, with intervals of IFR possible thanks to rain, mist
and low cigs. Winds shifting from southeast to north by late
Tuesday night.

Clouds will stick around for much of the day on Wednesday Sub-
VFR conditions possible during this time. Conditions should
clear to VFR by Thursday during the late morning to early
afternoon hours.


Southeasterly channeling on the Potomac below US 301 has
subsided, with south to southeast winds 5-12 kt expected for the
remainder of the night and into Monday morning.

SCA up for lower Potomac and middle Chesapeake on Monday
afternoon and night as southeasterly flow ahead of approaching
low increases. Have added the middle Potomac to this SCA since
channeling could occur once again. Carried the headline through
Monday night with the other zones, although it`s possible the
channeling subsides with sunset. SCA may expand to all zones on
Tuesday as low approaches and winds shift to east and then
perhaps northeast, finally turning north and diminishing later
Tuesday night as the low passes. Note that the southeastern
waters we cover, most likely the middle bay, could actually
become southerly if the low passes to the north of those areas.
Rain and fog will be a concern as well during this period.

As the storm system from Tuesday exits Wednesday morning, some
gusty winds are possible across the marine areas, with SCA
criteria winds possible during that time. Gusty winds also
possible Thursday morning, but not looking as strong as on
Wednesday at this time.


While we had significant rain a week ago, its been pretty dry
since then. That said, 1-2 inches of rain looks likely across
the region Monday night through Wednesday, which could
lead to some minor hydrology issues and cause river and stream
rises, potentially up to flood stages. Will be closely
monitoring this for potential need of any flood watches over the
next day or so.


As low pressure approaches the waters from the southeastern
U.S. late Monday into Tuesday, a persistent onshore flow will
develop. This will yield increasing tidal anomalies, and the
possibility of coastal flooding toward the middle part of the
work week.


MARINE...Small Craft Advisory from 1 PM Monday to 6 AM EDT Tuesday for


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