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Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
313 PM EDT Fri Jun 22 2018

Synopsis...
a warm front will slowly approach the region tonight before
passing through Saturday. Low pressure will move into the
Great Lakes during this time, and the cold front associated
with the low will approach Sunday before passing through
Sunday night. High pressure will return for Monday and Tuesday
before moving offshore during the middle portion of next week.

&&

Near term /through tonight/...
a surface boundary bisects central Virginia this afternoon with
a moist east northeast flow from the Atlantic to the north of
said boundary. The area of rain that plagued locations along and
east of the Blue Ridge this morning has waned a bit as it
entered a more stable airmass to the north. Temperatures have
held pretty steady thanks to the cloud cover and rain coverage,
ranging in the middle 60s to near 70 degrees area wide.

The aforementioned boundary will continue lifting northward
through tonight, as showers and thunderstorms develop and press
eastward this afternoon and evening. Areas that have experienced
heavy rain over the last 24 hours are poised to see additional
rainfall through this evening. As such, we have issued a Flash
Flood Watch until late tonight for portions of central and
northern Virginia, and southern portions of eastern West
Virginia.

The potential also exists for some strong thunderstorms along
the extreme southern portions of our cwa, mainly across central
Virginia and southern Maryland this evening. Models do indicate this is
where the best instability will reside, with the NAM showing
0-6km shear values 30-35kts and MUCAPE 1000-1500 j/kg. GFS holds
the boundary a bit further south, thus keeps much of our County Warning Area on
the stable side. Regardless, there will be some potential for
some stronger storms across these areas this evening, and will
be dependent on the location of the boundary.

The showers and thunderstorms this evening will favor a more
progressive nature with the increased flow aloft, but combined
with the rainfall already received over the past 24 hours, a
continued flooding threat remains overnight for the areas
highlighted in the Flash Flood Watch. High res guidance
indicates a downward trend in precipitation coverage after
midnight, as activity moves off to the east northeast. Overnight
lows will remain in the 60s for most.

&&

Short term /Saturday through Sunday night/...
the front will reside near the Potomac River Saturday morning,
lifting northward as a warm front through the day. This will
provide a warm and unstable airmass over the County Warning Area. At the same
time and upper level low will be rounding the lower Great Lakes
with vorticity advection passing overhead. This combined with
the warm front ushering in a warm and moist airmass, scattered showers
and thunderstorms will develop mid morning Onward. Given the
aforementioned set up and ingredients, some of these storms
could be strong.

The stalled boundary will reside nearby on Sunday, with the
warm and humid air mass sticking around as well. Additional
showers and storms will be possible during Sunday afternoon with
daytime heating. The flow aloft will favor more of a westerly
trajectory as opposed to Saturday, with not as much favorable
lift aloft. The stalled boundary looks to translate to a cold
front and move through the region later on Sunday, helping bring
a bit more stable environment to the region Sunday night, as
well as drier conditions. With the threat for precipitation and
ample cloud cover, do not expect temperatures to stray too far
from normal or slightly above through the period.

&&

Long term /Monday through Friday/...
Canadian high pressure will build overhead for Monday through
Tuesday, bringing dry conditions along with sunshine and low
humidity. Max temps will be in the lower to middle 80s for most
locations with min temps in the 50s and 60s.

The high will move offshore Wednesday through Friday and a west
to southwest flow will cause a warming trend in temperatures
along with a return of more humid conditions. The increased heat
and humidity may lead to a few showers and thunderstorms each
day, especially during the afternoon and evening hours.



&&

Aviation /18z Friday through Wednesday/...
IFR to MVFR forecast through the overnight period with episodes
of LIFR conditions possible as well. The terminals will see
showers move out of the area early this afternoon, as additional
showers and storms develop and approach from the west southwest.
The strongest activity and greatest impacts to aviators will be
at cho where better instability and higher rainfall rates are
expected to reside. Will maintain rain chances at all terminals
through much of the night before tapering off after midnight
from west to east. Expect restrictions to remain overnight with
ample moisture remaining over the area.

Gusty easterly breezes this afternoon and evening will slacken
overnight, turning light and southerly Saturday morning.
VFR/MVFR cigs will hold on during the day on Saturday.
Additional showers and thunderstorms will be possible through
the remainder of the weekend, bringing periods of sub-VFR
conditions.

High pressure will bring VFR conditions Monday and Tuesday. The high
will move offshore Wednesday, and this may trigger a few showers and
thunderstorms, especially during the afternoon and evening
hours.

&&

Marine...
a frontal boundary south of the waters will continue lifting
northward today and into Saturday. Gusty easterly winds ahead of
the front have helped bring Small Craft Advisory conditions to
the waters today, and this is expected to persist in to this
evening. A few storms may traverse the lower tidal Potomac and
Chesapeake Bay this evening, some of which may be strong,
requiring the issuance of special marine warnings should
conditions warrant.

Winds are forecast to slacken towards Saturday morning
as the gradient weakens. Showers and storms may impact the
waters Saturday and Saturday night, potentially producing
locally gusty winds.

High pressure will build in behind a cold front Monday through
Tuesday. The high will move offshore Wednesday. A Small Craft
Advisory may be needed for portions of the waters Monday and Monday
night...and again from the return southwest flow Wednesday.

&&

Hydrology...
ongoing flood warnings are abound across central and northern
Virginia this afternoon thanks to persistent moderate to heavy
rainfall overnight and early this morning. The heavier rain has
moved off to the north and east, but numerous Road closures
remain across the affected areas. Heed any and all Road closure
signs and do not venture in to any flooded areas.

While the earlier rains are moving out of the area to our north
and east, additional showers and thunderstorms are expected to
develop later this afternoon and into this evening across the
same areas that received the heaviest rainfall since yesterday.
As a result, a Flash Flood Watch is in effect late this
afternoon through tonight for portions of central and northern
Virginia, including portions of the Shenandoah Valley and
adjacent eastern West Virginia counties. Additional rainfall
accumulations of one to two inches are expected across these
areas with localized higher amounts where stronger showers and
storms impact.

&&

Tides/coastal flooding...
water levels running one to two feet above astronomical normals
this afternoon. Easterly breezes will promote water levels
continuing to run at or just above this level through tonight.
A coastal flood advisory is in effect this afternoon and through
early Saturday morning during times of high tide at Dahlgren,
the District of Columbia, Arlington County, the city of
Alexandria, and the shoreline in St. Marys County.

Tonights high tide cycle is expected to be the highest
astronomically, with locations near Annapolis and Baltimore
potentially reaching minor flood stage overnight. Will allow the
evening shift to further analyze this threat and follow up with
any necessary advisories as needed.

Additional tidal flooding will be possible again Saturday night
and early Sunday during the high tide cycle. After which, winds
will turn westerly during the day on Sunday and shifting
northwest Monday, helping lower the threat for tidal flooding.

&&

Lwx watches/warnings/advisories...
District of Columbia...coastal flood advisory until 8 am EDT Saturday for dcz001.
Maryland...coastal flood advisory until 4 am EDT Saturday for mdz016.
Coastal flood advisory from 6 PM this evening to 2 am EDT
Saturday for mdz017.
Virginia...Flash Flood Watch through late tonight for vaz025-026-036-037-
503-504-508.
Flash Flood Watch from 6 PM EDT this evening through late
tonight for vaz029-038>040-050-051-055>057-502-507.
Coastal flood advisory until 4 am EDT Saturday for vaz057.
Coastal flood advisory until 8 am EDT Saturday for vaz054.
WV...Flash Flood Watch through late tonight for wvz505-506.
Marine...Small Craft Advisory until midnight EDT tonight for anz530>543.

&&

$$
Synopsis...bkf
near term...bkf
short term...bkf

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