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Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
320 am EDT Fri Oct 18 2019

Synopsis...
low pressure will depart through the Canadian Maritimes today as
high pressure builds in from the Ohio Valley. The high pressure
center will move overhead tonight, then pull offshore Saturday
as low pressure moves across the north-central Gulf Coast and
into the southeastern United States. This low will move
northeastward offshore of North Carolina or Virginia on Sunday,
then meander offshore through Tuesday until another cold front
approaching from the Ohio Valley pushes across the region. High
pressure is expected to return for the second half of next week.

&&

Near term /through tonight/...
low pressure will continue to lift northeast through Nova Scotia
today while high pressure builds toward the area from the Great
Lakes. A northwest flow is expected between these features, and
winds will be gusty, but not nearly as gusty as Thursday. Wind
gusts for most areas will be around 20 to 30 mph, but less
toward central Virginia.

A dry northwest flow will lead to plenty of sunshine along with
seasonable temperatures. Highs will be in the upper 50s to
Lower/Middle 60s for most locations. A stratocu deck will affect
the Allegheny Highlands this morning and a few cu will spill
east, but these should dissipate throughout the day.

High pressure will settle overhead tonight, and winds will
diminish quickly around sunset. With light winds and clear skies
overhead, this will provide a good setup for radiational
cooling. Min temps will range from around 30 in the colder
valleys to the middle and upper 30s for most other locations. A
freeze warning is in effect for western Maryland, eastern WV,
and extreme northern Virginia. These areas will be right underneath
the high. A frost advisory is in effect for locations northwest
of Interstate 95. Areas of frost are expected for these
locations. Farther east, patchy frost is possible but confidence
is too low for any headline at this time since they will be
farther from the center of the high.

&&

Short term /Saturday through Sunday night/...
high pressure will gradually slide offshore Saturday, but it
will remain close enough for more dry and seasonable conditions.
Surface low pressure will track through the Gulf Coast states
into the southeastern conus, and the upper-level low in the
southern stream will also track through these areas. High
clouds ahead of this system will gradually increase Saturday
afternoon into Saturday evening.

The low will track toward the North Carolina coast Saturday
night and then off the mid-Atlantic coast Sunday. Warm and moist
air ahead of this system will overrun the cooler air near the
surface, and there will be lift from the upper-level shortwave
energy associated with the surface low. Therefore, rain is
possible across the area later Saturday night into Sunday.
Confidence remains low in regards to how far north and west the
rain will make it and also how much rain will fall. This is due
to divergence in the guidance that still exists. It does appear
that a period rain is increasingly likely across central
Virginia to southern Maryland as well as the Washington and
Baltimore Metro areas. Less confidence for rain exists farther
north and west.

The low will move away from the area later Sunday and high
pressure will return for Sunday night, bringing a return of dry
conditions. Low clouds and fog may develop underneath the
nocturnal inversion Sunday night.

&&

Long term /Monday through Thursday/...
the long term period will start with broad/sturdy ridging over the
eastern North Pacific extending into the western conus, with a
large and deep trough digging across the central Continental U.S..
ridging will also be centered over or near Florida and the
Bahamas, extending northward along the eastern Seaboard.
Embedded within this ridge will likely be the remnants of an
area of low pressure offshore of the mid-Atlantic (the low
expected to originate over the Gulf of Mexico and possibly
become tropical storm Nestor as it moves into and then offshore
of the southeastern us). At the surface, a strong low pressure
and cold front will be pushing east across the Midwest while
high pressure retreats across northern New England and the
Canadian Maritimes. This should result in a surface high
pressure wedge locally over the mid-Atlantic in the Lee of the
appalachian crest. Given the ridging building locally, any rain
should be gone well prior to daybreak Monday.

As the cold front to the west approaches Monday night into Tuesday,
increasing clouds and rain chances are expected. The upper trough
looks pretty strong, which makes sense given the upstream ridging
and amplified flow pattern overall. This may very well lead to
another soaking rain event, especially given strong southerly 850
hpa flow advecting moisture into the region ahead of the front.
Enhanced synoptic lift may even result in a few heavier
downpours or rumbles of thunder as the upper trough GOES
negatively-tiled. As to be expected several days out, there is
some uncertainty as to the exact timing and strength of the
front, but overall there appears to be a pretty high chance of
rain on Tuesday.

A large area of high pressure is expected to follow during the
second half of next week accompanied by mainly dry weather until
perhaps the very end of the week when another system approaches.

&&

Aviation /07z Friday through Tuesday/...
VFR conditions are expected through Saturday. Rain is likely
along with subvfr conditions later Saturday night into Sunday,
especially for the eastern terminals. High pressure will return
Sunday night, but low clouds and fog may develop underneath the
nocturnal inversion.

Could see some patchy sub-VFR fog Monday morning in the wake of
sunday's rain as a wedge of high pressure builds overhead.
Otherwise VFR likely later Monday into Monday night in light
northerly flow.

Winds should turn southeasterly and increase a bit at the
surface late Monday night into Tuesday ahead of a strong
approaching cold front. Low-level wind shear and sub-VFR
conditions in rain are possible Tuesday, with winds shifting to
westerly behind the front Tuesday night into Wednesday (in
tandem with a return to vfr).

&&

Marine...
a Small Craft Advisory remains in effect for the waters today
due to northwest winds gusting around 20 to 30 knots. Winds will
quickly diminish toward sunset as high pressure builds toward
the waters. The high will build overhead tonight before moving
offshore Saturday. A return southerly flow will develop Saturday
into Saturday evening but winds should remain below Small Craft Advisory
criteria for most areas.

Low pressure will pass by to the south and east late Saturday
night before moving offshore Sunday. An Small Craft Advisory may be needed for
portions of the waters during this time. High pressure will
build toward the waters Sunday night, but and Small Craft Advisory may still be
needed for portions of the waters.

Generally light winds are expected Monday as surface high
pressure ridges across the waters and low pressure meanders
out at sea. A strong cold front will approach from the west
Tuesday, and some Small Craft Advisory level gusts are possible in southeast flow ahead
of the front (along with an increasing potential for soaking
rain and lower visibilities). Winds will shift to the west
behind the front Tuesday night into Wednesday and may again
approach Small Craft Advisory levels. There's a low chance of gusty winds
accompanying heavier downpours immediately ahead of the front
Tuesday, as well.

&&

Tides/coastal flooding...
tides are running below normal as of early this morning, with no
flooding issues anticipated over the next 24 to 36 hours.

Onshore (easterly) flow is expected to increase ahead low
pressure approaching from the south Saturday night into
Sunday, resulting in increasing water levels. There remains a
moderate level of uncertainty as to the extent and duration of
onshore flow, as that will be reliant upon the exact strength
and track of approaching low pressure. Therefore, the confidence
in exact water levels is low, though the confidence in seeing
at least minor flooding at the more vulnerable tidal sites seems
likely.

Light northerly flow should cause water levels to decrease a bit
heading into Monday, before onshore flow (southeasterly) flow
resumes Monday night into Tuesday, leading to a subsequent rise
in water levels once again.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...frost advisory from 2 am to 9 am EDT Saturday for mdz004>006-
503>507.
Freeze warning from 2 am to 9 am EDT Saturday for mdz003-501-
502.
Virginia...frost advisory from 2 am to 9 am EDT Saturday for vaz025>027-
029-030-036>040-050>053-055-501>506.
Freeze warning from 2 am to 9 am EDT Saturday for vaz028-031.
WV...frost advisory from 2 am to 9 am EDT Saturday for wvz505-506.
Freeze warning from 2 am to 9 am EDT Saturday for wvz050>053-
055-501>504.
Marine...Small Craft Advisory until 6 PM EDT this evening for
anz530>543.

&&

$$
Synopsis...dhof
near term...bjl

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