Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 270759

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
359 AM EDT Thu Jun 27 2019

High pressure will remain overhead through Friday before moving
offshore Saturday. A cold front will pass through Sunday into
Sunday evening before high pressure returns for Monday. High
pressure will move offshore for the middle portion of next week.


After some patchy river valley fog dissipates, the remainder of
the day should be mostly sunny. The surface high will be moving
overhead while the mid level ridge axis approaches from the
west. Thus there will be slightly better moisture/instability in
the southwestern portion of the CWA, which will see a slightly
better chance of a pop-up shower or storm. However, terrain
circulations could help initiate convection just about anywhere
over the mountains. To the east, subsidence should win out.
Expect high temperatures similar to a couple degrees hotter than
Wednesday...meaning lower 90s in many locations. Heat indices
may push into the mid to upper 90s across southern parts of the
CWA, where dew points will be a little higher.

Any convection will collapse quickly with sunset. Patchy river
valley fog will be possible again. It will be warm and muggy
with lows in the 60s to around 70, with mid 70s in the urban


The surface ridge will move to the east Friday, while the 850 mb
high moves near the Outer Banks. This will lead to a bit more
southwesterly flow, and temperatures and humidity will creep up
a bit more. Highs will be in the lower to mid 90s, and the heat
index may locally top 100 in the I-95 corridor (though still
looks to be under advisory criteria). Instability will be more
evenly distributed with only weak capping in place. There likely
won`t be any large scale triggers, so expect the highest chance
of pop-up showers and storms to be over the mountains. Some of
these could drift to the east later in the afternoon, or sea-
breeze collisions could also initiate convection. Regardless,
we`re only looking at small/brief areal coverage, though any
stronger storm could have gusty outflow. Would expect another
strong diurnal trend, with the overnight hours being dry but

Low pressure will be moving into New England on Saturday ahead
of a closed upper low moving southeast from Hudson Bay. However,
other than gradually falling heights, it doesn`t appear there
will be any large-scale ascent over our area during the day. In
addition, low level flow will be westerly, which also might help
cap convection. So while a pop-up storm can`t be completely
ruled out, much of the day may turn out dry. Convection which
develops over Pennsylvania (better forcing) may ride the
northwest flow aloft and reach our area during the late
afternoon or evening. The surface cold front may approach late
at night, and it could have a few showers along it as well. It`s
possible Saturday could turn out to be the hottest day of the
stretch (or at least be similar to Friday), but the westerly
winds may help lower dew points a bit, so am still not expecting
heat indices much above 100.


A cold front will pass through Sunday afternoon into Sunday
evening. Hot and humid conditions are expected ahead of the
front, so there will be the chance for a few showers and
thunderstorms both ahead of and with the frontal passage.
Moderate instability along with moderate shear profiles suggest
that stronger to perhaps even severe thunderstorms are possible,
but confidence is low at this time due to a downsloping
westerly component to the low- level flow.

High pressure will briefly build overhead for Monday behind the
cold front. There is still some uncertainty with guidance as to
how strong the high will be. If the high is weaker, then it
will slide offshore quicker and a return flow will lead to more
humid condtions along with the chance for thunderstorms.
However, if the high is a bit stronger then it will remain in
place for a bit longer before sliding offshore. This would bring
dry and less humid conditions. For now, the forecast will lean
toward the second scenario given the fact that ensemble guidance
is in good agreement with anomalously low heights off the New
England Coast (which would increase confluence aloft overhead).

The high will move offshore Tuesday through Wednesday and a
return flow will allow for humidity to return during those days.
An upper-level ridge will remain centered to our southwest, so a
northwest flow aloft is expected. The increased humidity along
with a northwest flow aloft will lead to chances of showers and
thunderstorms both days. However, it does appear that much of
the time both days will be dry.


High pressure will progress over the region today then move
offshore Friday and Saturday. VFR conditions and light winds
will prevail much of the time. Any concerns will revolve around
morning fog and isolated to widely-scattered thunderstorms.
There`s a little better chance of both Friday than today, mainly
at MRB and CHO. A front will be approaching from the north late
Saturday, so there will be another chance of thunderstorms,
although coverage is uncertain at this time -- but certainly
not widespread.

A cold front will bring the chance for showers and thunderstorms
Sunday afternoon and evening. Dry conditions are most likely
Monday before a return of humidity increases the chances for
afternoon and evening showers and thunderstorms Tuesday and


High pressure will move overhead today and offshore Friday.
Expect light winds today, with prominent diurnal/onshore
direction this afternoon. Winds will be a bit more southwesterly
Friday, but there still could be afternoon onshore flow. As low
pressure moves into New England Saturday, the gradient will
tighten and westerly flow will increase. It may come close to
SCA conditions, mainly on the narrower waters (and nearshore)
across the northern Bay and upper Tidal Potomac. While a stray
storm can`t be ruled out Friday and Saturday, most of the
activity should stay over land.

There is a chance for thunderstorms Sunday through Sunday
evening, and some of those storms may produce gusty winds.

A cold front will pass through the waters Sunday afternoon into
Sunday evening. A north to northwest flow behind the boundary is
expected and a Small Craft Advisory will likely be needed for
at least portions of the waters into Monday.

High pressure will build overhead later Monday before moving
offshore during the middle portion of next week. Winds should
remain below SCA criteria for most of the time.




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