Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 131437

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1037 AM EDT Sun May 13 2018

A front will meander in or near the area through through much
of next week. Another cold front will then approach from the
northwest late in the week.


Surface frontal boundary this morning resides from near
Cumberland MD to Culpeper VA and towards the Virginia
Tidewater. To the northeast of this boundary, cooler
temperatures, low clouds, areas of fog, and some morning
showers/isolated thunder are present. To the southwest, partly
to mostly sunny skies have developed and temperatures have
warmed into the 70s already.

An interesting setup presents itself this afternoon, and one
that has a conditional threat of severe weather. The surface
boundary is expected to remain more or less stationary through
the day, as a wave of low pressure moves along the boundary from
Ohio this morning, east-southeastward and into central Virginia
by late this evening. In the warm sector, temperatures should
have little problem warming up through the 80s and perhaps
locally near 90F again. Surface dew points will maintain
readings in the low to mid 60s as well. In addition, relatively
steep mid level lapse rates of around 7C/KM still exist in this
sector. This combination will lead to 1000-1500 J/KG of MLCAPE
by this afternoon. To the northeast of this boundary, while
there is expected to be little to no surface instability,
there`s indication that elevated instability will move aloft,
and MUCAPE values near 1000 J/KG may exist by later this
afternoon and evening. There is ample wind shear across the
region, although it is displaced northward from the best
instability, with 0-6KM values ranging from 25-30 knots in the
warm sector to 40-45 knots across Maryland. The issue is that a
strong capping inversion will likely exist across much of the
warm sector this afternoon, prohibiting development. However,
morning convection over Ohio/PA has likely set down some outflow
boundaries, and that along with the frontal convergence may be
enough to spark shower/thunderstorm development across OH/WV/PA
again this afternoon, and spread eastward into eastern WV/MD/VA.
If this does occur, a severe threat exists, with damaging winds
and large hail the main threats in the warm sector, large hail
on the cool side, and perhaps a very low risk of an isolated
tornado if a cell can move along the frontal boundary. Locally
heavy rain will also occur.

Convection will wane overnight, with lows from the upper 50s to
low 60s.


Most guidance indicates the boundary will start making progress
back to the northeast on Monday, though it may be quite a
struggle, so only have somewhat warmer temperatures currently
forecast. Worst case scenario, highs Monday end up fairly close
to today. That said, there will be another shortwave coming
through, and the northeastward progress of the front will likely
influence any severe weather threat. CAPE looks robust, but that
is assuming the front makes as much progress as currently shown,
which is a little questionable. Again, SPC has marginal for
their outlook of severe weather, which again seems reasonable
given uncertainty.

Any showers/t-storms Monday evening should again dissipate by
night`s end. At least right now, guidance is strongly in
agreement that the front will finally make real progress on
Tuesday, allowing temps to surge back above normal across the
whole region. In fact, readings may reach low 90s across most of
the forecast area, and with greater humidity than the last hot
day, it may feel downright summerlike. However, it doesn`t look
to last, as the front will start heading back south after a wave
of low pressure passes by to the north Tuesday night. As this
wave moves by, the risk of showers and thunderstorms will again
increase, and we will again have to watch for any severe weather
threat. Lows Tuesday night will remain mild.


A downward trend in convective activity is shown on the
ensemble guidance for Wed and Thu based on ensemble CAPE and QPF
distributions, but this doesn`t mean there won`t be risk of
showers and t-storms. This likely a result of weaker forcing as
stationary front washes out.

Late Thu night through Fri evening, ensemble guidance show
better convective potential due additional moisture streaming
northward from the northern GMEX between a mid-upper level trof
over the OH/TN valleys and a strong western Atlc subtropical
ridge. Given abundant moisture (PWs>1.5 inches) and
unidirectional flow, potential exists for training convection
and localized flooding especially in the I- 95 urban corridor.


Sub-VFR conditions in low ceilings and visibilities will be the
main concern this morning at all sites except CHO. Conditions
gradually improve somewhat this afternoon before lowering again
tonight. Another risk of shower/thunderstorm activity is
expected this afternoon and evening, and there is potential for
some storms to contain frequent lightning, heavy rain, and
isolated gusty winds and hail.

Morning low ceilings/visibilities likely again Monday morning
before some improvement by the afternoon. There is another risk
of showers/storms again in the afternoon.

A downward trend in convective activity is shown on ensemble
guidance Wed and Thu, but isold t-storm activity is possible.


Risk of significant wind on the waters is fairly low through
Monday, but increases as a front lifts north of the waters
Tuesday. Otherwise, occasional thunderstorms passing the waters
today through Monday could bring gusty winds, but these should
not last long. Special marine warnings cannot be ruled out at
times, however.

Possible SCA conditions Fri. Sct-nmrs showers/t-storms Wed-
Fri. Mariners can expect higher winds and waves near t-storms.




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