Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
1040 am EDT Thursday Jul 2 2015
a stationary front will remain over central Virginia and southern
Maryland through today...and linger over the area through the
weekend. With several waves of low pressure will move east along
the front as an active pattern continues. High pressure will
return for the first half of next week.
Near term /through tonight/...
nearly stationary front is stretched east-to-west across central
Virginia this morning with a weak wave of low pressure over the
Chesapeake Bay. Showers are becoming more scattered in nature late
this morning...but are expected to become more numerous this
afternoon ahead of vorticity maximum approaching from Ohio Valley. This
will be targeted to southern areas. Cloud cover is fairly
extensive and should limit instability...but breaks seen in visible
imagery may allow pockets of slightly higher instability. With most of
County Warning Area on north side of boundary...would not expect a widespread
strong thunderstorm threat. However with deep layer shear around
30 knots...there is a chance of a few stronger storms given
sufficient instability...mainly in southern areas. Given wet soils
across the area...will also have to watch for repetitive storms
which could produce localized flooding issues with precipitable waters at or above 1.5
inches. Main changes with morning update involved adjusting probability of precipitation
based on latest guidance. Also blended in latest lamp
temperatures...which lowered maximum temperatures a degree or two with the
Precipitation coverage in greater question tonight...as there may
be a lull after departing wave...especially depending on how far
south the front sinks. Lows tonight will generally be similar to
Short term /Friday through Saturday night/...
right now...the best chance of precipitation across the area
would appear to be Friday...as a decent shortwave and vorticity maximum
moves across the area supporting a more notable low pressure wave
at the surface along the stalled front. Uncertainty remains as to
how long it lingers Friday night...with some guidance allowing it
to pull away fast enough so that the region dries out...while
other models keep the showers over the area. For now kept highest
probability of precipitation during the day...but the possibility of decent precipitation
during Friday night is definitely of note and if there is better
consensus in the future...probability of precipitation may certainly end up significantly
higher. Temperatures will be cool Friday with many areas failing
to reach 80...while lows will remain similar to recent days.
Fourth of July...stationary front lingering over the area (with
shortwaves riding along it) makes for a complicated forecast.
Consensus among 00z GFS/ECMWF/NAM is for a morning low to cross
southern Maryland/waters with a secondary shortwave to cross the Potomac
Highlands late in the afternoon. Both of these features will shift
the frontal system...to an uncertain extent. Felt the likely probability of precipitation
for the southern half of the County Warning Area was a bit high...so went chance most
places in the afternoon. With shortwave entering the west in the
peak heating...went likelies for the Potomac/Allegheny Highlands.
Maximum temperatures limited by strato-cumulus and any precipitation...generally low 80s
(with dewpoints in the middle 60s...expect a bit of a heat index to
the middle 80s).
Saturday night...key for evening pop is progression of weak short
wave from the central Appalachians and remaining instability. Those
details will become more apparent with future model runs...but
current best guess is to have scattered thunderstorm activity
Long term /Sunday through Wednesday/...
frontal boundary south of the area drifts farther south Sunday
and Monday as upper ridge builds east over the Midwest. Any isolated
to possibly scattered thunderstorms both day look to be diurnally
Quiet weather Tuesday under high pressure and near normal temperatures.
Next cold front approaches region from the Midwest Wednesday and
slows with more active weather into late next week.
Aviation /15z Thursday through Monday/...
fairly light winds from the north this morning shifting easterly
by tonight. VFR ceilings and visible outside of any showers. The showers
have been decreasing in coverage and should see the threat for
lower visibilities decrease through the midday hours. There will be a
renewed chance for showers and storms later this
afternoon...especially at cho. Even outside of showers...some haze
may linger with visibilities 5-9 sm. Forecast a bit more uncertain
overnight in regards to showers and potential impacts to aviation.
Low pressure may bring some steadier rain to the region by
Friday possibly into Friday night...with potential for a more
sustained period of MVFR or even IFR cigs/vis. Timing and
northward extent of this is still in question and so confidence is
Stationary front lingers just south of District of Columbia metros
Saturday...before drifting farther south Sunday and Monday.
Scattered thunderstorms through Saturday into the evening across
the terminals. Morning northeasterly flow 10 to 15 knots as low passes southern Maryland.
Diurnally isolated sun/Mon. Quiet weather Tuesday.
winds should stay below Small Craft Advisory levels today...with northerly winds
below 10 knots gradually shifting to easterly. A few heavier
showers or even an isolated thunderstorm are possible
today...which could produce gustier winds. The greatest chance for
stronger storms will be over the lower tidal Potomac and lower Maryland
Winds might increase to Small Craft Advisory levels later Friday and Friday night
as a low pressure wave passes by to the south and southeast with
more widespread rain showers. Confidence is not high so did not
issue an Small Craft Advisory just yet...but it may be needed.
Will need to watch a weak Low Pass southern Maryland Saturday morning for
northeasterly gusts around 20 knots. Scattered thunderstorms possible all
day/evening. Light onshore flow then Sunday and Monday as front
dips farther south of the area. Quiet Tuesday.
District of Columbia...none.