Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 220133
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
933 PM EDT SAT MAY 21 2016
Low pressure near the mouth of the Chesapeake Bay will move
northeast toward the Canadian Maritimes through Sunday. A second
area of low pressure will develop off Cape Hatteras Sunday and head
northeast through Monday. High pressure will build for the middle
of the week.
.NEAR TERM /THROUGH TONIGHT/...
Light rain impacting northeastern corner of MD and the central VA
area this evening. Expecting to see increase in coverage of
showers southwestern areas of the CWA second half of the night as
shortwave trough swings through the region. Drizzle and fog
possible over remaining areas until the rain showers reach there
close to daybreak and possibly later Sunday morning. Temps on
track this evening with lows still looking to reach the U40 to
.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
The 500 mb trough axis sharpens Sunday into Monday as shortwave
energy continues to undercut the low. There will be enough energy
to spawn a second coastal low by late Sunday. Unfortunately, have
needed to make some pessimistic adjustments to the forecast.
Cannot accurately time showers specifically in such a pattern, but
am becoming increasingly convinced that there will be measurable
rainfall each day. Have increased PoPs to likely most areas. Even
have a bit of 80 percent probability in the Potomac Highlands
In regards to thunder chances...there will be a meager amount of
instability in the mountains Sunday (no shear). The CAPE setup
looks a little better areawide on Monday as heights will be lowest
at that time. Am keeping thunder a slight chance at best (for one
thing, dewpoints rather low), in the mountains with diurnal
heating Sunday and areawide on Monday.
Lowered high temperatures tomorrow given the expected cloud
cover. Although Monday won`t be a sunny day, there should be at
least a few glimpses. Rainfall won`t be all day, either. The
added insolation may be enough to bump temps upward. Guidance
fairly insistent on upper 60s/lower 70s. Low temperatures have
been consistently on the higher side due to atmospheric moisture.
No substantive changes made there.
.LONG TERM /TUESDAY THROUGH SATURDAY/...
Couple of quieter weather days Tuesday and Wednesday, as influence
of vertically-stacked closed low shifts off to the NE and mid-level
heights rise. Only real exception would be Tuesday morning, where a
few lingering showers are possible (especially across eastern
portion of CWA). Depending on how quickly we clear out, highs
Tuesday might finally be near normal (M/U70s). With the help of
downsloping westerlies we warm a few degrees Wednesday, though
remain dry, with most places into the L80s by afternoon.
By late Wednesday, developing Bermuda high offshore will begin to
promote southerly flow over the area, which will increase low-level
moisture rather quickly. Though, absence of large scale lift or
significant upper level feature through at least Saturday suggest
mesoscale forcing mechanisms will be primary driver of any diurnally
driven thunderstorm activity through Saturday. As such, best
thunderstorm chances will remain over the higher terrain...with
lower chances to the east. While instability will be moderate-to-
high, kinematic flow is poor (flow generally less than 30 kts at all
levels), which would keep any pulse-type stronger storms generally
.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
Varying conditions this evening, jumping between IFR and MVFR cigs
and vis. Expecting mainly dry or possibly drizzle at TAF sites
through remainder of the night. Guidance hints at improvement to
VFR for a period Sunday morning but have stay more conservative
and will keep at MVFR for now. Winds generally NW-N less than 10
There won`t be much change in the pattern Sunday-Monday. Numerous
showers possible each day, but timing restrictions explicitly a
challenge. Have MVFR forecast through the end of the TAF. Lower
restrictions certainly possible. Further, ceilings should be on
the rise, perhaps reaching VFR category.
VFR conditions expected Tuesday and Wednesday with high pressure
remaining in control. A few isolated thunderstorms possible Thursday
afternoon as humidity increases over the area.
Occasional gusts to 20 knots on the middle Chesapeake. Otherwise,
sub-SCA conditions expected as mixing looks poor with the upper
low approaching. That covers the synoptic pattern for both Sunday
and Monday. In terms of direction, these winds will mainly be
coming from the north.
With high pressure under control, westerly winds will remain light
Tuesday and Wednesday. Winds remain sub-SCA Thursday, though
they shift to southerly as Bermuda high develops offshore.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>534-