Area Forecast Discussion

FXUS61 KLWX 250130
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
930 PM EDT Sat Sep 24 2016
High pressure will build in for tonight and Sunday before moving
off the coast Monday. A cold front will pass through the area
overnight Monday into Tuesday. An upper-level low may impact the
area during the middle portion of next week.
Exact location of cold front is difficult to pinpoint this
evening due to diffuse surface features, but is likely toward
southern VA. A secondary boundary resides over PA, behind which
dew points are in the mid 40s. The boundary will make some inroadsacross
northeastern sections of the CWA tonight. A couple of small
isolated showers remain near the central Blue Ridge, and these
should dissipate/push southeast in the next couple of hours. Based
on model RH fields, the clearing line is nearing its southern
extent for the night. This low level moisture and onshore/upslope
flow may result in some patchy drizzle and fog for the southern
third of the CWA tonight. These features may hold low temperatures
in the 60s. Further north, clearing skies, decreasing winds, and
lowering dew points should result in the coolest night of the
season thus far, with 50s widespread and 40s in the
colder/northern locales.
On Sunday, high pressure drifts by to the north and flow becomes
more easterly. Temps will rebound only into the lower 70s in most
areas due to cool air aloft in northern areas and persistent
clouds further south. Some drizzle/mist may linger in central
Virginia during the morning.
High pressure continues drifting eastward away from us Sunday
night and Monday as the next frontal boundary approaches from the
west. With persistent east/southeast flow the available moisture
will increase and the front may be slow enough and forcing strong
enough to allow for a decent rain across the area Monday night
into early Tuesday. Right now guidance is a bit uncertain on exact
timing but certainly Monday night looks wet...with uncertainty on
just how fast it arrives late Monday and how fast it departs
Tuesday. Lows will be cool again Sunday night (50s to high 40s) with
highs Monday as the front approaches again stuck in the lower 70s.
Monday night will be milder with the front crossing the
area, with lows staying in the 60s for many spots. Clouds
breaking Tuesday will likely allow for 70s to return once again.
Cold front moves away from our region Tuesday night, with some
showers possibly lingering to the eastern part of our CWA. Conditions
become dry overnight Tuesday into Wednesday.
A cutoff upper level low will push south from the Great Lakes
Wednesday into Thursday night or even late Friday. Guidance is in
disagreement on location of this upper level low and this will
affect the forecast in the long term. GFS keeps the upper low
moving over the Mid-Atlantic early on Thursday with the ridge
building behind it on Friday and northerly flow through early
Saturday. This scenario brings showers over our CWA Wednesday
night into Thursday, and then dry into Sunday. On the other hand,
the Euro keeps the upper level low to our west and lingering to
our southwest through early Friday before moving back north. This
scenario allows for southerly or easterly flow advecting moisture
into our region and bringing precipitation chances Wednesday
night into Saturday before finally drying out late Saturday into
Sunday. Temperatures will be affected by the position of the upper
lever low, but for now forecasting near normal.
Clouds have mostly cleared away from the northern terminals, but
recent obs of FEW015-025 at DCA/BWI lend some concern that clouds
may try to redevelop. This hasn't been indicated in guidance, so
have left out of the TAFs for now. Low level moisture reaching CHO
this evening, and cooling overnight should allow MVFR cigs. Kept
these cigs through 12Z due to climo, disregarding guidance showing
improving conditions overnight. Could also be some patchy mist or
drizzle around CHO later tonight. 
Generally VFR Sunday-early Monday, although low cigs could be a
concern at CHO again Sunday night. Then sub-VFR with approaching
front and associated SHRA Monday night into early Tuesday. Winds
generally around 10 knots or less through Tuesday...starting off
northeasterly tonight...turning southeasterly Sunday...and
southerly Monday...then westerly Tuesday.
VFR conditions expected Tuesday night into Wednesday, then sub-
VFR conditions possible late Wednesday into Friday as upper level
disturbance brings showers into the region.
Other than a stray 20 kt gust around Pt. Lookout, winds have been
sub-SCA this evening. A secondary boundary pushing southward and
cool air advecting over the warm bay waters could result in a
surge of winds on the northern part of the Bay after midnight, so
have left SCA in effect and extended until 10 AM based on hi-res
guidance. However, it looks rather marginal. Trimmed the SCA for
the southern MD waters back to 2 AM however, as no guidance shows
the stronger surge of winds reaching these waters after the gust
potential diminishes this evening.
High pressure likely to bring lighter winds Sunday before
southerly channeling ahead of cold front bring another threat of
SCA for Monday and then cold advection brings more potential SCA
gusts Tuesday behind aforementioned front.
Winds diminish Tuesday night into Wednesday night. Possible small
craft advisory conditions late in the week an as upper level
disturbance moves nearby.
High pressure building southward just to the northeast of the area
will result in northeasterly flow through tonight, then easterly to
southeasterly flow late Sunday into early next week. Anomalies have
dropped some this afternoon, but the overall pattern will not allow
much more water to drain out of the Chesapeake Bay/Tidal Potomac
River. Anomalies will likely begin to increase again starting later
tonight. Minor coastal flooding appears possible by the Sunday
night high tide cycle.
Unusually warm weather has persisted over the area for
quite some time. Autumn-like temperatures are finally making an
appearance over the area, and DCA may fall below 60 degrees
early Sunday morning for the first time since June 9th (total of
106 days through September 23rd). The record most consecutive
days at or above 60 degrees in the Washington DC area is 112 set
in 2012.
MARINE...Small Craft Advisory until 10 AM EDT Sunday for ANZ530>533-
     Small Craft Advisory until 2 AM EDT Sunday for ANZ534-537-543.