Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 300758
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
358 AM EDT Fri Sep 30 2016
Upper level low pressure over Kentucky will begin to lift
northward towards the Great Lakes today and into the weekend. The
low will pass by to the north early next week as it moves
offshore. High pressure will build into the area late in the
weekend and into early next week.
.NEAR TERM /THROUGH TONIGHT/...
Have canceled the remaining portions of the Flash Flood Watch...as
the threat for repeated rounds of heavy rain has ended over these
Current radar analysis shows pockets of light...to at times
moderate...rainfall across much of the area. Drizzle has also been
observed in areas where showers are not occurring. Expect these
trends to continue into today...as persistent onshore flow remains.
Ensemble/Deterministic guidance is in fairly good agreement with
bringing another round of showers (and perhaps some isolated
thunder) into the southern portions of the forecast area by mid-to-
late afternoon. HRRR-TLE shows decent probabilities for an inch over
a three hour period with this activity...with very low probabilities
of 2 inch/hr rates. Thus...expect only a localized flooding threat
if training convective cells manage to develop. Strong wind field
remains...so a few stronger storms are possible across the south.
Elsewhere...including the DC/Balt metros...wet weather
continues...though, mainly in the form of patchy light showers or
drizzle. Some moderate showers could work into the area by this
evening as the activity across the south possibly pivots into the
area. Rain rates with this activity would be rather light.
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
Upper-level low will continue to track northward towards the Great
Lakes and then pass by to our north as it moves eastward toward the
weakening ridge over the Atlantic Saturday into Sunday. Impacts
associated with the low will begin to lessen Saturday...with only a
few spotty showers lingering (mainly across the northern half of the
CWA). By 00Z Sunday...dry slot expected to have finally worked its
way through the area...as frontal boundary pushes off to the east.
This will finally eliminate precipitation chances into Sunday...for
all but upslope areas along the western slopes. Drying out Sunday
with highs in the M/U70s.
.LONG TERM /MONDAY THROUGH THURSDAY/...
Surface high pressure remains over New England through mid-
week...with mostly dry and seasonable conditions across the Mid-
Atlantic. Highs in the 70s early next week...though, as onshore flow
develops around the surface high by mid-week...high temperatures
will fall into the 60s.
Tropical Cyclone Matthew expected to be located somewhere in the
Atlantic by mid-week. Extreme run-to-run inconsistencies and model
spread continues...as is usually the case this far in advance. Far
too early to discuss possible impacts to Mid-Atlantic region.
.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Upper level low will continue to impact the area through Friday with
onshore flow causing low ceilings at most terminals. Periodic
drizzle/showers through tonight...with IFR conditions prevailing.
Though, brief LIFR ceilings are possible in the heaviest showers.
Improving conditions Saturday as upper-low begins to lose its
influence over the area. Prevailing VFR conditions Saturday through
early next week as high pressure builds into the area.
Solid (to high-end) SCA winds currently being realized across much
of the Chesapeake Bay...with winds closer to SCA criteria up the
tidal Potomac. HRRR/NAM/GFS BUFKIT soundings show decent mixing
continuing through early evening...with SCA winds remaining.
Extended the SCA headline for the Chesapeake Bay into early Saturday
morning...as guidance indicates SCA gusts lingering awhile longer
there. Gradient relaxes for all locations by early Saturday...with
winds remaining below SCA criteria into early next week as high
pressure builds into the area.
Water levels running up to 2 feet above astronomical predictions
early this morning. With the continued east wind, do not expect much
change. If anything, the departures may increase slightly. As a
result, we are in the midst of a prolonged tidal flooding event. For
the most part, minor flooding is being realized within a couple
hours of high tide. However, there are a couple of exceptions,
namely St. George Island and Annapolis, where moderate flooding has
occurred (or in Annapolis` case is forecast).
Confidence is sufficient to extend most Advisories and Warnings for
both tide cycles on Saturday. There are a couple of exceptions: (1)
the mid tidal Potomac, where the shoreline is high (will only carry
Advisories through the tide cycle tonight); (2) Southern Baltimore,
where the tide cycle occurs Saturday evening (will only go through
Saturday day). Skill in water level forecasting isn`t great beyond
36 hours. Consequently, have a Coastal Flood watch for Saturday
evening in St. Marys and Anne Arundel Counties, where moderate is
still forecast, but will less confidence.
On the upper tidal Potomac, at least minor tidal flooding should
occur through at least Saturday evening. Am awaiting the latest
model output from the HEC-RAS, and will decide from there.
A gradual decline in water levels should transpire Saturday night
and Sunday. It is unclear at this time whether an additional tide
cycle would be affected. There is plenty of time to figure that out.
DC...Coastal Flood Warning until midnight EDT tonight for DCZ001.
MD...Coastal Flood Advisory until midnight EDT tonight for MDZ013.
Coastal Flood Warning until noon EDT Saturday for MDZ014-017.
Coastal Flood Watch from Saturday afternoon through Saturday
evening for MDZ014-017.
Coastal Flood Advisory until 8 PM EDT Saturday for MDZ016-018.
Coastal Flood Advisory until 2 PM EDT Saturday for MDZ011.
VA...Coastal Flood Advisory until midnight EDT tonight for
Coastal Flood Advisory until 8 PM EDT Saturday for VAZ057.
MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ530>534-
Small Craft Advisory until 7 PM EDT this evening for