Scientific Forecaster Discussion
fxus61 klwx 290808
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
408 am EDT Mon may 29 2017
upper level low pressure will drift east across the north side
of the Great Lakes this week. In the meantime, weak high
pressure will prevail over the mid Atlantic outside of weak
cold frontal passages Tuesday night and Wednesday night. Low
pressure approaches from the west this weekend.
Near term /through tonight/...
early this morning, low pressure centers are located near the
Great Lakes and east of the Delmarva. A warm front links the two
lows but curves south of the lwx County Warning Area. Thus most of the area is
locked in a stable marine layer of low clouds, fog and drizzle.
Some observations have briefly hit 1/4 mile, so will have to
keep an eye on dense fog advisory potential, but on the whole,
this is more of a low stratus situation.
A cold front trailing the low to our west will push eastward
today, overtaking the stalled warm front. This front will act to
scour out the low clouds through the morning hours, but perhaps
taking until early afternoon in the Baltimore area. With the
low offshore and high pressure remaining to the northeast, it is
looking like the front will get hung up around the Chesapeake
Bay this afternoon. A temperature (as well as cloud) gradient
will be in place across northeast Maryland...and have thus shaved
highs several degrees east of Baltimore. Elsewhere, highs should
surge into the 80s. In addition, the frontal convergence zone
may spark a few showers and perhaps a thunderstorm across the
eastern quarter of the area.
The front becomes more nebulous tonight, drifting to the south.
Onshore flow will be re-introduced, bring a return of lower
clouds, especially the northeastern quadrant of the area. Fog
and reduced visibility will be possible, whether due to low
clouds (east) or radiational cooling of the moist airmass
(west). Low pressure will develop along the frontal zone to our
south, which could bring a shower threat to the far southeast,
but otherwise dry. Lows will be in the upper 50s to mid 60s.
Short term /Tuesday through Wednesday night/...
upper level low pressure will continue to spin over Ontario
Tuesday and Wednesday with the jet stream located just to our
northwest. Surface features will continue to be less defined
though, as weak fronts (more pressure troughs than huge airmass
differences) waver across the region.
The first will push southeast on Tuesday afternoon. With low
stratus to start the day, it is uncertain how quickly and how
far east it will erode. West of the marine air, modest
instability will develop. Deep layer shear increases in
magnitude with northward extent. Thus there is some risk of
strong/gusty thunderstorms developing ahead of the front. Storm Prediction Center
has placed northern parts of the area in a marginal risk.
This front will lose its definition as it slips southward
Tuesday night, ending the chance for showers. A final and
slightly more well- defined front will push south Wednesday
afternoon and evening. While low level convergence isn't great,
some scattered showers and storms will still develop along the
front. The front will align better with diurnal heating to our
west, so expect a decreasing trend for any storms that enter the
area. Once again though, deep layer shear will be strong, so
any organized storms could have a gusty wind threat. A marginal
risk is in place for the northwestern half of the area.
Highs both days will be in the 70s and 80s with lows in the 50s
Long term /Thursday through Sunday/...
Canadian high pressure will dominate Thursday as a trough
passes aloft. Warm air will begin returning northward on Friday,
but by Saturday the next cold front is already dropping back
south across the region as another trough rotates around the
stalled upper low over southern Canada. That front looks to
stall south of the region by Sunday as the upper low starts
moving further east, with the front perhaps lifting back north
as a warm front early next week. We will have a chance of
showers and thunderstorms with the front dropping south across
the area late Friday into Saturday. By Sunday, with the
potential for the front to stall south of US, the weather is
uncertain, but if high pressure builds in strongly enough from
the north, it could be dry. Temperatures will mostly be cooler
than normal, with the warmest day likely Friday.
Aviation /08z Monday through Friday/...
IFR/LIFR conditions through daybreak with an onshore flow. How
quickly cigs lift this morning will depend if the warm front is
able to lift north, or if cold frontal passage will be required
to mix out the clouds. Regardless, improved conditions are
likely by early afternoon, except perhaps near mountain where the
front may become hung up. A few showers may redevelop around
Baltimore this afternoon, but not enough coverage to put in the
The front will not have a lasting effect though, with onshore
flow redeveloping tonight. Low MVFR or IFR likely in Baltimore
area, and possible at the rest of the terminals except cho. It's
uncertain how quickly these clouds will erode on Tuesday as
well. Where they do, thunderstorms will be possible during the
afternoon and evening as the next boundary drops southeast.
It's possible the low cloud pattern repeats Tuesday night as
the front washes out. A more well-defined cold front will push
through late Wednesday with scattered showers and thunderstorms
Mainly VFR later this week with high pressure returning.
Thunderstorm chances are maximized late Friday into staturday
with a passing front.
overall light flow is expected on the waters through Wednesday.
Expect variable wind directions as weak boundaries waffle
across the area, though onshore flow may persist more often than
not until a more defined cold front passes through Wednesday
night. With the frontal passage after sunset, am not thinking
strong winds will be found in its wake. Also can't completely
rule out a few thunderstorms each day, mainly focused in the
afternoon and evening, although it's uncertain how many reach
the waters at this time.
Sub-Small Craft Advisory winds expected later this week. Thunderstorm chances
increase late Friday with an approaching front.
water levels remain high and should continue above normal for
the next few cycles due to the persistent onshore flow. Likely
to continue reaching minor flood at many sites and advisories
remain out into early this afternoon. Guidance also suggests
advisories will need to get extended or reissued for the next
cycles late tonight and tomorrow. After Tuesday, northerly flow
may finally allow anomalies to start falling.
District of Columbia...coastal flood advisory until 2 PM EDT this afternoon for
Maryland...coastal flood advisory until 1 PM EDT this afternoon for
Coastal flood advisory until 10 PM EDT this evening for mdz017.
Virginia...coastal flood advisory until 2 PM EDT this afternoon for