Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Baltimore Maryland/Washington District of Columbia
529 am EDT Thursday Aug 21 2014
a stationary front will waver across the middle Atlantic as ripples
of low pressure move along the boundary. The last impulse will
traverse the front Friday night...which will push it south as a
cold front. High pressure from New England will down down the East
Coast this weekend.
Near term /through tonight/...
update...an earlier spec weather statement for locally dense fog has
been replaced west/ a dense fog adzy for much of these same areas...
espec a tier of counties either side of the northern Virginia Blue Ridge/Maryland
catoctins. This includes portions of the northern shen valley as well.
Rain from last evening...calm winds and a moist boundary layer
contributing to an increasingly dense - though shallow - low
stratus layer over the mountain valleys and Piedmont regions.
the upper-level low pressure pinwheel a few hundred miles to our
north will slingshot another wave of moisture across the region
later today...before dissipating and being becoming part of a
larger upper trough. Our moist airmass will undergo very little
change over the next couple of days. This modified sub-tropical
regime has a corridor of dry air atop a dense and very moist
boundary/surface layer. Northwest flow aloft will carry waves of embedded
vorticity maxes...which are essentially areas of lift...but in an
intermittent and random nature. Later today/this evening...a more
compact area of upper energy will be shoved quickly down across
the upper Midwest and over the middle-Atlantic region.
Like Wednesday however...the upper low will force this wave down toward
the area but kind of stable coastal block will dissipate the energy
as it approaches the Atlantic coastline. But before then...plenty of
moisture and marginal instability will be available for the incoming
shortwave energy to develop convection. One of the key factors will
be our high temperatures...again expected to repeat in the M-u80s. These
values combined west/ surface dewpoints in the u60s/l70s will be more than
enough to at least break the weak cap and force for the
conditionally unstable atmos to develop scattered showers/tstms. A
handful of isolated showers could dot the map over the coming
hours...but not until middle-late afternoon is more organized/widespread
development expected. Timing/locations/intensities will depend on
early outflows and storm momentum factors but the low level shear
will certainly be enough /20-30kt boundary layer/ to sustain a
As west/ the convection on Wednesday aftn/eve...the activity level will
quickly drop off after sunset and west/in a couple hours afterward.
Overnight lows will drop only into the u60s/l70s again since
dewpoints won't budge - setting up for another wave of showers/thunderstorms
on Friday afternoon.
Short term /Friday through Friday night/...
most short/medium range models...including the tail end of some local
WRF members are in good agreement on a Friday afternoon and into late evening
hours heightened level of shower/thunderstorm activity across portions of the
area. Probability of precipitation are higher than the garden-variety isolated afternoon regime.
This could mean that a more stratiform/dissipated region of earlier
convection will drop a regional-scale batch of light/moderate rain
over the area...espec across the western half of the County Warning Area as models
Increasing cloud cover from late tonight into early Friday could also
inhibit convective potential and limit it to areas further S and east
where temperatures are expected to peak in the m80s. Model adjusted values
are giving l80s for highs...accounting for the cloud cover
increase...a light northerly wind that is expected to turn more Ely over
time and certainly into the late night hours - which is a more stable
Long term /Saturday through Wednesday/...
500 mb pttn will be even more amplified in the extended period...W/ ridging in the
center of the Continental U.S. And troffing along the eastern Seaboard. That will
permit surface hipres over the Canadian Maritimes and downeast Maine to build
down the coast. Therefore...am becoming incrsgly confident that the wet
pttn of this weak will break.
Sat will be the transition day. Will still have Ely surface low and modest
instability. Meanwhile...the jet stream will still be ovhd...proving a zone
of shear. With a maritime air mass in the low level and lift west/in the
column...need to paint a gloomy picture. Will be holding probability of precipitation at
chance...and emphasizing points east of the blurdg /upslp flow/. Am not
going as low on maxt as possible. Since am not going for a washout...will
be holding out some hope that there will be some opportunity for temperatures
to rise from dewpoints. Those dewpoints should be upper 60s to near 70f...so
itll be a muggy day even west/o sunshine.
Will be pushing precipitation back into the mountains Sat night-sun as ridging builds.
Still have prolonged Ely flow...so sun will be the day when air temperatures will
be struggling to rise above water temperatures. There may be enough air mvmt so
that fog wont be an issue Sat night. If it turns out that flow dcpls
instead...it will be a whole different story.
Surface hipres will run the show for the rest of the extended. Am much more
generous west/ sunshine. Maxt upper 70s-lower 80s/min-T 50s to lower 60s
still below climatology...but reaching time of yr when nrmls starting to
Aviation /10z Thursday through Monday/...
ground/valley fog has settled over parts of the region...espec along
the northern Virginia Piedmont in typical fog prone valleys and areas that
received rain last evening. The metropolitan areas will only see a brief period
of high-end visibility ground fog and potentially a brief MVFR/IFR stratus
deck from the moist overhead envrmnt toward sunrise. Skies after
sunrise will waver in/out of partly sunny/mostly cloudy since we
still have a moist atmos. An upper-level disturbance will swing
across the region later today...potentially carrying another wave of
showers/thunderstorms to the area. Conds will improve again
overnight...though a few isolated showers and more dense cloud cover
is expected into early Friday. More shower/thunderstorms expected on Friday
afternoon/evening as well.
Sat-sun...loclds and visibilities may be an issue as a marine air mass spreads
inland. Think flg restrictions will be more pronounced and prolonged
Small Craft Advisory conditions not anticipated through the end of the work week...as
winds have dropped off overnight. A few thunderstorms possible this afternoon/evening
over the waters...calm again overnight west/ another round expected on
Ely flow will dominate this weekend as hipres builds down the coast. There
are some hints that winds may touch Small Craft Advisory crit at times...primarily in
the Middle Bay. That all depends upon strength of the surface high and
relative lopres off the Carolinas. Not too confident at a tight
p-gradient at this time...and have capped winds below Small Craft Advisory crit.
water levels about 3/4 to 1 feet above astro norms. That placed Annapolis
at risk for the early morning tide cycle. Gauge readings came back
around midngt...and we/re topping off just above minor criteria.
Departures elsewhere should not trigger any advisories...but a few
caution stages should be reached. Only other concern may be
Alexandria/swdc...where a 1.1 feet departure would tip the scale.
Think we may be able to sneak that one out...but admittedly its
gonna be close.
We will have southerly flow today becoming light tonight. Fortunately the 2nd
tide cycle of the day is the lower of the two astronomically.
Guidance suggesting we will have just enough opportunity for a Little
Water to ooze out of the estuary by then. So...tomorrow mornings
tide will be high...but am not anticipating reaching criteria at this time.
However...we are working on a margin of inches...which is pushing
current skill level...so its advisable to continue to monitor the
Flow will be northerly Friday before onshore /Ely/ flow returns for the weekend.
Thus...if water doesnt evacuate by the end of the week...there could
be more nuisance issues this weekend.
District of Columbia...none.
Maryland...dense fog advisory until 9 am EDT this morning for mdz003-004.
Virginia...dense fog advisory until 9 am EDT this morning for vaz025>031-
WV...dense fog advisory until 9 am EDT this morning for wvz051>053.