Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
359 am EDT Tuesday Sep 2 2014
Bermuda high pressure will persist today ahead of a weak cold
front that will cross the area tonight. High pressure will then
build from the Midwest through Wednesday then persist offshore
Thursday into the weekend. A cold front will enter the region in
the Saturday night time frame.
Near term /through tonight/...
another muggy overnight west/ much of the regional observation showing
dewpoints in the l70s - plenty humid for an early sept night.
Smaller scale ripples in the lower atmos caused some scattered
showers/thunderstorms over the northestern quadrant of the County Warning Area earlier in the
overnight which have since dissipated...but evidence of the
instability that can be tapped even overnight. The 00z kiad sndg
from last night shows over 2k j/kg surface cape...though the typical
nighttime decoupling took away much of the ll convergence.
Little airmass change over the next 12-18hrs will basically keep
the overhead atmos in maintenance Mode - allowing isolated
showers/thunderstorms to develop west/ even weak/disorganized forcing. A
handful of showers now reaching The Spine of the Appalachians. Though
this activity will likely look much different over the next few
hours...The Arm of low level energy causing them may develop more
activity further east into the midday. Near term WRF derivatives
largely point toward a late afternoon collection of thunderstorms developing
near the Blue Ridge and sliding east through the remainder of the evening. A
cold front will be a few hours behind this activity...so the
afternoon/evening thunderstorms will likely be forced by a pre-frontal trough/
differential heating boundary.
It has been a hot/humid past few days...and today will likely be
the apex of the current stretch. Winds will be mainly west-southwest...but west/
some downsloping later component to the winds. These winds may
also do well in preventing convection for a large part of the day.
Dry/unidirectional flow is typically not good for convergence...
so a low level boundary would be necessary for effective
initiation. Heating won't be an issue however...W/ temperatures quickly
rising right back into the 80s through the morning hours and l-m90s by the
peak heating of the middle afternoon. An influx of dense low-middle cloud
decks from the west during the middle afternoon may help to cut down on
the higher temperatures...and so would any ongoing showers/tstms.
Dewpoints in the u60s/l70s will also create another hot/humid
environment west/ heat indices well into the u90s west/ a few locales
potentially poking just above the 100f degree mark in the urban
After the showers/thunderstorms have rolled through later this evening...the
front will begin to clear out the area from the west. Skies will
start to clear and winds will turn northwesterly. This passage though won't
be particularly strong...W/ winds only in the 5-10kt range at best
and only about a 5f degree dewpoint drop overall...so the humidity
will remain elevated as we head into Wednesday.
Short term /Wednesday and Wednesday night/...
dry and stable on Wednesday. No probability of precipitation in the forecast. Another warm and
relatively humid day...as highs will again approach the 90f degree
mark across much of the area. Dewpoints will still be in the
l-m60s...creating heat indices right around the ambient temperature
Wednesday night...high pressure center over the area shifts
offshore with a return flow developing in the overnight hours.
Dewpoints quickly raise back to near 70f...might as well get areas
of fog wording in the grids. With the high persisting
offshore...an unstable Gulf airmass brings the middle-Atlantic right
back into an active pattern by Thursday. Chances for diurnal
driven scattered thunderstorms. Should low level flow be more southeasterly
than southerly...terrain enhancement would also be a factor in
thunderstorm development. Similar pattern for Friday with the next
Maximum temperatures Thursday and Friday around 90f though cloud cover will
be a factor.
Long term /Thursday through Monday/...
another weak cold front approaches Friday night. Likely probability of precipitation for
thunderstorms west from the Blue Ridge Saturday as there is good
consensus between the 00z GFS and 00z European model (ecmwf). Question is how
quickly the front pushes through the area. The European model (ecmwf) lingers the
cold front over southeastern zones through Sunday.
Aviation /07z Tuesday through Saturday/...
after some earlier shower/thunderstorms passed near the District of Columbia/balt metropolitan
areas a few hours ago...skies have generally cleared out. More
clouds on the way from the west...as well as a few showers that
will likely dissipate toward dawn. A west-southwest wind during the daytime
hours...W/ scattered showers and thunderstorms expected to develop late today -
surrounding the 00z time period - west/in a couple hours either side.
Some earlier activity possible...after 18z but timing/placement
confidence is low at this time. A cold front will push this activity off
the coast late tonight and clear out the area for Wednesday.
Fog Wednesday night under high pressure. Light southerly return flow
Thursday and Friday with mainly afternoon chances for
thunderstorms. Cold front enters the area late Saturday with
thunderstorms ahead of it.
a relatively light southwesterly flow over the waters tonight...the higher
gusts near 15kt over the Lower Bay zones. This will continue into
the daytime hours...though more of a westerly direction to the flow through-
out the day. Scattered thunderstorms possible late this aftn/eve...so another
potential for convective marine hazards. A cold front will weakly
pass through the region overnight...turning winds to northwesterly but still
lower than Small Craft Advisory crit. Wednesday will see high pressure build into the region
and drop wind speeds down even further.
Light and variable flow Wednesday evening gives way to southerly flow as
high pressure moves offshore. Will have to watch for channeling
again (probably Thursday evening) but as of now there are no Small Craft Advisory
headlines expected. Cold front approaches the waters Saturday with
good chances for thunderstorms.
District of Columbia...none.