Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
242 PM EDT Friday Aug 29 2014
high pressure over the northern middle-Atlantic this afternoon will
move offshore for the weekend. A warm front will lift north
through the area late Saturday. A cold front will approach during
the first part of next week.
Near term /through tonight/...
its a delightful afternoon...and that will continue into the evening. Hipres
ovhd west/ temperatures in the 70s and dewpoints in the Lower-Middle 50s /even
some upper 40s/. The surface ridge will ease offshore...but still have plenty
of ridging h7-5. Will develop a light southeasterly flow tonight...enough to bring in some
lower clouds aftr midngt. Will keep patchy fog reference to the central
shen valley and centrl foothills. Am not real confident that will take
place. The anticipated moisture advection should bump dewpoints to near 60f...
and thus min-T Lower-Middle 60s reasonable.
Short term /Saturday through Sunday night/...
believe that the ridging aloft will be the primary weather influence Sat.
Guidance suggesting that a warm front will enter County warning forecast area middle-lt afternoon.
Subsequently instability increases bhd fnt. GFS curiously attempting to
fire off thunderstorms and rain along warm front while strong middle-level divergence
/subsidence/ dominates. Shear none. Instead...believe that primary
focus for any storms would be orographically induced. Have broken or nearly
broken clouds for the day...but focused probability of precipitation on the mountains since cape
builds through the evening...chance thunderstorms and rain will linger later into the night.
The ridging starts to break down Sunday...W/ deeper southwesterly flow
prevailing. That will support a stream of precipitable water at or above 2 inches to ride
through County warning forecast area coincident west/ decent instability. While exact corridor of
cape/moisture likely will adjust west/ future model runs...suspect the main
idea will remain the same. Hence...expect decent covering of thunderstorms and rain...1st
in the mountains by midday...and dropping into a developing Lee trough along
the I-95 corridor by end of the afternoon into the evening. Have likely probability of precipitation
in grids...and possible heavy rain wording in severe weather potential statement.
Temperatures will be moderating on Sat. Will be staying west/ the warmer maximum
maxt. MOS mean maxt not far off from going forecast sun...and changes
minimal. Similarly...changes to min-T Sat and Sun night minor. In
general...favor the warmer met which has better support from forecast
While it will be hot by sun...W/ dewpoints at or above 70f...resulting
heat index values should remain under 100f. Therefore...do not
anticipate any heat headlines...even though its a Holiday weekend.
Long term /Monday through Friday/...
the upper level subtropical high over the southeast will migrate
westward to over the Mississippi Valley Monday. This will change the
direction of the upper level flow locally to be a bit more
northeasterly...which will result in a marginal enhancement of
instability Monday evening. At the surface...a well defined Lee
trough forming in moist southerly flow will further promote
instability...especially along the Virginia and Maryland Piedmont regions. Precipitable waters
Monday will be slightly less than Sunday...but with values still
around 2 inches and plenty of instability localized flash flooding
remains a threat. Despite a nicely curved hodograph...weak
shear...0-6 km shear values of less than 20 kts...should keep the
severe potential low Monday evening. Thunderstorms should begin to
die out after sunset as they loose their diurnal enhancement.
High pressure off the coast will ridge down across the southeast US
Tuesday. Meanwhile...a deep low pressure system well north of the
Great Lakes will swing a cold front south from the Ohio Valley to
the New England states. This front will struggle to get into the
middle-Atlantic Tuesday due to the high building to our south. The
result will be a more westerly/downsloping component to the low
level flow. This should help to suppress most convection
Tuesday...especially compared to Sunday and Monday. If thunderstorms
do form...the most likely location will be along the
Mason-Dixon...where the flow will be more southerly than across the
southern part of the County Warning Area.
The front will finally reach our area Wednesday...though with little
moisture left it should be mostly dry. The primary impact from the
frontal passage...which should occur from north to south over the
course of the day Wednesday...will be a drop in humidity and
slightly cooler temperatures for the second half of the workweek. Expect
highs near 90 Monday and into the lower 90s Tuesday...the warmest
day of the week...before dropping into the upper 80s for the second
half of the workweek.
Aviation /18z Friday through Wednesday/...
VFR through much of the valid taf period. A light southeasterly flow developing after dark may
support advection of a lower cloud deck aftr midngt. Not completely
sold on that...especially if there will be any subsequent flgt
restrictions. Fllwg lead of prvs taf...confining MVFR to cho
only. By afternoon...ceilings likely bkn035-040 west/ southeasterly winds at or below 10 knots.
Sun-Tue...sct-nmrs thunderstorms and rain...largely diurnally driven. Local flgt
restrictions possible...but cannot pinpoint where at this time.
Will be Post-frontal passage by Wednesday. VFR.
variable winds at or below 10 knots as hipres ovhd. Ridge will mv east of the
Delaware-Maryland-Virginia...supporting a southeasterly flow tonight and Sat...and southerly flow by
Sat night. Winds should remain at or below 10 knots through Sat. Gradient does increase
Sat ngt-sun...but it doesnt look like gradient flow will mix to the
surface. Have capped winds at 15 knots.
Have a thunderstorms and rain threat lt sun into sun evening. A high wind threat will
accmpny these storms. Special marine warnings possible...continuing through Monday.
District of Columbia...none.