Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
352 am EDT sun Aug 30 2015

high pressure remains over the region through the middle of the
week and then weakens through the end of the week.

Near term /through tonight/...
synoptically...not much has changed in the past 24 hours. Hipres ridge
still resides over the eastern Seaboard...perhaps a little bit further east
than Sat morning. Aloft there is a S/southwesterly drift...which is evident from
middle-upper level clouds streaming into Virginia. The last few frames of satellite loop
suggests that there may be some thinning to the moisture...and am still
hopeful that we/ll be mostly clear at sunrise. These clouds /and the fact
that we hvnt fully dcpld/ have interfered west/ fog formation...and do
not anticipate much at dawn.

Not only is there hipres at the surface...but this ridge extends up through
500 mb...parked off hse. Thus...subsidence should limit instability for much of
the County warning forecast area. Lower heights across the Appalachians combined west/ terrain circulations
suggest that if rain showers/thunderstorms and rain develop the mountains will be the location. At this time
threat looks a little less than it did from yesterday/S cycle...and have trimmed
the areal covering through dusk. The threat never was all that great...20-
30 probability of precipitation at best.

One diff over Sat will be the skycover. Think there will be a fair amount
of middle-upper level cldcvr spreadinga crs area...mainly during the afternoon.
In spite of this...air mass will be capable of supporting maxt a cpl degf
warmer. Metropolitan balt/DC/f-Burg should have a 90 degf day. Over the
blurdg/shen valley/ptmc Highlands...mid-upr 80s will be common. Southwesterly flow
will support dewpt advection to the middle 60s-- still not bad but a little bit

Positive vorticity advection approaches area tonight from the SW as a subtle surface boundary drops southward
from the Great Lakes. These two features suggest that not only will probability of precipitation
linger past sunset...but the chance will spread east from the mountains toward I-
95. Like the afternoon...probs not high /20-30 percent/ and quantitative precipitation forecast levels low.
Clouds more certain than precipitation. A majority of the time could be just
mostly cloudy west/o any rain. What does falls may be not much more than
sprnkls...especially during the overnight hours. Hard to provide specific
timing other than holding back across the District of Columbia/balt burbs through middle


Short term /Monday through Monday night/...
detail issues linger into the surface boundary dspts in/near northern
County warning forecast area and positive vorticity advection/moisture continues to overspread area from the SW. Need to keep
broken clouds and low end probability of precipitation in the forecast for much of the day due to
this weak/disorganized forcing...although confidence in getting wet
quite low. 500 mb trough axis will progress east of area by lt day...and will
offer up clearing during the middle-lt afternoon into Ely evening in its wake. Should
be mostly clear by overnight Monday into Ely Tuesday.

Am not going quite a warm for maxt Monday due to clouds and precipitation chances. dewpoints threaten to reach the upper 60s...cant say the same
for min-T forecast Monday night.


Long term /Tuesday through Saturday/...
with departing vorticity maximum to our east...500 mb height rises and very poor
lapse rates...with nearly moist adiabatic profiles above local...will
result in a dry day across County Warning Area. The one exception would be in our
far western counties...where a stray afternoon shower or
thunderstorm could develop. Main story Tuesday will be the
heat...with maximum temperatures at or above 90 east of Blue Ridge and in u80s
west of Blue Ridge...or 5-10 degrees above climatology in many spots.
Dewpoints remain in m60s...which will keep heat indices near the

Weak upper level disturbance moves through Wednesday increasing the
chances for showers/thunderstorms across the region. Weak lapse
rates remain...leading to long skinny cape profiles...which will
prevent storms from realizing maximum cape. Any showers/storms would
weaken quickly near sunset...with strong convective Reliance on

Our area remains in north to northwest flow through the remainder of
the period...with chances for diurnal showers and thunderstorms each
day. Lack of additional lifting mechanisms beyond daytime
heating...will generally keep activity isolated and the best chances
west of Blue Ridge. Very little relief from the heat with
temperatures remaining 5-10 degrees above normal...though continued
northwest flow could allow for a slight cooling trend by next

Aviation /08z Sunday through Thursday/...
mrb has been in/out of MVFR visibility overnight...and think that will be the
site to watch. Do not expect IFR. Cho may yet see brief restrictions
as well...but will need to get rid of cldcvr first.

VFR conds and light southerly flow will prevail through the daylight hours. There
may be a cpl mountain shra/tsra...but none should impact terminals. Precipitation
chances spread east tonight but still remain west of District of Columbia/balt hubs. Confidence
in anything affecting mrb/cho too low to mention in taf at this time.


Monday...low probability rain showers/thunderstorms and rain linger into the day...however low
confidence precludes taf mention.
Tuesday...VFR conditions under high pressure.
Wed-Thu...low afternoon shower/thunderstorm chances return...with
brief periods of sub-VFR near activity.


southerly channeling has been relatively active...necessitating Small Craft Advisory to be
issued for the core of the Maryland chesepeake Bay. WRF guidance suggests that the
winds should be calming slightly overnight. Have expiration at 4am...and do not
anticipate extending it.

Southerly flow will continue through Monday. Think there will be another southerly channeling
event tonight...although latest forecasts suggest that Small Craft Advisory wont be required. Yesterday
the same could be said for last night/this will need to monitor
this further.

Winds expected to remain sub-Small Craft Advisory from Tuesday through remainder of
period. Afternoon thunderstorm chances return to the waters on
Wednesday and continue through the remainder of the workweek.


Lwx watches/warnings/advisories...
District of Columbia...none.
Marine...Small Craft Advisory until 4 am EDT early this morning for


near term...heights
short term...heights
long term...mse

National Weather Service Glossary of Abbreviations