Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
328 PM EDT Wednesday Sep 2 2015
surface trough will move over the region through this evening. Weak
high pressure builds overhead Thursday before a backdoor cold front
moves into the area Friday. High pressure then returns for the
weekend into early next week.
Near term /through tonight/...
radar lit up by 16z as shortwave energy hit the 2000 j/kg MUCAPE air mass.
There/S still no shear to speak of...but instability...positive vorticity advection and steep
low level lapse rates have worked in concert to produce scattered-numerous thunderstorms and rain west
of the blurdg. Echoes primarily in the lwx County warning forecast area. Some of these
echoes have been tall. Per previous analysis...although severe weather not
expeceted... if a threat were to come from these...it would be from a wet
microburst. Do have heavy rain in these cells as well. Precipitable water running
between 1.5-1.75 inch. While ffg high enough so that shudnt be an issue
either /weve been in a dry spell past month or two/...repeated
echoes may cause local ponding.
Bumped probability of precipitation up into the likely range for rest of the afternoon. Am
confining hiest probability of precipitation in vicinity of the shortwave...which will take it S of District of Columbia by
sunset...and out of the County warning forecast area by midngt. Am thinking the clouds will
thin in the subsidence in the wake of the vorticity...so will have mostly clear
skies overnight. Since grnd will be wet...air mass humid...and winds still
light...that makes for a good fog recipe. Have patchy fog for much of
Virginia/ptmc Highlands by dawn.
Have made no adjustments to min-T...which will range from middle 60s to lower
Short term /Thursday through Thursday night/...
500 mb ridge axis will reside across the MS valley Thursday...which places County warning forecast area in
the northwest flow. However...the air mass will be just as warm...humid...and
unstable as today. Would expect diurnally drive thunderstorms and rain to develop alng the mountains
during the afternoon. Theres consistent model evidence that these storms
will emerge from the mountains and track into the coastal plain by evening.
Timing specific vorts problematic at this point...but would think
that all of this activity will clear County warning forecast area before dawn.
Hvnt made any big adjustments to temperatures...as persistence should prove
to be a good forecasting model.
Long term /Friday through Wednesday/...
main weather feature during the long term will be a backdoor cold
front that moves through Friday afternoon and evening...providing
rain chances for the entire area. At this time...kept probability of precipitation at chance
everywhere...as not all guidance suggests widespread coverage. While
cape will be low/moderate...generally at or above 1000 j/kg...ebs nearly
nonexistent...so expect pulse-type activity and heavy rainers as
individual cells move very slowly. Cloud cover...onshore flow...and
frontal passage should work in concert to keep temperatures a few
degrees less than previous days. However...still expect temperatures
a few degrees at or above normal. Precipitation chances should dwindle quickly
after dusk Friday night with loss of instability.
Very little day-to-day variability in sensible weather through the
remainder of the extended...as high pressure builds in Saturday and
remains through the middle of next week. With warm middle-level
temperatures...lapse rates will be very poor...consequently chances
for precipitation during this period appear very low and generally would
only be confined to the higher terrain in the west.
Elsewhere...expect high temperatures near normal through the
weekend...with dewpoints falling into the u50s by Sunday. By
Monday...temperatures start creeping back above normal and by
Tuesday some areas could be back into l90s.
Aviation /19z Wednesday through Monday/...
numerous thunderstorms and rain in vicinity of mrb/cho...and these storms appchg iad. Dca should be
on the edge of the activity. Expect brief IFR west/in any storm. Some
strong wind gusts possible.
Once storms clear...should be plenty of residual low level moisture as skies
clear. MVFR fog potl...especially mrb/cho. Will need to evaluate extent of
Forecast should be relatively simlr tmrw...except all activity should be
several hours later. Lt day/Ely evening thunderstorms and rain...producing local restrictions.
Sub-VFR possible Friday afternoon as backdoor cold front moves
through the region. VFR conditions return Saturday through the
extended under high pressure.
numerous thunderstorms and rain across the land at this time. These should stay west of the
waters...but wont completely rule out a storm or two reaching the
ptmc. Strong wind gust potl west/in a few of these storms.
Northwest flow through Thursday. Another round of scattered thunderstorms and rain possible late day into the
evening...mostly staying west of the marine area but the ptmc river will
be on the edge of the hiest potl region.
Winds expected near Small Craft Advisory Friday afternoon through Saturday
evening...as backdoor cold front moves through the region. Cold
front will also bring chance for showers and thunderstorms over the
waters Friday afternoon and evening. Winds should subside below Small Craft Advisory
late Saturday into Sunday morning and through the extended.
District of Columbia...none.