Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
205 PM EST Friday Nov 27 2015
high pressure will sink southward away from the region tonight and
Saturday. A cold front will push south across the area starting
late Saturday and continuing through Saturday night. A weak wave
of low pressure will pass south of the area Sunday. High pressure
will try to build in from the northeast Monday before another
storm pushes a cold front across the area Wednesday.
Near term /through Saturday/...
as of 2 PM...a ridge of surface high pressure extended from
western North Carolina northeastward across the Delaware-Maryland-Virginia and into
the western Atlantic. Aloft a ridge is now in place...and with
warmer temperatures in the middle levels compared to
yesterday...surface temperatures have also turned out warmer than
yesterday. Highs will probably crack 70 in a few places.
High presure will sink southward tonight as a cold front
approaches from the northwest. High clouds will continue to pass
overhead in slowly increasing depth and the gradient will
increase a bit so right now we do not expect a repeat of the
patchy dense fog we saw this morning. Lows will stay warmer as
well...with most places staying in the 40s.
The front now appears even slower. Though the pressure trough
appears over US by 18z...the 1000 mb Theta-E gradient not
slip south into the County Warning Area until very late in the day. This suggests
that we will stay pretty warm once again and may only stay a
little cooler than today because of cloud cover. Thus...went above
most guidance and have highs well into the 60s again for Saturday.
As the front does finally slip southward...showers will slowly
advance southeast into the County Warning Area...but many places will probably
stay dry through sunset.
Short term /Saturday night through Sunday night/...
cold front finally slips southward across most of the County Warning Area Saturday
night. This is just in time for a weak wave of low pressure to
slide eastward along the front...making rain more probable than it
appeared earlier. Thus...have increased probability of precipitation to likely across most
of the County Warning Area...especially northern areas...for Saturday night...with
best probability of precipitation after midnight. Rain amounts look light...mainly a tenth
to a quarter inch. Temperatures will slide back into the 40s behind the
Cold front should be near the southern border of the County Warning Area Sunday
morning...making only slow progress during the day. Warm/moist air
aloft will overrun the front...allowing for cold rain on the north
side of the boundary. Models do show a trend of the dry air slowly
filtering in from the north...but the GFS is sort of out on its
own in showing Sunday totally dry. There is enough support from
ensembles to justify retention of the current 40 pop on
Sunday...and actually increasing it to 50-plus over the west/SW
portion of the forecast area where the dry air will take the
longest to reach.
Sunday night is low confidence with simulation solutions all over
the place. The one common theme seems to be development of a wedge
as the strong surface high centers itself in the perfect place over
southern Quebec. What is not clear are precipitation chances. Again the op GFS
is dry as a Bone while the European model (ecmwf)...many gefs ensemble
members...and the overall consensus hint at continued rain
chances. While I did leave far northestern Maryland dry...a small chance of
rain was introduced for the Baltimore metropolitan...increasing as you go
further south and west. Depending on the strength of the cold air
and just how far north rain actually gets...a little light
freezing rain cannot completely be ruled out for the Monday
morning commute along the Maryland/PA border. The probability is too
small to mention in the forecast yet though.
Long term /Monday through Friday/...
stalled frontal boundary remains to the south through midday
Monday...keeping highs generally in the 40s...with onshore flow
and chances for overrunning showers. Boundary lifts north through
the area as a warm front late Monday...while an area of low
pressure ejects northeastward from the Central Plains toward the
Great Lakes. Passage of warm front will lead to an increase in
return flow and subsequent low-/mid-level moisture and allow
temperatures to return to the M/u50s Tuesday and Wednesday.
Cold front...associated with aforementioned low pressure over
Great Lakes...approaches our western border late Tuesday and moves
through the area Wednesday...bringing the highest chances for
precipitation during the long term. Cold air could filter in fast
enough to allow for some changeover to light snow showers across
our far west.
While there are a few noticeable outliers among the different
ensemble guidance...general consensus moves precipitation out by
late Wednesday. Cooler and drier air filters into the region
behind the front...with high pressure building over the area
through the end of the workweek.
Aviation /19z Friday through Wednesday/...
VFR expected through the next 24 hours. Small risk of patchy fog
again tonight but with front approaching...think gradient keeps it
stirred up enough to avoid it. As front slips south later on
Saturday and rain moves in Saturday evening...ceilings and visible appear
likely to fall to at least MVFR if not outright IFR. Winds tonight
will remain south to southwest less than 10 knots...then shift
northwest behind the front Saturday afternoon and Saturday night
at similar speeds.
Potential for MVFR ceilings/visibility in rain Sunday and Sunday night. IFR
ceilings possible during that time.
Stationary boundary to our south Monday will allow for some
overrunning showers over mainly the southern half of the
area...with sub-VFR conditions possible in these areas. Periods of
sub-VFR spread across all terminals by late Monday into Tuesday as
warm front lifts over the area and then through Wednesday as cold
front passes through.
sub-Small Craft Advisory conditions expected through Saturday night. Fog should not
make a significant reappearance tonight as there should be just
enough wind to disperse any that tries to form...but S to SW winds
should still stay below 10 knots overall. Front crosses the waters
later Saturday into Saturday night with an increasing chance of
light rain and mist. Winds shift northwest but should stay below Small Craft Advisory.
Cold front settles just south of the lower Maryland waters by Sunday.
Gradient winds pick up with mixing...yielding potential for 15
knot gusts Sunday night. Capped just below Small Craft Advisory criteria at this
time due to uncertainty of the strength of The Wedge/pressure
Mainly sub-Small Craft Advisory conditions expected Monday through
Tuesday...though pressure gradient may keep winds near Small Craft Advisory early
Monday...before it relaxes and winds subside.
Cold front will pass through the waters Wednesday...with a
potential for Small Craft Advisory conditions.
District of Columbia...none.