Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
929 PM EDT Friday Oct 31 2014
low pressure over Indiana this evening will cross South Carolina
Saturday before strengthening offshore while heading towards Cape
Cod Saturday night into Sunday. High pressure will build over the
area into Monday with the next cold front arriving late Wednesday.
Near term /until 6 am Saturday morning/...
as of 01z...upper low center is approaching Louisville per water
vapor imagery. The upper low will shift southeast overnight and cross
South Carolina Saturday morning. The 1008mb surface low is currently
over Columbia South Carolina...confirming the direction the storm
is expected to take. The leading lobe of the upper trough is over NC
(where some thunderstorms are) with the surface low well off the NC/Virginia
border...likely around 1004mb.
The eastern flank of the trailing...more potent low is producing
precipitation over the Allegheny Highlands. Our observer at 3700ft in
southwestern Pendleton Colorado reported snow sticking to cars at 8pm.
Elevations above 3000 feet will see a wet dusting to an inch or
possibly overnight...not enough for an advisory.
Otherwise...clouds thicken and lower. Rain from the off shore low
will spread in from the ocean late tonight. This coverage of
precipitation will slowly close through Saturday morning.
Short term /6 am Saturday morning through Sunday/...
previous discussion still valid...
upper vorticity/short WV over eastern Tennessee Sat morning...moving east of the SC CST during
the afternoon. This will make The Highlands the only area in the forecast
area west/ the possiblity of seeing snowfall..but the 1000-850md tchkns's
aren't that low: ~1320m. This means the atmospheric column just
isn't that cold...and any precipitation that falls in the frozen variety will
be very dependent on elevation. During Sat accumulations would be
slushy and minor. Blv that there will be a period of rain east of the mountains
just about everywhere Sat.
Highs ranging from the l50s east of I-95 to the u30s in The Highlands.
By Sat night...shortwave energy would have shifted off the CST...inducing
cyclogenesis NE of Cape Hatteras. In terms of precipitation east of the
aplchns...that pretty much will mark the end...as drier air will work
in on the northwest side of the cyclone. Winds may be the biggest
impact...but the nocturnal timing may prevent the full 30-40 knots
available h9-8 from mixing down. Will be keeping gusts 25-30 miles per hour.
Across the mountains...the air mass will be cooling sufficiently for some snow.
However...the best dynamic cooling will pass S of area. This leaves only
a marginally favorable air mass west/ warm grnd. Snow totals 1-2" in the higher
elevs. It is not out of the qstn that accums could be enough to
reach advisory level...but this is not a slam dunk and will leaving this to
be decided later.
Lows Sat night range from the m20s in the mountains to the l40s along the
Long term /Sunday night through Friday/...
blv winds may end up being the biggest story from the upcoming system.
Some snow flakes could continue to fall for a couple of hours after
sunrise along the Allegheny Front...but all of the precipitation will be
wrapped up across the area by middle-morning Sunday. The story will
then be the cold and wind. A 100+ knots jet centered around 20000 feet
will pass overhead Sunday. Expect a decent amount of these winds to
mix down to the surface...with frequent gusts in the 30 to 40 knots
range possible for the higher elevations and over the Bay...and
gusts of 20 to 30 kts possible elsewhere...during the day Sunday.
Deep layer northwesterly flow with a fetch well up into eastern
Canada will bring seasonably cool air to the mid-Atlantic. Highs
Sunday will be around 50...which is about 10 to 15 degrees below
normal...though still a couple of degrees above record low maximum
Sunday night will see the coldest temperatures of the fall so far...with
freezing and subfreezing temperatures expected for most of the County Warning Area
as high pressure settles into the area. Wind speeds will also be on
the decline Sunday night due to the decreasing pressure gradient as
the high pressure center approaches.
High pressure will settle along the southeast coast for the first
part of the workweek and temperatures will moderate toward normal as light
return flow fills in. Highs Tuesday and Wednesday should be well
into the 60s with plenty of sunshine.
A cold front may approach the area Wednesday night and Thursday.
This does not appear to be a strong cold front...so confidence is
low in precipitation coverage at this point. Operational European model (ecmwf)
actually keeps US mostly dry while GFS brings in some light rain.
Overall have low end chance probability of precipitation in the forecast with rain as the
only expected p-type due to warm low level temperatures. High pressure
should build back in by the end of next workweek.
Aviation /01z Saturday through Wednesday/...
skies have gone overcast over much of the area as low pressure over Michigan rotates
moisture into the middle Atlantic. Ceilings are presently VFR...but these are
expeceted to slowly lower overnight. MVFR conds expeceted Sat. Possible IFR for
kbwi/kmtn with morning rain showers.
Low pressure strengthens off the coast Sat. Nearly winds will gust
up to 20-25 knots.
Winds will provide primary impact Sat ngt-sun...as an elevated core
of 40+ knots winds cross terminals. The low level jet will reside about 2k feet above
surface. Should have 25-30kt gusts in northwest flow at grnd level...maybe a little
higher than that sun mrng-midday. Any ceilings would be MVFR-VFR Sat night
and should clear by/soon aftr sunrise Sun morning.
VFR Sun afternoon into Tuesday.
northerly flow increases through the night...Small Craft Advisory begins at 2am...expands
to all waters by 8am Saturday.
With the strengthening low moving off the coast on Saturday...small
craft conditions will be met with northerly winds gusting to
around 30 knots.
We have issued a Gale Warning for Sat night.
A Gale Warning will likely be needed for portions of the water
during the day Sunday and potentially into Sunday evening. A Small
Craft Advisory may be needed into early Sunday morning.
blowout conditions are expected Saturday evening through Sunday.
It is uncertain as to how far below MLLW the water levels will
drop at this time...but a northwesterly gale should drop them to
at least one foot below MLLW.
District of Columbia...none.
Marine...Small Craft Advisory from 2 am to 8 PM EDT Saturday for
Gale Warning from 8 PM EDT Saturday to 6 am EST Sunday for
Small Craft Advisory from 6 am to 8 PM EDT Saturday for anz530-
Small Craft Advisory from 8 am to 8 PM EDT Saturday for anz535.