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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
335 PM EST Sat Dec 27 2014

high pressure will slide east over the Atlantic tonight. A cold
front will move into the region on Sunday before stalling...then
low pressure will move along the stalled front Sunday afternoon
and evening...pushing the front south of the area. Another low
pressure system will move across the Carolinas Monday and Monday
night. Strong high pressure will build over much of the country
by Tuesday...and gradually push eastward during the rest of the
work week.


Near term /through Sunday night/...
tons of sun today helping much of the area reach the highest temperatures
in a couple of weeks - some areas since the beginning of Dec. Warm
for this time of year...many locations - away from the waters - are
well into the 50s and 60s - some areas will top out in the m60s
shortly. Locations along/closer to the cooler bodies of water like
the Chesapeake Bay and tidal Potomac River still have the more
moderated conditions in the u40s/l50s.

Only a few more hours left of the generally clear a
solid shield of low-middle cloud decks approach from the west -
extending from the Gulf Coast to southeastern Canada. As the clouds approach
temperatures will drop steady back down into the 40s later tonight but halt
in that range once the dense cloud cover and precipitation arrive during
the overnight hours. Min temperatures tonight will only drop into the
u30s/l40s across much of the area.

Not quite the Standard middle-latitude cyclone expected for the next
couple of days to affect the East Coast. Currently...two separate
systems and linked together by their placement along the upglide
region of an elongated upper trough. The northern-stream system will
continue to speed off to the NE across the Great Lakes...dragging a
cold front and light precipitation across the upper Midwest. The front is
connected to a developing system over the deep south...W/ much more
moisture associated west/ it. This feature will eventually make its way
over the remainder of the southeast and just into our area by early
sun. The deep nature of the upper trough will wane somewhat as we
head into the daytime hours Sunday...which will have an effect of how
much precipitation we get. All guidance members have begun to back off a
bit on expected qpfs and overall intensity. Looking like a day-long
period of generally light rain west/ only about a quarter-inch or less
total rainfall anticipated through late sun. By sunset sun evening...only
some light residual activity hanging around into the late night hours
west/ the potential for some light rain/snow mix on the back side of
this activity...espec the higher terrain for a brief period.


Short term /Monday through Monday night/...
as the first upper wave pulls away early Monday...precipitation will
likely be ongoing in the vicinity of the 850 mb front...which should
be south of the metropolitan area. Soundings by 15z at dca show a lot of
drying...especially above 700 mb and around 850 mb. But the 15z
sounding at nhk is saturated between 700-900 mb. Knowing that exact
placement on day 3 is unlikely...have carried better than 50/50 probability of precipitation
Monday morning in the southern half of the forecast area...tapering
to chance or even slight chance in the afternoon. There could be a
brief window of snow potential in the morning along the Allegheny
Front but that would not amount to much.

Monday night...all the models bring back in some middle-level
moisture...with north/NE flow in the low levels overlain by deep and
strong westerly flow aloft. Previous forecaster mentioned the model
differences...mainly in quantitative precipitation forecast...and those differences still persist
with the 12z model runs. Given this...I lowered probability of precipitation slightly...but
did not remove them as there is an upper shortwave approaching
overnight. More than likely...any precipitation that does occur would be
light. Lows are near freezing west of there is a mention
of rain/snow along and west of the Blue Ridge...and all snow on the
Allegheny Front. Kept the less than one inch accumulation out there
from the previous forecast.


Long term /Tuesday through Saturday/...
NAM/sref and now 12z GFS are trying to hint at a third wave of low
pressure developing along a stalled front off the Carolina coast
late Monday night into Tuesday. The surface low track is well south
of what would be ideal for precipitation in our area /between Norfolk and
Cape Hatteras is the typical location/ but the best positive vorticity advection at 500 mb is
further north across our area. It is an open wave so any low
pressure area that develops should move quickly offshore before any
significant precipitation occurs. For now have low end probability of precipitation to
account for the chance of some light precipitation. Will have to be watched
in future updates...especially if middle-upper disturbance is able to
undulate the flow in the low-levels and result in favorable upslope

Temperatures will take a nosedive Tuesday night as high pressure
with Arctic origins takes hold to the west of the local area. With
850 mb temperatures between -8 and -11 c...high temperatures should only
top out in the 30s...5 to 10 degrees below normal for this time of

The cold airmass will linger over the area Tuesday night through
Wednesday night. There may be a few flurries Wednesday as a
shortwave trough passes but there is not nearly enough confidence to
include that in the forecast at this time. Overnight lows will
depend on how strong the winds are and if there are any
clouds...though at this point neither night looks to be a classic
widespread radiational cooling night. Even so...dew points in the
single digits mean that anywhere winds do go calm and clear will
likely freefall into the teens.

Temperatures will moderate some Thursday into Friday ahead of
another cold front. At this point it looks like temperatures should
warm enough for a mostly rain event except perhaps in the higher
elevations with this next system...with only the 00z European model (ecmwf) showing
temperatures cold enough for snow at any time late in the forecast
period /and that is due to a suspiciously strong surface low amidst
a relatively weak 500 mb perturbation late in the week/.


Aviation /21z Saturday through Thursday/...
solid VFR conditions through late tonight past 00z and into the 06-12z
time frame. Generally dense middle-level decks moving in toward dawn
sun...then scattered showers moving across the region throughout the
daytime hours sun west/ low-end VFR ceilings and some MVFR into the late
night hours.

Despite the precipitation chances Monday and Monday night...ceilings look
mostly high enough to be VFR. There is however some threat of lower
ceilings east depending on very low-level moisture inflow. Precipitation could
restrict visibility at times...mainly at cho. Winds look generally light.

Mainly VFR expected Tue-Wed...wind north 10-15 kts Tuesday PM...becoming
lighter 5-10 kts Wednesday.


light winds continue this afternoon/evening as high pressure moves off the
coast. A cold front will move through early sun west/ scattered showers
sliding over the region from Sun morning into the late evening hours. Winds
will generally stay at or below 10kt.

Marine winds should be generally light Monday and Monday night...but
could exceed 10 knots in the Bay near the Maryland/Virginia border Monday night
with a decent northerly fetch. Chance of rain on the waters both
Monday and Monday night.

Northerly channeling behind a cold front and ahead of an approaching
area of high pressure could result in Small Craft Advisory level
winds Tuesday night. Winds slacken Wednesday into Thursday as the
gradient relaxes with high pressure nearby...then could come close
to Small Craft Advisory levels again late in the week due to
southerly channeling ahead of a cold front.


klwx WSR-88D is officially back in operation as of a few moments
before this discussion was sent.


Lwx watches/warnings/advisories...
District of Columbia...none.


near term...gms
long term...dfh

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