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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
707 PM EST Wednesday Dec 24 2014

Synopsis...
strengthening low pressure will lift into the Great Lakes
tonight...bringing a cold front across the region. Mild southern
high pressure will move in for the end of the week. A weak cold
front will push in from the northwest Sunday. Then...low pressure
from the deep south will ride up along that front and over the
region early next week. Colder high pressure will follow for middle
week.

&&

Near term /through Thursday/...
areas of dense fog have developed across central Maryland southward into Loudoun Colorado.
As a result an advisory has been issued. Warm moist over cold grnd west/
light winds.

Surface low will continue to deepen and head north tonight as cold front pushes east ahead
of a well defined upper trough axis. The attendent cold front will reach mountains
near 00z...and should be to the western shore of the Bay in vicinity of 06z.
Present indications are corridor of precipitation minima between best isent lift along the Bay
and cold front in the Midwest will be transitioning over County warning forecast area lt taftn-
Ely evening. Have captured this guidance indication west/in database...but
didnt go below chance probability of precipitation. While support along fnt will still be
favorable...W/ moisture transport from 50 knots low level jet...have opted to lower probability of precipitation due
to covering concerns.

While air mass still unseasonably moist west/ precipitable water up to 1.5 inches...rafl
thus far not significant...and any addtl rain should be spread out. Given
low level wedge...guidance indicators suggest not as much instability present. Will
scale back rafl intensity to light and thunder back to schc. Theres
still more than enough dynamics/shear present for gusty showers to
potentially punch through. Will keep Wind Advisory in place for mountains...although
based on current surface pttn...window of opportunity looking more
scant.

Breezy conds will reside bhd cold front...W/ better mixing in place. No big
adjustments from prvs forecasts here...as mean layer suggests g25 knots or so.
Source region of air mass not that cold...and will have downsloping winds.
Therefore...maxt tmrw should be simlr to today...or perhaps a pinch
warmer. Morning clouds from positive vorticity advection will quickly erode. Mosun for the rest of the
day.

Upslope conds good for continued cldcvr along the western slopes...but lack
of moisture not favorable for producing precipitation. Mountain probability of precipitation Thursday still capped at
chance...and have ptype mix ra/sn.

&&

Short term /Thursday night through Saturday/...
after the multi-day period of influence from another monstrously
large low pressure system...by Thursday night - we will be on the cusp
of a multi-day reprieve...at least a couple of days. In the wake of
the exiting low...surface high pressure will build in to our south...
which is an important key because it will spread warmer temperatures up the
eastern Seaboard into the weekend. Friday will be clear and quiet. After
a breezy/windy Thursday...winds will drop off to a light southerly flow.
Although not an incredibly warm day...maximum temperatures on Friday should reach
5-10f above average for late Dec.

While the High Rolls off the Carolina coast into Saturday...southerly flow
will again bring in slightly warmer air west/ maximum temperatures a bit higher
than Friday...topping out in the M-u50s. Another low pressure wave
will set up over the Southern Plains/lower MS valley. Gulf Coast moisture
will again be tapped for the early stages of this next system...but
in terms of northward extent - it will have trouble west/ secondary
waves coming in from the northwest. 12z runs of the GFS/Euro are in the
closest alignment west/ each other in the past few days as to the
interactions/progressions of these features. A strong enough upper
jet will transport the sheared edge of the Gulf moisture up across
the upper Midwest late Sat night. However...a potent upper low
sliding just south of Hudson Bay will take this northern moisture up and
over the region before the southern system can get fully organized.

&&

Long term /Saturday night through Wednesday/...
high pressure along the middle Atlantic coast will hang on into
Saturday night. Dry conditions will linger. There is a chance for
a few upslope snow showers in the higher parts of the Potomac
Highlands. Just twenty four hours ago...the Euro and GFS models
were both hinting of a cold frontal passage Saturday into Saturday
night. Since then...both models have backed off on this idea.

The cold front should push across the area Sunday before sliding to
the south and stalling across the Carolinas by Sunday night. Some
light stratiform rain will accompany the front Sunday and into
Monday with the front just to our south. The 00z Euro model stills
wants to hang onto the idea of a low developing along the stalled
front but now the GFS just wants to push elongated strands of energy
along the surface front and out to sea. Consider this...high
uncertainty on whether we see additional periods of light rain or
perhaps light snow in our County Warning Area by Monday night.

There is a slight chance of some departing rain or snow showers
Tuesday...should a low develop along the front. Otherwise...dry
conditions will be brought about with high pressure building later
Tuesday through Wednesday.

&&

Aviation /00z Thursday through Monday/...
IFR or lower conditions this evening ahead of approaching front.

LIFR ceilings across the area through at least 05z. Currently have ceilings
improving to IFR or MVFR as the front/showers move through.
However...improvement time could be 2-3 hours later than in taf.
Not expecting VFR till late tonight. Low confidence on timing of ceilings.

Visible varying from less than 1 mile to 3 miles across the taf
sites. Will keep visible down through the night till frontal passage...improving
to p6sm late tonight. The 1 mile or less could continue 1-2 hours
later than in taf.

Highest confidence in forecast is winds...southerly flow this
evening...becoming SW then west overnight with frontal passage. Possible occasional gusts
with west flow late tonight...then more persistent gusts 20-25 kts
during the day tomorrow. VFR conditions as well through the day with
scattered cumulus.

High pressure builds in as the potent low pressure system moves off
the coast late Thursday. The high will keep conditions quiet/solid VFR
into the weekend.

&&

Marine...
daytime rafl has produced a stable layer that has prevented winds from
making it to the Waters Edge. For the most part...Small Craft Advisory has yet to
materialize. Not real confident on when low level stability will erode...
other than it should sometime this evening since the p-gradient will be incrsg.
Therefore will leave Small Craft Advisory in place as is. Low level jet just ahead of cold front will
contain higher winds...and a fraction of that should mix down in vicinity of
rain showers. Smw likely the best vehicle to address that threat. Am doubting
if it will be widespread.

Cold front east of waters by Thursday. While glw conds not out of the qstn...it
would take ideal mixing. Sticking west/ mean mixed layer...and carrying Small Craft Advisory
through the day.

Light winds to close out the week as a large surface high develops to
our south and calms down the area wind field. Southerly channeling
possible into late Sat west/ the high moving off the coast and another
low pressure system approaching from the west.

&&

Tides/coastal flooding...
time of high tide has passed for lower chesapeaker Bay with water
levels slowly coming down now. As of 7 PM...levels below any
flooding thresholds so allowed the advection to expire there. Continuing
northern Chesapeake Bay western shore as Annapolis currently
showing in solid minor levels...and the remaining areas expecting to
reach minor levels with 1.5 feet anom possible.

A cold front crosses the water tonight. Direction not all that
great for blowout conditions. We will need to see how much water
is truly pushed out. May need an advisory for anothr cycle...especially
Middle Bay. On the flip side...the am cycle is the lower of the two
astronomically.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...Wind Advisory until 6 am EST Thursday for mdz501.
Dense fog advisory until 1 am EST Thursday for mdz004>006-011-
503>508.
Coastal Flood Advisory until midnight EST tonight for mdz011-
014-508.
Virginia...Wind Advisory until 6 am EST Thursday for vaz503-504.
Dense fog advisory until 1 am EST Thursday for vaz505-506.
WV...Wind Advisory until 6 am EST Thursday for wvz501>503-505-506.
Marine...Small Craft Advisory until 5 PM EST Thursday for anz530>543.

&&

$$

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