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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
352 am EST Sat Jan 31 2015

Synopsis...
high pressure builds across the region through tonight. Low pressure
will approach from the west Sunday...and cross the region Sunday
night. High pressure will return for the first half of next week.

&&

Near term /until 6 PM this evening/...
high pressure under a building ridge will bring dry and initially
windy conditions to the middle Atlantic. Two things for this
morning...wind chills and gusty winds. Wind Chill Advisory for the
western slopes of the Potomac Highlands remains in effect until 9 am
to account for wind chills between -10 and -15 degrees. As temperatures
warm this morning...and winds diminish...expecting wind chill values
to improve to above zero by middle morning. While the remainder of the
area is not under a Wind Chill Advisory...areas along the Mason
Dixon and west of the Blue Ridge could see a brief period of wind
chill values below zero through 8 am.

As for the winds...expecting gusts into the 20s through daybreak
over eastern areas with the positioning of the low level jet and the tighter
pressure gradient...teens or even no gusts to the west where the
pressure gradient has slacken. Winds will gradually then diminish
throughout the day as the center of the high passes just to the
south. As the winds back to the south...temperatures will begin a warming
trend...with many areas reaching near 30 today...and even into the
middle 30s over central Virginia.
&&

Short term /6 PM this evening through 6 PM Sunday/...
surface high nudges east as an area of low pressure over the
southwestern US becomes organized and approaches the area from the
west. 00z model suite in decent agreement with the placement of the
low over Kentucky by 00z Monday. Dry weather in place tonight as the ridge
becomes more zonal in nature. Plenty of subsidence aloft will keep
skies mostly clear for the first half of the night...with increasing
cloud cover for the second half ahead of the low. Thus generally
expecting a quick drop in temperatures this evening...with a more gradual
trend for the latter half of the night.

Introduced probability of precipitation into the far northwest areas by daybreak Sunday to account
for light precipitation development along the isentropic lift. As the lift
pushes north...along with a shortwave aloft lifting across the area
into PA...spread the probability of precipitation to the east...mainly along the Mason-Dixon
line. Temperatures below freezing during the morning should keep any precipitation
in the form of snow...with up to a couple tenths of an inch accumulate
possible.

For the afternoon...precipitation spreads east and south...with only southern Maryland
possibly staying dry till sun evening. P-type for the afternoon remains
tricky as the track of the 850mb low now brings 850mb temperatures to
around -2 degrees celsius by 18z sun...and right near freezing by
00z. Both the NAM and GFS BUFKIT soundings indicating precipitation up to 18z
as snow...but with the warmer layer aloft sneaking in...could see a
period of sleet mixing in...mainly over Virginia...and even possibly
transitioning to all rain over central Virginia. The one thing of note is
that with surface temperatures pushing above freezing as warm air advection sets up with
southerly flow...the freezing rain threat during the day looks to be
minimal. Areas just north of Baltimore metropolitan could see high temperatures
just hit freezing...but with colder 850mb temperatures there...generally
should expect snow and not freezing rain through 00z Monday. For areas that
remain all snow or mostly snow for Sun afternoon...could see up to one
inch accumulation.
&&

Long term /Sunday night through Friday/...
the surface low prognosticated to track from the middle ohvly to the northern sections of the
County warning forecast area Sun night. This solution on the same Page as the past day or so of
forecast runs...and represents the more northern track. The 800 mb low also a little
further north...into centrl PA. Given this solution...there will be nothing to
impede warm air aloft from overspreading County warning forecast area. Therefore...expect melting
of some if not all hydrometeors. The key will be surface temperatures. Thats
proving a bit trickier...not in the overall trend /which will be
non-diurnal warming/ but in pegging specifics with respect to surface freezing. Am not
convinced we/ll be able to scour out all cold air in the valleys. Area
of concern stretches across northern Maryland and in the ptmc Highlands...and
includes the balt northern burbs/Frederick/I-81 north of okv. Have held onto
ptype of freezing rain/S in these areas...W/ a chgover to rain S of there. Am a
bit uncertain about local temperatures for an addtl sliver include
Howard/Montgomery/Loudoun counties...but dont have enough hard evidence
to prevent a forecasted warm-up overnight.

Probability of precipitation will be at their hiest overnight when the low tracks nearby...fllwd
by a relatively pronounced cold front passage during the morning commute. Temperatures should
be steady/falling most of the day...tempered a pinch by compressional
warming in northwesterly flow. Due to that offset...should be able to get precipitation
away from District of Columbia before thermal conds support snow. Elsewhere...will be switching
back to a rain/snow Mode as cold air advection aloft will bring temperatures below freezing above the
surface. Will also have the return of upslope shsn...but at this point am
keeping only minimal addtl accums.

Speaking of which...do not see significant snow amts west/in County warning forecast area based on
current storm track. Held northern Maryland/ptmc Highlands higher than latest raw
guidance would suggest. Current thinking is that final tallies would be
below watch/warning values. Since forecast erring on the high side and that
doesnt meet watch criteria...will be continuing severe weather potential statement mention only.

County warning forecast area will have a cpl days under hipres. Monday night-Tuesday will be abnormally
cold though...as 800 mb temperatures bttm out around -12c in the wake of the
deeping coastal low. Latest MOS guidance a touch warmer for Monday night min-T.
This still suggests a large swath of County warning forecast area /nrn Maryland and west of blurdg/
will start Tuesday in the single digits...and all but centrl Virginia will be
stuggling to rise above freezing Tuesday. Return flow will reach area Tuesday night
though...for a considerably warmer Wednesday.

There may be a weak...moisture-starved system dropping southward from the
northern stream Wednesday...which could be the focus for ripples from eastward through the
end of the weak. Have kept some cldcvr to accompany fnt...but removed
precipitation. Similarly...present indications are suggest the fnt would then set up far enough
away from /ie...southeast of/ County warning forecast area Thu-Fri. Threat of precipitation looks minimal.
&&

Aviation /09z Saturday through Wednesday/...
high confidence VFR conditions through Sun morning.

Northwest flow with gusts 20-25 kts will gradually diminish today...with
gusts dropping into the teens middle-late morning...then becoming more occasional
in nature for the afternoon. Winds should drop below 10 kts 21-23z...and
becoming light/vrb tonight. Exception...gusts already ended kcho and
will be occasional in nature this morning at kmrb.

S flow generally 10 kts or less sun. Precipitation could impact kmrb as early
as 15z...as -sn...with precipitation becoming more likely for the afternoon at all
terminals. -Ra/pl for kcho...-sn/pl elsewhere. Any snow accums prior
to 00z Monday will be less than one inch.

IFR conds likely Sun night as lopres tracks across area. Its looking more
like a cold rain for cho/dca. Ptype at BWI/iad a little more
uncertain...but rain should dominate. Mrb stands best chance at receiving
wintry precipitation. Cold front passage during morning push. Will be experiencing strong northwesterly
flow most of the day.

VFR expeceted Monday ngt-Wed.
&&

Marine...
winds gradually diminishing early this morning. Will be converting
Gale Warning to Small Craft Advisory to account for observation having reported for multiple
hours of gusts less than 34 kts. Small Craft Advisory in effect for all waters through
19z...then just on the Chesapeake Bay and southern portion of the
tidal Potomac until 23z. Sub-Small Craft Advisory conditions then tonight through Sunday.

Of note for this morning...with sustained winds above 20 kts...seas
around 3 feet...and air temperatures in the 20s...possible light freezing spray
resulting in minimal ice accums on the open waters of Chesapeake Bay.

Lopres will track just barely north of the waters lt Sun night...fllwd by
cold front passage Monday morning. Winds will pick up on Monday. Mixed layer will easily support
Small Craft Advisory...and may even support the return of glw. Will add that potl to
syn and severe weather potential statement. Expect a gradual decrease of winds Monday night. Tuesday and Wednesday should
be quieter as hipres settles ovrhd.
&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...Wind Chill Advisory until 9 am EST this morning for mdz501.
Virginia...Wind Chill Advisory until 9 am EST this morning for vaz503-504.
WV...Wind Chill Advisory until 9 am EST this morning for wvz501-503-
505-506.
Marine...Small Craft Advisory until 6 PM EST this evening for
anz530>534-537>543.
Small Craft Advisory until 2 PM EST this afternoon for anz535-
536.

&&

$$
Synopsis...Sears/hts
near term...Sears
short term...Sears
long term...heights
aviation...Sears/hts
marine...Sears/hts

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