Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
939 PM EST Wednesday Mar 4 2015

Synopsis...
a cold front will cross the region tonight. Associated
precipitation will continue into Thursday as snowfall due to the
passage of a core of stronger winds in the jet stream. High
pressure builds overhead Friday...then persists through the
weekend.

&&

Near term /through Thursday/...
cold front has moved through most of the County Warning Area...with light
northerly winds established as far east as the I-95 corridor. The
front will continue to move through the rest of the County Warning Area late this
evening. Surface temperatures behind the front are cooling...albeit
gradually at the start. Most of the County Warning Area remains in the 40s...with
some upper 30s over Maryland. The notably colder air is just to the
north and west of the County Warning Area...with temperatures near or below freezing
across southwestern PA and central WV. Main issue for tonight will
be how quickly this colder air is able to move into the region and
promote precipitation to turn from rain to snow/sleet overnight.
Have not made changes to the snow forecast with the colder air so
close at hand.

Dense fog has also been an issue this evening with abundant low
level moisture and light winds ahead of the front. Earlier
advisory for Baltimore/Washington metros has been cancelled as
slightly drier air and light northwest flow has allowed visibilities to improve
gradually. Advisory for Albemarle/Orange counties in effect until
midnight. Fog will also be prevalent over the rest of the Piedmont
and southern Maryland late this evening until the frontal passage.

Overnight...am keeping forecast consistent in transitioning from rain
to snow west/ a short period of pl in between. Exact timing of each still may
need to be adjusted by an hour or two either way. Bttm line...by
sunrise xpctg all snow by sunrise tmrw north of shd-dca-nak. Have a snpl
or plra mix to the S.

&&

Short term /Thursday night/...
will take the transition to snow from shd-dca-nak at the start of period
southward to edge of County warning forecast area by middle morning. Upward vertical velocity aligns very well west/ snow
growth region a cpl hours bhd transition to snow. Screaming message
here...even though it may not look bad upon waking up...expect conds
to get worse during the day. Will have the rrq of 160 knots upper jet...
snow growth region...and fgen forcing all crossing County warning forecast area during the
midday hours-- a little erlr to the north and later central Virginia/southern Maryland. That
is when the bulk of the forecasted snow will accumulate. Small tweaks
here could have big impacts on storm ttls. Did not massage forecast as any
changes would be minor with respect to possible forecast errors.

Guidance holds onto pl a bit longer in southern Maryland. Ultimately that will
impact snow ttls...potentially by cutting them down. Current forecast
represents best assessment at this time. Recognize that this
aspect of the forecast subject to more sizable errors than the rest
of the County warning forecast area.

Back edge of snow should mv southeast by late day. Added a chance snow Ely
evening to accnt for lingering activity/timing diffs. After that
itll all be a temperature forecast. Temperatures will already be dropping due to cold air advection
during the day Thursday...and that drop will only be enhanced at night west/
the loss of daylight. Used MOS mean west/ a small contribution from
prvs forecast for min-T. Will have plenty of lows inthe single digits and
teens...and will need to be considering subzero wind chill factor
by dawn Friday morning.

&&

Long term /Friday through Wednesday/...
not much has changed from previous forecasts for the extended
period. After an extremely cold start Friday morning...temperatures
will be very slow to rise through Friday...with weak cold air advection and
residual snow pack. Very weak southerly flow commences late Friday
night...but the snowpack and strong radiational cooling will keep
nighttime lows well below normal.

Warming begins Saturday...and despite the passage of a weak cold
front Saturday night...temperatures will slowly warm through the
rest of the weekend and through the early/middle part of next week.
For now kept the extended pop free due to large model divergence.

&&

Aviation /03z Thursday through Monday/...
mainly IFR conditions tonight. Rain will gradually change to a mix
snow and sleet and eventually all snow late tonight into early
morning. Adjustments to timing possible with later updates
tonight. Generally north winds tonight and gradually increasing to
around 10kt. Wind gusts up to around 20kt expected Thursday.

Conds will be imprvg middle-lt afternoon. VFR possible by Thursday evening...and likely by
Thursday night.

Expect VFR conditions Friday as winds calm and ceilings rise. VFR
conditions expected through the rest of the weekend.

&&

Marine...
winds have shifted to the north over most of the waters. Wind
speed will gradually increase tonight. Small Craft Advisory GOES into effect at 1
am and continues until Thursday night.

Small Craft Advisory likely extends into Friday before high pressure builds overhead.
Do not anticipate marine concerns through the remainder of the
weekend.

&&

Hydrology...
rain is ongoing across the northern half of the forecast area...and
will spread southward this evening. Prior to the changeover to
wintry precipitation...additional rain will be between 1/4 and 1
inch. With no direct sunshine...snowmelt has been slow but steady.

The only stream reactions through middle afternoon have been in the
North Branch Potomac...North Fork South Branch Potomac...and Wills
Creek basins. The North Branch Potomac at Kitzmiller has risen above
action stage and will be monitored for possible flooding all night.
Other streams in those basins are showing some ice effects and have
risen quite a bit...but are still well below bankfull.

Poor drainage flooding and ponding are likely where normal drainage
patterns are disturbed by snowpack and/or ice. However...localized
minor out-of-bank flooding of streams cannot be ruled out...
especially in northern Maryland and The Highlands.&&

Lwx watches/warnings/advisories...
District of Columbia...Winter Storm Warning from midnight tonight to 9 PM EST
Thursday for dcz001.
Maryland...Winter Storm Warning from 7 PM this evening to 7 PM EST
Thursday for mdz003-501-502.
Winter Storm Warning from 10 PM this evening to 7 PM EST
Thursday for mdz004>006-011-503>508.
Winter Storm Warning from midnight tonight to 9 PM EST
Thursday for mdz013-014-016-018.
Winter Storm Warning from 3 am to 10 PM EST Thursday for
mdz017.
Virginia...Winter Storm Warning from 7 PM this evening to 7 PM EST
Thursday for vaz028-503-504.
Winter Storm Warning from 10 PM this evening to 7 PM EST
Thursday for vaz025>027-029>031-040-501-505>508.
Winter Storm Warning from midnight tonight to 9 PM EST
Thursday for vaz038-039-050>055-502.
Winter Storm Warning from 3 am to 10 PM EST Thursday for
vaz036-037-056-057.
WV...Winter Storm Warning from 7 PM this evening to 7 PM EST
Thursday for wvz050>052-055-501>506.
Winter Storm Warning from 10 PM this evening to 7 PM EST
Thursday for wvz053.
Marine...Small Craft Advisory from 1 am Thursday to 6 am EST Friday for
anz530>543.

&&

$$
Synopsis...heights
near term...kcs/hts
short term...heights
long term...mse
aviation...kcs/mse
marine...mse/hts/kcs
hydrology...

National Weather Service Glossary of Abbreviations