Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
356 am EST sun Feb 1 2015
low pressure moves through the region through Monday. High pressure
will return for the first half of the week. A cold front is
expected to move into the region Wednesday night.
Near term /until 6 PM this evening/...
surface high pushing east this morning as the next low pressure system
approaches from the west. Light returns currently appearing on radar
just to the west...associated with the initial surge of isentropic
lift and a passing shortwave. However...high dew pt depressions
right now should keep anything from reaching the ground. Have slight
chance probability of precipitation for light snow over the western Allegheny Front...but no
accumulation really expected.
Heading into today...some discrepancies in the models on the
handling of the precipitation. There continues to be some suggestion of precipitation
development later this morning/early afternoon...which if it does
occur...would be in the form of light snow. However...again a very
dry airmass needs to be overcome. Southerly flow sets up...but is
weak...and with the low level jet not in place till tonight...it could very
well end up being dry across much of the area during the day. Best
chance for any precipitation would be along the Mason-Dixon line and west of the
Higher confidence of precipitation moving in by late this afternoon...and have
spread probability of precipitation east and south to account for this. As for p-type...a
good surge of warm air advection finally arrives with the southerly flow...which
will work to push back the snow/rain line to the north. Initially
should see sleet mixing in...thinking mainly 20-00z time period...as
850mb temperatures warm up quicker than surface temperatures. Areas along the
Mason-Dixon line expected to remain snow...with up to an inch of
Short term /6 PM this evening through 6 PM Monday/...
low pressure moves into the Ohio River valley by 06z this
evening...with the biggest surge of warm air advection and moisture riding the
southerly flow out ahead of the low. This evening will be the
biggest transition period for precipitation...and also the lowest confidence
of p-type as there remains some question as to how quickly the
warmer air can erode the persisting cold air...especially along the
Mason-Dixon line. Using a combination of the 00z GFS and NAM BUFKIT
soundings...continued with the mention of freezing rain central and western
Maryland...and south into portions of northern Virginia and northern parts of District of Columbia
metropolitan...where temperatures are hovering right near freezing. Confidence
would lean that if anything the temperatures warm up quicker...and thus a
shorter time period of freezing rain. But with the potential for at least a
period of freezing rain resulting in a trace to a couple hundredths of an inch
of ice...have gone ahead and extended the Winter Weather Advisory to
incorporate northwest Howard...northwest Montgomery...western Loudoun...and the
remainder of the eastern WV Panhandle...as well as
Hampshire...Frederick Virginia...and Clarke. While the timing of the
advisory currently exists from this afternoon through tonight...the main
potential for ice accumulate will be between 23-09z tonight.
Elsewhere...precipitation changes over to all rain as 850mb temperatures warm well
into the single digits above zero...and BUFKIT soundings indicating
wet bulbing finally being achieved by 06z. With multiple shortwaves
working through tonight as the surface low passes over the
mid-Atlantic...can expect periods of moderate to heavy precipitation.
The surface low pushes off the New Jersey coast by midday Monday...while the
strong upper level trough finally swings through the region Monday
afternoon. Continued probability of precipitation everywhere for Monday morning...then tapered
off from west to east...with the thinking all precipitation should clear the
area by Monday evening except with the chance for upslope snow showers.
Strong surge of cold air advection with the northwest flow on the backside of the low will
increase winds by Monday afternoon...with gusts into the 20s. This will
also result in a non-diurnal temperature trend...mainly for western areas
as they will see their daytime highs in the morning. Over the
eastern areas...before the cold air advection moves in...could see highs in the
Long term /Monday night through Saturday/...
strong northwest flow kicks in Monday evening as coastal low bombs out off Cape
Cod. The combo of a tightening p-gradient and good mixing from cold air advection will
easily provide mean mixed layer winds greater than 30 knots. This
gradient will be subsiding/dcplg slightly through the evening...but it will
still be a brzy /and chilly/ night. Scattered lingering upslope shsn at start
of period will diminish as hipres builds. Will have pt-mostly cloudy inthe mountains but clear
elsewhere. Temperature guidance not quite as cold as prvs runs. Will lean on the
cooler side of latest data. Wind chills not to advisory level...but single
digits should be common.
Hipres will be nearby Tuesday...W/ return flow beginning Tuesday night. 800 mb thermal
trough still an influence Tuesday /-10 to -12c/. Maxt adjustments
minimal...maybe a degf or two higher across Maryland/northern Virginia. Centrl Virginia the
only place that safely will get above freezing. Min-T Tuesday night wont be quite
as harsh...due partly to a building middle cloud deck.
Lopres in the northern stream drops from S-central cnda across New England
Wednesday-Wednesday night. Its not exactly clear how much moisture will be associated west/
it...but for the middle Atlantic its outpacing any upa support. Therefore
will be banking on a system that will be struggling to produce precipitation on
its own. After a relatively mild day Wednesday...will be keeping schc snow
confined to the mountains Wednesday night.
Temperatures will be dropping Thursday /15 degf cooler than maxt Wed/...which could
provide an environment for diurnal shsn. Too far out to pinpoint
covering...but it would be a situation for brief...minimal accumulate /if
any/...but sudden onset shsn. Will have Middle-Range clouds and low end probability of precipitation.
Hipres should close out the week. No other hazards anticipated at this time.
Temperatures gradually moderating toward climatology...but not sure if we will get
Aviation /09z Sunday through Thursday/...
VFR conditions this morning. Conditions diminish as the precipitation moves
in...with IFR becoming likely by Sun night with the heaviest precipitation.
Initial precipitation -sn/pl/ra mix...becoming all rain tonight everywhere.
Transition timing in tafs could be +/- 2 hours. Any snow accums at
the terminals today would be less than one inch.
S winds less than 10 kts today/tonight will shift to the northwest Monday
and increase...with gusts mixing in by Monday afternoon. VFR conditions
as well by Monday afternoon as the precipitation exits the region.
VFR should prevail Monday night through at least Wednesday. Biggest impact will be the
winds Monday night...which will be from the northwest and may gust up to 35-40 knots
during the evening push.
Another cold front will cross lt Wednesday. May have widely scattered shsn reducing visibility
on Thursday...along west/ more gusty winds.
sub-Small Craft Advisory conditions on all the waters through 06z tonight. Have
issued a Small Craft Advisory over the central Chesapeake Bay and the lower upper
tidal Potomac to account for increasing southerly flow late
tonight...and then northwest flow by tomorrow afternoon. Possible lull in the Small Craft Advisory
gusts as the winds shift...but this will be short lived.
As coastal lopres strengthens lt Monday...it will increase the p-grad.
Combined west/ deepening favorable mixing in cold air advection...winds should exhibit a
marked increase late Monday afternoon into Monday night. Will have mean 35-40 knots west/in
the mixed layer. Have raised glw for the 2nd half of the afternoon into the
evening. Small Craft Advisory conds will continue thereafter...and may leak into Tuesday. Building
hipres will gradually result in diminishing winds.
District of Columbia...none.
Maryland...Winter Weather Advisory from noon today to 8 am EST Monday for
Winter Weather Advisory from 3 PM this afternoon to 8 am EST
Monday for mdz004>006-503-505-507-508.
Virginia...Winter Weather Advisory from noon today to 8 am EST Monday for
Winter Weather Advisory from 3 PM this afternoon to 8 am EST
Monday for vaz505.
WV...Winter Weather Advisory from noon today to 8 am EST Monday for
Marine...Gale Warning from 3 PM Monday to 1 am EST Tuesday for
Small Craft Advisory from 1 am to 3 PM EST Monday for anz533-