Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 091938

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
238 PM EST Mon Jan 9 2017

A ridge of high pressure extends from the DelMarVa to the
Carolinas this afternoon and evening. The high will slowly move
offshore Tuesday as a warm front moves north across the
Appalachians. Several disturbances will glide along this boundary
over the mountains through the middle of the week.


Plenty of clouds across the region this afternoon. Most of them
are of the mid-high variety in response to jet energy and mid
level warm advection/isentropic lift. The jet streak will progress
northeast of the area tonight, which will allow for partial
clearing. Otherwise, there is not an appreciable change in the
pattern tonight. Temperatures aloft do warm, so low temperatures
won`t be quite as cold. Blended MOS seems to have a decent handle
on the situation.


A weak vort lobe crosses the area Tuesday morning; otherwise,
dynamics remain well northwest of the area on Tuesday. That is
significant since the lack of dynamics suggest there is little to
no forcing for precipitation. Those trends have been identified by
model guidance in recent runs. GFS remains the most aggressive in
terms of developing precipitation in the forecast area tomorrow.
It also remains an outlier.

If precipitation were to fall, precipitation type would be
interesting. Not only are critical partial thicknesses initially
cold, ground temperatures likely to be frozen by recent cold. The
cold air will be retreating during the course of the day, but its
speed still a bit uncertain. Low levels likely to be reluctant to
warm. Have taken a non-diurnal trend, but have opted to trim a
couple degrees off the warm end due to ground conditions.

Have taken a reserved PoP approach for Tuesday. Am keeping chance
PoPs across the Potomac Highlands and north-central Maryland.
Thermally, precip would be snow at first, gradually changing to
sleet, freezing rain, and then rain over the course of the day.
PoPs too low to consider any additional action; threat remains in
Hazardous Weather Outlook.

Odds for precipitation improve tonight (likely/categorical by
overnight hours) as vorticity advection increases and boundary
drops close to forecast area. Do not believe this boundary makes
it through-- it has practically no wind shift and meager thermal
gradient. Am hold temperatures nearly steady due to saturation and
a steady south flow. A couple thousand feet up, winds forecast to
be 40-50 kt...especially east of the Blue Ridge. A concern will be
whether any of this will intersect the ridges (mixing will be poor
for downward vertical transport).

The first wave will exit by Wednesday morning, but a second wave
will arrive by Wednesday afternoon/evening. Again, the highest
PoPs will be at night across the north (MD/WV) nearer the stalled
boundary. Am keeping a warming trend with respect to temperature
forecasts due to continued south flow and waves of warm advection.


High pressure offshore will allow for southerly flow over our area
on Thursday... which will be mostly dry with high temperatures
reaching the 60s in most areas behind a quick moving warm front. Dry
conditions continue into Thursday night before a cold front
approaches on Friday increasing the chance of precipitation to our
area... but actual timing is uncertain and that may affect Friday`s
temperatures and therefore p-type. For now guidance suggest an early
fropa on Friday, with high temperatures being colder than Thursday.

The cold front will stall near our area Friday into parts of Sunday
with waves of low pressure tracking east along the front... so
unsettled weather possible through Sunday. But temperatures will be
right on the edge... so frontal position will impact the p-type...
needless to say the uncertainty is high. Guidance disagrees Sunday
into Monday as to what will happen with this front... will it
continue to be stalled over our area or will it push north as a warm
front? In summary... starting warm and drier on Thursday... becoming
unsettled Friday into maybe Monday...with wintry precip possible at

VFR conditions expected through the valid TAF period. Any clouds
this afternoon and tonight will be mid-high variety. Clouds
thicken tomorrow ahead of an upper level disturbance.
Precipitation chances real low, and have been left out of
forecasts. If there were any precip, MRB has the best chance.
Precip type would be interesting due to slowly retreating cold
air (snow to sleet to freezing rain to rain). Again, believe all
terminals will be warm enough for just rain by the time precip
does make it to area on Tuesday evening.

Several items of possible impact Tuesday night: advance of warm
air over cold ground with likely rain suggests that flight
restrictions (at least MVFR if not IFR) probable. In addition, a
40-50 kt low level jet leads to a low-level wind shear threat.

The first round of precip will exit Wednesday morning, but a
second round likely to arrive by Wednesday evening. Flight
restrictions quite possible in between since warm air/cold ground
setup will continue.

Drier but breezy conditions Thursday into the night... then
becoming unsettled Friday into Saturday with a frontal system near
our area. So... sub-VFR conditions possible mainly Friday and

South flow on the waters this afternoon, and it will remain that
way for the next several days. Am not seeing the signal for
southerly channeling on the Bay tonight, so have cancelled the
Small Craft Advisory. However, the gradient will increase on
Tuesday ahead of an upper level disturbance. A 40-50 kt low level
jet will set up, but mixing will be poor. Am keeping the Small
Craft for all waters during the day Tuesday, and extending it for
the Bay/lower Potomac Tuesday night.

The mixing remains poor through midweek, and the gradient relaxes
as well by Wednesday. The only questions will be how much wind
lingers in the morning, and how much mixing will tap the second
low-level jet Wednesday night.

Drier but breezy conditions Thursday into the night... with a
Small Craft Advisory possible. Then becoming unsettled Friday
into Saturday with a frontal system near our area. No Small Craft
Advisories expected for Friday and Saturday as winds will be
below the threshold.

MARINE...Small Craft Advisory from 8 AM Tuesday to 6 AM EST Wednesday
     for ANZ530>534-537>543.
     Small Craft Advisory from 8 AM to 7 PM EST Tuesday for ANZ535-


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