Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 151430

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1030 AM EDT Sun Apr 15 2018

A backdoor cold front is dropping south across the Mid-Atlantic
this morning and will stall out across the area this afternoon.
Tonight, a strong low pressure over the Midwest will move into
the eastern Great Lakes, dragging a strong cold front eastward
across the Mid-Atlantic by Monday morning. High pressure will
gradually build back in for Tuesday and Wednesday before another
low pressure system crosses the region Thursday.


The backdoor front is quite sharp, having pushed south of CBE,
MRB, IAD, and nearly NHK. Across the Washington metro at 10 AM,
temperatures ranged from 46 at Ft. Meade to 74 in Manassas. The
main forecast updates at this time were to account for these
near term trends. Cloud cover near and south of the front
remains scattered to broken, so it`s possible the added
insolation slows its progress somewhat (and that does seem to be
the case the past couple hours). Isolated light showers are
moving into southern Maryland, but overall have slowed down the
timing of PoPs today. A substantial dry layer seen in the 12Z
IAD sounding will take some time to erode, so any showers
through mid-afternoon should remain relatively light. Will
consider potential Flash Flood Watches as 12Z guidance comes in.

Previous discussion:

Guidance continues to waver as to just how far the front gets,
with the aggressive NAM sending it all the way to
Charlottesville and Harrisonburg before turning back north
overnight tonight, so while temps are mild now, they will be
cooling off and our highs have likely already occurred. Some low
clouds and even patchy drizzle are possible behind the backdoor
front, but other than the dramatic drop in temps, it isn`t
quite the main story. The other strong cold front, associated
with low pressure to our west and a potent negatively tilting
trough aloft, will be approaching the region today, and
increased divergence ahead of the front plus whatever
instability can be mustered south of the backdoor front will
cause showers to start breaking out, most likely no later than
midday. With the CAPE available especially south of the backdoor
front plus increasing shear, thunder will be possible, and with
the rapid rise in PW`s, some heavy rainers could be had, but
the main forcing will not arrive until tonight, so think the
risk of excessive rain or severe weather this afternoon is not
that great. That said, something isolated in both regards could
still occur.

Tonight, as the powerful cold front crosses the region, the
backdoor front will reverse and come back north as a warm front.
It likely won`t make it back into the northern Shenandoah
Valley, but the I-95 corridor will likely end up warming again
just before the main cold front passes. This combined with the
very strong forcing from the surface front and upper trough and
the high PW`s over 1.5" will likely lead to plenty of rain and
embedded thunder. Potential exists for a QCLS feature as well as
perhaps an isolated tornado, with that risk generally confined
to regions that end up getting back into the warm sector.
However, the risk will extend right to the warm front itself as
it will be a source of shear. Rainfall amounts generally look to
be on the order of 1-3 inches, with highest amounts near and
wets of the Blue Ridge where orography and isentropic lift are
maximized, but localized totals of 3 inches could occur across
the metro. For now will continue to carry flood potential in the
HWO, but will wait one more cycle before issuing any flood
watches. Lows tonight are tricky given the warm front lifting
back north just ahead of the main cold front... places that see
temps falling most of today could see them rising much of
tonight, the reverse of the usual diurnal pattern.

Latest guidance suggests the main hazards will clear the region
by 8AM if not sooner, but lingering showers behind the front,
associated with the upper trough and wraparound moisture, will
likely persist much of the day. These should be much lighter,
however. With temperatures cooling rapidly, especially aloft,
rain will mix with and change to snow at the higher elevations,
and minor accumulations may occur. Temps will probably be steady
or falling much of the day behind the cold front with the
continuing clouds and showers.


Upslope flow behind the surface low and deep trough aloft will
combine to promote continued snow showers along the Allegheny
Front, with an inch or two possible by Tuesday morning.
Elsewhere, a chilly breeze will bring temps down into the 30s
across much of the region, though freezing temperatures are
questionable given the winds may stay up. However, some areas in
the southern Shenandoah Valley could decouple, so will add
mention of potential freeze to the HWO. With the trough lifting
out a bit and winds relaxing behind the low as it pulls further
away Tuesday, clouds and upslope snow will diminish, but temps
will remain quite chilly for mid April, with highs in the 40s
and 50s. With a better shot of decoupling under lighter winds on
Tuesday night, will also add in risk of freeze to HWO.


High pressure centered off of the southeast coast remains in
control for the early part of the day on Wednesday keeping dry
conditions over our area. Warm air advection due to
southwesterly flow will allow the temperatures to become more
near normal, with highs reaching the 60s and 70s.

Clouds will be increasing as a low pressure system moves into
the southern Great Lakes on Wednesday night, and eventually
pushes a cold front over us sometime on Thursday. Timing and
exact track of this low and the fropa is still somewhat
divergent among guidance, but they do agree that moisture will
be limited. Therefore rain showers expected Wednesday night and
Thursday with total mounts less than a tenth of an inch.

Gusty northwesterly flow behind this front will allow for some
snow showers west of the Allegheny Front and dry everywhere else
Thursday night. A high pressure system will build over us from
the northwest on Friday and remain in control into Saturday with
northwesterly flow keeping below normal temperatures, reaching
the 50s and 60s. both days.


Backdoor front sliding southwest across the region. IFR to LIFR
has spread into BWI/MTN, with MVFR to IAD/DCA/MRB. IFR has
really slowed its southwestward progress and thus its uncertain
if the latter three terminals drop further during the daylight
hours (advection suggests they eventually could). Vis
restrictions have developed in the deeper marine airmass at BWI
and MTN, but may not occur at other terminals until significant
rainfall develops tonight with the cold front sliding eastward
toward us. Rain can be heavy, isolated thunder is possible, and
gusty winds are also a threat tonight as the front barrels
through, aside for the low cigs and reduced vis. Conditions
improve a bit on Monday behind the front, but showers will
linger, so MVFR will remain possible. Should improve to VFR
Monday night and remain so thru Tuesday night as high pressure
gradually builds back in.

VFR conditions are expected on Wednesday with high pressure in
control. Sub-VFR conditions are possible sometime Wednesday
night onto Thursday with another front moving across. VFR
conditions are expected to return sometime Thursday night into
Friday as high pressure builds into the region. Gusty
northwesterly winds are expected on Friday, with some gusts
reaching up to 20 knots.


SCA winds are generally focused north of a backdoor cold front
that is drifting south over the waters. These spread over all
waters by afternoon. Tonight, as the front lifts back north and
a strong low level jet could bring gusty south winds back to
portions of the waters. Have maintained gale over waters for
tonight with potential for 35+knot gusts in heavy rain and
embedded t-storms as cold front crosses the region. SCA will
likely be needed Monday behind the front but held off issuing
for now. Winds will gradually relax as we head through Tuesday.

Winds should remain below small craft advisory criteria
Wednesday and Thursday. A front will move across us on Thursday
and winds could increase above SCA criteria behind it sometime
Thursday night or Friday, and may require an advisory.


Strong negatively tilted upper trough and surface cold front
expected to cross region Sunday night, with widespread heavy
rain and embedded t-storms. PW`s rise to around 1.5", quite high
for April, thanks to a very strong fetch from the Gulf and
Atlantic. With the forcing and potential training, rainfall
could locally exceed 3 inches in a few hours, so flooding is a
concern. However, fast storm motion and the recent dry weather
may mitigate the threat, so have continued with just HWO mention
without an actual watch. Will have a more detailed assessment
for this threat by afternoon.


Water levels will be near astronomical normals on Sunday.
Onshore flow followed by a strong southerly push Sunday night
could lead to an influx of water causing minor flooding on late
Sunday night or Monday. Guidance suggests that minor flooding
events could last into Monday night, maybe Tuesday.


MARINE...Small Craft Advisory until 11 PM EDT this evening for
     Gale Warning from 11 PM this evening to 8 AM EDT Monday for
     Small Craft Advisory until 11 PM EDT this evening for ANZ533-


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