Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180831

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
331 AM EST Sat Feb 18 2017

High pressure will linger over the area today. A weakening low
crosses the area tonight. A backdoor cold front will enter the
area Monday into Tuesday, followed by another disturbance
Tuesday night into Wednesday.


High pressure can be found off the coast early this morning.
Nonetheless, with nearly clear skies, little wind, and dewpoints
in the 20s, temperatures have been able to radiate efficiently.
Have been making some near term adjustments. Don`t believe we`ll
drop too much further from where we are now, and am using that
as a guide.

In spite of the cold start, ample warm advection will lead to a
noticeable temperature jump compared to yesterday. Most guidance
suggesting 70 within reach in DC. That would be quite a rise; am
not sure if the airmass is capable of supporting that much
warmth near/north of the Potomac. (Around Charlottesville is a
different story.)  ECMWF has been a better performer of late,
and it suggests mid-upper 60s, lower 70s central Virginia. Have
bent forecast in that direction.


An upper level disturbance along the Gulf Coast will head our
way tonight. Aside from the vorticity, there`s not much else
supporting this feature. The positively tilted trough weakens
and is absorbed into the mean flow as it passes south of the
forecast area. QPF is light (a few hundredths) and spotty. PoPs
highest in the mountains, but no higher than chance there. Will
be keeping the tip of southern Maryland dry. Many areas will see
nothing more than a few sprinkles, as the low-levels don`t ever
really saturate.

Any lingering clouds Sunday morning will quickly mix out as a
large scale ridge axis approaches Sunday. Concerns will shift to
shortwave energy reinforcing the trough across New England.
Temperatures at 850 mb won`t be as warm as today, and will only
be dropping as a backdoor cold front slips toward us. It will be
in the neighborhood by Monday.

Am keeping Sunday high temperatures similar to today, as the
low level airmass doesn`t really change up to that point. Monday
will be significantly more challenging though-- both high and
low temperatures. Believe that the core of the warm air in
Virginia will remain. Have much less confidence in what will
happen in northeast Maryland. Temperatures will drop; the
questions are by how much, and where that dividing line will be.
For the time being, am using the Potomac River Valley as the
dividing line.


A ridge axis (both surface and aloft) passes across the region on
Tuesday, with return flow commencing and increasing humidity. A weak
upper wave runs headlong into what`s left of the ridge on Tuesday
night, and there could be enough moisture and instability to spark a
few showers -- nothing major though.

Surface warm front passes through around that time as well, ushering
in another unseasonably warm stretch. Reaching 70 degrees is
plausible on Wednesday, and even more likely on Thursday and Friday.
Went above guidance throughout the warm stretch and my gut feeling
is that I may still be a couple degrees too cool. However, there is
some guidance showing more clouds and even isolated or scattered
showers during the late week, so I am hedging a bit toward that

The next storm system arrives right at the end of the forecast
period (next Friday night), but it currently appears we would be
south of the triple point and thus the heavier rains would again
miss us to the north; though it could bring temps at least
closer to late-February expectations by next weekend.


VFR for the valid TAF period, and for much of the weekend. An
upper level disturbance will pass by tonight, but low levels
remain dry enough that the impact will be increasing mid-high
clouds this afternoon-evening, thickening to a 5000-6000 ft
stratocumulus deck with perhaps a few sprinkles. MRB stands the
best chance to have the ground moistened. By Sunday the
disturbance will be east of the terminals, with Sunny skies
returning. May have some 15-20 kt gusts Sunday afternoon.

A backdoor cold front will drop south toward the area Monday.
Its unclear at this point whether any marine stratus will advect
inland. Currently the forecast indicates no. Would still keep
that in the back of the mind as a worst-case possibility.

VFR Tuesday with building thick high clouds. Lower clouds are
possible for a time Tuesday night with possible MVFR or lower in
scattered showers. Whether that happens or not, VFR returns on

Light winds on the water early this morning, and that will be
the case for much of the day. Any wind would come from the
southwest today, but at speeds 10 kt or less.

A disturbance will cross the area tonight, with perhaps a few
sprinkles. The larger impact will come Sunday in the wake of
this low, when northwest flow increases. Believe that the cold
water/warm air combo will prevent winds from mixing across the
marine area. For that reason, am not forecasting winds higher
than 10-15 kt.

The flow will turn north/northeast Monday as a backdoor cold
front slips toward the waters. It remains unclear how far south
this boundary will make it. Since area won`t be clear from this
discontinuity, am once again keeping winds under Small Craft
Advisory thresholds.

Winds should remain below headline criteria Tuesday through
Thursday. Some potential for southerly channeling on Friday which
could lead to reaching advisory criteria in gusts.




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