Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 klwx 261426 

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
1026 am EDT Wed Apr 26 2017

low pressure off of the Delmarva will slowly move northeastward
away from the mid-Atlantic coast today. A cold front will
approach the region and weaken over the area Thursday night. A
backdoor front may slide into the area late Saturday into
Sunday. A cold front will pass through the region early next


Near term /this afternoon/...

The closed low off the Delmarva is the primary weather story
today. This is moving very slowly away - held in place by high
pressure in the Gulf of Maine. On visible satellite you can see
the demarcation between clouds and clear - a parallel line through
Fauquier County would be a decent estimate. South of that line
there is a lot of sunshine and temperatures are climbing
decently, while north of it skies are overcast. And in the
extreme NE of the forecast area low clouds and drizzle continue
to make the sun seem 93 million miles away.

Temperatures this afternoon will range from the upper 60s in NE
Maryland to the low 80s in the central Shenandoah Valley.


Short term /tonight through Friday night/...

Ridging develops over the area tonight...leading to light winds and
relatively clear skies. With dewpoints in the u50s/l60s...expect
lows near 60f...with the possibility of fog due to radiational

Very warm with increasing humidity across the area Thursday as
southerly return flow strengthens ahead of a weak cold front. Front
will begin to washout as it approaches our area...bringing the
possibility of showers/elevated thunderstorms to the higher terrain
late Thursday. Only a low chance that a shower or storm pushes east
of the Blue forecast thermodynamic profiles suggest
capping inversion would generally suppress convection.

Warm and humid weather continues Friday. Warm air aloft will lead to
a capping inversion near 800 mb that will likely suppress convection
across the much of the area...especially with a lack of a focusing
mechanism. One exception could be across the higher terrain where an
isolated shower/thunderstorm could develop by late afternoon.
Though, poor mid-level lapse rates across this area would likely
limit parcel accelerations and prevent necessary ingredients for


Long term /Saturday through Tuesday/...
there is (unfortunately) tremendous spread in the weekend forecast.
The synoptic setup by that time will be for a warm (or backdoor
cool) front stretched east-west across the mid Atlantic and the Ohio
Valley, to low pressure which will be in the Southern Plains. The
quandry is how far south will the front make it. The GFS, supported
by the gefs mean and a number of its ensemble members, suggests
somewhere around southern Pennsylvania or northern Maryland...which
would obviously keep the forecast area on the warm side of the
boundary. Some gefs members are still further north than the mean.
The ECMWF, on the other hand, represents the furthest south of the
ensemble spread, although some gefs members support it, too. Based
on this solution, the forecast area will be impacted by a backdoor
front, which would drop temperatures by around 10 degrees, most
notably across the northern portion of the forecast area. Will seek
some sort of middle ground, although will still favor warmer
solutions until forecast mean comes into some sort of consensus.

By Sunday, it seems apparent that we will be under the influence of
a backdoor front. The only question here will be how cool will it
get. Errors from the Saturday forecast will only multiply, so will
not deviate from the model blend at this time.

There are precip implications to the above, as warmer weather will
yield greater instability, and the presence of a surface boundary
potentially could yield thunderstorm development. Will be keeping
the Saturday forecast as chance showers/thunderstorms. Am thinking
it may be dubious for storms to have a chance on Sunday (they'd be
shunted to the south instead). Will utilize showers instead.

Either way, by Monday the front would be dragged north of the area
again as low pressure intensifies and heads toward the Great Lakes.
That would pave the way for a late Monday or Monday night cold
frontal passage.


Aviation /14z Wednesday through Sunday/...

With the exception of cho only very slow clearing is expected
this afternoon. At major airports ceilings are in IFR/low MVFR
range. Some improvement is possible this afternoon, but BWI
would be the last to experience it.

Sub-VFR could return tonight as fog develops over the area
(especially across the ne). VFR returns during the day Thursday.
Cold front approaches the area late Thursday with the chance
for isolated showers/thunderstorms. Dry Friday with VFR

There is a large degree of uncertainty in the weekend forecast, as a
frontal boundary will be meandering around the area. Showers and
thunderstorms will be likely, but areal coverage will be a question.
If the front does make it south of the terminals, then there will be
the opportunity for low clouds to work inland. Lots of opportunities
for flight restrictions, but details unclear at this time.


light winds expected through tonight as low pressure continues
to move away from the area. Strengthening southerly flow
Thursday ahead of a cold front could lead to low-end Small Craft Advisory wind
gusts. Low confidence precludes issuance of Small Craft Advisory at this time
range. Light winds and dry weather expected Friday.

A front likely will be meandering across the waters. As a result,
the pressure gradient likely to not be strong. Direction of winds
unclear, dependent upon what happens with the front. If the front
does sink south, then a northeast wind may pick up Saturday


Tides/coastal flooding...

The overnight tide cycle is the higher of the two astronomically.
Since these forecasts are right at threshold, that will be the only
cycle of concern. Will need to monitor trends for tomorrow
night/Thursday morning, and perhaps again Friday morning.


Lwx watches/warnings/advisories...
District of Columbia...none.


National Weather Service Glossary of Abbreviations