Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 291408

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1008 AM EDT Sat Apr 29 2017

A frontal boundary just north of the Mason-Dixon Line will move
south this afternoon and remain over the region through early
Sunday...before lifting north as a warm front late Sunday afternoon.
A cold front will then cross the mid-Atlantic late Monday. Weak low
pressure moves into the region Tuesday before a surface trough
develops Wednesday as strong low pressure develops over the
Tennessee Valley and heads towards the mid-Atlantic Thursday and


Considerable uncertainty remains regarding the forecast for the
remainder of the day and tonight. While the airmass boundary
(i.e. dew points in the mid 60s) remains north of the area, a
couple rounds of decaying convection overnight have sent outflow
boundaries across the area, muddling the surface picture. There
is enough instability aloft (1000 J/kg MUCAPE on 12Z IAD
sounding) that showers and thunderstorms continue to generate.
On the whole, models have not handled these processes well,
degrading confidence in their output for the remainder of the
day. The question remains if the current convection keeps the
regeneration trend going, or if the building mid-level ridge
and weakening low level jet allow a drier period during the
midday to mid afternoon. With the boundaries laid out farther
south now, that could provide focus for strong storm development
father south later today as well, of course pending midday
trends. As mentioned below, ingredients will be in place in the
region for strong to severe storms, but triggers (and locations
thereof) to battle the ridge and effects of morning storms
preclude higher confidence.

Previous discussion:

There is ample ridging in place today, with plenty of warmth
supporting temps to rise to/above 90 degrees. In light of this heat,
MUCAPE forecast to increase to 2000-3000 J/kg...or higher...with
minimal CIN. But, the northern portion of the forecast area will
first need to recover from the early morning rainfall. Have shaved a
degree or two off high temps across northern Maryland and eastern
West Virginia. In turn, this would provide a differential heating
boundary. Believe that would be enough in this volatile air mass for
late day showers/thunderstorms to develop. While heavy rain would be
primary threat (due to atypical precipitable water for late April),
would not rule out localized strong wind gusts or large hail in
taller storms (and thunderstorms today likely to be quite tall). SPC
concurs, placing most of the forecast area in Marginal risk.

Thunderstorms will gradually decrease in coverage upon loss of
diurnal heating. However, there will be enough fuel lingering to
justify chance PoPs overnight.


Similar considerations apply to Sunday`s forecast, ie: the lingering
surface boundary and a warm/unstable air mass to its south. An
east/onshore flow across northeast Maryland may provide a
stabilizing influence, and focus storms in the Potomac Highlands,
where terrain/upslope will provide another lifting mechanism. This
time, instability will wane with sunset.  And, since flow will be
onshore, have no reason to dispute slightly cooler high temperatures
per SuperBlend...which still are about 10 degrees above normal.

On Monday, a deep low pressure system will work from the Plains to
the Great Lakes, which will drag a cold front toward the forecast
area. GFS/ECMWF timing is a little closer then previous, though
differences remain. Focus would be on the afternoon-evening hours.
However, instability does not look nearly as strong as during the
weekend, but there still will be ample shear and favorable
timing for a few strong thunderstorms.


Per the 29/00Z GEFS, the H5 low over the upper Great Lakes early
Tuesday lifts northeastward towards Quebec, followed by slight
height rises through Thursday.  On Thursday, a digging H5 trough
moves into the Tennessee Valley, and slowly makes it way towards the
northeast U.S.

The surface reflection of the upper levels reveal strong northwest
flow on Tuesday, followed by a saddle point in the surface pressure
field and stalled frontal boundary as low pressure develops in the
Tennessee Valley on Thursday and slowly makes its way to the
Chesapeake Bay region.

It should be noted that the deterministic 20/00Z ECMWF has a closed
upper low for the Thu/Fri system as opposed to an open, albeit slow-
moving, wave per the 29/00Z GEFS solution.  This is to be expected
with the GEFS as the various solutions are damping out details of
the position of the upper low, and the 29/00Z GFS deterministic has
a substantial, deep, cut H5 low.  Both the 29/00Z GFS and ECMWF seem
to be in harmony with similar solutions.

At this time, the sensible outcome of this strong low pressure
system looks to be a considerable, slow-moving Nor`easter rainstorm
for the region in the Thursday-to-Friday timeframe.


Outflow from overnight storms have led to development of new
storms this morning. Overall coverage is small though and
persistence of storms is low, so will have to handle with
amendments. High pressure ridging may build enough to allow for
a break during the midday hours versus additional scattered
storms, but overall confidence in the forecast is low.

Ample daytime heat will lead to scattered late day
thunderstorms, with local flight restrictions and gusty winds
likely. Areal coverage once again the mitigating factor.

Threat of flight restrictions will linger overnight/into Sunday
morning, with areas of fog possibly developing. Daytime Sunday will
carry a renewed thunderstorm risk, although areal coverage and
strength of storms potentially could be less.

A cold front will cross the terminals Monday afternoon/evening, with
yet another opportunity for thunderstorms/local aob IFR.

Primarily VFR should prevail Tuesday-Wednesday.


A low level jet was able to partially mix down early this
morning across southern portions of the waters and resulted in a
few 20 kt gusts. However, this is expected to weaken, and have
thus held of on a SCA for now.

There will be several opportunities for thunderstorms this weekend,
each of which could carry locally strong wind gusts. It will be
tough to pinpoint timing with a boundary nearby, but morning
activity may wane a bit before the threat renews late today
into tonight, and again on Sunday. Finally, a cold front will
arrive Monday night.

In advance of the front, the gradient wind will increase from the
south. That will be the first chance at Small Craft Advisories.
Marine Warnings possible prior to that.

Northwest flow on Tuesday will likely bring small craft conditions
to the waters.  As low pressure moves through the region in the
Thursday-to-Friday timeframe, additional small craft advisories may
be required.


Water levels have retreated to near normal, with no complications
anticipated this weekend. That should be the case for much of the
week. By the end of the week though, a coastal low is likely to
cause elevated water levels once again.


The time periods to watch will be high temperatures today, and
warm overnight lows this weekend. Here are the record highs/warm
lows through the weekend:

Washington DC...
Saturday 29 April...91 (in 1974)/68 (in 1956)
Sunday 30 April...92 (in 1942)/67 (in 1983)

BWI Airport...
Saturday 29 April...91 (in 1974)/67 (in 1956)
Sunday 30 April...92 (in 1910)/63 (in 1983)

Dulles Airport..
Saturday 29 April...87 (in 1996)/62 (in 1996)
Sunday 30 April...86 (in 2007)/64 (in 1983)

Also of note...the warmest night time low at DCA in April is 69
degrees. That is being challenged as DCA only dropped to 70 this




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