Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 290128 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
928 PM EDT Sat Mar 28 2020

A front will stall out near the Appalachians into southern
Virginia through tonight. The front will then try to lift north
as a warm front early Sunday, before a cold front moves through
from the west late Sunday. High pressure will be in control
Monday into Tuesday. A low pressure will then approach from the
southeast U.S. Tuesday night before departing off the mid-
Atlantic coast on Wednesday.



Biggest challenge for tonight is going to be how low cigs get
and how low visiblities drop. Short-term guidance is all over
the place with MOS guidance suggesting vsbys remain in the
2-4sm range while GLAMP and other hi-res models indicating
visibilities dropping to less than half a mile. Not expecting
any measurable precip other than some light drizzle.



Short-term models like the HRRR indicate isold supercell
potential possible especially along and north of Interstates 68
and 70. Even if warm front fails to lift into PA, there appears
to be sufficient elevated instability that supercells could
still produce large to very large hail. Convective initation
remains questionable especially in southern areas where there is
lack of frontal convergence. Isolated to scattered
thunderstorms are still expected in the northern half of the
fcst area.

Previous afd...

Sunday will start with ample low clouds and fog attempting to
lift. How quickly it does so may be the key to the forecast.
Believe there will be a sharp inversion, with a warm nose which
could be as warm as 19C near 925 mb.

A cold front will approach during the day, and cross the region
during the afternoon, during peak heating (18-00 UTC). There
will be ample shear across the region. Instability will be the
question. If the wedge holds, then instability will be minimal
and the front would either come through dry or as a few showers
(with perhaps an elevated rumble of thunder). But, if the wedge
does manage to mix out, then MUCAPE may exceed 1000 j/kg. Given
the other ingredients in place, that would be supportive of
robust thunderstorms. There are guidance members on each side of
the fence.

After collaboration with SPC, decided that the conditional
threat great enough to at least mention somehow...hence
Marginal Risk of severe. Am maintaining a chance of precip
crossing the area in the database, with a slight chance of
thunder. It may be that both solutions are correct; ie:
cool/stable air across northern Maryland while moist/unstable
air present across central Virginia and southern Maryland. Will
be monitoring later cycles to discern these details.

Sunday night into Monday a deep cyclone will build across the
Great Lakes. The area will experience high pressure but westerly
flow and cold advection. Lows will be in the 40s again by
Monday night.


An upper level trough over the southern Plains will drop
southward through the southern parts of the mid-Atlantic and the
Carolinas Tuesday into Wednesday. A surface low associated with
this trough will move to our south Tuesday afternoon and into
Wednesday morning. The main area of precipitation should be
situated to the south of our forecast area over the Carolinas
and the southern portions of Virginia. As the main forcing
remains to our south, the precipitation that we observe in our
region will likely be caused by a combination of overrunning
precipitation and an easterly flow off of the ocean. If this
system tracks further northward, our region has the potential to
experience heavier precipitation mainly late Tuesday and into
Wednesday morning. Precipitation and an easterly flow will lead
to daytime temps running near to slightly below normal for this
time of the year in the 50s and lower 60s with overnight lows in
the 40s

High pressure builds into our region behind the exiting low
Wednesday afternoon and remains over our area through Thursday.
Light winds out of north will lead to cooler conditions over our
area. Temperatures will continue to hover in the 50s with overnight
lows in the 40s on Thursday.

Another cold front is forecast to approach and move through our
region Friday and into early Saturday. Models have some decent
agreement on the subtropical jet moving northward and phasing with
weak jet over the midwest and mid-Atlantic. This phasing will
combine with a frontal passage to bring precipitation to our region
once again. The main limiting factor will be a westerly flow
transporting drier air into our area. We will need to monitor the
strength of the jets to determine further hazards for end of next


Flight conditions straddling between MVFR and IFR, but across
BWI/MTN the marine flow spreading inland has resulted in
conditions on the verge of LIFR. Low clouds and fog will
continue to spread inland, with LIFR widespread overnight.

Conditions will slowly improve Sunday. How quickly this
transpires still in doubt. A cold front will be crossing the
terminals this afternoon. If the clearing comes by midday, then
strong thunderstorms possible along with the frontal passage.
Otherwise it may be a dry front.

VFR will return Sunday night, continuing through Monday.

SubVFR conditions are possible late Tuesday and into Wednesday due
to rain and an easterly flow. VFR conditions should return Wednesday
afternoon and continue through Thursday as high pressure builds over
our region.


Light northeast winds (at or below 10 kt) will continue across
the waters through tonight. Low clouds and fog will be
thickening through the night. Winds will likely veer south
tomorrow as the marine wedge lifts. Its unclear how quickly this
will happen, but any clearing would be followed by a threat of
thunderstorms with gusty winds late Sunday afternoon.

Conditions will improve Sunday night, but winds shouldn`t be
high enough to need Small Craft Advisories. No Small Craft
issues expected on Tuesday as well, but Small Craft Advisories
may be needed on Wednesday due to a northerly flow.




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