Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 klwx 220141 

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
941 PM EDT Sat Jul 21 2018

an area of low pressure will move north across the Delmarva
Peninsula. Meanwhile, a broad and slow moving low pressure
system will drop from the upper Midwest into the southern
Appalachians by the end of the weekend. This low will continue
to influence the mid Atlantic during the early part of the
coming week.


Near term /until 6 am Sunday morning/...
low pressure will continue moving northward through the
Delmarva this evening and tonight, reaching northeastern PA by
Sunday morning. Widespread heavy rain has occurred and will
continue to occur over the next several hours, before rain moves
northward and out of the region after midnight. Widespread
amounts of 2-5" have already fallen along and east of a line
from Spotsylvania-Fauquier-Jefferson counties eastward, with
localized amounts of 5-7" in Fairfax, Prince William, Stafford,
Spotsylvania, and Charles counties. This has led to numerous
reports of flooding and flash flooding. Please refer to warning
products for additional information. As the low moves northward,
the axis of additional heavy rain will shift northward into
central and northern Maryland and will need to monitor for
additional flooding concerns. Flash Flood Watch will remain in
effect until 2 am.

After main area of rain exits, off and on light rain or drizzle
could continue in convergent surface flow. Light fog and low
clouds will also be likely much of the night.


Short term /6 am Sunday morning through Monday night/...
for Sunday and Monday, upper low will be drifting south to our
west. This mean copious moisture will continue to be drawn into
the region with nearly constant chances of showers and
thunderstorms. While there will be some diurnal component,
especially to convective elements, forcing and moisture will
support chances of rain through the nights as well. Details
regarding timing and locations of heaviest rain will be ironed
out as finer scale features are identified. Unidirectional flow
and/or southeast low level flow into the terrain could enhance
the threat for locally heavy rain at times. High temperatures
will be below normal in the 80s, while lows remain in the 60s
to lower 70s due to the high dew points.


Long term /Tuesday through Saturday/...
the upper low will remain anchored to our southwest through
Thursday, thanks to a blocking upper-level ridge over the northern
Atlantic. Multiple fluxes of moisture from the southeast and south,
along with the meandering trough of low pressure will induce several
rounds of showers and thunderstorms through Thursday night.
Expecting the shower and thunderstorm activity to peak during the
warmest part of the afternoon each day. Although we are currently
not anticipating widespread flooding, precipitable water values approaching and
exceeding 2 inches will provide plenty of moisture for heavy
rainfall with any showers or storms. This could potentially
lead to some flooding concerns over localized areas through this
period, if heavy rain sits over one area long enough. With
constant cloud cover throughout this period, expect high
temperatures in the mid 80s, with lows only in the low to mid

The upper level ridge over the northern Atlantic should begin to
break down and move east late Thursday into Friday. This will allow
for that stubborn upper-level trough to move out of the area.
However, the chance of showers and a few thunderstorms still linger
Friday and Friday night because of the added lift of a front. High
temperatures will be closer to normal as we could see a southwest
wind develop along a weak boundary as it moves east.


Aviation /01z Sunday through Thursday/...
low pressure is moving up the coast with widespread rainfall
(except cho, which may largely miss out). Conditions have
largely deteriorated into IFR and will persist at least into the
early morning hours. Some improvement is likely after the rain
exits. Some low level wind shear is possible as a low level jet moves overhead
through this evening, but surface winds 10-15 kt may limit the

Conditions improving somewhat Sunday into Tuesday, but risk of
showers/T-storms will remain into Tue. Otherwise, expect broken
cigs that are VFR during the day but could drop at night. Patchy
fog also can't be ruled out due to the moist atmosphere.

Showers and thunderstorms each day Tuesday through Thursday. The
warmest time in the afternoon into the early evening hours would be
the more prominent time to encounter these showers and
thunderstorms. Occasionally MVFR or perhaps IFR could develop with
the showers and thunderstorms through the period. Winds generally
southeast to south at 5 to 10 knots with higher gusts in
thunderstorms Tuesday through Wednesday night. Winds could become
southwesterly at 5 to 10 knots Thursday ahead of the trough.


low pressure is advancing toward the waters from the south, with
Small Craft Advisory conditions common and gusts 25-30 kt. Gale Warning remains
in place for the Bay and lower tidal Potomac into early Sunday

Small Craft Advisory remains in effect for Sunday afternoon in the mid Bay and
Sunday night north through Pooles Island due to increasing
southerly flow. This regime will continue through Monday.

Small craft advisories likely Tuesday. Small craft advisories
possible Tuesday night and Wednesday. Winds southeast to south at 10
to 15 knots gusts 20 to 25 knots Tuesday. Winds southerly around 10
knots gusts to 15 to perhaps 20 knots Tuesday night and Wednesday.


widespread rain amounts of 2-5" with localized 5-7" amounts are
expected into this evening. This magnitude of rain coupled with
rates of up to 2" in one hour have resulted in and will continue
to result in numerous flooding incidents with localized flash
flooding. The main areas of concern have been from Fairfax
County southward to Spotsylvania County, but as rain shield
shifts northward, this will spread into central Maryland.
Additional flooding concerns are possible into this week, but
uncertainty remains high.


Tides/coastal flooding...
minor coastal flooding forecast at most sites into early Sunday
morning, with the exception of have de Grace. Moderate coastal
flooding is ongoing at Straits Point this evening.

There may be a momentary decrease in water levels early Sunday,
but they will rise again later Sunday into Monday with southerly
flow in place.


Lwx watches/warnings/advisories...
District of Columbia...Flash Flood Watch until 2 am EDT Sunday for dcz001.
Coastal flood advisory until 8 am EDT Sunday for dcz001.
Maryland...Flash Flood Watch until 2 am EDT Sunday for mdz003>006-011-013-
Coastal flood advisory until 2 am EDT Sunday for mdz016-018.
Coastal Flood Warning until 2 am EDT Sunday for mdz017.
Coastal flood advisory until 5 am EDT Sunday for mdz014.
Coastal flood advisory until 6 am EDT Sunday for mdz011.
Virginia...Flash Flood Watch until 2 am EDT Sunday for vaz052>055-057-505-
Coastal flood advisory until 2 am EDT Sunday for vaz057.
Coastal flood advisory from 1 am to 6 am EDT Sunday for vaz054.
WV...Flash Flood Watch until 2 am EDT Sunday for wvz053.
Marine...Small Craft Advisory from 6 PM Sunday to 6 am EDT Monday for
Gale Warning until 4 am EDT Sunday for anz530>534-537-539>543.
Small Craft Advisory until 2 am EDT Sunday for anz535-536-538.
Small Craft Advisory from 2 PM Sunday to 6 am EDT Monday for


long term...cjl

National Weather Service Glossary of Abbreviations