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Area forecast discussion
National Weather Service Wakefield Virginia
729 am EDT Thu Jul 27 2017

Synopsis...
high pressure remains off the northeast coast today as a cold
front tracks into the Ohio Valley. Low pressure affects the
region Friday through Saturday...with a cold front pushing
across the region Friday night. Low pressure lingers near the
mid Atlantic coast Sunday through at least Monday night.

&&

Near term /until 6 PM this evening/...
sfc high pressure resides over the area early this morning as an
mesoscale convective system decays across srn Illinois/in and a secondary mesoscale convective system intensifies over
cntrl MO invof a cold front. Meanwhile, flat upper level flow is
allowing low-mid level clouds to stream across the region. The
clouds may scour out briefly around mid-late morning but should
fill back in during the afternoon. Weak Lee troughing develops
over the area this afternoon as the approaching cold front
tracks across the Midwest into the Ohio Valley. Shortwave energy
from decaying Illinois/in mesoscale convective system is expected to track east-southeast through today
and eventually interact with the Lee trough boundary...allowing
showers and thunderstorms to develop mainly around/after 800 PM.
Did slow arrival of precip into the Piedmont late this
afternoon/early evening based on GFS/sref model solutions best
reflecting current mesoscale convective system activity/precip coverage early this
morning, and it may need to be slowed down more depending how
conditions unfold today. Limited chance thunderstorms to no
higher than 30-40% in the Piedmont late this afternoon in case
terrain-based convection can survive as it moves off the
mountains. Overall, today will be dry with breezy southwest
winds and high temperatures in the upper 80s to possibly 90f in
spots (low-mid 80s beaches).

&&

Short term /6 PM this evening through Saturday night/...
shortwave energy/moisture interacting with the Lee trough will
bring showers/thunderstorms to the region this evening into the
overnight hours. Slow storm movement and pwats increasing to
1.50-1.75 inches will create the potential for moderate to heavy
rainfall. Main impacts with any storms tonight will be heavy
rainfall and locally strong wind gusts. The cold front continues
to slowly track east-southeast towards the Appalachians overnight. Low
pressure develops along this boundary on Friday as the cold
front pushes into far NW counties late aftn/early evening.
Widespread thunderstorms should be anticipated Friday night as
the front finally moves through the region, and especially as an
upper trough digs over the Ohio Valley as it intensifies. This
will enhance instability/lift and some storms could become
severe. Pwats increase to around 2.00 inches, and mbe vectors
in BUFKIT confirm slow storm motion. This will create the
potential for very heavy rainfall and localized minor flooding.
Therefore the primary impacts with thunderstorms Fri/Fri night
will be very heavy rainfall, locally strong to severe wind
gusts, and localized minor flooding. Models are just now keying
in on the upper trough becoming a more organized and
intensifying low, which brings caveats to the overall forecast.
If the trough digs deeper into the Ohio Valley, this could slow
the ewd progression of the cold front into the mid Atlantic
region, and/or it could focus heavier precipitation along nrn
periphery of sfc low/deformation axis farther south within the
forecast area instead of keeping the bulk of heavy rains north
of the area. Due to timing and placement inconsistencies/uncertainty
between model solutions, have opted to hold off on issuing a
Flood Watch but will continue to mention potential within severe weather potential statement.

The cold front is expected to stall over far southeast Virginia/NE NC on
Saturday before shifting southeast of the area late Saturday
night. Sfc low lingers near the mid Atlantic coast through
Saturday night with the upper trough sitting overhead. This will
keep showers/storms focused along the coast with possible
wraparound moisture moving into the area from the north on
Saturday. Precip chances expected to wane Saturday night as the
upper low shifts ewd. Once again, primary threats from any
thunderstorms will be moderate to heavy rainfall and locally
strong wind gusts.

Widespread precipitation, cloud cover, and cooler Canadian air
with the upper trough will keep temperatures 5-10 degrees below
normal Fri/Sat. Highs in the low-mid 80s Fri...lower 80s Sat.
Low temperatures gradually cool off with each passing night.
Warm and muggy Thu night with lows in the low-mid 70s. Lows Fri
night lower 70s (upper 60s possible far nw). Lows Sat night
lower 60s northwest to mid-upper 60s southeast.

&&

Long term /Sunday through Wednesday/...
models have a shortwave riding east-northeast along the boundary, but the
latest data suggests the high pressure over the mountains
suppresses the bulk of the moisture southeast of the Wakefield forecast area.
This low is progged to deepen out over the Gulf Stream by Tue,
which will bear watching this time of the year. Kept the
forecast dry through the period except for some slight chc pops
invof the Albemarle Sound Mon night and Tue where a limited
amount of moisture spreads north.

Temps below normal to start with readings returning to near
normal by mid week. Highs 80-85 sun, low-mid 80s Mon, mid-upr
80s Tue and upr 80s-lwr 90s Wed. Lows in the 60s Mon/Tue, mid
60s-lwr 70s Wed.

&&

Aviation /12z Thursday through Monday/...
a leeside trough develops over the region this afternoon as a
cold front crosses the Midwest into the Ohio Valley today.
Shortwave energy/moisture interacting with the Lee trough will
result in the development of thunderstorms primarily this
evening into the overnight hours. Main impacts will be heavy
rainfall, locally strong wind gusts, and brief periods of
ceiling/visible reductions to MVFR/IFR. With storms not expected to
impact taf sites until closer to 28/0000z, have kept thunder and
ceiling/visible reductions out of 06z taf issuance and will likely do
the same for the 12z issuance as well. The cold front slowly
crosses the mid Atlantic region on Friday with more widespread
thunderstorms anticipated. Very heavy rainfall, locally strong
wind gusts, and rapid ceiling/visible reductions to IFR (possibly lifr)
will be the primary impacts. Some storms could become severe
with gusts in excess of 50kt due to additional instability/lift
from an upper trough digging over the Ohio Valley as it
intensifies.

The cold front is expected to stall over far southeast Virginia/NE NC on
Saturday before shifting southeast of the area late Saturday
night. This will keep showers/storms focused along the coast
with possible wraparound moisture moving into the area from the
north. Once again, primary threats from any thunderstorms will
be moderate to heavy rainfall and locally strong wind gusts.
With rainfall intensity waning, any brief ceiling/visible reductions
will likely be MVFR.

Upper trough sits over the mid Atlantic region Sunday through
Monday...exiting the coast late Monday night/Tuesday morning.
Periods of clouds and showers/storms should be anticipated.

&&

Marine...
no headlines in the short term, winds are southerly and 10-15 kt
on avg early this morning with seas 2-3 ft and waves around 2 ft
in the Bay. S/SW flow around 10 kt through early this aftn will
back slightly in direction to S/southeast with speeds increasing to 15
kt later this aftn and tonight. Probably have a few hrs where
occasional gusts to 20 kt develop, but appears too marginal to
raise headlines for the Bay. Waves avg 2-3 ft in the Bay with
3-4 ft seas for coastal waters. Similar conditions Fri, except
winds will tend to be a few kt less.

A fairly strong cold front (by mid Summer standards) will affect
the region over the weekend and at some pt Small Craft Advisory headlines will
likely be needed. The timing remains rather uncertain however,
as the models are quite different with respect to the evolution
of sfc low pressure along the Delmarva and slowly translating
offshore sun/Mon. Overall, the models are a little slower at the
arrival of the stronger northwest winds Sat and the bulk of the Small Craft Advisory
conditions could hold off until late Sat night or Sunday as
winds shift to the north. Latest European model (ecmwf)/Gem would keep strong winds
of 20-25 kt going through Mon. Have blended the solutions for
now, but went higher than superblend since it has too much
averaging and never really brings winds much higher than 15 kt
at any given time. Have seas building to 5-6 ft and Bay waves to
3-4 ft Sat night through Sun night.

&&

Tides/coastal flooding...
water levels in the Upper Bay (mainly eastern side adjacent to
the Maryland eastern shore) approached minor flooding thresholds
early this morning and will continue the coastal flood
statement mainly for Cambridge through 10 am. An additional
statement may be needed for tonight.

&&

Akq watches/warnings/advisories...
Maryland...none.
NC...none.
Virginia...none.
Marine...none.

&&

$$
Synopsis...bmd
near term...bmd
short term...bmd

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