Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200743
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
343 AM EDT Fri Oct 20 2017

.SYNOPSIS...
High pressure will remain across the Mid Atlantic through this
weekend. A low pressure system will affect the region early
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A very weak cold front currently pushing south across
Pennsylvania will cross the region early this morning. This
front is keeping the wind a bit more stirred up, so temps have
not dropped as cool as recent mornings, and there is a bit less
fog. The front should result in little if any cooling today,
with its main effects being a wind shift and a bit more
gustiness over the waters. Otherwise, it should remain mostly
clear with temperatures returning to the 70s in most spots.

Tonight, high pressure builds overhead once again, so we should
have a decent inversion and a bit more potential for patchy
fog. Otherwise, mostly clear and cool with lows mostly in the
40s.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
High pressure will slowly shift east off the coast Saturday and
remain there Sunday. Winds will gradually become southerly and
temps aloft will warm a bit, but otherwise, mostly sunny skies
and light winds should prevail, as we generally remain under the
light gradient close to the center of the high. High clouds
will likely increase Sunday, and by Sunday night, the next
system may bring enough moisture and lift to spark a shower in
the mountains late at night, but that`s about it though the
short term. Highs will be in the 70s, with Sunday probably just
slightly warmer than Saturday, and lows in the 40s Saturday
night, rising to the 50s Sunday night as the clouds and
southerly flow ahead of the next front keep it mild and better
mixed. Patchy fog will be a concern late at night and early in
the morning hours, probably more so Saturday night/Sunday
morning than Sunday night.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Main story of the long term period is a powerful upper
trough/front.

Phasing between northern and southern streams seems imminent
given amplification of flow upstream and blocking downstream.
The juxtaposition of the right-entrance region of the upper-jet,
strong mid-level vorticity advection and frontogenesis in the
low levels would suggest high probability for widespread and
potentially heavy precipitation. Given antecedent drier-than-
normal conditions and progressive (quick moving) nature of the
system, widespread flooding seems unlikely. The rain should
instead prove beneficial. Ensemble mean QPF is around 1-1.25",
mainly focused in 00z Tuesday through 12z Wednesday.

The tight gradient between deep troughing to the west and
ridging to the east is expected to result in a stribg southerly
low-level jet (marked by 50 knot 925 mb flow). This may result
in gusty winds reaching the surface, particularly with any
heavier showers or embedded convective elements. Timing and
strength differences are still evident amongst the latest
deterministic/ensemble guidance, but gusty winds and heavy
downpours seem most likely sometime in the Tuesday morning
through Tuesday evening timeframe.

Cooler temperatures and the potential for showers are expected
during the middle of next week in the wake of the surface front
as upper troughing stalls overhead. Some ensembles hint at
coastal low development late next week, but historically it
becomes difficult to see substantial impacts from such a system
once the surface front has already rolled through.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Patchy fog possible at MRB/CHO/IAD over the next few mornings,
but odds are low and should generally be MVFR at worst.
Otherwise, VFR through Sunday night all terminals with light
winds under high pressure.

Sub-VFR possible Mon AM (fog/low CIGS), then again late Mon
night with lower CIGS/developing showers ahead of an approaching
cold front. LLWS possible by Tuesday with strong flow (40-45
kts) around 2kft. Models differ on timing but agree on overall
setup. Widespread restrictions likely Tuesday with showers (some
heavy) expected. Gusty surface winds possible.

&&

.MARINE...
A weak passing cold front early this morning will stir the
winds up over the waters. Soundings show some sustained winds of
20-30 knots just above the surface, and while upstream areas
near shore will probably have a surface inversion, just off
shore, especially as one heads further downstream, that
inversion should be weaker if present at all given the warm
water temps, so expect some of that wind to mix down. As temps
warm and the winds just above the surface diminish during the
day Friday, gusts should drop below SCA criteria. After midday,
winds should generally stay below SCA criteria under high
pressure through Sunday night. There is a small risk of
southerly channeling Sunday night as the next front approaches,
but right now think odds are pretty low as low level jet stays
pretty weak.

SCA likely Monday as high pressure departs and southerly flow
develops. High end SCA seems likely Tuesday with strong
southerly LLJ. Gales possible as well, especially in heavier
convective elements ahead of an approaching cold front.

&&

.TIDES/COASTAL FLOODING...
Tidal anomalies will increase early next week as southerly flow
develops. Chances for minor tidal flooding increase as
southerly flow strengthens ahead of an approaching cold front
Monday night into Tuesday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until noon EDT today for ANZ531>534-537-
     539>541-543.

&&

$$
SYNOPSIS...RCM
NEAR TERM...RCM
SHORT TERM...RCM
LONG TERM...DFH
AVIATION...RCM/DFH
MARINE...RCM/DFH
TIDES/COASTAL FLOODING...DFH



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