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Area Forecast Discussion (AFD)

Issued by Baltimore/Washington (LWX)

FXUS61 KLWX 201415 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
1015 AM EDT Fri Oct 20 2017

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



Tranquil weather under high pressure sfc and aloft. MaxT will be
in the mid 70s under light NW winds.

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 


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.


Main story of the long term period is a powerful upper 
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.


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.


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.


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.